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ORIGIN AND EVOLUTION OF THE NICARAGUAN SILICIC

ASH-F LOW SHEETS

presented by

ELA LITA ESTRADA VIRAY

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ORIGIN AND EVOLUTION OF THE
NICARAGUAN SILICIC ASH-FLOW SHEETS

By

Ela Lita Estrada Viray

A THESIS

Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of

MASTERS OF SCIENCE

Department of Geological Sciences

2003

ABSTRACT
ORIGIN AND EVOLUTION OF THE NICARAGUAN SILICIC ASH-FLOW SHEET
By
Ela Lita Estrada Viray

Abundant silicic ash-ﬂow sheets occur in Nicaragua yet little is known about their
geochemistry, volume, and age. Their origin is a matter of considerable interest because
the generation of silicic magma is generally attributed to the assimilation and/or melting
of an underlying continental crust. However, no continental crust occurs in this area. Two
models on the generation of these ignimbrites are evaluated: (1) fractional crystallization
of basalt or andesitic melt, and (2) partial melting of previously emplaced arc-related
igneous rocks. Chemical and mineralogical variations within and among the seven
recognized silicic ash-ﬂow sheets cannot be explained by ﬁactional crystallization alone
but rather by partial melting of previously emplaced arc-related igneous rock. This arc-
related igneous rock was partially melted by the injection of new maﬁc magma from the
metasomatized mantle wedge. Continued production and ascent of both the silicic and
maﬁc magmas into the magma chamber allowed mingling between the two magmas, and
before even reaching compositional and thermal equilibrium, were erupted as ash-ﬂow
sheets with distinct chemical variations. REE patterns of the silicic ash-ﬂow sheets mimic

the REE trends of the modern arc lavas in Nicaragua indicating a genetic relationship.

ACKNOWLEDGEMENT

I would like to express my gratitude to all those who gave me the possibility to
complete this thesis and my Master’s degree at Michigan State University. There is not
enough space on this page to properly thank even one of the following people for their
enormous help, understanding, and support. I can only hope each of you will realize how
important you were, and you are, to me...

I am deeply indebted to my advisor, Dr. Thomas A. Vogel, for giving me the
chance to be his student, and whose guidance and encouragement helped me in all the
time of research for and writing of this thesis. I would like to thank my co-advisor, Dr.
Lina C. Patino, who kept an eye on the progress of my work and was always available
when I needed her advises regarding my project and my stay at MSU. I would also like
to thank, Dr. Bill Cambray, who did not hesitate to be a part of my committee and for
providing me with valuable comments on this thesis.

I am grateﬁil to all of my colleagues in the Department of Geological Sciences. I
am sincerely thankful to Karen Tefend for being my “big sister” and for all the support.
Many thanks to Melissa Wilmot, Dave Szymanski, the petrology group, and my fellow
graduate students for giving me the feeling of being at home away from home.

Special thanks are due to my former professors who have encouraged me to go to
graduate school: Dr. Caloy Arcilla, Dr. Ed Listanco, and Dr. Kelvin Rodolfo.

I have to thank my roommate Catherine who had been extremely tolerant and
understanding of whatever I’m doing or up to. My utmost thanks to my friends Dan, Pam,
Monina, Winchelle, Ma-an, Chris, Amy, Kate, and Eric for always being there for me
when I needed a ﬁiend to talk to.

Last but certainly not the least, I am forever grateful to the love and care of my

family, wala ako sa Ia'nalalagyan k0 ngayon kung wala sila.

iii

TABLE OF CONTENTS

List of Tables .................................................................................... v

List of Figures ................................................................................... vi
Introduction ..................................................................................... 1
Regional geology and extent of silicic volcanism in Nicaragua ........................ 3
The silicic ash-flow sheets in Nicaragua ................................................... 7
Sampling methods ...................................................................... 7
Sample preparation ..................................................................... 7
Petrography and mineralogy ........................................................... 9

Geochemistry ........................................................................... 11

Whole Rock Geochemistry ................................................... 11

Major element variations ............................................ 11

Trace element variations ............................................. 12

Mineral Chemistry ............................................................. 15

The generation of the silicic ash-ﬂow sheets in Nicaragua ............................. 18

Fractional crystallization ............................................................... 18

Partial melting off previously emplaced arc-related igneous rock ............... 22

Magma mingling ........................................................................ 25

Model for the generation of the Nicaraguan sicilic ash-ﬂow sheets ............. 28

Comparison with Central American Volcanic Arc ................................. 30

Conclusion ....................................................................................... 31

Appendices ....................................................................................... 32

Appendix A: Tables .................................................................... 32

Appendix B: Figures ................................................................... 61

References ....................................................................................... 93

iv

LIST OF TABLES

Table 1. Sample locations in Nicaragua

Table 2. Whole rock major and trace element concentrations for pumice fragments in
Nicaragua. Oxides listed in wt%, trace elements in ppm (parts per million). bd:
below detection limit; XRF : trace element analyzed by XRF.

Table 3. Major element concentrations for matrix glass, melt inclusion, maﬁc enclaves,
plagioclase, and pyroxene ﬁ'om select pumice fragments using electron

microprobe analyses.

Table 4. Four possible cases during stage 3 in Figure 27 (see Figure 28 for cartoon)

LIST OF FIGURES

Figure 1. Map of study area: Circles represent sampling location of the 7 different ash-
ﬂow sheets; red triangles represent Quaternary volcanoes along the Central
American Volcanic arc; numbers in mm/yr represent subduction rate of Cocos
Plate under Nicaragua. Map modiﬁed from Roger et al. (2002).

Figure 2. Sampling location of the seven different ash-ﬂow sheets in red circles.
APOYO: Apoyo, MONTE-: Monte Galan, LAS-SI: Las Sierras, SAN-M: San
Rafael, OSTOCA, Ostocal, COYOL: Coyol, LAS-MA: Las Maderas.

Figure 3. (A) Active quarry site for ignimbrite (Ostocal unit). (B) Typical pumice
fragments of various sizes embedded in ﬁne-ash matrix.

Figure 4. Photomicrograph of typical glomeroporphyritic texture in the pumice
fragments. (A) Under plane polarized light and (B) under crossed polars. (Apoyo
unit, sample # 020617-10).

Figure 5. (A) Typical oscillatory zoning in plagioclase crystal (Apoyo unit, 020617-2c).
(B) Typical sieve/resorved texture in plagioclase suggesting chemical dissolution
(Las Sierras unit; 020620-6b).

Figure 6. (A) Corroded plagioclase cores observed in Las Maderas unit (020622-5). (B)
Melt inclusions found in plagioclase phenocryst San Rafael unit (020619-1t).

Figure 7. Phenocrysts of clinopyroxene and orthopyroxene (Apoyo unit, 020617-2c).
Distinction between these 2 pyroxenes is difﬁcult from petrography alone.

Figure 8. Banded pumice fragment observed under the microscope (Apoyo unit, 020618-
5a).

Figure 9. Maﬁc enclave within a silicic pumice fragment (Las Maderas 020622-5). (A)
under plane polarized light and (B) under crossed polars. The phenocrysts content
of the enclave are the same as the host pumice but are smaller in size.

Figure 10. Intrusive-like feature of Fe-Ti oxide within an enclave into the silicic glass
host (Ostocal unit, 0206 24-2c).

Figure 11. Dove-tail texture observed in plagioclase crystals within the enclave
indicating quenched crystallization (Ostocal unit, 020624-2c).

Figure 12. TAS classiﬁcation diagram of LeBas et al. (19866) of pumice fragment from
the seven recognized ash-ﬂow sheets in southern Nicaragua: ApoyoO , Monte
Galan 31 , Las SierrasA, San Rafael * , Ostocal 0 , Coyol =33 , Las Maderas [J .
All samples plottled have been normalized to 100% volatile free.

vi

Figure 13. Major element variation against Si02 (wt. %) among ash-ﬂow units in
Nicaragua. ApoyoO , Monte Galan x , Las SierrasA , San Rafael 'A' , Ostocal O ,
Coyol {h , Las Maderas El . Filled area represents trend for low Ti02 Quaternary
Nicaraguan lavas.

Figure 14. Si02 histogram of the seven silicic ash-ﬂow sheets. Mean and standard
deviation are for the high silica pumice fragments only.

Figure 15. Trace element concentration of the silicic ash-ﬂow sheets versus Si02
content. ApoyoO , Monte Galan 3t , Las SierrasA , San Rafael * , Ostocal o ,
Coyolcca , Las Maderastj . Filled area same as Figure 13. No Ta data for
Nicaraguan lavas.

Figure 16. Cumulative frequency curves (CFC) of trace element ratios of the seven ash-
ﬂow units. CFC’s of most of the ash-ﬂow sheets show little variation in the trace
element ratios, but are distinct from each other. Others show distinct breaks, e. g.
Ostocal, Las Sierras (HF/T h). ApoyoO , Monte Galaer , Las SierrasA , San
Rafael * , Ostocal O , Coyol {'9 , Las MaderasCl .

Figure 17 . Trace element ratios plotted against distance along the Central American
Volcanic Arc (CAVA) (starts in Mexico-Guatemala border, 01cm and ends in
central Costa Rica, ~1100 km). Red line represent all CAVA lavas and ﬁlled area
represent Nicaraguan lavas. ApoyoO , Monte Galan at , Las Sierras A , San
Rafael ﬁr , Ostocal 0 , Coyol :3: , Las MaderasE] .

Figure 18. REE trend for the seven ash-ﬂow units are similar and deﬁne a slight MREE
depletion pattern (Dy and neighboring elements; normalization values from Sun
& McDonough, 1989). Filled area same as Figure 13.

Figure 19. Dy/Lu <1 in some of the silicic ash-ﬂow sheets suggest amphibole in the
source melt. ApoyoO, Monte Galanx , Las SierrasA , San Rafael* , Ostocalo,
Coyollﬂa , Las MaderasE] .

Figure 20. Calculated Eu anomaly for the seven silicic ash-ﬂow sheets versus Si02

contents. ApoyoO, Monte Galan x , Las SierrasA , San Rafael ﬁr , Ostocal 0 ,
Coyol r3: , Las Maderas U .

Figure 21. Primitive mantle-normalized trace element pattern of the seven ash-ﬂow
sheets showing distinct island-arc signature (normalization values from Sun &
McDonough, 1989). Filled area same as Figure 13.

vii

Figure 22. Relative ages of the seven silcic ash-ﬂow sheets in Nicaragua compared to
U/Th ratio. Older units Las Maderas and Coyol have low U/Th ratio similar to
Plank et al.’s (2003) observation in the Miocene Nicaraguan lavas. The youngest
unit, Apoyo, has high U/T h ratio similar to the Quaternary Nicaraguan lavas (Data
from http://www.rci.rutgers.edu/~carr/cageochem080800.txt). ApoyoO , Monte
Galan 2:, Las Sierras A , San Rafael * , Ostocal O , Coyol Ii} , Las Maderas E],
Nicaraguan Quaternary lavas «- , Nicaraguan Miocene lavas+ .

Figure 23. Plagioclase composition for all analyses with respect to distance ﬁ'om the core
for each crystal in the pumice fragments of the seven ash-ﬂow sheets. Each
symbol represent one plagioclase crystal analyzed within the given ash-ﬂow unit.

Figure 24. Anorthite content of plagioclase rims versus Si02 content of whole pumice

fragment. ApoyoO , Monte Galan 31 , Las Sierras A , San Rafael ﬁ' , Ostocal 0 ,
Coyolcﬂr , Las Maderas [j .

Figure 25. Analyzed pyroxenes of the silicic ash-ﬂow sheets have a wide range of Mg#.
ApoyoO, Monte Galanﬂ , Las SierrasA , San Rafael ‘k , Ostocal O , Coyol =3= ,
Las Maderas [j .

Figure 26. Calculated temperatures of Las Sierras, Las Maderas, Ostocal and Apoyo
units using the 2-pyroxene program QUILF version 6 (Andersen et al., 1993).
Other units do not have opx or were not analyzed. Apoyo O , Las SierrasA ,
Ostocal O , Las Maderas El .

Figure 27. Whole pumice fragment and matrix glass (encircled areas) composition of the
ash-ﬂow sheets showing distinct large compositional gap. Gray area represents
maﬁc matrix glass, stippled area represents silicic matrix glass, vertical pattern
represents maﬁc enclaves and horizontal pattern represents melt inclusion in
plagioclase.

Figure 28. MgO vs Si02 of the whole pumice fragment and matrix glass (encircled
areas) of the ash-ﬂow sheets. Symbols same as Figure 25.

Figure 29. Model for the genesis of the silicic ash-ﬂow sheets in Nicaragua. Cross-
section is not to scale. See text for explanation.

Figure 30. Cartoon depicting 4 possible scenarios during stage 3 in Figure 27. See text
for explanation.

Figure 31. DyN/LuN vs LaN/LuN for Nicaraguan ash-ﬂow sheets in comparison with
some Costa Rican ash-ﬂow sheets (Si02 >65 wt%). Costa Rican data from
Hannah et al. (2000) and Szymanski et al. (2002).

viii

Introduction

Abundant silicic ash-ﬂow sheets occur in Nicaragua yet little is known about their
geochemistry, volume, and age. Studies of volcanism in this area have been concerned
primarily with the basaltic and basaltic andesite lavas, and these voluminous silicic
ignimbrites have been neglected. Their origin is a matter of considerable interest because
generally, the generation of silicic magma is attributed to the assimilation and/or melting
of an underlying continental crust. However, no continental crust occurs in this part of
Nicaragua.

There are two widely accepted but contrasting models for the origin of silicic
magmas in arcs that have been proposed by several workers. The ﬁrst model is fractional
crystallization of basaltic or andesitic melts (Sisson and Grove, 1993; F eely and
Davidson, 1994; Grove et al., 1997; Brophy et al., 1999; Hildreth and F ierstein, 2000).
This model requires a high percentage of fractional crystallization (as high as 68%) to
generate high-silica magma, and in the process would produce cumulate rocks. Ifthis
model were to apply to the silicic magmas in Nicaragua, the cumulate rocks would have
been produced at depth and have not yet been exposed at the surface since no cumulate
rocks are observed in the region. Alternatively, the second model is partial melting of
previously emplaced arc-related igneous rocks to explain the origin of silicic magmas in
an evolved arc setting (e. g. Beard and Lofgren, 1991; Roberts and Clemens, 1993;
Tamura and Tatsumi, 2002; Smith et al., 2003). Dehydration melting experiments on
basaltic rocks and their metamorphosed equivalents, amphibolite and eclogite, at various
pressures yield partial melts with composition similar to island-arc tonalities and dacites

(Beard and Lofgren, 1991; Rapp et al., 1991; Wolf and Wyllie, 1994; Sen and Dunn,

1994; Rapp and Watson, 1995). Tamura and Tatsurni (2002) considered a calc-alkaline
andesitic magma to be a source. Inﬂuxes of hot basaltic magmas from the mantle wedge
would reheat, partially melt and remobilize the still hot and ponded andesitic magma to
produce silicic rocks (Tarnura and Tatsunri, 2002).

The silicic ash-ﬂow sheets in Nicaragua, like any other ignimbrites, each
represent an instantaneous partial evacuation of the magma chamber. Any chemical
heterogeneity in the original magma bodies would then be preserved in the ash-ﬂow
sheets and therefore, their evolution can be inferred. Two major processes have been
attributed as to why chemical heterogeneities occur in silicic ash-ﬂow sheets: (1) involves
large-scale differentiation of an originally homogeneous magma body (i.e. Mittlefehltd
and Miller, 1983; Baker and McBirney, 1985; McBirney and Nielson, 1986; de Silva and
Wolff, 1995); and (2) effects of discrete magma batches emplaced in a magma chamber
that were generated by partial melting and melt extraction with distinct compositional
groups unrelated to crystal fractionation (i.e. Marsh, 1984; Bergantz 1989; Sawyer,
1994; Eichelberger and Izbekov, 2000; Eichelberger et al., 2000). However, these
discrete magma batches may subsequently be modiﬁed by magma mixing, magma
mingling, assimilation, and/or crystal fractionation as they ascend to a high-level
chamber and/or to the conduit, and possibly even during eruption. Furthermore,
Eichelberger et al. (2000) pointed out that the heterogeneity in erupted materials may be a
result of chamber recharge either by: (1) maﬁc magma intruding a more silicic magma,
characterized by effusive eruption (i.e. Pinatubo, Karymsky, Ruapehu, New Zealand;

Pallister et al., 1992; Nakagawa et al., 1999; Eichelberger and Izbekov, 2000); and (2)

silicic magma intrudes a maﬁc magma body, commonly associated with pyroclastic
eruption (i.e. Katmai, Alaska; Eichelberger and Izbekov, 2000).

The purpose of this study is to: (1) document the chemical and mineralogical
variations among and within the silicic ash-ﬂow sheets in Nicaragua; and (2) to evaluate
these data with respect to the models proposed above for their origin and evolution.
Major and trace element whole-rock analyses, petrography, mineral chemistry and
analyses of matrix glass, enclaves, and melt inclusions in plagioclases are used to

evaluate the models for the genesis of these silicic magmas.

Regional geology and extent of silicic volcanism in Nicaragua

Nicaragua is located along the western margin of the Caribbean plate in Central
America (Figure 1). The Caribbean plate is composed of various blocks and microplates
and Central America is divided into two main tectonic regions, the Chortis and the
Chorotega blocks (Dengo, 1969; Escalante, 1990). The Chortis block has a pre-Mesozoic
continental-type crust and encompasses southern Guatemala, Honduras, Northern
Nicaragua and the western Nicaraguan Highlands (e. g. Dengo, 1969; Pindell and Barrett,
1990). This block serves as the basement for the northern part of the Central American
Volcanic Arc (CAVA). The Chorotega Block on the other hand, is underlain by a
thickened basaltic crust, which many workers believe is part of over-thickened oceanic
plateau on the western edge of the Caribbean Plate emplaced during the Late Cretaceous-
Paleocene and constitute most of the southern CAVA (Pindell and Barrett, 1990; Di
Marco et al., 1995; Kerr et al., 1997, Meschede and Frisch, 1998). It was not until the

Eocene-Oligocene that the CAVA began to form in the western part of the Caribbean

plate as a result of the amalgamation of the Chortis and the Chorotega blocks (Cigolini
and Chaves, 1986; Meschede and Frisch, 1998).

The boundary between the Chortis and the Chorotega Blocks is still a matter of
controversy. South of Nicaragua, the Santa Elena-Hess Escarpment lineament is believed
to mark the boundary between the two tectonic terranes (Bourgois et al., 1984). Hauff et
al. (2000) however suggested that the structural boundary may lie south of the
escarpment based on the remarkable similarities on trace elements and isotopic contents
of the alkaline and picritic rocks ﬁom Tortugal and the alkali basalts from Santa Elena,
both located in northern Costa Rica. Moreover, ophiolitic rocks studied in detail by
Weinberg (1992), which are commonly observed offshore of Panama and Costa Rica but
are not found in onshore outcrops, indicate that the suture between the two terranes is
located in central Costa Rica. On the contrary, major and trace element data ﬁ'om front
arc Tertiary-Recent volcanic rocks in Nicaragua, along with Sr, Nd, and Pb isotope data
suggest that the boundary passes through Blueﬁelds in the Caribbean side of Nicaragua
and through the Nicaraguan Depression on a NW-SE trending direction (see Figure l in
Nystrom et al., 1993). Recent geophysical studies using modeling of seismic wide-angle
measurements across the Paciﬁc convergent margin of Nicaragua, gravity, earthquake
and borehole data, as well as coincident seismic reflection proﬁle, also argue that the
suture between these two tectonic blocks is located at the northeastern border of the SE-
NW-trending Nicaraguan Depression (Walther et al., 2000). Their argument was based
on their reconstructed tectonic evolution of the Nicaraguan margin. They suggested that
the collision of an oceanic plateau (i.e. Galapagos hotspot plume or Mid-Cretaceous

superplume) in the Upper Cretaceous time left the former trench and margin area in deep

water. This is followed by the subsequent trench jumping by about 70km to the SW,
which left a portion of ophiolitic crust and upper oceanic mantle underneath the
Nicaraguan Depression (Figure 13 in Walther et a1, 2000). If this is the case, then most of
the observed silicic volcanism in Nicaragua is associated with the oceanic Chorotega
block.

The Tertiary-Recent volcanism in Nicaragua is attributed to the present day
subduction of the Cocos Plate at 91 mm yr'l (DeMets et al., 1990) in a northeasterly
direction beneath the Caribbean Plate. Seismic data deﬁne a steeply dipping Benioff zone
in Nicaragua with a dip increasing from 25° in the seismogenic zone to 84° between 100
and 220 km depth (Burbach et al., 1984; Wilson, 1996). The associated chain of
volcanoes is located in the western edge of the Nicaraguan Depression, which is a 50-km
wide extensional structure formed in the Plio-Pleistocene and has been deﬁned as a half-
graben parallel to the trench that hosts Lake Nicaragua and Lake Managua (McBirney
and Williams, 1965; Weinberg, 1992; Elming et al., 2001). There are 21 Quaternary
volcanic centers in Nicaragua, which are closely spaced with an average of 25 km
between central craters (Van Wyk de Vries, 1993). As compared with the other eruptive
centers in CAVA, the volcano summits in Nicaragua are relatively lower with smaller
volumes of erupted materials (Van Wyk de Vries, 1993). The volcanic deposits in
Nicaragua are chemically bimodal, dominated by basaltic to basaltic andesite ﬂows, and
dacitic to rhyolitic ash-ﬂow sheets and air-fall deposits (Ehrenborg, 1996). Slab
signature is strongest in the Nicaraguan lavas as compared with the other CAVA lavas.
This signature is manifested by the high Ba/Th and Ba/La ratios as well.” the abundance

of 10Be (Carr et al., 1990; Patino et al., 2000; Morris et al., 1990). An interesting

characteristic of the Nicaraguan lavas is the high U/Th ratio of the younger ﬂow units
(<7Ma) relative to the older lavas and to the rest of the CAVA lavas (Plank et al., 2002).
This has been interpreted as a result of the closure of the Panama gateway, which led to
the “carbonate crash” at 10 Ma followed by high organic carbon burial in the sediments
(Lyle et al., 1995; Plank et al., 2002).

The silicic ash-ﬂow sheets for this study cover an area of approximately 6,400
km2 (Figure 1 and 2). The oldest pyroclastic ﬂow unit is represented by the Coyol
Group, which is believed to have originated from El Limon caldera, a >3 0km diameter
structure formed between 12 and 16 Ma (Ehrenborg, 1996). K-Ar dates of the Coyol
Group ranged from 4.3 to 24.7 Ma (Ehrenborg, 1996), whereas deep sea tephra
stratigraphy combined with Ar-Ar dating obtained an age of 12.3 to 18.4 Ma (Sigurdsson
et al., 2000). The youngest ignimbrite is dated at 23,000 yrs BP by radiocarbon dating
and found along the periphery of the Apoyo caldera (Sussman, 1985). Sussman (1985)
estimated that the volume of the last erupted materials from the Apoyo caldera is 10.7
km3 dense-rock equivalent (DRE). The stratigraphy of the Tertiary volcanic rocks in
Nicaragua is described by Ehrenborg (1996). He noted three major volcanic events: (1)
rhyolitic shield volcanism in the Oligocene, which produced the Highland ignimbrites;
(2) extrusion of basaltic to andesitic magmas along the Paciﬁc coast and the construction
of the Coyol volcanic arc during the Miocene; and (3) creation of the modern volcanic

arc during the Pliocene-Pleistocene after the southwestward shift of arc volcanism.

The silicic ash-ﬂow sheets in Nicaragua
Formatting note

The images in this thesis are presented in color.
Sampling Methods

More than 300 whole pumice fragments and air fall deposits were collected
during 2 ﬁeld seasons, July 2001 and June 2002. Table 1 lists the sampling locations for
all the samples collected. The majority of these samples were taken ﬁ'om active quarries
(Figure 3A) and road cuts, which are easily accessible. The samples represent the
variation among glassy pumice fragments present within an outcrop (Figure 3B).
Sampling was based on color, size, degree of welding (if present), and the types and

amount of phenocrysts in the observed hand samples.

Sample preparation

One hundred seventeen pumice fragments were selected and analyzed for mineral
and element composition for this project. The basis for sample selection process for the
analyses is the same as the sampling methods done in the ﬁeld —- to sample the variation
present. Whole pumice fragment major element and selected trace element compositions
were analyzed by X-ray ﬂuorescence spectrometry (XRF) for the all selected samples.
Additional whole pumice fragment trace and rare-earth element compositions were
analyzed using Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA ICP-
MS). Based on the variation in chemical composition of these analyzed samples, a small
number of pumice fragments were selected and analyzed for phenocrysts major element

contents using the Electron Probe Micro Analyzer (EPMA) at University of Michigan.

Fused glass disks were used for both XRF and LA ICP-MS analyses. The disks
were made ﬁom whole pumice fragments that were powdered using an aluminum ﬂat
plate grinder after passing them through a chipmunk. Three grams of ﬁnely ground
pumice fragment powder were diluted by adding 9.0g of lithium tetraborate (LizB4O7)
and 0.5g of ammonium nitrate (NH4NO3) as an oxidizer. These were then mixed and
melted in platinum crucibles at 1000 °C of oxidizing ﬂame for >20 minutes while being
stirred with an orbital mixing stage. The melt was poured into platinum molds making the
glass disk for analyses. The ﬁrsed disks were analyzed using a Rigaku S/Max X-ray
ﬂuorescent spectrograph. XRF major element analyses were reduced by a fundamental
parameter data reduction method (Criss, 1980) using XRF WIN software (Omni
Instruments), while XRF trace element data were calculated using standard linear
regression techniques. For LA ICP-MS trace element data, a Cetac LSX200+ laser
ablation system was used coupled with a Micromass Platform ICP-MS and using
strontium, determined by XRF, as an internal standard. Trace element data reduction was
done using MassLynx software. Prior to any calculations, the background signal was
subtracted from the standards and samples. Element concentration in the samples were
calculated based in a linear regression method using MST-612, BHVO, W-2, JB-l, JB-2,
JB-3, JA-2, JA-3, BER, SN], and RGMl standards.

For electron microprobe analyses, select pumice fragment thin sections were used
based on a wide range of chemical compositions. The minerals analyzed were
plagioclase, clinopyroxene, orthopyroxene and in addition, the matrix glass, enclaves and
melt inclusions present in the plagioclases. Analyses were performed on a Cameca SX

100 EPMA equipped with ﬁve wavelength spectrometers using an accelerating potential

of lSkV, a 6m beam spot size, counting time of 3min/mineral, and a lOnA beam

current.

Petrography and mineralogy

Petrograhic analyses were done on thirty selected thin sections of pumice
fragments. They represent a wide range of chemical compositions and color (e. g. white,
gray, and black). The lighter colored pumice fragments are typically more silicic than the
darker pumice fragments. They contain varying amounts of plagioclase, clinopyroxene,
orthopyroxene and Fe-Ti oxide phenocrysts, either in clots or as isolated crystals.
Crystals larger than 0.5mm were considered phenocrysts and smaller crystals were
considered part of the groundmass.

Petrographically, there is no visible distinction among the ash-ﬂow sheets except
for the abundance of phenocrysts content. Generally, the amount of phenocrysts present
in the pumice ﬁ'agments decrease with increasing silica content. Crystal-rich and black
pumice fragments are usually basaltic to andesitic in composition. They contain 10- 55%
glass, 25-50% plagioclase, 5-lO% orthopyroxene, 10-20% clinopyroxene, and 5-10% F e-
Ti oxides. Crystal-poor samples, which are dacitic to rhyolitic, have the same phase
assemblages but in considerably fewer amount. They contain 70-90°/o glass, 5-10%
plagioclase, 1-5% orthopyroxene, 2-10% clinopyroxene, and 1-5% Fe-Ti oxides. The
nearly aphyric samples are usually the white pumice fragments and the phenocrysts occur
within glomerophyric clots (Figure 4).

Plagioclase crystals, which are the most abundant phenocrysts, occur as medium-

to coarse-grained, euhedral to subhedral crystals, and most exhibit albite twinning and

oscillatory zoning (Figure 5A). Often, these crystals have embayed-, sieved-, or resorbed-
textures (Figure 5B) while some plagioclases have corroded cores (Figure 6A). These
textures indicate disequilibrium between the crystal and the melt. A few plagioclases
contain melt inclusion (6B) and some of these inclusions are devitriﬁed.

Phenocrysts of pyroxene are less abundant than the plagioclase and occur mostly
as medium-grained subhedral crystals. Usually, clinopyroxene and orthopyroxene either
occur together as clots or in aggregates with the plagioclase. It is difficult to distinguish a
clinopyroxene from an orthopyroxene based on optical properties alone (Figure 7) and
can only determined through microprobe analyses. Generally, the clinopyroxenes are
more abundant than the orthopyroxene in any given pumice fragment in the ash-ﬂow
sheets.

The F e—Ti oxides (e. g. ilmenite, magnetite) occur as minor phases, either in
aggregates with other phenocrysts or as euhedral and isolated crystals. Prismatic apatite
crystals occur as an accessory mineral found within the plagioclase.

A few banded pumice fragments have been observed within the silicic ash-ﬂow
sheets (e. g. Apoyo unit; Figure 8). They usually have gray color and their composition is
transitional between the silicic and the maﬁc pumice fragments. These banded pumice
clasts are interpreted to be the result of mingling between the silicic and maﬁc magmas.
Incomplete mixing was probably due to viscosity contrast between the two magmas as
manifested by the ﬂow banding present.

Maﬁc enclaves occur within some of the pumice fragment units (i.e. Las Maderas,
Ostocal; Figure 9). These enclaves are mostly >2mm in size and contain the same

mineralogy as the host pumice but are more crystal-rich and the phenocrysts sizes are

10

relatively smaller. Some enclaves show an intrusion-like feature of F e-Ti oxide into the
host pumice fragment (Figure 10) These features are indicative of mingling between the
silicic and maﬁc magmas. Within these enclaves are dove-tail textured plagioclases

(Figure 11), which suggest quenched crystallization process after the two magmas came

in contact.

Geochemistry
Whole Rock Geochemistry

Major element variations

Whole pumice fragment major and trace element data are listed in Table 2. All
major element values used in the discussion and plots have been normalized to 100%
volatile free because of secondary hydration of glassy pumice. Total iron is calculated as
Fe203m.

The compositions of the pumice fragments in the ash-ﬂow sheets from Nicaragua
vary from basalt to rhyolite, with Si02 ranging from 50.92 to 71.47 wt% (Figure 12). The
concentrations of the major element oxides, except the alkalies, decrease with silica
content and deﬁne distinct chemical trends for these ignimbrites (Figure 13). Most of
these ignimbrites are enriched in potassium and fall within the medium- to high-K calc-
alkaline ﬁeld, with K20 range from 0.5 to 5.71 wt. % (Figure 12). Based on these
chemical variations and stratigraphic location, seven distinct ash-ﬂow sheets were
recognized. These ash-ﬂow sheets are Apoyo, Las Sierras, and 5 informally named units
referred here as Coyol and Las Maderas of the Coyol Group (Ehrenborg, 1996), Ostocal,

Monte Galan, and San Rafael. The pumice fragments with lower Si20 content in any

11

given ash-ﬂow units occur within a trend similar to the Quaternary Nicaraguan lavas
(Figure 13).

Considering all units, the Apoyo ash-few sheet has the widest range in Si02
content from 50.9 to 69.1 wt %. It is however dominated by high-Si02 pumices (mean of
the high Si02: 67.78 wt. %; Figure 14) with distinctly low Ti02 and K20 trend than the
other ash-ﬂow units (Figure 12). The Las Maderas unit, like Apoyo, has low-Si02 pumice
fragments and is dominated by dacitic pumices (mean of the high Si02: 64.03; Figure
14). This unit has a distinct high A1203 and K20 content relative to the other units (Figure
13). The Coyol ash-ﬂow sheet, like Apoyo and Las Maderas, contain both low- and high-
Si02 pumice fragments (Figure 14) but samples are mostly trachydacites (Figure 12).
This unit has the highest K20 content for all the ash-ﬂow sheets with a distinguishing
trend, which range from 2.04 to 5.71 wt. % (Figure 13). The Las Sierras ash-ﬂow sheet
has the highest Si02 content for all the units sampled, and range from dacitic to rhyolitic
composition, 66.95 to 71.47 Si02 wt. % with a mean composition of 70. 16 wt. °/o (Figure
12, 13, 14). The Monte Galan unit is mostly of rhyolitic composition (average: 70.18 wt.
% Si02; Figure 14) and in many aspects resembles the major oxide content of the Las
Sierras ash-ﬂow sheet (Figure 13). Ostocal and San Rafael units both are dominated by
dacitic tuffs with an average composition of 67. 15 and 64.28 wt. % Si02, respectively

(Figure 13) but each have a distinctive Ti02 and F e203m content (Figure 13).
Trace element variations

Trace element abundances for all the seven ash-ﬂow sheets in Nicaragua are listed

in Table 2. Within an ash-ﬂow sheet, the pumice fragments show little variation in trace

12

element abundances. However, a weak linear relationship of some trace elements with
silica content occurs in some units (Figure 15).

Cumulative frequency distribution curves (CFCs) of incompatible trace element
ratios among the seven the ash ﬂow sheets show small but distinct variation (Figure 16).
Such distinctions among these ignimbrites ﬁirther demonstrate the occurrence of seven
compositionally different ash-ﬂow sheets in the region. Las Sierras and Monte Galan
have similar La/Yb ratio but they show distinction in their Hf/Th ratio. All other ash-ﬂow
units fall on a distinct CFC and this is consistent with each unit having a separate source.
Distinct “breaks” or “gaps” in incompatible trace element ratios (i.e. Hf/Th; Ba/Nb) on
the CFC plots are also observed within some of the ash-ﬂow units (i.e. Las Sierras,
Ostocal; Apoyo, Las Maderas). These “breaks” or “gaps” imply that ﬁactional
crystallization is not a dominant process but rather played a minor role in the chemical
heterogeneity of these ignimbrites. If fractional crystallization were the dominant
mechanism, then the trend for any of these ash-ﬂow sheets would fall along a single
continuous curve. The “breaks” or “gaps” suggest that another process (i.e. different
source area) may have contributed for such incompatible trace element trend.

Trace element ratios of the seven ash-ﬂow sheets mimic the trace element ratios
of the low Ti02 Quaternary front arc lavas in Nicaragua (Figure 17). The Ba/La and
La/Yb ratios of the ignimbrites are consistent with the modern arc lavas (Carr et al.,
1990) along with Ba/Rb and Hf/Th ratios (Figure 17). Rare earth element (REE) trends
for these silicic rocks are similar and deﬁne a slight middle rare earth element (MREE)
depletion (Figure 18). A DyN/LuN ratio <1 further supports this observation particularly

in the younger ash-ﬂow sheets (Figure 19). REE ratios of the older ash-ﬂow sheets (i.e.

l3

Coyol and Las Maderas) show an increasing LaN/LuN with increasing DyN/LuN ratio,
while younger ignimbrites (i.e. Las Sierras, Monte Galan) have relatively constant
LaN/LuN with increasing DyN/LuN ratio (Figure 19). This difference could be a ﬁrnction
of the degrees of melting that each unit has undergone.

Some ash-ﬂow sheets like Ostocal, Las Maderas, and Las Sierras units contain a
small negative Eu anomaly (0.67 to 0.90). Eu anomaly is a measure of Eu/Eu*, which is
calculated from the linear equation of the line connecting Sm and Tb on a REE chondrite
normalized plot (for equations, refer to Hannah, 2000). Such negative Eu anomaly in
these units may result from plagioclase fractionation. However the Apoyo unit contains a
slight positive Eu anomaly (1.09 to 1.13) (Figure 20), which may reﬂect plagioclase
accumulation. Spider diagrams normalized to primitive mantle concentration (Sun and
McDonough, 1989) show enrichment in the large-ion lithophile elements (LILEs) and
depletion in the high ﬁeld strength elements (HF SEs) (Figure 21) for all units, which is
consistent with the composition of melts associated with subduction zone environment.

Relatively young ash-ﬂow sheets correspond to high U/Th ratio while older units
tend to have lower U/Th ratio. This is similar to the observation made by Plank et al.
(2002). They related this change from low to high U/Th ratio resulting from the
“carbonate cras ” at 10 Ma, where the istmus of Panama changed the circulation patterns
in the eastern Paciﬁc Ocean (Ler et al., 1995). This is supported by the fact the youngest
ash-ﬂow sheet in this study is the Apoyo unit, with an age of 23,000 yrs BP (Sussman,
1985) and has a high U/Th ratio of 0.45 to 1.30. The oldest units are from the Coyol
Group, the Coyol and Las Maderas ash-ﬂow sheets, with an age of Early Miocene

(Ehrenborg, 1996) and have a low U/T h ratio of 0.35 to 0.52 and 0.29 to 1.09,

14

respectively. From here, it is inferred that the other units, Monte Galan, Las Sierras, San

Rafael, and Ostocal correspond to an age younger than the Coyol Group since they have a

higher U/Th ratio (Figure 22).

Mineral Chemistry

Electron nricroprobe analyses of phenocrysts from select pumice fragments are
listed in Table 3. The plagioclase analyses were obtained from the core, nriddle and rim.
Analyses in the middle of the plagioclase were usually halfway between the core and the
rim of the crystal. Representative clinopyroxene and orthopyroxene grains were analyzed
and at least 2 glass analyses from each pumice fragment and from the maﬁc enclave (if
present) were obtained. A number of melt inclusions present in the plagioclase were also
analyzed.

The range of plagioclase compositions for all the seven ash-ﬂow sheets is from
Anzs to Aﬂ91 (Figure 23). Some plagioclases show a more calcic rim and a sodic core
(e. g. Monte Galan, Ostocal) while others shows calcic core with very low sodic rim (e.g.
Apoyo) (Figure 23). Both normal and reverse zoning in these plagioclases were
observed in any given ash-ﬂow unit. The composition of the plagioclase rims would be
expected to be in equilibrium with the last liquid. Except for Ostocal, all ash-ﬂow sheets
have plagioclase grains with rims that display a wide range of composition (Figure 24).
They vary from as low as 20% to as high as 70% An content within a single pumice
fragment (i.e. Monte Galan and Apoyo). This large variation indicates that the
I plagioclases were not in equilibrium with the last crystallizing liquid/s prior to the
eruption of these ignimbrites. Neither the coexistence of both normal and reverse

zonations of the plagioclase phenocrysts nor the large variations in An content in the

15

plagioclase rims within a pumice ﬁ'agment can be explained by simple closed-system
crystallization. It would imply that at least 2 different magma bodies, a high-silica and a
low-silica magma, are in contact and did not reach compositional equilibrium. Another
indicator of these compositional disequilibrium is the high An content (up to 91% An) of
some of the plagioclase cores, particularly in Apoyo and Las Sierras units (Figure 23)
because silicic rocks typically have plagioclases with An content less than 50%.

For clinopyroxene phenocrysts, the composition range from W044,5.32,4 Emg,4.32,5
F829_o-13_2 while orthopyroxene composition range from W09_6-2_o En67_14o_7 Fsso_5-3o,1.
Orthopyroxenes were not observed in some ash-ﬂow sheets (i.e. San Rafael, Monte
Galan and Coyol). It is possible that they are present but not analyzed due to inability to
recognize an orthopyroxene from a clinopyroxene through optical properties. The Mg#
for both clinopyroxene and orthopyroxene among the seven ash-ﬂow sheets show a wide
range of variation as shown in Figure 25. The Ostocal and the San Rafael units have a
distinctive Fe-rich orthopyroxene and clinopyroxene respectively, while Las Maderas and
Apoyo ash-ﬂow sheets have Mg-rich clinopyroxene. This indicates that each unit has
distinct source composition unrelated to the other units. Figure 26 shows the calculated
temperature at which the phases of the last crystallizing liquid were last in equilibrium
using the program QUILF version 6 (Andersen et al., 1993). The temperature was
calculated using two pyroxene thermometer in clinopyroxene + orthopyroxene bearing
pumice fragments. Each ash-ﬂow unit has a distinct temperature, and even within a single
ﬂow unit, 2 different temperatures have been calculated. The calculated temperature are:

Apoyo: 910i 19 and 9251- 39; Las Maderas: 941 3:71 and 1012 i73; Las Sierras: 959 i

69 and 969i144; and Ostocal: 1214 i139 and 1086 i 274. Even though the uncertainty of

16

the calculated temperatures is high for some ignimbrites (i.e. Ostocal), they still indicate
thermal distinction of the source magma among the ash-ﬂow sheets.

Matrix glass composition for the pumice fragments, including the maﬁc enclaves
and melt inclusions in plagioclases vary with the Si02 of the whole pumice fragment and
range fiom 47.0 to 76.6 wt %. Figure 27 and 28 show the plots of MgO and K20 versus
Si02 for the matrix glass, maﬁc enclave, and melt inclusion compositions together with
the whole pumice fragment content for each ash-ﬂow unit. The matrix glass in the maﬁc
enclaves found in Las Maderas and Ostocal units have lower Si02 and K20 contents than
the matrix glass of their respective host pumice fragment. The MgO content of the
Ostocal maﬁc enclaves is higher than the host pumice fragment but Las Maderas maﬁc
enclaves contain both high and low MgO contents. Melt inclusions in plagioclase
phenocrsyt have lower Si02, K20 and MgO content than the host pumice fi'agment. The
Si02 and K20 contents of the matrix glass overlap the composition of the whole pumice
fragment for Coyol, Monte Galan and San Rafael units (Figure 27 and 28). A large
compositional gap occurs in the whole pumice fragment and in the matrix glass data for
any of the ash-ﬂow sheets, including the maﬁc enclave in Las Maderas and Ostocal units
and the melt inclusions in San Rafael ash-ﬂow sheet (Figure 27 and 28). This
compositional gap represents at least two evolving magma batches that did not attain
thermal equilibration and were not stored in prolonged contact before eruption. This
scenario is similar to what occurred in the Aniakchak Caldera and the enclave-bearing
Mount Dutton eruptions, both in Alaska, where a large difference in the whole rock and

melt compositions are observed (Eichelberger et al., 2000).

17

The generation of the silicic ash-ﬂow sheets of southern Nicaragua

Abundant silicic magmas are typically rare in island arc environment because
they are generally thought to be generated by assimilation and/or melting of underlying
continental crust. The seven silicic ash-ﬂow sheets observed in Nicaragua may therefore
provide excellent means to infer their origin in this island arc environment. Distinct
mineralogical and chemical heterogeneities occur among and within the silicic ash-ﬂow
units and these can be used to determine magmatic processes that occur in the magma
chamber prior to eruption since ash-ﬂow tuffs represent instantaneous evacuation from
the magma chamber. Two models have been proposed to generate these silicic ash-ﬂow
sheets: (1) fractional crystallization of basalt or basaltic andesite and (2) partial melting
of previously emplaced arc-related magma. Subsequent magma mixing, magma
mingling, and/or crystal fractionation may modify the produced melt. Each of these

models will be evaluated individually.

Fractional Crystallization
The evolution of silicic magmas by fractional crystallization of basaltic or basaltic

andesite melts was proposed by several workers (Sisson and Grove, 1993; Feely and
Davidson, 1994; Grove et al., 1997; Brophy et al., 1999). In order to attain high-silica
magmas, a large amount of fractionation is involved (~68%; e. g. Grove et al., 1997). For
this to be produced, cumulate rocks such as anorthosite would have to be generated as
well. However, no cumulate rocks have been reported in Nicaragua. Even in areas where
the roots of silicic volcanic rocks are thought to be exposed, such as in the Cordillera de

Talamanca in southeastern Costa Rica, cumulate rocks are rare (Alvarado and Carr,

18

1993). Brophy et al. (1999) proposed that silicic calc-alkaline rocks might originate by
fractionation near the crust-mantle boundary, which would explain the scarcity of
exposed cumulate rocks. The base of a thickened crustal province, like the CLIP, might
serve as an ideal density trap for magmas to pond (e. g. Hauff et al., 1997, Sinton et al.,
1997, Hauffet al., 2000).

Each ash-ﬂow sheet has a distinct CFC trace element ratio trend (Figure 16). Even
within some ash-ﬂow sheets such as Ostocal and Las Sierras, “breaks” or “gaps” in their
trace element ratio CFC occur (e. g. Hf/Th) and these “breaks” or “gaps” suggest that
another process such as a different source area for each trend, may have contributed to
such distribution trend.

The seven silicic ash-ﬂow sheets lack signiﬁcant Eu anomaly (Figure 18).
However, this does not necessarily mean that plagioclase fractionation did not occur.
Most calc-alkaline rocks, such as the silicic ignimbrites of Nicaragua, are produced under
an oxidizing environment where Eu behaves as Eu3+ and therefore Eu would behave like
the other REES and no Eu anomaly would be observed even if plagioclase fractionated.
Nonetheless, Las Sierras and Las Maderas units contain slight negative Eu anomaly
(Eu/Eu“ <1) with increasing 8102 (Figure 20), and would imply that ﬁactional
crystallization may have played a minor role in their production and that an oxidizing
state may not be truly the case. The Apoyo ash-ﬂow sheet, the only unit with a small
positive Eu anomaly (Eu/Eu* >1), suggests the effect of plagioclase accumulation.
However, the Eu/Eu* of the more maﬁc and more silicic pumice fragments fall into two

distinct groups that can not be related by simple fractional crystallization (Figure 20). If

19

 

 

 

the silicic tuffs fractionated from the more maﬁc ignimbrites, a continuous linear trend
would be expected, but this is not the case for the Apoyo unit.

Oscillatory zoning in plagioclase is a widespread phenomenon observed in many
volcanic rocks (e. g. Anderson, 1984; Kuritani, 1998; Ginibre et al., 2002, Halarna, et al.,
2002). The observed chemical zonations in plagioclase phenocrysts from the seven silicic
ash-ﬂow sheets can be used to elucidate magma-chamber and emplacement processes.
Plagioclase phenocrysts in any of the ignimbrite units commonly display abrupt
ﬂuctuations in An content (up to 25% in Apoyo unit) (Figure 23). The abrupt shifts from
a calcic core to a sodic rim (e. g. Apoyo) or the trend from a more sodic core into a calcic
rim (e.g Monte Galan, Ostocal) could correspond to the well-developed dissolution
surfaces in the plagioclase crystals (e. g. embayed, resorbed textures). The coexistence of
both normal and reverse zonations of the plagioclase phenocrysts within the pumice
fragments, the large variations in An content in the plagioclase rims from a given unit
(Figure 24), and the very high An content of the core (>50 An%) for all the units except

Ostocal imply that each ash-ﬂow unit can not be generated by simple crystal fractionation
(Figure 23). If ﬁ'actional crystallization of the melt produced these plagioclases, then they
should have normal uninterrupted zonation, little variation in the plagioclase rim
composition since they should be in equilibrium with the last crystallizing liquid, and an
An content of <50%, typical of silicic melts. The observed characteristics of the
plagioclase phenocrysts however suggest compositional and thermal disequilibrium
within the magma chamber contradicting the process of ﬁ'actional crystallization.

The observed compositional discontinuity within both the pumice fragments and

the matrix glass, including the maﬁc enclaves and melt inclusions in the plagioclases

20

(Figure 27 and 28) suggest chemical and thermal disequilibrium in the magma chamber
(e. g. Eichelberger et al., 2000), in accordance with the plagioclase phenocrysts. If these
silicic ash-ﬂow sheets were related by progressive fractional crystallization and/or even
by assimilation, it is impossible to have a discontinuity in melt composition without a
corresponding discontinuity in temperature (Eichelberger et al., 2000). This large gap of
unrepresented compositions can be accounted for by some special mechanism such as
sidewall crystallization with efficient expulsion and collection of melt, producing a
gravitationally stable silicic over maﬁc layering (McBirney, 1980; Turner and Gustafson,
1981; McBirney et al., 1987; Bacon and Druitt, 1988) or by incomplete mixing of a
silicic and maﬁc magma in the chamber (e. g. Snyder and Tait, 1998; Eichelberger et al.,
2000). Sidewall crystallization has been envisioned to be an effective mechanism to
produce ryholitic magma such as those erupted during the 1912 Katmai eruption in
Alaska (Hildreth and Fierstein, 2000). However, should fractional crystallization generate
the trends within the seven high-silica rocks in Nicaragua, individual glass matrix
analyses should show higher Si02 and K20 with lower Mg0 when compared with the
whole pumice ﬁ'agment analyses. But most of the silicic ash-ﬂow sheets contradict this
except Apoyo, Las Maderas and Ostocal (Figure 27 and 28). Some of the matrix glasses
are more maﬁc than the whole pumice fragment indicating that fractional crystallization
did not generate these matrix glasses. Such variations in the matrix glass of the silicic
ignimbrites are best explained by mingling of at least 2 different magma type, a silicic
and a maﬁc magma and this will be discussed more in detail below.

The presence of crystal-rich maﬁc enclaves (Figure 9) within the silicic pumice

fragments (eg. Las Maderas, Ostocal) is another indicator of disequilibrium in the magma

21

chamber, a process unrelated to fractional crystallization. These maﬁc enclaves record
the intrusion of a mostly liquid maﬁc magma into a mushy silicic magma (e. g.
Eichelberger et al., 2000), supporting the process of magma mingling. Such interaction is
shown in Figure 10, where an oxide mineral of the maﬁc enclave “invades” the matrix
glass of the silicic host.

In summary, trace element variations among the ash-ﬂow sheets, Eu/Eu*,
plagioclase zonations, compositional gaps, and maﬁc enclaves can be used to reject the
model of fractional crystallization ﬁ‘om a low-silica magma to a high-silica magma as a

dominant mechanism to produce any of the seven silicic ash-ﬂow sheets in Nicaragua.

Partial melting of previously emplaced arc-related igneous rock

Partial melting of previously emplaced arc-related igneous rocks has been cited as
a mechanism to generate high-silica rocks in an island arc setting (Beard and Lofgren,
1991; Roberts and Clemens, 1993; Wolf and Wyllie, 1994; Nakajima and Arima, 1998;
Tamura and Tatsumi, 2002; Smith et al., 2003). Dehydration melting experiments of
basaltic compositions carried out by Beard and Lofgren (1991) under low pressure have
yielded rhyolitic melts coexisting with the anhydrous assemblage plagioclase +
orthopyroxene + clinopyroxene + Fe-Ti oxides consistent with the phenocrsyt
assemblage of the seven silicic ash-ﬂow sheets in Nicaragua. Although the rhyolitic melts
produced by Beard and Lofgren (1991) have the same decreasing Ti02, CaO, MgO, and
Fe203 with increasing K20 and Si02, their melts are more silicic and have lower
potassium than the Nicaraguan ash-ﬂow sheets. Tamura and Tatsunri (2002) argued that

20-3 0% dehydration melting of solidified calc-alkaline andesite in the crust rather than

22

basaltic magmas play an important role in producing high silica rocks (e. g. Izu-Bonin
arc). They suggest that water-saturated andesites solidify at depth and never erupt to the
surface and these andesites remelted to produce the high-silica deposits in Izu-Bonin.
Major elements and phenocryst assemblage of the silicic ash-ﬂow sheets in Nicaragua are
similar to the rhyolitic rocks in Izu-Bonin arc (Tarnura and Tatsumi, 2002) but the K20
content is relatively enriched. This enrichment in K20 may be attributed to either the role
of amphibole in the source and/or the inﬂuence of sediment-derived ﬂuids, which will be
discussed in detail below.

As discussed above, each of the silicic units are not produced by fractional
crystallization alone and are independently generated. The increasing Rb and Zr and
decreasing Sr with Si02 (Figure 15) could therefore indicate that there has been an
involvement of an older, altered oceanic crust. In most systems where plagioclase is
ﬁ'actionating, Sr is greatly reduced and the Sr value approaches zero. However, the Sr
content of any of the ash-ﬂow sheets is high (>100; Figure 15) indicating the
involvement of an altered crust. Such crust must have a total crustal thickness of 15-20
km («0.5 Gpa), where amphibole is a stable phase, in order to generate silicic magmas
(i.e. Smith et al., 2003). The breakdown of amphibole at some temperature between the
wet and dry solidi (~ 900° C) triggers dehydration melting of the lower crust (i.e. Beard
and Lofgren, 1991; Smith et al., 2003). Smith et al. (2003) pointed out that once a ﬂuid is
present, heat can propagate relatively fast by convection to produce large batches of
silicic magmas. The observed slight depletion in MREEs in the younger ash-ﬂow units
(Figure 18) indicates the stability of amphibole in the source magma, which has a strong

afﬁnity for the MREEs. This is apparent from the observed DyN/LuN ratios in the younger

23

silicic ash-ﬂow sheets, which are <1 suggesting that amphibole is in the source rock
(Figure 19). Such dehydration melting of an amphibole-rich source also explains the
enrichment in K20 content of the ash-ﬂow sheets as mentioned above. The relative low
LaN/LuN ratios of the pumice fragments suggest a high degree of melting or from a source
with low LaN/LuN ratio, while increasing LaN/LuN ratios with increasing DyN/LuN ratios
imply crystal fractionation of a melt derived from an amphibole-rich source. If the
subsequent differentiation did not occur, the DyN/LuN ratio would remain constant even
under varying degrees of melting.

It is important to note that efﬁcient partial melting of amphibole-bearing
previously emplaced arc-related igneous rock to generate the silicic ash-ﬂow sheets is
governed by periodicity and multiplicity of basaltic intrusion from deep source into the
lower crust (e. g. Petford and Gallagher, 2001; Annen and Sparks, 2002). Continuous and
multiple production of basaltic magma from the melting of the mantle wedge due to the
ﬂuids coming off the dehydrating, subducted Cocos plate would ascend to the lower crust
and effectively partially melt the previously emplaced arc-related magma. Segregation
and ascent of partial melt would then produce the silicic ash-ﬂow sheets.

Because the silicic ignimbrites contain high amounts of Ba/La ratio similar to the
modern Nicaraguan lavas, which is interpreted to indicate high sediment inﬂux (Carr et
al., 1990; Patino et al., 2000), it implies that the source rock of the silicic ash-ﬂow sheets
were produced by the same manner. This is complimented by the high degrees of melting
as shown by the low LaN/LuN ratio in the silicic ash-ﬂow sheets. These sediment-derived

ﬂuids are often enriched in “ﬂuid-mobile” elements such as Ba, Rb, K, and Sr (Tatsumi

24

et al., 1986), which explains the K20 enrichment in the silicic tuffs accompanied by the

consumption of amphibole during partial melting.

Magma Mingling

Ash-ﬂow sheets such as observed in Nicaragua, represent an instantaneous partial
evacuation of the magma chamber and therefore any variation indicates heterogeneity in
the pre-eruptive magma. Mafic enclaves and large compositional gaps in eruptive
products in the ash-ﬂow sheets are thought to indicate maﬁc-silicic magma interaction,
and many workers have suggested that such magma mixing/mingling could trigger the
eruption (Sparks et al., 1977; Eichelberger, 1978; Stimac et al., 1990; Pallister et al.,
1992; Eichelberger et al., 2000; Eichelberger and Izbekov, 2000). Magma
mixing/mingling can occur effectively during the injection of either a maﬁc or silicic
magma into the chamber and/or during the ascent into the conduit (F eeley and Dungan,
1996; Eichelberger ct a1, 2000; Kuritani, 2001).

The occurrence of banded pumice within the silicic ash-ﬂow sheets resulted from
incomplete mixing of the coexisting magmas (Figure 8) as they ascend through the
conduit. Crystal-rich maﬁc enclaves (Figure 9) present in the pumice ﬁ'agments of Las
Maderas and Ostocal units indicate mingling of the coexisting magmas. These maﬁc
enclaves record the partial disintegration of the invading basaltic magma, strewing out
blobs of basaltic debris as it came in contact with the rhyolitic magma (e. g. Eichelberger,
1978; Eichelberger et a1, 2000). The quenched textures exhibited by the phenocrysts (e. g.
dove-tail texture in plagioclase) in the enclaves (Figure 11) are the result of undercooled

crystallization of the maﬁc magma. The interaction of these distinct magmas is also

25

manifested by the “intrusive-like” feature of Fe—Ti oxide phenocrysts in the enclave into
the glass matrix of the silicic host (Figure 10).

Chemical and thermal disequilibrium induced by the interaction of maﬁc and
silicic magmas are recorded by textures and composition of the plagioclase phenocrysts
in the silicic pumice fragments. Such disequilibrium resulted in: (l) embayed, resorbed
and sieved crystal forms (Figure SB) indicating incomplete dissolution, (2) compositional
reverse zoning (Figure 23), (3) abrupt shifts of both sodic plagioclase into a more calcic
composition, and calcic composition into a more sodic plagioclase (Figure 23), (4)
remarkably anorthitic core (>An7o, e.g. Apoyo, Las Sierras; Figure 23), and the wide
range in anorthite content of the plagioclase rims for a given pumice fragment (Figure
24).

A strong argument in favor of magma mingling in the silicic ash-ﬂow sheets in
Nicaragua is the remarkable large compositional discontinuity within the pumice
fragments and their matrix glass, the maﬁc enclaves and the melt inclusion compositions
(Figure 27 and 28). These large differences in melt compositions demonstrate the lack of
thermal equilibration and require that the two mingling magmas were not stored in
prolonged contact (Eichelberger et al., 2000). The similarity of the silicic matrix glass
with the silicic host pumice fragment in terms of Si02 and K20 for some of the
ignimbrite units (i.e. San Rafael, Coyol, Monte Galan) and although no maﬁc host
pumice liagment can be compared with the observed maﬁc matrix glass, maﬁc enclaves,
and melt inclusion compositions, is a signiﬁcant evidence that a silicic magma intruded a
maﬁc magma. This is comparable to the erupted Aniakchak ignimbrites in Alaska, where

the invading silicic magma interacted with the maﬁc magma in a limited way and weakly

26

contaminated each other (Eichelberger et al., 2000). The silicic replenishment case is
further supported by the nearly aphyric texture of the silicic pumice fragments. As
Eichelberger et al. (2000) pointed out, these silicic ignimbrites are crystal-poor not
because the parent magma was sitting at the roof of the chamber and heated from below
by a more maﬁc magma, but because they had just arrived from the deep source, after
they were produced from partial melting of a previously emplaced arc-related igneous
rock This silicic intrusion scenario is the reversal of the enclave-bearing ignimbrites
where maﬁc recharge is envisioned (i.e. Las Maderas, Ostocal). Eichelberger et al. (2000)
made a distinction that maﬁc recharge are enclave-bearing and the enclaves are enriched
in silica and potassium relative to their host rocks and the contact between the magmas is
prolonged because the maﬁc entrant is initially denser than the resident magma.
However, even for the enclave-bearing units in Nicaragua, the silicic and maﬁc matrix
glass, and the silicic and maﬁc host pumice respectively, are still similar in silica and
potassium content indicating silicic recharge. Nonetheless, it is possible that the enclaves
came from the roof or walls of the chamber and were just picked up by the intruding
silicic magma as it ascend to the conduit. Further investigation on these cases needs to be
done but is not within the scope of this study. For whatever case applies for the enclave-
bearing units, either a maﬁc recharge or a silicic entrant, it is still conclusive to say that
magma mingling occurred.

Overall, the preservation of maﬁc enclaves in the silicic host pumice fragments,
the disequilibrium features of the plagioclase phenocrysts, and the large compositional
gap in the matrix glass and the pumice fragments can be used as evidence that magma

mingling played an important role in the evolution of the silicic ash-ﬂow sheets in

27

Nicaragua. The mingling of the magmas induced chemical and thermal instability in the

chamber and thereby possibly triggering the eruption of the ignimbrites.

Model for the evolution of the Nicaraguan silicic ash-ﬂow sheets

Chemical and mineralogical compositions within the seven silicic ash-ﬂow sheets
in Nicaragua suggest that they are the product of partial melting of previously emplaced
arc-related igneous rocks and are independently generated. Figure 29 is a cartoon of the
evolution of the Nicaraguan silicic ash-ﬂow sheets. During stage 1, calc-alkaline magmas
are produced by melting of the mantle wedge due to the ﬂuids from the dehydrating,
subducted Cocos plate. These magmas were produced by the same process as the modern
arc lavas being enriched in Ba/La ratios due to the high ﬂuid inﬂux, which induces higher
degrees of melting of the mantle wedge (low La/Yb). These magmas were emplaced at
the base of the crust, approximately 15-20 km where amphibole would be stable. At stage
2, rising maﬁc magmas partially melts the amphibole-bearing previously emplaced
igneous rocks to produce the silicic melt. The metasomatic characteristics of the mantle
melt (i.e. high in ﬂuid-mobile elements) and the presence of amphibole in the source rock
(DyN/Luu <1) could explain the enrichment of potassium in the silicic ignimbrites. As
these pockets or blebs of silicic magmas ascend, they mingle with other rising and/or
ponded maﬁc magmas (stage 3) at the magma chamber and/or during the ascent to the
conduit.

Four possible cases during stage 3 in Figure 29 have been conceptualized and are
shown in Figure 30 (Table 4). In case A, both the produced silicic magma and the
intruding maﬁc magma ascend into the potential magma chamber without mingling. This

scenario produces both the silicic and maﬁc pumice fragments in a single ﬂow unit. For

28

case B, some silicic magma stays behind in the chamber, and had ample time to mingle
with intruding maﬁc magma and immediately erupts as mingled ash-ﬂow sheets. These
pumices are characterized by the presence of maﬁc enclaves within the silicic pumice
fragments, with calcic plagioclase rims, and with matrix glass more maﬁc than whole
pumice fragment (low Si02 with low K20 and high MgO wt %; e. g. Las Maderas,
Ostocal). In case C, newly formed and ascending blebs of silicic magmas intrude pockets
of ponded maﬁc magma, mingles and gets erupted. Case C is distinguished with abrupt
shifts of plagioclase composition from a more calcic core to a sodic rim (e. g. Las Sierras),
and the glass composition in terms of Si02 and K20 content is similar to the whole
pumice fragment with distinct compositional gap (i.e. San Rafael, Coyol, Monte Galan).
The process for these three scenarios was fast enough such that chemical and thermal
equilibrium between the two magmas was not attained. The mingling of the magmas
might have triggered the eruption and the immediate evacuation of the magma chamber
caused caldera collapse. The fourth scenario, case D, involves an evolving intermediate
magma that is continuously replenished from the source. All four cases could

simultaneously occur but not in the same magma body.

29

Comparison with Central “American Volcanic Arc

The silicic ash-ﬂow sheets in Nicaragua are clearly enriched in Si20 and K20
compared to the low Ti02 modern Nicaraguan arc lavas (Figure 13). It is important to
note that the trace element variations (e. g. Hf/Th, Ba/La) of the silicic ignimbrites mimic
the trace element variations in the modem arc (Figure 17). LILE and HF SE
concentrations of the silicic ash-ﬂow sheets are relatively higher than the modern
Nicaraguan lavas (Figure 21) but the REE patterns are similar (Figure 18). Compared
with the rest of CAVA lavas, which represent Quaternary front arc volcanism from
Mexico-Guatemalan border down south to central Costa Rica, the Nicaraguan ash-ﬂow
sheets and modern arc lavas also show higher Ba/La ratio (Figure 17) indicating higher
sediment inﬂux (Carr et al., 1990). The presence of amphibole in the source rock of the
silicic ignimbrites of Nicaragua manifested by DyN/LuN ratio <1, is similar to the Costa
Rican ignimbrites (Figure 31; Hannah, 2000; Szymanski et a1, 2002) although the range
for Nicaragua is smaller compared to Costa Rica. The increasing LaN/LuN ratio with
increasing DyN/LuN ratio suggests differentiation after emplacement of the melt. The
DyN/LuN ratio differences result from different source rocks of the melts while the lower
LaN/LuN ratio in the Nicaraguan silicic ash-ﬂow sheets may be due to higher degrees of

melting or from a source with lower LaN/LuN ratio, followed by crystal fractionation.

3O

Conclusion

Seven chemically distinct ash-ﬂow sheets were sampled in Nicaragua. Chemical
and mineralogical variations among and within these silicic ash-ﬂow sheets indicate that
they cannot be explained by fractional crystallization alone but rather were produced by
partial melting of previously emplaced arc-related igneous rock at depths where
amphibole is stable (15-20 km). This arc-related source was partially melted by the
injection of new maﬁc magma from the metasomatized mantle wedge generated via
melting induced by ﬂuids released from the dehydrating, subducted Cocos slab.
Segregation and ascent of the partial melts produced the silicic ash-ﬂow sheets.
Continued production and ascent of both the silicic and mafic magmas into the chamber
allowed mingling between the two magmas, and before even reaching compositional and
thermal equilibrium were erupted as silicic ash-ﬂow sheets with distinct chemical
variations. REE pattern of the silicic ash-ﬂow sheets mimic the REE trend of the modern
arc lavas in Nicaragua indicating a genetic relationship. The enrichment in K20 in the
ignimbrites can be explained by the presence of amphibole in the source rock as
manifested by the behavior of the MREE, in particular the DyN/LuN ratio of <1 , coupled

with the inﬂuence of sediment-derived ﬂuids (high Ba/La).

31

APPENDIX A

Tables

32

Table 1

Sample Location

Sample
0107161a-e
010716-2a-h
010716-8a-e
010718-1a-o
010719-1a-g
02061 74 a-p
02061 7-2a-h
020617-3a-e
0206174a-e
020617-5a
020617-6a-f
020618-1a-d
02061 B-Za-c
02061 8-3a-j
020618-4414
02061 8-6a-g
020619-1a-w
020620-1a-n
020620-2a-e
020620-3a-e
0206204a-c
020620-5a-h
020620-6a-e
020620-7a-n
020620-8a-b
020620-9a-k

020620-10a-h
020620-1 1a-n

020622-1a-k
020622-2a
020622-2641
020622-3a-b
020622-41”
020622-6
ozoszz-sa-g
020623-1a-d
ozoszs-za-g
020624-1a-n
020624-2a-k

Map Name (1 :50,000)

Las Banderas
Las Maderas
Malpaisillo
Malpaisillo
Masaya
Masaya
Masaya
hﬁasaya
Masaya
Masaya
Masaya
Masaya
Masaya
Masaya
Masaya
Masaya
San Rafael Del Sur
Malpaisillo
Malpaisillo
Malpaisillo
Malpaisillo
Malpaisillo
Malpaisillo
Malpaisillo
La Paz Centro
La Paz Centro
La Paz Centro
La Paz Centro
Las Playitas
Las Playitas
Las Playitas
Las Playitas
Las Maderas
Las Maderas
Las Maderas
Las Maderas
Las Banderas
Las Banderas
Las Banderas

UTM Coordinates
613626 1362313
602120 1381700
550798 1397432
538300 1371500
600063 1317781
602582 1314027
601042 1316332
600414 1318125
601695 1319418
601695 1319418
601851 1319296
603668 1320690
603668 1320690
603354 1320701
604046 1321759
603288 1324370
568404 1316865
537726 1394134
538520 1394320
539786 1394607
539786 1394607
539786 1394607
539940 1394639
544295 1395855
538624 1373036
540470 1372277
536651 1370009
536496 1367451
602970 1384427
602988 1384055
602870 1383797
602764 1383579
602613 1383207
602150 1381805
602150 1381805
605021 1369890
605021 1369890
613695 1362284
612602 1362213

33

Table 2

 

Apoyo Ash-ﬂow Sheet

Sample 010110-13 010110-11: 010110-10 010110-10 010110-11 010110-19 020011-10 020011-10
Si02 66.90 65.78 65.88 64.86 64.12 65.44 49.96 65.67
1102 0.52 0.51 0.55 0.56 0.52 0.57 0.80 0.57
~20. 15.10 15.45 15.39 15.62 15.59 15.56 18.89 15.60
F020,", 4.20 4.08 4.35 4.54 4.50 4.74 10.57 4.47
14110 0.13 0.13 0.14 0.14 0.13 0.14 0.15 0.14
M90 1.12 1.10 1.22 1.37 1.11 1.32 4.12 1.21
Get) 3.76 3.87 3.92 4.07 3.68 4.21 10.30 3.96
N020 4.34 5.49 3.98 3.88 4.23 3.98 2.70 3.95
K20 2.09 2.04 2.02 1.89 1.99 1.91 0.49 1.99
P20. 0.14 0.14 0.15 0.15 0.14 0.15 0.13 0.14
Totals 98.30 98.59 97.60 97.08 96.01 98.02 98.11 97.70
Cr (XRF) bd bd bd bd bd bd bd bd
Ni(XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn(XRF) bd bd bd bd bd bd bd bd
Rb 38.40 37.70 44.20 41.40 37.00 38.50 44.70 41.90
Sr 354.80 351 .10 326.40 320.50 350.80 316.60 270.80 278.40
Zr 155.60 155.30 173.90 154.30 140.30 151.00 189.30 185.60
Ba 1433.68 1408.55 1565.81 1483.25 1351.77 1530.04 1596.08 1660.37
La 13.34 13.44 14.12 12.95 12.04 13.89 17.20 18.62
Co 30.19 29.53 31.68 27.98 25.68 29.36 34.27 35.60
Pr 3.94 3.89 4.09 3.54 3.39 3.82 5.46 5.36
Nd 15.26 15.53 16.46 13.75 13.40 14.68 24.92 23.95
Sm 3.64 4.00 4.26 3.36 3.18 3.42 6.33 6.05
Eu 1.28 1.27 1.25 1.08 1.15 1.15 1.82 1.78
Gd 3.58 3.82 3.64 3.19 3.02 3.38 6.79 6.56
Tb 0.61 0.62 0.65 0.50 0.49 0.55 1.18 1.12
Y 20.66 23.13 22.97 18.54 17.55 19.11 45.18 42.35
Dy 3.49 3.66 3.64 2.88 2.86 2.90 7.24 6.87
Ho 0.75 0.83 0.86 0.64 0.61 0.65 1.54 1.48
Er 1.94 2.24 2.25 1.89 1.77 1.75 4.64 4.47
Yb 2.56 2.64 2.67 2.40 2.24 2.43 4.84 4.56
Lu 0.39 0.43 0.43 0.34 0.34 0.36 0.79 0.74
V 69.49 74.59 63.26 93.33 86.17 76.34 18.96 28.69
Cr 3.71 4.14 4.45 3.98 3.09 4.05 3.43 4.29
Nb 7.25 8.12 8.99 6.64 5.23 5.83 5.44 5.82
Hf 3.56 3.66 3.87 3.36 3.13 3.41 5.55 5.13
Ta 0.45 0.45 0.52 0.40 0.36 0.42 0.37 0.39
Pb 5.38 5.12 6.49 5.92 4.88 5.78 7.97 9.08
Th 2.83 2.94 3.09 3.08 2.79 3.10 3.79 3.85
U 3.11 3.17 3.52 3.31 2.71 3.16 3.01 3.38
EuIEu‘ 1.12 1.04 0.97 1.07 1.19 1.09 0.89 0.91
Distance 754.80 754.80 754.80 754.80 754.80 754.80 754.80 754.80

34

Table 2 continued

 

Apoyo Ash-ﬂow Sheet

Sample 020011-11 020011-1n 020011-10 020011-20 020011-21: 020011-21: 020617-3d 020011-45
Si02 66.70 67.31 66.30 65.96 55.67 65.89 66.37 65.61
Ti02 0.50 0.52 0.50 0.51 0.81 0.56 0.51 0.57
A120. 15.19 14.95 15.16 15.63 17.01 15.02 15.33 15.38
F020,", 3.81 3.98 3.76 4.15 9.12 4.37 3.95 4.57
1.1110 0.12 0.13 0.12 0.13 0.19 0.13 0.12 0.14
M90 0.97 1.10 0.97 1.08 3.20 1.14 1.00 1.20
Ga!) 3.45 3.56 3.47 4.02 7.79 3.66 3.58 3.96
N020 4.00 3.93 3.99 3.98 3.23 3.99 3.97 3.91
K20 2.17 2.16 2.16 1.99 1.06 2.07 2.10 1.99
P20. 0.12 0.13 0.13 0.14 0.17 0.14 0.12 0.15
Totals 97.03 97.77 96.56 97.59 98.25 96.97 97.05 97.48
Cr (XRF) bd bd bd bd bd bd bd bd
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn(XRF) bd bd bd bd bd bd bd bd
Rb 39.20 44.80 52.40 58.20 57.60 55.50 61.10 62.10
Sr 301.40 273.40 286.80 222.00 201.10 221.00 214.00 220.50
Zr 168.10 190.40 194.10 208.10 211.70 206.70 211.90 210.00
Ba 1456.80 1666.62 1760.40 1967.93 1942.43 1999.46 1990.14 2003.82
La 16.07 18.46 16.27 16.63 16.65 17.38 16.88 17.31
Co 31.20 35.37 36.86 38.50 39.35 38.23 38.85 39.11
Pr 5.13 5.75 5.31 5.38 5.51 5.70 5.55 5.74
Nd 23.28 26.62 22.85 23.07 23.08 24.76 23.78 24.67
Sm 6.04 6.81 5.99 5.92 5.80 6.51 6.15 6.25
Eu 1.75 1.95 1.66 1.65 1.56 1.72 1.63 1.65
Gd 6.54 7.45 6.09 6.18 6.15 6.95 6.30 6.26
Tb 1.09 1.23 1.04 1.06 1.05 1.12 1.04 1.07
Y 42.46 46.77 38.41 39.86 40.01 41.58 39.48 39.52
Dy 6.87 7.62 6.28 6.44 6.49 6.78 6.63 6.43 ,
Ho 1.51 1.64 1.35 1.39 1.41 1.47 1.42 1.41
Er 4.53 4.92 4.10 4.28 4.36 4.66 4.33 4.49
Yb 4.58 5.08 4.45 4.46 4.60 4.95 4.76 4.80
Lu 0.75 0.83 0.71 0.75 0.73 0.82 0.77 0.77
V 75.91 13.59 31 .28 32.38 28.44 28.20 22.26 35.93
Cr 2.88 4.00 9.00 16.38 5.97 5.19 5.21 6.00
Nb 4.67 5.69 7.34 7.33 7.49 7.59 7.25 7.29
Hf 5.04 5.92 5.26 5.84 5.80 5.93 5.89 5.99
Ta 0.31 0.38 0.57 0.70 0.70 0.80 0.76 0.68
Pb 6.94 8.47 12.65 14.13 15.23 14.59 15.36 14.38
Th 3.50 3.97 3.31 3.58 3.66 3.65 3.62 3.74
U 2.63 3.18 3.57 4.07 4.11 4.13 4.02 4.01

EuIEu‘ 0.90 0.69 0.87 0.87 0.84 0.84 0.84 0.84
Distance 754.80 754.60 683.30 663.30 683.30 683.30 683.30 683.30

35

Table 2 continued

 

 

Apoyo Ash-flow Sheet

Sample 020011-40 0200114. 020011-00 020011-00 020011-00 020011-00 020011-01 020010-1c
Si02 65.75 66.40 65.24 50.84 67.41 65.48 55.08 65.86
1102 0.55 0.52 0.54 0.80 0.50 0.54 0.82 0.57
1020, 15.37 15.36 15.50 17.62 15.23 15.33 17.11 15.45
F020,", 4.42 3.94 4.32 10.32 3.95 4.21 9.19 4.57
Mn0 0.13 0.12 0.13 0.19 0.12 0.13 0.19 0.14
M90 1.13 1.03 1.13 4.84 1.03 1.13 3.25 1.20
Cao 4.06 3.80 3.84 9.75 3.74 3.74 7.75 4.11
Map 3.94 4.01 3.89 2.79 3.92 4.01 3.45 3.96
K20 1.98 2.04 2.02 0.73 2.13 2.08 1.00 1.97
P20. 0.14 0.13 0.14 0.13 0.13 0.14 0.19 0.14
Totals 97.47 97.35 96.75 98.01 98.16 96.79 98.03 97.97
Cr (XRF) bd bd bd bd bd bd bd bd
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn (XRF) bd bd bd bd bd bd bd bd
Rb 32.40 50.80 53.40 49.70 51 .70 49.30 65.50 64.30
Sr 345.70 248.90 252.40 245.50 248.80 251.60 209.90 195.20
Zr 146.90 190.80 191.40 190.20 189.50 192.80 216.90 221.20
Ba 1385.55 1817.29 1811.91 1837.73 1880.30 1830.66 1893.77 1882.10
La 13.62 14.98 14.85 15.11 14.61 14.71 14.66 14.91
00 29.16 34.62 33.52 34.11 34.60 34.12 32.45 32.73
Pr 3.78 4.90 4.81 4.93 4.93 4.90 4.55 4.47
Nd 15.46 20.59 20.29 20.79 21.23 20.26 18.46 18.25
Sm 3.73 5.19 5.19 5.44 5.38 5.28 4.57 4.58
Eu 1.19 1.40 1.42 1.44 1.45 1.45 1 .03 1.01
Gd 3.72 5.26 5.36 5.44 5.37 5.35 4.44 4.65
Tb 0.61 0.90 0.90 0.93 0.94 0.90 0.75 0.81
Y 23.77 36.98 37.28 37.47 35.69 36.83 31 .52 32.90
Dy 3.66 5.58 5.59 5.66 5.69 5.54 4.67 4.85
No 0.69 1.10 1.13 1.11 1.26 1.14 0.93 0.99
Er 2.20 3.41 3.51 3.56 3.79 3.51 3.00 3.11
Yb 2.68 4.00 4.08 4.27 4.23 4.17 3.68 3.80
Lu 0.42 0.62 0.64 0.65 0.65 0.64 0.57 0.58
V 80.80 26.17 23.25 21.72 22.61 23.11 54.17 58.14
Cr 3.77 4.50 4.17 4.23 5.12 5.00 5.41 4.79
Nb 7.54 6.80 6.68 6.89 6.78 7.19 6.59 6.85
l-lf 3.67 4.76 4.84 5.03 5.04 4.96 5.59 6.00
Ta 0.48 0.48 0.51 0.56 0.68 0.53 0.57 0.58
Pb 5.98 11.16 11.01 11.82 14.72 13.47 13.62 13.66
Th 2.14 1.93 2.00 2.05 2.92 2.07 2.72 2.81
0 1.80 1.90 1.91 1.97 3.82 2.20 2.50 2.41
Err/Eu“ 1.03 0.85 0.86 0.84 0.85 0.87 0.72 0.69
Distance 656.00 656.00 656.00 656.00 656.00 656.00 656.00 656.00

36

Table 2 continued

 

Apoyo Ash-ﬂow 011001

Sample 020616-21) 02061646 02061641 020616-39 020616411 020616-44: 020616-46 020610-41
5102 05.49 05.07 00.35 05.09 00.42 00.09 00.33 00.51
rio2 0.44 0.59 0.57 0.55 0.50 0.53 0.57 0.51

A120. 15.74 14.97 15.10 15.40 15.20 15.42 15.27 15.05
0020,", 4.29 4.47 4.31 4.42 4.44 4.33 4.71 3.09
Mac 0.12 0.14 0.13 0.14 0.14 0.13 0.14 0.13
M90 1.33 1.23 1.09 1.24 1.13 1.10 1.31 1.05
Ca0 4.30 3.02 3.00 3.90 3.09 4.01 4.01 3.45
Nazo 3.70 3.92 3.90 3.91 3.09 3.95 3.90 3.99
K20 1.95 2.12 2.10 2.00 2.11 2.07 2.00 2.21
9.0. 0.11 0.14 0.14 0.15 0.14 0.14 0.15 0.14
Totals 97.47 90.07 97.49 97.70 97.12 90.37 90.51 90.99
Cr (XRF) bd bd bd bd bd bd bd bd

Ni (XRF) 00 00 00 bd bd bd 00 00

Cu (XRF) bd bd bd bd bd bd bd bd

Zn (XRF) bd 00 bd bd bd bd bd bd

Rb 40.00 40.70 41.10 47.70 45.40 40.70 40.00 40.20
Sr 204.40 250.10 254.10 201.30 203.50 201.10 203.00 250.30
Zr 170.90 103.00 107.00 102.00 103.40 104.40 100.50 103.40
Ba 1792.50 1037.70 1940.93 1921.95 1009.50 1001.42 1091.91 1057.00
La 14.79 14.92 15.40 15.79 14.90 15.30 15.23 15.21
Ce 32.10 34.42 37.21 34.97 34.29 34.00 35.17 35.11
Pr 4.00 4.93 5.09 5.30 4.07 4.92 5.02 4.07

Nd 20.43 20.99 21.51 22.00 20.70 21.20 21.52 21.22
3m 5.30 5.34 5.52 5.77 5.37 5.43 5.31 5.15

Eu 1.47 1.50 1.50 1.04 1.44 1.50 1.00 1.51

00 5.49 5.45 5.57 5.90 5.44 5.00 5.03 5.40

Tb 0.90 0.92 0.92 0.90 0.95 0.90 0.94 0.90

v 30.21 37.74 30.30 37.07 35.05 37.03 35.09 35.00
Dy 5.35 5.40 5.02 5.90 5.70 5.07 5.70 5.09

Ho 1.07 1.13 1.20 1.20 1.25 1.29 1.20 1.20

Er 3.40 3.43 3.02 3.93 3.05 4.00 3.00 3.05

Yb 3.97 4.10 4.20 4.20 4.24 4.29 4.30 4.21

Lu 0.02 0.04 0.05 0.00 0.07 0.70 0.10 0.09

v 20.04 29.11 22.04 40.05 24.07 25.43 24.75 21.25
Cr 4.47 4.05 4.73 5.04 3.94 4.30 3.30 4.00

Nb 5.95 0.03 0.07 0.02 0.70 0.07 0.07 0.04

111 4.00 4.07 5.03 4.03 4.90 4.99 5.10 4.91

Ta 0.40 0.53 0.03 0.57 0.01 0.02 0.03 0.02

Pb 9.90 11.22 14.00 12.00 13.55 13.05 14.29 14.70
111 1.91 2.02 3.03 2.01 2.93 2.92 2.92 3.01

u 1.70 1.90 3.92 3.40 3.01 3.70 3.00 3.04

swam 0.07 0.09 0.07 0.90 0.04 0.07 0.94 0.90

Distance 050.00 050.00 050.00 050.00 050.00 050.00 050.00 050.00

37

Table 2 continued

 

Apoyo Ash-ﬂow Sheet
Sample 020010-00 020010-00 020010-01
Si02 66.53 65.59 86.95
Ti02 0.46 0.47 0.42
N20, 14.86 15.16 14.78
F020,", 4.22 4.62 3.94
MnO 0.13 0.14 0.12
M90 1.15 1.32 1.06
000 3.79 4.26 3.68
Nazo 3.79 3.62 3.60
K10 2.11 1.95 2.17
P20. 0.12 0.12 0.11
Totals 97.16 97.25 96.83
Cr (XRF) bd bd bd
Ni (XRF) bd bd bd

‘ Cu (XRF) bd bd bd
Zn (XRF) bd bd bd
Rb 49.20 49.20 48.40
Sr 243.30 250.50 259.90
Zr 192.70 189.20 185.50
Ba 1892.57 1883.81 1860.38
La 15.11 15.32 15.54
00 35.13 35.74 35.90
Pr 5.00 4.98 5.13
Nd 21.30 21.31 21.59
Sm 5.38 5.34 5.38
Eu 1.46 1.54 1.57
Gd 5.61 5.44 5.64
Tb 0.96 0.93 0.96
Y 36.96 35.91 36.70
Dy 5.83 5.90 5.80
Ho 1.28 1.28 1.30
Er 3.87 3.95 3.90
Yb 4.26 4.33 4.18
Lu 0.69 0.69 0.70
V 25.63 26.13 22.14
Cr 5.09 5.18 4.97
Nb 7.17 6.82 6.78
Ht 5.16 5.08 5.12
Ta 0.80 0.62 0.68
Pb 14.64 13.86 15.30
Th 3.10 3.03 3.04
0 4.22 3.79 3.81

EulEu‘ 0.85 0.91 0.91
Distance 683.30 683.30 683.30

38

Table 2 continued

 

Monte Galen Ash-ﬂow Sheet

Sample 020620-100: 020620-109 0206204011 020620416 020620-111 02062041111 02062040 W
0102 05.32 00.91 00.11 00.23 00.25 07.33 07.30 07.05
1102 0.59 0.40 0.44 0.44 0.42 0.45 0.50 0.51
A1202 14.79 14.34 14.24 14.29 14.10 14.37 14.29 14.30
0020.", 4.41 3.97 3.00 3.03 3.00 3.93 3.73 3.70
MnO 0.10 0.14 0.15 0.15 0.14 0.14 0.14 0.14
M90 0.00 0.00 0.55 0.57 0.53 0.59 0.50 0.04
CaO 2.00 2.23 2.05 2.12 2.02 2.19 2.29 2.30
N020 3.09 3.44 3.70 3.19 3.70 3.55 3.23 3.07
K20 2.05 3.23 3.22 3.07 3.40 3.47 3.22 2.92
P20; 0.10 0.10 0.09 0.09 0.09 0.10 0.11 0.11
Totals 95.97 95.42 90.41 90.50 90.51 90.12 95.37 90.30
Cr (XRF) 00 bd bd bd bd bd 00 00
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) 00 bd bd 00 bd bd bd bd
Zn (XRF) bd 00 bd bd bd bd 00 bd
Rb 41.20 43.20 50.10 30.30 20.50 33.00 31.40 27.00
Sr 490.00 527.10 473.10 304.00 301.90 307.00 302.90 310.50
Zr 123.00 110.10 145.70 123.00 125.20 120.00 129.50 121.50
Ba 1095.24 1070.00 1347.55 991.04 957.90 905.10 991.05 029.05
La 12.12 10.17 20.70 11.40 11.10 11.30 11.51 14.23
c. 25.24 20.44 35.30 22.35 21.00 22.35 22.09 21.22
Pr 3.70 5.00 7.30 3.73 3.05 3.73 3.72 4.22
Nd 10.75 22.21 33.11 17.00 17.47 11.05 10.15 20.35
Sm 4.27 5.50 0.30 4.99 4.90 5.03 5.20 5.01
Eu 1.21 1.00 2.25 1.50 1.49 1.49 1.52 1.07
Gd 4.12 5.51 0.14 5.79 5.51 5.03 5.04 0.05
Tb 0.07 0.00 1.30 1.01 0.90 0.90 1.00 1.10
v 24.59 34.70 50.40 30.94 30.01 30.00 37.44 50.02
Dy 4.00 5.49 0.01 0.23 0.19 0.19 0.41 7.70
Ho 0.00 1.21 1.99 1.39 1.33 1.34 1.39 1.01
Er 2.32 3.50 5.51 4.24 3.92 4.02 4.15 5.24
Yb 2.40 3.55 5.07 4.19 4.01 3.94 4.11 5.10
Lu 0.37 0.53 0.09 0.09 0.05 0.00 0.09 0.00
v 130.44 147.40 101.90 252.92 244.20 252.01 244.12 243.00
Cr 3.00 3.30 3.31 4.02 4.00 4.05 4.72 5.09
Nb 2.90 2.02 3.40 4.11 4.07 4.11 4.25 3.01
111 3.53 3.35 4.27 4.20 4.24 4.30 4.40 3.57
Ta 0.21 0.21 0.20 0.20 0.27 0.27 0.29 0.20
Pb 4.41 4.54 5.00 4.57 4.00 4.00 4.97 4.03
Th 2.97 2.00 3.70 2.44 2.47 2.50 2.51 1.07
u 1.10 1.30 4.10 1.72 1.09 1.09 1.77 3.30
EulEu‘ 0.92 0.94 0.07 0.90 0.91 0.90 0.09 0.00
Distance 075.00 075.00 015.00 075.00 075.00 075.00 717.00 717.00

39

Table 2 continued
Monte Galen Ash-ﬂow Sheet

Sample 020020-00 020020-00 020020-09
0102 07.90 00.73 00.39
no2 0.49 0.50 0.40
A1203 14.47 14.34 14.31
F020,", 3.04 3.74 3.40

 

MnO 0.14 0.14 0.13
M90 0.01 0.02 0.57
eao 2.45 2.29 2.22
N020 3.07 3.99 3.99
K20 2.09 2.93 3.05
9,0. 0.11 0.11 0.11
Totals 90.57 95.39 90.71
Cr (XRF) bd 00 00
Ni (XRF) 00 00 bd
cu (XRF) bd bd bd
Zn(XRF) 00 00 bd
Rb 34.90 37.00 37.10
Sr 257.70 235.90 242.30
Zr 210.70 234.50 220.50
Ba 1297.15 1455.25 1355.40
La 14.09 17.40 15.01
Ce 34.00 42.17 30.52
Pr 5.10 5.90 5.43
Nd 23.31 27.05 24.52
Sm 0.37 7.15 0.75
Eu 1.01 2.05 1.00
Gd 7.00 7.73 7.12
1'0 1.23 1.30 1.24
Y 47.75 49.00
Dy 7.43 7.79
Ho 1.09 1.70
Er 4.70 5.10
Yb 4.90 5.21
Lu 0.70 0.01
,v 59.04 00.00 39.03
er 4.40 0.94 4.44
Mb 0.31 0.95 0.52
111 5.53 0.31 5.00
Ta 0.51 0.57 0.57
Pb 0.31 7.25 0.91
Th 2.70 3.15 3.00
u 2.01 2.07 2.11

EulEu“ 0.86 0.88 0.86
Distance 717.00 717.00 717.00

Table 2 continued

Las Sierras Ash-ﬂow Sheet

Sample 01011030 01011040 010710-30 01011035 010110-10 010710-10 010110-10 010110-11
Si02 66.72 67.79 67.71 64.75 66.86 68.42 67.15 67.15

T102 0.54 0.50 0.52 0.58 0.51 0.48 0.49 0.53

A120. 15.06 14.68 14.61 15.40 14.52 14.29 14.44 14.73

F010,", 4.07 3.67 3.84 5.15 3.98 3.65 3.69 4.08

 

14110 0.14 0.14 0.14 0.15 0.14 0.14 0.14 0.15
10190 0.70 0.60 0.65 1.18 0.67 0.53 0.56 0.62
C80 2.70 2.44 2.58 3.61 2.57 2.29 2.34 2.39
N820 2.99 3.25 3.59 3.34 3.45 3.42 3.58 3.03
K20 2.82 3.05 2.95 2.40 2.92 3.03 2.96 2.85
P20. 0.12 0.11 0.11 0.15 0.12 0.10 0.10 0.11
Totals 95.86 96.23 96.70 96.71 95.74 96.35 95.45 95.64
Cr (XRF) 21.50 1153.70 20.90 27.50 bd bd bd bd
Ni (XRF) 21.30 148.50 bd bd bd bd bd bd
Cu (XRF) bd 46.10 bd 42.40 bd 24.90 bd bd
Zn (XRF) 104.70 88.90 104.50 93.60 95.40 104.00 124.00 95.30
Rb 41.30 30.00 38.40 32.30 58.40 65.80 67.80 46.80
81' 244.70 209.60 233.90 227.20 390.60 394.80 376.20 419.90
Zr 225.60 226.10 227.30 232.40 209.00 212.40 218.20 208.10
Ba 1383.10 949.20 1314.10 1328.30 1580.00 1700.00 1570.00 1960.00
La 16.51 14.72 16.47 13.96 21.10 21.70 22.00 21.90
Ce 37.66 35.06 34.76 33.68 39.20 40.50 39.50 38.70
Pr 5.61 4.94 5.41 4.70 6.56 6.72 6.87 6.52
Nd 24.72 22.72 24.68 21.65 29.20 29.00 30.90 29.60
8111 6.57 6.20 6.76 5.83 7.05 7.15 7.56 7.27
E11 1.87 1.65 1.71 1.58 1.75 1.77 1.83 1.77
Gd 6.67 6.60 6.97 5.90 6.78 6.90 7.39 6.89
Tb 1.14 1.11 1.21 0.98 1.16 1.15 1.26 1.13
Y 46.48 43.22 48.56 38.48 48.40 47.50 57.50 45.30
Dy 7.20 6.77 7.61 6.02 7.01 6.74 7.84 6.52
Ho 1.53 1.40 1.59 1.26 1.52 1.46 1.74 1.40
Er 4.59 4.26 4.86 3.82 4.40 4.28 5.22 4.00
Yb 5.02 4.63 5.11 4.22 4.78 4.51 5.95 3.94
Lu 0.74 0.68 0.77 0.61 0.77 0.70 0.98 0.64
V 12.60 78.32 31.95 37.23 68.50 75.30 64.50 77.10
Cr 14.37 10.93 10.36 16.47 11.30 13.20 11 .30 11.00
Nb 9.95 8.27 9.20 9.16 5.43 5.62 5.48 5.10
Hf 4.41 5.30 5.60 5.06 5.26 4.90 5.51 5.43
Ta 0.46 0.48 0.49 0.48 0.30 0.32 0.33 0.31
Pb 7.32 5.17 4.72 5.00 8.64 10.40 8.22 8.83
T11 2.70 2.99 3.20 2.94 4.65 4.66 5.10 5.00
U 3.07 2.06 2.27 2.21 2.02 2.31 2.03 1.46

511le 0.90 0.83 0.79 0.86 0.80 0.80 0.77 0.80
Distance 717.00 717.00 717.00 717.00 675.00 675.00 675.00 675.00

41

Table 2 continued

 

Las Sierras Ash-ﬂow Sheet

Sample 010110-11 010710-11 010710-1L 010718-1m 010110-10 020020-10 020020-111 020020-11
Si02 64.07 67.68 69.32 69.25 68.34 65.83 66.99 67.85
110, 0.52 0.49 0.43 0.42 0.44 0.58 0.50 0.50
~20, 14.65 14.38 14.23 14.15 14.56 15.04 14.36 14.41
F020,", 3.78 3.82 3.33 3.27 3.54 4.64 3.82 3.74
Mn0 0.14 0.14 0.08 0.07 0.08 0.14 0.14 0.14
M90 0.59 0.62 0.64 0.66 0.75 1.24 0.68 0.60
000 2.27 2.34 2.18 2.19 2.52 3.96 2.38 2.39
N020 3.44 3.35 2.96 2.92 3.19 3.78 3.83 3.78
K20 2.77 3.02 3.85 3.89 3.56 2.07 3.00 3.03
P20. 0.10 0.11 0.08 0.07 0.06 0.15 0.12 0.12
Totals 92.33 95.95 97.10 96.89 97.04 97.43 95.82 96.56
Cr (XRF) 1179.90 23.60 22.30 21.70 bd bd 29.10 bd
Ni (XRF) 133.00 19.50 bd bd bd bd bd bd
Cu (XRF) 20.90 16.50 46.10 bd bd bd 36.70 bd
Zn (XRF) 98.20 74.00 90.30 67.50 64.00 67.70 70.50 -
Rb 59.40 41.40 39.30 44.80 51.80 48.50 43.10 50.90
Sr 340.50 447.30 436.90 284.60 258.00 265.30 311.60 263.50
Zr 252.30 176.40 151.90 179.80 186.10 182.10 158.10 185.00
Ba 2140.00 1720.00 1680.00 1564.20 1591.40 1581.20 1538.00 1813.31
La 20.30 27.10 19.50 14.24 15.72 13.69 13.15 14.49
00 46.80 36.90 35.90 31.60 31.37 32.23 29.24 33.78
Pr 6.29 7.93 6.20 4.71 4.89 4.65 4.56 4.70
Nd 26.90 36.30 28.70 20.45 21.76 19.64 19.99 19.68
Sm 6.31 9.20 7.27 5.19 5.39 4.93 5.24 5.10
Eu 1.62 2.09 1.73 1.41 1.33 1.37 1.43 1.32
Gd 5.44 8.84 6.64 5.10 5.54 4.78 5.12 5.19
Tb 0.92 1.45 1.05 0.84 0.94 0.78 0.82 0.89
Y 33.60 61.10 36.00 32.94 38.21 31.73 32.29 35.82
Dy 5.12 8.38 5.86 5.13 5.75 4.94 4.95 5.63
110 1.09 1.82 1.18 1.05 1.21 1.02 1.07 1.08
Er 3.08 5.15 3.25 3.25 3.72 3.21 3.34 3.37
Yb 3.07 5.10 2.95 3.77 4.27 3.60 3.55 3.84
Lu 0.46 - 0.83 0.46 0.55 0.64 0.51 0.55 0.62
V 42.20 87.10 117.00 32.50 17.05 27.21 83.75 36.83
01 14.80 10.20 10.90 13.49 10.84 15.48 16.40 9.35
Nb 7.42 4.36 3.87 6.52 5.99 6.86 5.51 6.45
Ht 6.19 4.92 4.29 3.66 4.45 3.71 3.12 4.74
Ta 0.42 0.28 0.25 0.35 0.38 0.36 0.30 0.56
Pb 13.40 6.99 6.43 10.16 7.64 11.84 8.95 11.95
Th 5.82 4.47 3.94 2.42 2.74 2.40 2.08 2.01
0 2.46 1.11 1.07 2.89 2.56 3.21 2.71 2.09
EulEu‘ 0.86 0.74 0.80 0.88 0.78 0.90 0.89 0.81
Distance 675.00 675.00 675.00 656.00 656.00 656.00 656.00 683.30

42

Table 2 continued

 

Las Sierras Ash-ﬂow Sheet

Sample 020020-1111 020020-20 02002020 02002040 02002030 020020-40 020020-00 020020-01
Si02 68.00 68.56 67.99 69.09 68.96 66.48 67.35 67.40
T10, 0.48 0.48 0.49 0.40 0.42 0.52 0.52 0.50
A1203 14.34 14.33 14.38 14.03 13.86 14.87 14.29 14.71
Fe202m 3.62 3.69 3.72 3.26 3.42 3.95 3.91 3.69
Mn0 0.14 0.13 0.14 0.09 0.09 0.14 0.14 0.14
M90 0.58 0.58 0.59 0.63 0.66 0.63 0.68 0.56
000 2.34 2.37 2.35 2.38 2.30 2.59 2.48 2.38
Na20 3.62 3.77 3.86 3.05 3.00 3.31 3.64 3.65
K20 3.14 3.02 3.02 3.65 3.71 2.86 2.93 2.89
P20. 0.11 0.11 0.11 0.09 0.10 0.12 0.12 0.11
Totals 96.37 97.04 96.65 96.67 96.52 95.47 96.06 96.03
Cr (XRF) bd bd bd bd bd bd bd bd
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn (XRF) bd bd bd bd bd bd bd bd
Rb 52.20 54.10 52.80 57.10 50.40 65.50 65.70 61.20
Sr 244.90 251.80 252.20 250.10 244.60 197.00 195.90 217.30
Zr 191.70 192.20 192.20 188.90 190.70 223.30 223.40 215.70
Ba 1839.60 1895.17 1839.78 1851.54 1814.38 1882.63 1848.94 1852.88
La 14.59 15.14 15.40 14.75 14.73 15.13 14.75 14.99
00 34.70 35.26 36.21 34.31 33.90 34.93 33.46 32.36
Pr 4.87 4.98 5.21 4.84 4.79 4.91 4.64 4.75
Nd 20.20 20.77 22.07 20.12 20.35 19.85 18.56 19.09
Sm 5.32 5.17 5.56 5.36 5.16 4.95 4.50 4.82
Eu 1.37 1.35 1.45 1.36 1.35 1.05 0.96 1.11
Gd 5.31 5.39 5.65 5.17 5.22 4.73 4.39 4.67
Tb 0.90 0.89 0.92 0.87 0.89 0.80 0.78 0.79
Y 36.2 37.26 38.50 36.70 36.60 32.99 32.25 32.72
Dy 5.77 5.77 5.93 5.61 5.53 4.90 4.76 4.84
Ho 1.10 1.11 1.17 1.11 1.11 0.96 0.93 0.96
Er 3.50 3.57 3.58 3.54 3.39 2.97 2.98 3.10
Yb 4.11 4.21 4.12 4.02 4.05 3.68 3.53 3.83
Lu 0.64 0.65 0.67 0.62 0.61 0.57 0.57 0.59
V 24.45 28.83 31.56 29.18 29.19 65.02 59.91 68.28
Cr 10.27 12.76 10.46 12.81 9.38 13.58 12.95 12.29
Nb 7.17 6.78 7.01 7.35 6.66 7.31 6.80 7.26
H1 5.04 4.86 4.93 4.90 4.85 5.80 5.61 5.70
Ta 0.65 0.59 0.56 0.55 0.55 0.63 0.65 0.56
Pb 13.66 12.68 12.11 12.07 10.89 13.19 12.13 11.61
Th 2.17 2.02 2.11 2.08 2.08 2.72 2.58 2.75
0 2.30 2.16 2.15 2.10 2.09 2.65 2.48 2.41
EuIEu' 0.82 0.83 0.84 0.83 0.69 0.67 0.74
Distance 683.30 683.30 683.30 683.30 683.30 683.30 683.30 683.30

43

Table 2 continued
Las Sierras Ash-ﬂow Sheet

Sample 02062060 020620411 020620411 020620-79 020620-71

 

0102 00.27 00.97 07.07 07.03 00.30
1102 0.50 0.51 0.51 0.50 0.40
A120, 14.09 14.44 14.43 14.45 14.42
F020,", 4.35 3.00 3.01 3.77 3.00
Mn0 0.15 0.14 0.14 0.14 0.14
M90 0.73 0.04 0.01 0.03 0.59
CaO 2.09 2.47 2.43 2.40 2.44
N020 2.00 3.13 3.04 3.00 307
K20 2.00 2.01 2.90 2.77 2.07
P20. 0.12 0.11 0.12 0.12 0.11
101010 95.42 95.02 90.20 95.75 90.90
or (XRF) 20.00 00 00 00 00
Ni (XRF) 00 00 00 00 00
Cu (XRF) 00 00 00 00 00
Zn (XRF) 57.00 50.00 57.10 59.50 57.00
Rb 35.50 37.00 37.20 30.30 37.00
Sr 340.00 300.50 303.30 374.70 309.00
Zr 102.10 101.00 102.30 150.90 105.00
Ba 1450.00 1490.00 1400.00 1410.00 1500.00
La 14.70 15.00 15.10 14.20 14.00
00 29.00 30.00 30.00 29.10 30.50
Pr 4.2 4.24 4.29 4.03 4.25
Nd 17.10 17.30 17.30 10.00 17.20
Sm 3.97 4.17 4.05 3.00 3.95
Eu 1.34 1.30 1.35 1.31 1.37
00 3.70 3.74 3.77 3.00 3.70
10 0.01 0.03 0.02 0.59 0.01
v 24.90 25.40 25.00 24.20 25.00
Dy 3.03 3.71 3.01 3.53 3.53
110 0.79 0.00 0.77 0.75 0.77
Er 2.30 2.32 2.33 2.24 2.27
Yb 2.72 2.74 2.00 2.59 2.00
Lu 0.41 0.42 0.41 0.40 0.41
v 79.90 04.70 00.00 92.90 05.00
Cr 15.70 17.50 10.40 10.00 10.30
Nb 0.09 0.35 0.20 0.01 0.05
H1 3.02 3.50 3.02 3.41 3.73
10 0.44 0.45 0.40 0.41 0.40
Pb 0.90 7.30 7.14 0.50 7.03
111 3.01 3.01 2.07 2.77 2.94
u 2.00 2.99 2.01 2.07 3.00
EulEu" 1.11 1.09 1.10 1.11 1.14
Distance 754.00 754.00 754.00 754.00 754.00

44

Table 2 continued

 

3011 1001001 Ash-ﬂow 311001

Sample 020010-10 020010-11 020010-111 020010-10
sio2 04.15 03.42 02.35 03.25
1102 0.77 0.19 0.03 0.70
A1202 13.93 13.97 14.25 13.93
F020,", 1.05 7.97 0.50 1.53
MnO 0.21 0.20 0.21 0.20
M90 0.05 0.07 1.47 0.70
C00 3.49 3.43 4.40 3.27
N020 4.04 4.00 4.04 4.00
11,0 2.35 2.30 2.00 2.33
P20. 0.21 0.21 0.22 0.21
101010 97.05 97.10 90.41 90.34
Cr(XRF) 00 00 00 00
Ni(XRF) 00 00 00 00
Cu (XRF) 00 00 00 00
Zn(XRF) 00 00 00 00
Rb 50.20 49.70 05.20 03.00
Sr 251.10 231.40 540.10 500.00
Zr 109.20 195.90 157.70 150.00
Ba 1035.41 1901.51 1270.29 1311.05
La 15.47 15.97 23.00 21.02
00 35.30 30.05 40.94 40.14
Pr 5.13 5.27 0.07 0.14
Nd 21.01 22.33 30.10 27.25
3111 5.59 5.02 0.90 0.47
Eu 1.50 1.52 1.05 1.70
00 5.05 5.75 0.00 0.25
10 0.90 0.90 0.99 0.95
Y 37.03 37.41 37.24 34.10
Dy 0.00 0.12 5.79 5.50
140 1.30 1.31 1.27 1.19
Er 4.05 3.92 3.47 3.29
110 4.40 4.37 3.42 3.39
Lu 0.70 0.72 0.55 0.51
v 21.37 10.03 191.24 195.23
or 4.53 5.00 0.09 0.10
Nb 0.40 0.93 3.97 4.15
111 5.20 5.31 4.20 4.29
hi 0.02 0.03 0.20 0.30
90 14.40 15.30 3.90 4.10
T11 3.10 3.14 5.23 5.30
u 3.54 3.03 2.10 2.74

EulEu‘ 0.89 0.83 0.90 0.91
Distance 683.30 683.30 670.00 670.00

45

Table 2 continued
Ostocal Ash-flow Sheet

Sample 010716-1A 010716-13 010716-16 010716-10 020624-11: 020624-16 020624-111 020624-11

 

8102 05.01 02.02 05.09 04.00 04.33 04.11 05.49 05.55
no2 0.07 0.73 0.07 0.70 0.70 0.70 0.09 0.07
A1203 14.04 14.74 14.09 14.71 14.22 14.07 13.99 13.92
mom, 0.21 0.90 0.10 0.10 0.00 0.44 0.29 0.23
MnO 0.20 0.22 0.19 0.24 0.20 0.20 0.19 0.20
11190 0.00 2.43 1.04 1.39 1.10 1.03 0.91 0.00
oao 3.14 2.90 2.93 2.94 3.40 2.99 3.05 3.05
110.0 4.20 2.32 3.75 3.05 3.00 3.93 3.94 4.10
K20 2.23 1.01 2.19 2.00 1.90 2.01 2.10 2.10
P20. 0.2 0.29 0.10 0.14 0.20 0.10 0.10 0.10
101010 97.32 95.22 90.09 90.29 90.77 95.70 90.01 90.92
Cr(XRF) 00 00 00 00 00 00 00 00

Ni(XRF) 00 00 00 00 00 00 00 00

Cu (XRF) 00 00 00 00 00 00 00 00

Zn (XRF) 00 00 00 00 00 00 00 00

Rb 40.50 34.90 13.00 30.70 37.40 10.30 34.30 37.30
Sr 352.20 357.90 400.00 340.90 342.00 473.20 359.40 350.20
Zr 150.70 154.20 50.10 100.30 101.10 01.70 149.10 140.30
00 1532.43 1490.94 502.40 1499.01 1501.07 790.32 1359.07 1415.00
La 13.95 13.72 7.19 14.01 14.50 9.54 13.00 12.44
Ce 32.29 30.50 14.33 31.19 31.54 19.10 29.03 20.00
Pr 4.02 3.95 2.20 4.10 4.11 2.07 3.09 3.39
Nd 15.90 15.70 10.01 15.01 10.03 13.22 15.57 13.39
Sm 3.04 3.99 2.90 3.01 3.00 3.43 3.00 3.31

Eu 1.30 1.31 1.00 1.31 1.30 1.20 1.20 1.12
00 3.71 3.70 3.13 3.04 4.10 3.52 3.01 3.09
10 0.02 0.01 0.52 0.00 0.00 0.57 0.01 0.49
v 20.70 21.70 10.45 21.91 22.52 20.10 22.31 10.50
Dy 3.30 3.55 3.11 3.45 3.07 3.30 3.59 3.00
Ho 0.70 0.79 0.00 0.77 0.03 0.73 0.02 0.00
Er 2.00 2.15 1.09 2.02 2.34 2.07 2.22 1.01

v0 2.59 2.57 1.90 2.07 2.90 2.15 2.51 2.35
Lu 0.40 0.41 0.29 0.40 0.43 0.34 0.40 0.30
v 00.04 75.05 301.31 00.09 70.27 235.21 72.50 00.09
Cr 3.75 3.77 21.97 5.30 4.00 5.57 4.07 0.30
Nb 7.70 1.27 2.25 7.44 7.75 3.14 7.00 5.05

111 3.47 3.02 1.07 3.05 3.90 2.20 3.52 3.05
10 0.49 0.52 0.14 0.49 0.51 0.21 0.43 0.30

Pb 0.19 0.00 1.75 5.02 5.50 2.44 5.10 4.94

Th 2.90 3.03 1.10 2.93 3.22 1.05 2.72 2.94

u 3.00 3.15 0.01 3.10 3.10 1.22 2.02 2.79

EulEu‘ 1.09 1.00 1.12 1.12 1.12 1.10 1.00 1.13

Distance 754.00 754.00 754.00 754.00 754.00 754.00 754.00 754.00

46

8“

 

Table 2 continued

 

Ostocal Ash-flow Sheet

Sample 02062441: 020624411 020624-29 020624-211
0102 05.12 04.50 04.59 05.10
no. 0.00 0.09 0.70 0.00
111,0, 13.00 13.91 14.09 13.09
00.03‘“ 0.35 0.23 0.43 0.32
Mn0 0.23 0.20 0.40 0.20
M90 0.04 0.00 0.00 0.00
000 4.04 3.13 3.20 3.00
100,0 3.97 4.01 4.00 3.93
K20 2.10 2.11 2.10 2.13
no. 0.43 0.10 0.20 0.17
101010 97.42 95.04 90.05 90.54
Cr(XRF) 00 00 00 00
Ni (XRF) 00 00 00 00
Cu (XRF) 00 00 00 00
Zn(XRF) 00 00 00 00
R0 42.00 41.70 30.30 30.50
sr 325.00 334.00 355.00 340.30
Zr 100.40 103.20 157.50 100.00
30 1501.03 1509.90 1432.04 1459.40
La 14.07 14.30 13.03 13.91
00 31.50 31.03 29.04 30.35
Pr 4.20 4.12 4.05 4.00
Nd 17.12 17.10 15.03 10.34
Sm 4.32 4.24 3.07 3.01
Eu 1.34 1.33 1.30 1.33
00 4.12 3.99 3.97 4.01
10 0.00 0.04 0.03 0.03
Y 24.41 23.10 22.49 23.30
Dy 3.02 3.00 3.53 3.14
Ho 0.04 0.00 0.00 0.02
Er 2.43 2.10 2.13 2.19
v0 2.01 2.00 2.71 2.79
Lu 0.45 0.41 0.40 0.42
v 73.30 71.25 70.24 70.31
Cr 4.20 3.95 3.07 3.72
Nb 0.51 7.09 7.41 7.29
H1 3.03 3.72 3.52 3.01
10 0.47 0.49 0.49 0.49
Pb 5.41 5.04 5.02 5.20
111 3.00 3.10 2.94 3.01
u 3.00 3.00 2.92 2.90

EulEu“ 1.01 1.04 1.13 1.12
Distance 754.80 754.80 754.80 754.80

47

Table 2 continued

 

Coyol Ash-ﬂow Sheet

Sample 020022-10 020022-10 020022-111 020022-20 020022-20 02002240 02002240 02002240
8102 54.34 54.24 64.31 66.51 67.33 64.65 62.94 65.29
Ti02 0.86 0.86 0.64 0.64 0.61 0.68 0.73 0.64
A1203 16.39 16.38 15.33 15.29 15.14 16.00 15.59 15.42
F020,", 9.62 9.56 3.96 3.89 3.61 4.35 5.45 3.98
MnO 0.17 0.17 0.12 0.13 0.12 0.09 0.18 0.11
M90 3.00 2.83 0.98 0.50 0.31 0.77 0.66 0.41
000 7.24 7.45 1.74 1.88 1.63 2.12 3.22 1.95
N010 3.38 3.43 2.48 4.01 4.01 3.62 3.72 4.15
K20 1.99 2.03 5.43 5.21 5.27 4.17 4.19 4.72
P20. 0.62 0.82 0.16 0.18 0.15 0.12 0.36 0.19
Totals 97.61 97.77 95.15 98.24 98.18 96.57 97.04 96.86
Cr (XRF) bd bd bd bd bd bd bd bd
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn (XRF) bd bd bd bd bd bd bd bd
Rb 148.40 142.90 147.80 94.40 104.40 106.00 90.90 65.90
Sr 265.30 289.10 258.40 330.20 426.40 312.30 564.00 374.60
Zr 354.60 346.00 358.10 333.20 239.30 339.20 216.70 189.80
80 2219.12 2504.21 2488.68 2604.57 2324.41 2687.50 1962.11 1979.85
La 30.28 31 .03 30.07 22.45 29.48 31.71 24.84 22.00
Ce 69.35 68.96 67.19 49.40 63.14 64.01 45.70 45.86
Pr 8.56 8.96 8.64 5.96 8.62 8.94 7.23 7.00
Nd 33.85 35.36 34.00 24.16 35.41 35.16 30.74 30.77
Sm 7.14 7.86 7.56 5.40 7.81 8.08 6.98 7.24
Eu 1.69 1.94 1.81 1.84 2.02 1.90 1.65 1.90
Gd 6.68 7.13 6.82 5.20 7.01 7.16 6.78 6.70
Tb 1.04 1.14 1.09 0.81 1.05 1.11 1.07 1.06
Y 35.96 39.87 39.42 26.64 30.40 37.46 37.37 39.56
Dy 5.94 6.58 6.41 4.66 5.58 6.49 6.10 6.23
Ho 1.28 1.40 1.39 1.00 1.12 1.35 1.30 1.42
Er 3.74 3.87 3.94 2.74 2.96 3.70 3.76 3.99
Yb 4.23 4.56 4.29 3.01 2.97 4.08 3.75 4.13
Lu 0.64 0.66 0.72 0.42 0.38 0.61 0.57 0.62
V 23.65 36.55 39.55 43.81 78.14 39.84 83.57 97.11
Cr 3.73 3.60 3.19 4.43 3.80 4.06 5.12 4.35
Nb 10.45 10.33 10.52 9.41 8.11 10.12 6.38 4.81
H1 8.55 8.35 8.37 7.98 6.20 8.13 5.52 4.85
To 0.86 0.86 0.94 0.80 0.55 0.91 0.39 0.38
Pb 21.88 14.95 12.52 20.67 9.65 37.18 10.69 10.21
Th 12.04 11.73 12.02 11.11 8.89 11.37 7.73 4.71
U 5.86 5.81 5.93 4.36 3.09 5.05 2.80 2.18
EulEu‘ 0.79 0.83 0.80 1.13 0.89 0.80 0.78 0.88
Distance 670.00 670.00 670.00 670.00 670.00 670.00 675.00 675.00

48

Table 2 continued

 

Las Maderas Ash-Flow Sheet

Sample 010110-20 010710-20 010110-2c 010710-20 010110-21 01071049 01011020 02002240
Si02 62.97 61 .46 61.43 62.24 64.59 62.98 61 .60 60.37
T102 0.77 0.77 0.78 0.80 0.68 0.80 0.79 0.85
A120. 15.39 15.16 15.68 15.72 15.29 15.69 16.03 16.04
F020,", 6.26 6.05 6.09 6.40 5.12 6.87 6.96 7.61
Mn0 0.15 0.15 0.14 0.14 0.14 0.10 0.16 0.15
M90 1.86 1.76 2.20 1.13 0.93 0.88 1.17 1.46
CaO 4.19 4.01 4.04 3.90 2.68 4.05 3.98 4.16
N020 2.74 2.72 1.97 4.00 3.58 3.65 3.47 3.65
K20 3.81 4.00 4.14 2.84 3.55 2.97 2.57 2.28
P20. 0.36 0.36 0.35 0.37 0.17 0.47 0.36 0.57
Totals 98.50 96.44 96.82 97.54 96.73 98.46 97.09 97.14
Cr (XRF) 20.00 bd bd bd bd bd bd bd
Ni (XRF) bd bd bd bd bd bd bd bd
Cu (XRF) bd bd bd bd bd bd bd bd
Zn (XRF) 61.20 bd bd bd bd bd bd bd
Rb 34.50 11.60 36.60 41.30 39.60 43.60 38.50 24.70
Sr 382.20 544.80 360.00 328.10 321.10 329.90 362.10 454.40
Zr 150.00 64.30 152.60 163.20 162.80 164.60 149.10 81.40
Ba 1350.00 725.78 1455.56 1542.28 1522.58 1614.74 1444.39 776.70
La 13.90 9.40 13.88 14.36 14.38 14.62 13.40 9.36
00 27.80 16.05 30.48 31.88 32.12 33.05 29.73 18.46
Pr 3.87 3.13 4.02 4.07 4.24 4.15 3.90 2.73
Nd 15.90 14.66 16.18 17.01 16.77 16.28 15.48 12.20
Sm 3.68 3.82 4.06 4.24 4.03 3.98 3.82 3.16
Eu 1.27 1.25 1.33 1.27 1.35 1.32 1.28 1.12
Gd 3.45 3.81 3.80 4.07 3.92 3.88 3.57 3.37
Tb 0.57 0.63 0.64 0.67 0.63 0.64 0.58 0.57
Y 24.10 23.66 23.78 24.73 22.67 22.22 20.73 20.22
Dy 3.50 3.86 3.90 3.81 3.76 3.60 3.36 3.32
No 0.76 0.80 0.83 0.92 0.82 0.84 0.75 0.75
Er 2.16 2.26 2.44 2.72 2.27 2.12 1.93 1.98
Yb 2.46 2.22 2.74 2.78 2.91 2.69 2.47 2.15
Lu 0.39 0.34 0.42 0.45 0.42 0.41 0.38 0.33
V 90.70 253.27 65.70 53.95 62.04 61.73 65.21 212.51
Cr 14.60 21 .70 3.37 3.04 4.85 4.89 3.89 5.93
Nb 7.44 2.30 6.80 7.53 8.40 8.33 7.03 3.33
H1 3.39 1.93 3.57 3.83 3.89 3.59 3.52 2.22
Ta 0.42 0.16 0.54 0.59 0.53 0.58 0.47 0.22
Pb 5.33 2.70 5.52 6.62 6.23 6.51 5.39 2.57
T11 2.70 1.25 2.79 2.94 3.11 3.03 2.80 1.61
U 2.36 0.56 2.40 2.63 3.68 3.81 2.98 1.42
EuIEu' 1.13 1.05 1.07 0.97 1.09 1.07 1.11 1 .10
Distance 754.80 754.80 754.80 754.80 754.80 754.80 754.80 754.80

49

Table 2 continued

 

L00 140001110 Ash-Flow 511001

Sample 0200224 020022-00 02002241 02002249
510, 03.25 57.79 54.02 50.00
110, 0.70 0.00 0.70 0.01
A1,o, 15.71 10.05 10.01 10.04
r-'e,o,m 5.39 0.14 0.02 7.04
MnO 0.13 0.12 0.12 0.14
M90 1.34 1.92 2.00 1.05
000 4.03 0.75 7.30 7.54
010,0 2.01 2.00 3.02 3.31
K,o 3.03 1.00 1.01 1.95
12,0. 0.20 0.27 0.35 1.79
101010 97.25 90.50 90.09 90.35
Cr(XRF) 00 00 00 00
Ni (XRF) 00 00 00 00
Cu (XRF) 00 00 00 00
Zn (XRF) 00 00 00 00
R0 40.50 40.30 35.70 34.50
sr 327.20 330.90 351.00 357.40
Zr 102.20 102.70 152.00 149.00
30 1524.04 1551.40 1449.00 1410.90
La 13.99 13.02 13.99 13.00
Ce 32.29 31.72 31.05 29.03
Pr 4.07 4.04 4.17 3.90
Nd 10.14 15.59 10.04 15.70
Sm 3.92 3.04 3.90 3.02
Eu 1.29 1.21 1.32 1.22
00 3.70 3.03 3.97 3.75
10 0.00 0.59 0.04 0.00
Y 21.31 22.00 22.22 21.00
Dy 3.30 3.57 3.00 3.73
Ho 0.77 0.70 0.79 0.00
Er 2.09 2.10 2.13 2.09
Yb 2.50 2.09 2.49 2.57
Lu 0.39 0.30 0.39 0.41
v 74.04 00.29 02.77 70.94
Cr 4.14 3.02 3.92 2.09
Nb 7.14 7.99 7.42 0.00
111 3.02 3.50 3.00 3.50
10 0.40 0.50 0.40 0.47
Pb 0.15 0.09 5.42 5.09
111 2.94 2.95 2.03 2.01
u 3.30 3.59 3.13 2.00
EulEu' 1.00 1.09 1.07 1.00

Distance 754.80 754.80 754.80 754.80

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~00..._0.~0.n..-0.00~0
.00....0.~0.0..0.00~0
.00...0.~o.n...0.00~0
0.950

00.020... «.0... «02.0 30.....2 .0003. :00

0.0205 30.0 x50...
02.0.28 0 0.00»

52

..0.00«
mfg
3.00
«N50
00.00

00.3
00.00
1.250
0 «.00
00.50
.30...

05.0..
3.09

00.0
«0.0
v «.0
0N0
.0

0N0
0...0
0N0
0N0
0...0
.0

00.0
9.0
.0

00.0
00.0
00...
05.0
00.0
03.

05.0
00.0
0 «.v
9.0
05.0
ON!

00.0
r «.N
09.

0.20
0...N
00.0
05.0
000
0002

EN
«0N
00.0
00.0
0N0
O~uz

00.0
00.0
3.2

00.0..
00.N
00.0
0v.N
0 .....
Oa0

30
00.«
00.«
00...
mm...
ONO

N5.5
00.5
Ou0

0.0
00.0
00.0
....
00.0
00.2

00.0
00.0
00.0
0.0.0
000
002

00.0
0~.0
00....

.x.
NN.0
..0.0
N ...0
00.0
0:2

0«.0
0«.0
00.0
00.0
N...0
0:5

0N0
0:2

00. ..
VNéN
0N. ..
vmé
05d
”ONGE

F.
«0..
00..
00..
.0.~

«of.

N00
v0.5
nOu‘E

.00
00.0
00.0
00.0
~00
«o...

.00
00.0
00.0
00.0
00.0
«o.»

00.0
~00
«o...

.0..~ 0~.00 ~.0.00.0.~.~00~0
0.0 0.00 ..0.0u.0.-00~0
00.0~ 00.~0 ._0.0..0.0.-00.~0
.00. 00.00 20.00.300.800
0.0. -.~0 03.00.30.030
«0«.< «0.0 0.0.000
025.25 03!: «02.0 .50—«.52 05.0qu as
00.0. 00.00 0.0.0o.0.-00~0
0.0. 00.00 ..0.0o.0.-00~0
00.0. 00.0.. ._0.~o.0.-00~0
00.0. :00 20.50.30.000:
.00. ~20 ..0..u.00.-00~0
6«_< «0.0 0.0.000
.800 300.02 0280.2 00..

00.0~ 00.00 .00....0..0.00.0~00~0
0~.... 0000 >00.._0.0«..0..-00~0
.0«.< «0.0 0.0.000

5.3.0:. «.2: «02.0 320...: .260

0.0.0.0.... «aa.0 x505.
02.5.5". 0 9.0.0...

53

Table 3 continued

Plagioclase Analysis

Apoyo Ash-flow Shoot

020017-10-01-p1-c 55.38
020617-1o-c1-p1-Int 54.84
020617-1o-c1-p1-r 71 .55
020617-1o-c2-p1-c 56.39
020617-1o-c2-p1 4nt 55.1 0
02081740624314? 56.19
020617-1o-c2-p1-r1 56.26
020617-1o-c3-p1-c 52.94
020617-1o-c3-p1-lnt 55.85
020617-1o-c3-p1-r 56.98
020817-10-65—91-6 52.51
02061 7-1od-p1-r 68.58
020817-2c-cz-p1-c 44.77
020617-20-c2-p1-lnt 45.33
020617-2042-914 50.72
02061740031314: 56.28
020617-26—63-91-1nt 57.67
020817-2c-c3.p1-r 55.14
020617-2c-c3-p2-mp 52.45
020617-5a-c3-p1-c 54.26
02061758c3-p14nt 56.57
02061745843414 65.82
02061758-c7-p1-c 55.84
020617-58-cT-p1-lnt 53.18
02061768494114 53.55
020617-68-cz-p1-c 45.61
020617-68-62-p1-lnt 45.31
020617-68-c2-p1-r 50.53
020617-68-422-92-4: 46.86
020617-60-cz-pz4nt 50.02
02001781182924 55.43
020617-68-c3-p1-c 51.92
020617-68-c3-p1-lnt 53.47
02061748431014 50.93
Monte Golan Ash-ﬂow Sheet
Sample 810:
020620-109-c1-p‘l-c 59.19
020820-1og-c1-p14nt 59.27
020620-109-c1-p1-r 60.32
020620-109-cz-mp1-c 61.39
020620-109-c2-mp1-r 50.14
0206204 0943-91-1: 60.39
020620-109-c3-p1-lnt 59.85
020620-109-c3-p1-I' 68.66
020620-88-01-p1-c 57.25

N203
28.34
28.82
15.29
28.14
29.21
27.48
28.34
29.88
28.02
27.88
30.23
19.22
35.39
34.83
31.12
27.83
28.59
27.91
28.14
27.97
28.52
19.98
27.18
28.15
28.88
33.89
33.88
30.20
32.33
30.10
25.13
29.98
28.95
30.32

N20:
25.30
25.40
28.10
18.41
30.59
24.72
25.77
18.84
25.98

F020,
0.41
0.46
2.06
0.47
0.43
0.39
0. 43
0. 43
0. 47
0.37
0.59
1.32
0.51
0.77
0. 53
0. 34
0. 37
0. 46
0. 79
0.41
0.37
1.19
0. 46
0. 41
0. 38
0. 82
0.75
1.03
0.92
0. 79
0. 54
0. 59
0.45
0.84

F020,
0.31
0. 34
0. 37
3.63
1.33
0.30
0.33
1.89
0.46

54

680
10.40
10.83
2.60
9.95
11.18
9.57
9.78
12.51
9.92
9.30
12.76
4.79
18.90
18.82
14.47
10.00
8.83
10.68
11.35
10.55
6.48
5.31
9.38
10.15
10.98
17.59
17.71
13.39
16.00
13.46
7.75
12.35
11.77
13.41

080
7. 26
7. 09
6.98
5. 34
13.98
6.28
7.39
3.62
7.98

N820
5.51

5.16
2.13
5. 72
5. 22
6.03
5.92
4.41

5. 67
6. 04
4. 07
4.19
0.73
1.04
3. 36
5. 69
6.16
5.16
4.42
5.24
6.21

4.79
5.90
5. 44
5.19
1.41

1.33
3.56
2.34
3. 66
6. 43
4.30
4.78
3.80

7. 04
7.17
7.49
3.49
3. 23
7. 64
7.10
5.39
6.56

K20
0.17
0.17
2.28
0.19
0.11
0.19
0.18
0.12
0. 22
0.20
0.07
1.45
0. 06
0. 01
0.13
0.20
0.27
0.16
0.19
0.16
0.21
1.19
0.20
0.13
0.15
0. 00
0. 03
0. 09
0. 00
0.11
0. 22
0.10
0.13
0.09

K20
0.39
0.29
0.42
2.49
0.1 1
0.41
0.39
2.02
0.37

Total

100.22
100.28

95.91

100.85
101.25

99.84
100.91

100.28
100.16
100.57
100.24

99.53

100.37
100.79
100.32
100.35

99.88
99.51
97.33
98. 59
98.36
98.28
98.96
97. 46
98.92
99.31
99.02
98.80
98.44
98.14
95.50
99.23
99.55
99.40

Total
99.49
99.57

101.69

94.76
99.38
99.74

100.83
100.43

98.60

An 96
53.16
28.39
48.48
53.88
46.23
47. 24
60. 64
48.52
45.44
63.13
33.96
93.16
90. 86
69. 90
48.70
43.51
52.84
57.96
52.17
42.44
34.51
46.20
50.38
53.41
87.36
87.89
67.16
79. 05
66.60
39. 43
61.00
57.21
65.74
54.22

An 96
33.18
36.53
70.03
30.50
35.69
22.95
39.34
37.37
25.61

Table 3 continued

Plagioclase Analysis
Monte Galan Ash-ﬂow Sheet
Sample Si02
020620-Oa-c1-p1-lnt 57.67
020620-88-c1-p1-r 63.81
020620-6ac4—p1-c 54.05
020620-68-84-p1-int 58.62
020620-6a«c4-p1-r 65.29
Las Sierras Ash-flow Sheett
Sample Si02
020620-3a-c3-p1-c 61.26
020620-36-c3-p1-r 60.47
020620-38-04-p2-c 57.28
020620-3a-c4-p2-r 59.59
0206203a-p1-c 60.51
0208203a-p1-Int 61.09
020620-30-p1-r 58.89
020620-6b-c1-pZ-c 48.39
020620-6b-c1-p24nt 53.13
020620-6b-c1-p2-r 54.07
020620-68—02-91-0 49.97
020620-65-c2-p1-1nt 54.42
020620-60-cZ-p1-r 58.20
San Rafael Ash-flow Sheet
020619-1F-c1-p1-c 54.87
020619-1F-c1-p1-lnt 56.68
020619-1F-c1-p1-l' 66.29
020619-1F-c2-p1-c 59.21
020619-1F-c2-p1-lnt 57.26
020619-1qu1-7 57.65
020619-1F-c3-p1-c 57.71
020619-1F-c3-p1-r 56.62
020619-1l1-c1-p1-c 56.70
020619-1h-c1-p1-int 55.83
020619-1h-c1-p1-r 55.09
020619-1h-62-p1-c 57.06
020619-1h-c2-p1-r 57.88

Ostocal Ash-flow Sheet

Sample
020624-1c-c1-p1-c
020624—1c-c1-p1-int
020624-1c-c1-p1-r
020624-16-c3-p1-c
020624-1 c-c3-p1 -int
020624—1c-c3-p1-r

8102
57.96
56.27
55.23
54.53
54.62
57.68

N203
25.54
18.28
26.62
26.47
17.26

~an
23.98
24.83
27.00
25.49
24.28
24.17
25.04
32.69
29.42
28.82
31.33
28.70
27.12

N20:
26. 70
26.96
14. 00
25.36
27.14
28.21
25.96
26.95
26.87
27.09
26.19
26.68
19.01

A120,
25.45
26.52
26.46
27.63
27.80
25.63

F8203
0.41
1 .53
0.44
0.39
1 .72

F620;
0.31
0.36
0.36
0.33
0.29
0.29
0.29
0.76
0.69
0. 41
0. 46
0. 40
0.47

F9203
0.51
0.53
6. 59
0. 46
0. 53
0. 53
0. 66
0. 60
0.45
0.53
0.53
0.57
5.50

F8203
0. 38
0. 42
0. 43
0. 44
0. 41
1.05

55

080
7.57
3.91
11.18
8.80
3.64

(:60
5.72
6. 45
8. 94
7. 40
6.37
5.97
7.15
16.16
12.41
11.77
14.72
11.21
8.87

C80
11 .01
9. 44
3. 02
7.51
9.26
8. 84
8. 51
9.32
8.77
9.17
8. 94
9. 05
6.94

080
7. 54
8. 62
8. 96
10. 03
10.34
8.32

N320
8.79
5.19
5.07
8.31
4.70

N820
7.98
7.55
6.16
7. 00
7. 66
7. 74
7.09
2.36
4. 26
4. 81

3.15
5.05
6.32

N820
5.03
5.86
3. 68
6. 78
5.92
6.27
6. 31

5. 84
6. 24
5. 90
5. 94
6.13
4.16

N820
6.79
6.11

5. 99
5. 58
5. 49
5.86

K20
0.35
1.66
0.16
0.20
1.75

K20
0.55
0.49
0.35
0. 43
0 48
0. 53
0.37
0.06
0.12
0.10
0. 08
0. 16
0.20

K20
0.17
0.31
2.52
0.40
0.30
0.31
0.33
0.26
0.30
0.25
0.27
0.26
1.04

K20
0.31
0.25
0.16
0.17
0.15
0.39

Total
98.32
94.37
99.71
98.79
94.36

Total

99.81

100.15
100.07
100.24
99.59
99.79
98.82
100.41
100.03
99.98

99. 71

99.94
101.19

Total
100.29
99.82
96.10
99.72
100.41
99.81
99.49
99.60
99.32
98.77
96.95
99.95
94.52

Total
98.41
98.20
97.24
98.39
98.82
98.93

An “lo
54.42
43.00
25.60
76.79
82.54

An %
27.48
31.16
43.61
35.97
30.62
28.99
35.01
78.83
61.25
57.15
71. 71
54.56
43.16

An 96
46.23
23.84
37.09
45.56
43.01
41.89
46.11
42.96
45.50
44.27
44.18
39.59
38.50

An %
37.34
43.16
44.82
49.31
50.55
42.91

6I4s4

Table 3 continued
Plagioclase Analysis

Ostocal Ash-ﬂow Sheet

Sample
020624-1l-c1-mp1-c
020624-11-ct-mp1-r
020624-1l-c1-p1-c
0206244 i-c1-p1-int
020624-1 l-c1 -p1 4’
0206244 i-c3-p1 -c
020624-1 i-c3-p1 -int
020624-1 i-c3-p1 -r
020624—2c-c1-p1-c
020624-2c-c1-p1-r
020624-2c-c1-pZ-c
020624-2c-c1-pZ-r
020624-2c-c3-p1-c
020624-2c-c3-p1-lnt
020624-2c-c3-p1-r

Coyol Ash-ﬂow Sheet
Sample
020622-1e-c3-p1-c
020622-1e-c3-p1-lnt
020622-1e-c3-p1-r
020622-1e-c4-p1-c
020622-1e-c4-p1-lnt
020622-1e-c4-p1-r
020622-2c-c1-mp1-c
020622-2c-c1-mp1-ln1
020622-2c-c1-mp1-r
020622-2c-c2-p1-c
020622-2c-c2-p1-lnt
020622—2c-c2-p1-r
020622-2c-c4-p1-c
020622-2c-c4-p1-int
020622-2c-c4-p1-r

Si02
56.53
55.76
55.40
57.28
56.73
55.09
57.23
56.37
56.39
55.85
59.83
56.98
59.42
57.05
57.63

Si02
57.32
57.50
57.20
52.16
52.37
51.96
56.79
56.56
56.09
57.11
58.79
57.95
57.60
57.37
57.75

AI203
24.89
27.26
27.92
26.71
27.25
27.77
26.27
26.97
25.77
27.97
20.16
26.68
25.43
27.23
26.53

“203
25.93
25.98
26.19
28.77
28.89
29.31
26.53
26.59
25.45
26.70
27.19
25.94
26.51
26. 64
26.49

F8203
2.07
0.58
0. 41
0. 38
0. 40
0.42
0.39
0.55
1.90
0.77
8.28
0.81
0.41
0.39
0.44

F0203
0.47
0.42
0.41
1.14
1.19
1.08
0.67
0.70
1.52
0.56
0. 56
0. 43
0.39
0.63
0.46

56

080
8.54
9. 56
10.10
8.61
9.11
9.83
8. 33
9. 22
9. 81
10.54
7.35
9.79
7.74
9.51
8.74

CaO
8.01
7.699
7.88
12.48
11.85
12.54
8. 88
9. 05
8. 83
9. 00
9. 74
8.27
8.63
8.45
8.40

N820
5.38
5.73
5.51
6. 24
6. 26
5. 42
6. 29
5. 91
5.57
5.65
5.32
5.65
6.93
5.97
6.11

M20
6.23
6.32
6.33
4.01

4.11

4.08
5.78
5.77
5.51

5. 98
5. 84
6. 35
5.96
6. 04
6.11

K20
0.37
0.24
0.17
0.25
0.26
0.16
0.29
0.21
0.16
0.19
0.33
0.23
0. 31
0. 22
0. 24

K20
0.60
0.71
0.76
0.44
0. 44
0. 40
0.58
0.63
0. 55
0. 64
0. 59
0.65
0. 70
0. 66
0. 72

Total
97.77
99.13
99.51
99.44
100. 02
98.72
98.80
99.23
99.59
100. 97
99.26
100.15
100.25
100.37
99.69

Total
98.77
98.62
98.76
99.01
98.85
99.38
99.24
99.29
97.95
99.99
100. 71
99.59
99.79
99.79
99.93

An 96
45.61
47.30
49.83
42.63
43.86
49.50
41 .55
45.72
48.87
42.30
48.28
37.47
46.24
43.55
50.53

An 96
38.93
61.60
59.81
61 .50
44.34
44. 73
45.36
43.70
46.37
40.27
42.64
41.90
41.34 .
35.50
34.75

Table 3 continued

Plagioclase Analysis

Las Made!“ Ash-ﬂow Sheet
Sample Si02
020622-4e-c1-p1-c 57.89
0206224e-c1-p1-int 57.55
020622-Ae-c1-pt-r 56.66
0206224e-c2-p1-c 56.35
0206224e-cZ-p1 4nt 55.1 9
02062246-c2-p1-r 52.14
0206225-c1-p1-c 54.59
0206225-c1-p1-lnt 60.36
0206225-c1-p1-r 63.44
0206225-c2-p1-c 58.97
02082264221114 57.39
0206225-c3-p1-c 53.58
020622-5-c3-p1-lnt 53.07
020622-5-c3-p1-r 56.71
0206225-c5-p1-c 54.48
020622-5-c5-p1-r 61.54
020622-5-cS-p2-c 52.73
020622-5-cs-p2-r 55.42
020622-5-c6-p1-c 56.81
0206226-c6-p1-lnt 57.73
020622-5-c6-p1-r 56.71

AI203
26.01
25.41
26.39
27.30
27.97
29.70
28.44
24.17
22.45
26.27
26.83
29.48
30.03
27.51
28.33
24.96
29.52
27.83
27.29
26.81
27.44

F8203
0.37
0.38
0.39
0.50
0.82
0.74
0.77
0.94
0.84
0.48
0.59
0.58
0.71
0.51
1.27
0.90
1.04
1.32
0.54
0.40
0.51

57

080
7.53
7.35
8.33
9.26
10.12
12.69
11.00
8.21
5.89
7.95
9.16
11.84
12.41
9.68
11.42
6.21
12.86
10.61
9.37
8.87
9.56

N820
8.53
8.42
8.10
5.78
5.24
4.22
4.91

5.43
8.50
8.48
5.94
4.50
4.21

5.90
4.91

7.70
4.11

5.19
5.77
8.11

5.85

K20
0.83
0.85
0.67
0.60
0.46
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0.83
0.94
0.57
0.40
0.31
0.23
0.42
0.25
0.87
0.16
0.37
0.46
0.49
0.47

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97.94
98.55
99.78
99.59
99.72
100.04
99.95
100.06
100.67
100.32
100.27
100.66
100.74
100.66
101.99
100.44
100.74
100.24
100.42
100.33

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36.77
41.30
45.40
50.22
61.60
54.23
43.15
31.39
39.14
44.94
58.15
61.16
46.40
55.43
29.66
62.68
51.88
46.05
43.25
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Table 4: Four possible cases during stage 3 in Figure 27 (see Figure 28 for cartoon)

 

 

 

 

 

 

Possible Scenario at stage 3 Evidences Representative
Ash-ﬂow Sheet
Case A: ascent of both silicic - eruption of both silicic and maﬁc Apoyo Unit
and maﬁc magma without pumice fragments in a single ash-
mingling flow sheet
Case B: maﬁc magma - presence of maﬁc enclaves Las Maderas and
intrudes ponded silicic magma - calcic plagioclase rims Ostocal Units
at the chamber and/or during - with matrix glass more maﬁc
ascent than whole pumice fragment (low
Si02 With low K20 and high MgO
wt %).
Case C: silicic magmas - abrupt shifts of plagioclase Las Sierras Unit
intrude pockets of ponded composition from a more calcic
maﬁc magma core to a sodic rim
- glass composition in terms of San Rafael, Coyol,
Si02 and K20 content is similar to Monte Galan
the whole pumice fragment with
distinct compositional gap
Case D: evolving intermediate - normal zonation in plagioclase Any unit

magma that is continuously
Elenished from the source

 

- no maﬁc enclaves

 

 

6O

 

APPENDIX B

Figures

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63

 

Figure 3. (A) Active quarry site for ignimbrite (Ostocal unit).
(B) Typical pumice fragments of various sizes embedded in
ﬁne-ash matrix.

 

Figure 4. Photomicrograph of typical glomeroporphyritic
texture in the pumice fragments. (A) Under plane
polarized light and (B) under crossed polars. (Apoyo unit,
sample # 020617-10).

65

 

 

Figure 5. (A) Typical oscillatory zoning in plagioclase crystal
(Apoyo unit, 020617-20). (B) Typical sieve/resorved texture
in plagioclase suggesting chemical dissolution (Las Sierras
unit; 020620-6b).

 

 

 

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y: .
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v , .
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Figure 6. (A) Corroded plagioclase cores observed in Las
Maderas unit (020622-5). (B) Melt inclusions found in
plagioclase phenocryst San Rafael unit (020619-10.

67

 

Figure 7. Phenocrysts of clinopyroxene and orthopyroxene
(Apoyo unit, 020617-2c). Distinction between these 2 pyroxenes
is difﬁcult from petrography alone.

68

 

 

Figure 8. Banded pumice fragment observed under the
microscope (Apoyo unit, 020618-5a).

69

 

" - '- - ,"v r - .
Va u hound .va- l4 an annual...” A ~JJ¢I) .4 - -, (vi-u x ’ . if

 

Figure 9. Maﬁc enclave within a silicic pumice
ﬁagment (Las Maderas 020622-5). (A) under
plane polarized light and (B) under crossed polars.
The phenocrysts content of the enclave are the
same as the host pumice but are smaller in size.

70

 

 

Figure 10. Intrusive-like feature of Fe-Ti oxide within an enclave
into the silicic glass host (Ostocal unit, 0206 24-20).

71

 

 

Figure 11. Dove-tail texture observed in plagioclase crystals within the
enclave indicating quenched crystallization (Ostocal unit, 020624-2c).

72

 

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Figure 13. Major element variation against Si02 (wt. %) among ash-ﬂow units in
Nicaragua~ ApoyoO, Monte Galena , Las Sierras A, San Rafael * , Ostocal O ,
Coyol 5') , Las Maderas (:1. Filled area represents trend for low Ti02 Quaternary
Nicaraguan lavas.

74

 

Las Sierras

12'

8 'Nazs

4 .moan=70.16
std dev=0.99

o

0)

Frequency
A

Frequency

    
     
     
  

N=12 ‘
mean=64.03
std dev=1.27

  

 

A

San Rafael

 

Coyol

  
   

_ N=8
moan=67. 1 8
std dev=1.26

  

 

 

 

2
0
12 _ Ostocal . Monte Galan
' I
8 I- a
4 ' N=12 . N=11 .
mean=67.15 - moan=70.18 1
std dev=0.54 std dov=0.76
0 I
Apoyo 50 60 70 80
20 ' ' Si02
16 ' ‘

8 . N=35
mean=67.78
4 rstd dev=0.67
O

 

  
 
   

 

 

KOUODber

Figure 14. SiO2 histogram of

the seven silicic ash-ﬂow
sheets. Mean and stande

deviation are for the high silica

pumice fragments only.

75

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ash-ﬂow units are similar and deﬁne
a slight MREE depletion pattern (Dy
and neighboring elements;
normalization values from Sun &
McDonough, 1989). Filled area same
as Figure 13.

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the seven ash-ﬂow sheets showing
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93

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