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Origin of the Chemical Variation in
El Valle Central, Costa Rica

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Rachel Susan Hannah

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ORIGIN OF THE CHEMICAL VARIATION IN THE VALLE CENTRAL TUFF,
COSTA RICA

By

Rachel Susan Hannah

A THESIS

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

MASTERS OF SCIENCE

Department of Geological Sciences

2000

ABSTRACT

ORIGIN OF THE CHEMICAL VARIATION IN THE VALLE CENTRAL TUFF,
COSTA RICA

By
Rachel Susan Hannah

Costa Rican magmatism is somewhat unusual because of the occurrence of high-
silica volcanic deposits in an area where a continental crust is absent. However, the ash-
flow deposit of the Valle Central, Costa Rica consists in part of high-silica deposits. The
Valle Central Tuff covers about 7 85 km2 and has a volume of about 22 m3. Based on
analyses of pumice fragments, the tuff can be divided into three chemical groupings: a
low-silica group (54-62 wt. % SiOz), a high-silica group (63-69 wt. % SiOz), and a
mingled group (56-65 wt. % SiOz). Difl‘erent Eu/Eu", MREE, and HREE trends in the
high and low-silica groups show that these groups can not be related to each other

through fractional crystallization. The low-silica magma represents a mantle melt that

has undergone fractional crystallization, producing continuous variation in silica, ranging

from 53 wt % to 63 wt % SiOz. The high-silica magmas represent melting of amphibolite
at the based of a thickened oceanic crust. However, based on uniform potassium
enrichments and low variation of RE patterns, the high-silica magma and the low-silica
magma are related, and have a common source. We suggest that the high-silica melts
resulted from partial melting of a meta calc-alkaline rocks, which were similar to the low-
silica group within El Valle Central Tuff. The mingled group represents the physical and

chemical mixing between these two end-members.

ACKNOWLEDGEMENTS

I would never have finished (much less begun) this thesis without the help and
support of many people. First and foremost, family deserves kuddos for giving me
endless love and support. Mom and Dad, my sister Eleanor and her family, Drew and
Shaw, my brother John and his family, Patty, Katherine and Victoria, and last-but not
least- my sister Mary.. I love you all very, very much.

My friends helped with the grind of daily life. Maya Doe-Simki, Sarah Miltz,
Sarah Bork, and Natalia Hernandez-Gardiol gave me plenty of hugs, kisses, and parties,
(not to mention the odd romance novel) to bolster my spirits and offer distractions. Abe
Cambier became a treasured dinner and travel companion, and one of my best friends.
Marcin Kociuba dropped in and out for good wine and good conversation, and Ryan
Birkenfield commeserated on the joys and heartaches of a geologist fiom afar.

And perhaps most importantly, my advisors from Michigan State University.
Thomas A. Vogel taught me how to act and think like a scientist. Lina C. Patino offered
endless professional advice and support. Micheal Velbel, Duncan Sibley, and Bill
Cambray were always around to buy me a cup of hot chocolate. I also owe lots of
gratitude for our newcomer, Gary Wiessmann, in helping me figure out what to do next.

Finally, I would like to acknowledge the National Science Foundation, the
Geological Society of America, and Michigan State Honors College for providing me the

funding that made this project possible.

iii

TABLE OF CONTENTS

List of Tables
List of Figures

1.1ntrodnction
1.1 Purpose
1.2 Background
1.3 Geological Setting and extent of Silicic Volcanism in Mature
Island Arcs

2.Tlle Valle Central T1117

2.1 Previous Work

2.2 Stratigraphy

2.3 Geochemistry
2.3.1 Sampling
2.3.2 Methods
2.3.3 Major Elements
2.3.4 Trace Elements
2.3.5 Sr and Nd Isotopes
2.3.6 Petrography
2.3.7 Mineral Chemistry.
2.3.8 Comparison with Central American Volcanic Arc

3.Discnssion
3.1 Fractional Crystallization
3.1.1 Low-silica Group
3.1.2 High-silica Group
3.2 Partial Melting
3.3 Magma Mingling
3.4 K20 Trends
3.5 Origin of the Chemical Zonation in the Valle Central Tuff

4. Conclusions

Appendices
Appendix A: Tables
Appendix B: Figures
Appendix C: Equations

Bibliography
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94

LIST OF TABLES

Table 1. Sample locations within the Valle Central, Costa Rica.

Table 2. Major and trace element concentrations for pumice fragments in the Valle
Central Tuff. Oxides listed in wt%, trace elements in ppm (parts per million). D= below
detection limit.

Table 3. Major and trace element concentrations for phenocrysts in the Valle Central
Tuff.

Table 4. Phenocryst proportions in the low-silica group, based on point counting of two
thin sections with whole pumice content within each silica range.

Table 5. Partition coefficients used in multiple linear regressions for fi'actional
crystallization models.

Table 6a. Fractional crystallization models for the low-silica group; Step A and Step B.

Table 6b. Fractional crystallization models for the high-silica group; Step C and Step D.

LIST OF FIGURES

Figure l. Tectonic map of Central American Volcanic Arc. Red circles are active arc
volcanics, blue circles are back arc volcanics.

Figure 2. Simplified geologic map of Costa Rica.

Figure 3. A. Plane view of the central valley of Costa Rica and the extent of the Valle
Central tuff. San Jose, the capitol of Costa Rica is to the south-east of the tuff. The
volcanoes Platanar, Poas, Barva, Irazu, and Turrialba are along the active arc to the
northwest and northeast of the tuff. Modified from Tournon and Alvarado (1995). B.
Stratigraphic correlation of theValle Central deposits and deposits to the northwest, in the

Cordillera Central.

Figure 4. Photograph of the white pumice-fall unit that can be seen throughout the Valle
Central.

Figure 5. Photograph of the “La Garita Tuff,” unit, showing all three units. Includes
abundant fumerolic pipes. Person for scale

Figure 6. A. Variation in pumice clast color with silica and potassium content. Black
pumice and dark gray pumice are low in silica. White, light gray, and collapsed pumice
are high in silica. B. Classification of pumice samples into the low-silica group, high-
silica group, and mingled group within the Valle Central Tuff. Black and dark gray
pumice clasts belong to the low-silica group. Banded pumice clasts belong to the mingled
group. White, light gray, and collapsed pumice clasts belong to the high-silica group. All
figures use normalized major element values.

Figure 7. Major element oxide variation plots versus silica for El Valle Tufi‘.
Figure 8. Trace element variation plots versus silica for El Valle Tuff.

Figure 9. Spider diagram of representitive pumice clasts from the low-silica group,
mingled group, and high-silica group. Note that the high-silica group has a much greater
depletion in P, and is enriched in the large ion lithophiles (Rb, Ba, K, Pb) in comparison
to the low-silica group. Normalization factors fi'om Sun and McDonough (1989).

Figure 10. Rare earth element plot for representative samples from the low-silica group,
hiEll-silica group, and the mingled group. The REE pattern for all samples within thel
Valle Central Tuff is very well constrained, with small degrees of variation.
Nomalization factors from Sun and McDonough (1989).

Figu re 11. Sr and Nd isotope plot. Ranges indicate the isotopic character of different
mantle sources. The range within El Valle Central samples is tightly constrained,

vi

regardless of silica content. The Valle Central Tuff samples plot within the range of
isotopic variation seen throughout the Valle Central.

Figure 12. A. Photomicrograph of clinopyroxene phenocrysts from a black pumice clast
(54 wt. % SiOz within the low-silica group. B. Photomicrograph of plagioclase
phenocryst from the same sample; the pack-marked texture of the crystal is referred to as
“sieve” texture. C. Rounded vesicules in the brown glass, especially evident in nearly
aphyric samples of 62 wt. % SiOz. In all photomicrographs, the bar scale represents 1
mm. UXP= Uncrossed polars, or plane-polarized light. XP= View using crossed polars.

Figure 13. Phenocryst contents and variations in low-silica group black pumice. Dashed
lines in vertical axis indicates a change in scale.

Figure 14. A. Photomicrograph of a glomopheric clot from a white pumice clast from the
high-silica group. B. Photomicrograph of glass from an aphyric, white pumice clast from
the high-silica group. Note the stretched and collapsed vesicules within the glass. In all
photomicrographs, the bar scale represents 1 mm. UXP= Uncrossed polars, or plane-
polarized light. XP= View using crossed polars.

Figure 15. A. Photomicrograph of banded pumice fragments with abundant rotated and
broken phenocrysts. B. Photomicropgraph of remnant glomopheric clot surrounded by
white glass, typical of white pumice of the high-silica group, surrounded by brown glass
typical of the low-silica group C. Photomicrograph of glass in banded pumice fragments.
Note that the brown glass (59-60 wt. % $102) retains well rounded vesicules while the
white glass (69-70 wt. % SiOz) has stretched and collapsed vesicules. In all views, the
scale bar represents 1 mm. UXP= Uncrossed polars, or plane-polarized light. XP= View

using crossed polars.

Figure 16. Ternary diagram for feldspar. Note the decrease in anorthite content of
phenocrysts with increasing silica content in the whole pumice sample.

Figure 17. Pyroxene quadrilateral for the Valle Central phenocrysts. Low-silica gmup
and high-silica group augite phenocrysts have very similar compositions. High-silica
group pumice clasts have orthopyroxene. Rims on orthopyroxene crystals from mingled
pumice clasts are em'iched in PS.

Figure 18. A. Plots of F3 content in pyroxene phenocrysts with silica content in whole
pumice samples. The F5 content of pyroxenes in the low-silica group (55 wt.% $02) is
identical for clinopyroxene phenocrysts in the high-silica group (67 wt. % SiOz). Rims
fi'Oln mingled pumice samples are enriched in PS. B. K. values for all clinopyroxene
Phenocrysts versus silica content in whole pumice samples. Some phenocrysts in black,
grey, and banded pumice clasts from the low-silica group are in equilibrium with the
liquid. Phenocrysts in white and light gray pumice clasts are not in equilibrium with a
basaltic liquid, with K. values well below equilibrium values of 0.22 :1: .02.

vii

Figure 19. A. Photomicrograph in plane polarized light of a cluster of orthopyroxene
phenocrysts within a banded pumice fragment. B. Photomicrograph using backscattered
electron imaging to see reaction rims around the orthopyroxene phenocrysts. The
phenocryst cores are depleted in Fe and enriched in Mg compared to the phenocryst rims.

Figure 20. TAS classification of LeBas et a1. (1986) for the Central American Volcanic
arc, including the Valle Central Tufi'. CAVZ indicates basalts, basaltic andesites,
andesites, and rhyolites from CENTAM database, and from Kempter (1997). Note that
the Valle Central Tuff is enriched in K compared to other volcanic products from along
the Central American Volcanic arc.

Figure 21. Plots of Si02 vs. selected trace elements. CAVZ indicates basalts, basaltic
andesites, andesites, and rhyolites from CENTAM database and from Kempter (1997).
Note that El Valle Central Tuff is enriched compared to other volcanic products fi'om
along the Central American Volcanic arc.

Figure 22. A. Liquid lines of decent for batch fractional crystallization models. =Low-
silica Group. =High-silica Group. B. Spider diagram of observed and calculated (based
on multiple linear regression) for Steps A and B in A. C. Spider diagram of observed and
calculated (based on multiple linear regression) for Step C. D. Spider diagram of
observed and calculated (based on multiple linear regression) for Step D.

Figure 23. A. Plot of Eu/Eu* versus SiOz. Symbols as before. Note that the low-silica
group and A. Plot of Eu/Bu* versus Si02. Symbols as before. Note that the low-silica
group and the high silcia group plot in two distinct trends, implying that fractional
crystallization is not the controlling process on magma differentiation. Lines represent a
best fit line for each group. B. Ce is representative of other light rare earth elements,
incompatible in both the low and high silcia groups. C. Sm is representative of the middle
rare earth elements; incompatible in the low-silica group liquid, but behaving as a more
compatible element in the high-silica group liquid. D. Rb/Hf variation between the two
groups could indicate a crustal source for the high-silica group.

Figure 24. Element ratio-ratio plots as a test for magma mixing. Symbols as before. A
and C. Plots of Sr/Y vs Ti/P and Ta/T i vs. Y/Sm. Ta/T i ratios are multiplied by 1000 for
easier plotting. All the banded samples fall on a hyperbola defined by mixing the high
and low silica extreme compositions. B and D. Plots of Sr/Y vs. P/Y and Y/Sm vs.
Ti/Sm. A11 plots support magma mingling for the Mingled group. 10C and 10D do not
support magma mingling for the low or high silica group. Tic marks represent mixing
increments of 20%.

Figure 25 Schematic model of the evolution of El Valle Central Tuff

viii

INTRODUCTION

1.1 Purpose

The production of Silicic magmas in island arc settings has long been an enigma.
In most island arcs, high-silica igneous rocks are uncommon due to the absence of
continental crust (Hildreth, 1983; Fournelle et aL, 1994). However, there are some
exceptions, including some regions in the Aleutians (which contains Silicic rocks in the
absence of continental crust; Singer et al., 1992) and Costa Rica. Abundant Silicic
magmas were produced in the late Miocene and Pliocene in Costa Rica and erupted as
ash-flow sheets. These Silicic ash-flows are exposed several thousand square kilometers
in Costa Rica, yet few petrological studies have been done (Alvarado et al., 1992). One
of these Silicic ash-flow sheets is in the Valle Central, and is the subject of this

investigation.

1.2 Background

Ash-flow tufl‘s are ideal in studying the origin of compositional variation in
volcanic systems because they represent an instantaneous partial evacuation of a magma
chamber. Therefore, any variation present in a magma chamber is preserved unaltered on
the earth’s surface. In any series of lava flows, the genetic relationship of the magma
compositions has to be inferred, but in an ash-flow sheet there is no doubt of the original
composition within the magma chamber.

The common occurrence of compositionally and mineralogically zoned ash-flow

sheets provides strong evidence that large chemical, mineralogical, and thermal ranges

are common characteristics of magma bodies (for reviews see Smith, 1979; Hildreth,
1981; de Silva and Wolff, 1995; Mills et al., 1997). Much of the effort of recent studies
has been to evaluate the origin and evolution of zoned magma bodies by studying the ash
flow sheets. In almost all studies of zoned ash flow sheets, the inferred trend towards the
top of the parental magma bodies is increasing silica, decreasing temperature and
decreasing phenocryst content. Because of the effects of magma withdrawal processes
(Spera et al., 1986) and topography (Valentine et al., 1992) on compositional zonation in
ash flow sheets, it is not clear if the zoned ash flow sheets reflect the presence of
continuous chemical gradients or discrete layers in the magma body. Any conclusions
about thermal and compositional gradients in an ash-flow sheet, and subsequent
inferences about the origin and evolution of a magma body, is model dependent.
Workers have used models ranging from fractional crystallization of a basaltic andesite to
melting of previously emplaced island arc plutons to explain high-silica rocks in an island
arc setting (Sisson and Grove, 1993; Roberts and Clemens, 1993; Feeley and Davidson,
1994; Borg et. al., 1998).

Fractional crystallization models are difiicult to apply to the origin of high-silica
magmas because they require large amounts of fractional crystallization. For example, to
produce the high-silica magma fi'om a basaltic liquid would require 65% fractional
crystallimtion. Cumulate rocks would be produced by such high degrees of fractional
crystallization. In Costa Rica, it is difficult to test this model for young high-silica
deposits, as coarse-grained cumulate products would be produced at depth within the

crust and have not yet been exposed at the surface.

Several models for the origin of chemically and thermally zoned magma bodies
have been presented in previous studies. Most models that involve large-scale
differentiation of a homogeneous magnm are generally based on sidewall crystallization
(cf. Mittlefield and Miller, 1983; McBimey et al., 1985; Baker and McBirney, 1985; de
Silva and Wolfi‘, 1995). At the opposite end of the conceptual spectrum are models that
involve controls by partial melting and melt extraction, which were first proposed by
Marsh (1984) and expanded upon by Bergantz (1989) and Sawyer (1994). There is
recent petrologic evidence, reviewed in Mills et al. (1997), that many magma bodies may
have had distinct compositional groups that cannot be related by fractionation processes.
Chemically distinct magma batches that are unrelated by fractionation processes can
result from melting and extraction processes (Sawyer, 1994). Crystal fi'actionation,
assimilation and magma mixing processes can subsequently modify these magma
batches, and if enough time is available, the magma batches can become stratified in the
magma chamber according to their densities. An extreme case of the combining of
magma batches is the underplating of a Silicic magma body by an independently
generated mafic magma (Wiebe, 1994; Coleman et al., 1995; Metcalf et al., 1995).

Brophy et al. (1999) has proposed a model that could explain gaps in a fractional
crystallization trend (such as in silica content), even though fractional crystallization is
the dominant process controlling the differentiation of a magma. They propose that the
calc-alkaline series might originate by fractionation near the crust-mantle boundary. As
the solidification fi'ont descends through a magma body, high-silica liquids get trapped
within the crystal mush behind the solidification front. As convection ceases, the crystal

mush begins to behave as a solid, and fractures. High-silica magmas drain from the

interstitial boundaries of the crystals and migrate upwards, leaving behind the crystals.
This process can occur several times, leaving cumulate products of crystal fractionation
behind at a depth at which they seldom get exposed to the surface (such as in Costa Rica)
and explaining potential gaps a fractionating trend.

Most silicic calc-alkaline rocks are associated with continental arcs with
thickened continental crust and not island arc settings. In many of these continental arcs
there is evidence tlmt melting the lower crust produces felsic calc-alkaline magmas. Borg
and Clynne (1998) suggested that the majority of the felsic rocks in the composite
volcanic centers from the southern Cascades are due to partial melting of lower crust that
is compositionally similar to the calc-alkaline basalt in the region. Roberts and Clemens
(1993) have discussed the difficulty of generating high-K calc-alkaline silicic rocks fi'om
mantle sources because the K20 content of mantle melts is too low to produce
fiactiomtion in the high-K field. However, Price et al. (1999) use a combination of
melting of the lower crust and slab derived processes to explain the presence of high K
rocks at Edgemont Volcano, New Zealand. Singer et al. (1992) have shown that the
high-silica magmas fiom the Seguam volcanic center are produced by crystal
fiactiomtion in a strongly extensional environment. These studies provide a fi'arnework
in which to evaluate the origin of the high-silica volcanic rocks from Valle Central Tuff.

The purpose of this study is to document and explain the chemical variation in the
Valle Central Tufl; Costa Rica. Pumice samples collected fiom The Valle Central tuff,
Costa Rica, have a distinctive chemistry and provide us with an opportunity to address
the origin of a silicic magma in a mature island arc system. This study uses major and

trace element data, phenocryst composition and content, and Sr and Nd isotopes fiom

The Valle Central Tufl to provide insights on the magma genesis of high-K, high-silica
magmas in a mature island arc environment. A parallel thesis, completed by Wendy
Pérez at the University of Costa Rica, investigates the physical volcanology of the Valle

Central Tufl’, and its possible source.

1.3 Geological Setting and extent of Silicic Volcanism in Costa Rica

Subduction of the Cocos Plate under the Caribbean Plate produces the Central
American volcanic arc (e.g. DeMets et al., 1990; Figure 1). The Chorotega block
includes the southern part of the Central American arc, and extends from northern Costa
Rica to central Panama (de Boer et al., 1995). This part of the arc developed on oceanic
crust. The mature island arc crust is approximately 35 km thick under central Costa Rica
(Carr, 1984). The angle of subduction in the Chorotega block is steeper in the northwest
and shallower in central Costa Rica (Protti et al., 1995). In central Costa Rica the angle
of subduction is estimated to be 35°. The shallower subduction angle in central Costa
Rica is due to the subduction of the Cocos Ridge. The aseismic Cocos Ridge is
interpreted to contain traces of the Galapagos hot spot (Feigenson et al., 1993). It is
subducted to the south of the Valle Central of Costa Rica and the north of the Panama
Fault Zone (Kolarsky et a1, 1995).

In Costa Rica during the Cenozoic, most silicic volcanism was of limited extent.
In northwestern Costa Rica (Guanacaste volcanic belt; Figure 2) during the Plio-
Pleistocene, voluminous high-silica magmas were erupted as ash-flow sheets (Bagaces,
Rio Colorado, and Libera tuffs), which covered an area of 1600 km2 and preceded the

modem andesitic volcanic chain (Chiesa et al., 1992; Kussmaul et al., 1994; Kempter,

1997). These deposits were erupted fiom caldera sources now buried by the modern
stratovolcanos (Chiesa et al., 1992; Kempter, 1997). Based on the congruency of the
major and trace element data for the silicic and andesitic rocks at Rincén de la Vieja
(Figure 2), Kempter (1997) suggested that the silicic rocks were dominated by crystal
fiactionation of a common source, perhaps by repeated fractionation and replenishment
of discrete magma bodies. In central Costa Rica (Central volcanic belt; Figure 2) during
the Pleistocene, silicic magmas were erupted along the western flank of Platanar volcano
(Alvarado and Carr, 1993) and in the Valle Central (Fig. 2; Kussmaul et al., 1994).

The Valle Central Tufl‘ covers about 785 km2 and is roughly 22 km3 in volume
(Figure 3A) (Perez, in prep). The Valle Central lies in the center of Costa Rica between
the Cordillera de Talamanca and the Tilaran Range. There are five volcanoes
northwestern side of the tufi‘: Platanar (dormant), Poas (active), Barva (dormant), Irazu
(active), and Turrialba (active) (Figure 3A). These volcanoes are to the northwest of the
Cordillera de Talamanca and are the southernmost volcanoes of the Central American
Volcanic Arc. South of these volcanoes the shallow subduction of the Cocos Ridge
prevents further volcanism (Alvarado et al., 1992). Therefore, volcanic products fiom
this area, in particular those from the Valle Central, are useful in studying the efiects of a
slmllowly dipping subduction zone, and the possible effects of the Cocos ridge

subduction on magma chemistry.

THE VALLE CENTRAL TUFF

2.1 Previous Work

This ash-flow sheet has not been intensively studied. Williams (1952) published
the first work describing the overall geology of the Valle Central. He identified the ash-
flow tufl‘s and called them “glowing avalanche deposits.” Echandi (1981) renamed the
ash-flow deposits as Formacién Tiribi and Kussmaul and Speachman (1982) refer to the
deposits as the Avalanche Ardiente formation, following Williams’ work. Within this
formation Eclmndi defined three members: (1) El Miembro Nuestro Arno, characterized
by “hot mud flows” (2) El Miembro la Caja, characterized by an ash-flow tuff that is
poorly welded and (3) El Miembro Electriona, characterized by highly welded ash-flow
tufl‘s. However, Echandi presented little data on pumice types, and no chemical analysis
to support his subdivisions. The formation Orotl'na, a fine, light blue-gray tufl' appears to
be the distal part of the Valle Central Tufl' (Marshall et al., 1999; Perez, in prep).
Tournon (1984) also found three principal facies of the ash-flow unit based on welding
characteristics, color, and the nature of the pumice clasts and lithics. Kussmaul (1988)
analyzed four pumice samples that were high-K andesites transitional to shoshonitic
dacites with silica content ranging fi'om 59 and 67 wt %. A sample of black glassy scoria

had a basaltic andesite composition with a silica content of 54 wt %.

2.2 General Stratigraphy
Figure 3A is a geologic map that includes the Valle Central, Costa Rica, which is
the most densely populated area in Costa Rica. In the Valle Central sampling is restricted

to quarries and the occasional road-cut. The majority of sampling was done in active

quarries, where there was access to all different levels of the tufl‘; in some quarries, all
layers of the tufiv were exposed. In the central part of the Valle Central, the ash-flow
sheet is sandwiched between two lava flows, the Intercanyon lavas and postavalanche
lavas (Williams, 1952). In this study, Perez (in prep.) and I found no paleosols in any of
the deposits. Echandi (1981) has recorded their presence. Ash-flow tufl~ deposits from
the Valle Central have also been found in numerous boreholes (Echandi, 1981; Perez, in
prep).

Depending on location, the Valle Central Tull has different characteristics, based
on color, size, and degree of welding of pumice clasts present in the section. These
variations are not all necessarily vertical; there are lateral variations as well. Figure 3B is
a simplified stratigraphic section of volcanic units in both the Valle Central and in the
Cordillera Central. For the purposes of this discussion, I will distinguish between
sections of the Valle Central Tufi‘ that are in the Valle Central, and the sections that
extend west and north into the Cordillera Central.

Ubiquitous throughout the Valle Central Tuff is a crystalline and lithic-rich, white
ash- and pumice-fall deposit. This pumice-fall deposit ranges between 20 cm and 2 m in
thiclmess, depending on the location (Figure 3B). The pumice-fall deposit lies over both
the Intercanyon lava flows and on Tertiary sedimentary rocks (e. g. near the city of
Coldn), depending on the location (Figure 4). Within the Valle Central, above the white-
ash fall is a massive unit, VC 3, with a dark gray matrix and abundant large black pumice
clasts (the majority are 5 x 5cm, some as large as 12 x 4 cm), gray pumice, banded
pumice, and minor amounts of white pumice. Banding of pumice fragments was

observed both at hand sample and thin-section scale. This tufl” consists of 60% matrix,

30% pumice, and 10% lithics. In some locations it is evident that there is a densely
welded unit at the base, with small fiarnrnes. This unit has been described as a “tufi lava”
(Echandi, 1981); a dark-gray, welded ash-flow tufl‘. In some locations the tuff is
approximately 80% matrix.

The tuff in the west corner of the Valle Central, closer to the Cordillera Central,
has a slightly difl‘erent stratigraphy. First, the white ash and pumice fall unit is not
present. At the base ofthe tufl‘is a unit, VC 1, called “La Garita Tufl‘,” with the type
locality at Tajo La Garita. The La Garita tufl‘ is lithic rich; consisting of 60-70% matrix,
25-30% pumice fragments, and 10-15% lithics. The matrix of VC 1 is gray, poorly to
moderately welded, and consists of plagioclase and pyroxene phenocrysts. Pumice
fragments are black, gray, banded, and white. Black pumice fragments are very large
(the largestis30cmx32 cmx 11 cm,theaverage sizeis 12x4cm) and theyappearas
two types: crystal rich and crystal poor. Some of the vesicles in large black pumice
fragments have partially collapsed. Most black pumice samples are angrlar. Gray
pumice clasts are generally smaller (7 cm x 5 cm) and well rounded, by the white clasts
are the smallest (the largest sample is 5 cm x 2 cm, 2 cm x 2 cm on average). This unit
contains abundant fummerolic pipes, evidence that degassing took place (Figure 5). The
major difference between VC 1 and VC 3 facies is that the “La Garita Tuff’-type (V C 1)
has a larger percentage and size of white pumice clasts.

VC 3 extends into the northwestern part of the Central Valle, and overlies the VC
1 in most locations. However, at several locations there are lenses of fluidized
pyroclastic material between the VC 1 and VC3 (Perez, in prep). This material consists

of large, rounded black and white pumice clasts, small, banded pumice, and delicate

flattened pumice fragments. These deposits are referred to as VC2. The unit can only be
identified clearly in the quarries on the western edge of the Central Valley, and separates
the local VCl with the main body of the tufl‘, VC3. This unit shows clear evidence of
fluidization behavior, with little to no matrix trapped between the pumice clasts. Black
pumice clasts are both phenocryst rich and phenocryst poor. White pumice clasts are
phenocryst poor. As discussed below, the pumice clasts fi'om this unit have identical
chemical variations as pumice clasts from either VCl or VC3. For more detail on the
stratigraphy of the Valle Central Tufl, see Peréz (in preparation).

There are few localities where all three units of the Valle Central tufi‘ are present.
They occur on the northwestern edge of the Valle Central (Figure 3, Table 1; Tajo Rio
Grande, Tajo La Garita, Tajo Rio Virilla). Based on field observations, there are more
white pumice clasts (high-silica pumice) at the base of the tufl deposit, in VCl .
Therefore, respective to major element chemistry, the Valle Central Tuff is a zoned tufl,
with the base of the flow much more silica rich than the top.

Isopleth and isopach maps (Perez, in prep), together with stragraphical
correlation, suggest that different units within the Valle Central tuff erupted in one event,
or as pulses of the same event, mainly fiom one composite caldera located in the Barva
shield volcano. The ash-flow traveled at least 80 km to the west fi'om its source. The
original area of ash-flow deposits was estimated to be about 785 km2 (Perez, in prep).
The total volume of material erupted is equivalent to 50 km3 (22 km3 DRE). A major

uncertainty is the volume of tephra scattered very far fi'om the source.

10

2.3 Geochemistry
2.3.1 Sampling

Figure 3 is a geologic map that includes the Valle Central, Costa Rica. Samples
were taken during the 1997-1998 field season (early December through January). Table
1 is a list of each sampling location. The majority of samples were from active quarries,
where there was access to all levels of the tufl‘. Samples were taken from each unit, and
as discussed earlier, these units were distinguished based on physical characteristics of
the pumice within the unit. The pumice clasts sampled represented the variation among

glassy pumice clasts present within each unit.

2.3.2 Methods

Whole rock major-element and selected trace-element concentrations were
determined by X-ray fluorescence spectrometry (XRF) for all samples. Additional whole
rock trace element and rare earth element concentrations were determined for selected
samples, using laser ablation Inductively Coupled Plasma Mass Spectrometer. The
major-element chemical compositions of phenocrysts were determined by electron
microprobe analyses (EMPA) at Indiam University.

For whole rock chemical analyses, samples were ground with ceramic flat plate
grinder, after passing them through a chipmunk. Two different techniques were used for
preparation of glass disks: a high dilution fusion (HDF) and a low dilution firsion (LDF).
Glass disks were made by diluting finely ground rock powder with lithium tetraborate
and ammonium nitrate as an oxidizer. The proportion for HDF were 1 gm rock, 9 grns

lithium tetraborate and 0.25 grns ammonium nitrate; for LDF these proportions were

11

3:9:0.50. Only the LDF glass disks were used for LA-ICP—MS. These materials were
then mixed and fused at 1000" C in a platinum crucible for at least 30 minutes and then
poured into platinum molds. Glass disks were then analyzed by XRF and LA-ICP-MS
methods. Major and selected trace elements (Cr, Cu, Zr, Ba, Sr, Ni, Cu, Rb, Y, Nb, and
Zn) were analyzed by XRF on a Rigaku, SMAX. Other trace elements, including the rare
earth elements, were analyzed by laser ablation Inductively Coupled Plasrm Mass
Spectrometer (LA-ICP-MS), on a Micromass Platform ICP with a hexapole collision cell
(Patino et al., 1999). The laser system is a UV laser (Cetac LSX 200).

XRF major-element analyses were reduced by the fundamental parameter data
reduction method (Criss, 1980) using XRF WIN software (Omni Instruments). XRF
trace-element analyses were reduced by standard linear regression techniques. For LA-
ICP—MS results, calcium was used as the internal standard. Prior to any calculation the
background signal was subtracted from standards and samples. The concentration of the
REE in the samples was calculated based on linear regression techniques using BHVO-l,
W-2, JB-2, JA-3, BIR-l standards. Strontium and neoudyrnium isotopes were analde

in a VG Sector thermal ionization mass spectrometer at Rutgers University.

2.3.3 Major Elements

Raw major element and trace element data for over 70 pumice samples of The
Valle Central Tufl‘ are listed in Table 2. Data is from only the pumice samples; whole-
rockandash-falldataarenotusedbecausetheyarelithicrich. Pumceclastsare
hydrated, so all major element values are reported using normalized values in discussion

and figures. The Valle Central Tufl‘ pumice samples range from basaltic andesites and

12

trachy andesites to dacites, trachytes and trachydacites, with Si02 ranging fiom 53.9 wt.
%to 68.9 wt. %. Theyareem'ichedinpotassium, falling inboththe highKand
shoshonitic fields (Figure 6 and 20): K20 ranges fiom 2.20 wt. % to 5.49 wt. %.

Samples fi'om the Valle Central Tufl' tall into three groups based on both
chemistry and pumice types, as shown in a plot of K20 and Si02 (Figure 6A and 6B).
The first group, the “low-silica group” has silica ranging fiom 54 wt. % to 63 wt. %.
Black and gray pumice fiagments fall within the low-silica group. The second group, the
“high-silica group” has silica ranging fiom 65 wt. % to 69 wt. %. White, gray, and
collapsed pumice fiagments fall within the high-silica group. The high-silica group also
includes white pumice samples fi'om the plinian ash-fall seen at the base of the ash-flow
in the Valle Central. The third group, a “mingled group” has a chemistry that is
intermediate between the low-silica group and the high-silica group (silica ranging from
57 wt % to 66 wt %). The mingled group contains only banded pumice samples (banding
is observed both irt hand-sample and thin section scales). Using these three groupings,
major element oxide trends are shown in Figure 7.

There are many chemical difl'erences between the high-silica group and the low-
silica group. First, samples from the low-silica group have a large chemical variation
shown by the silica spread (54 wt. % to 63 wt. % Si02; Figure 6). Most major element
oxides have a linear relationship with silica (except Mn0, which has a change of slope
around 61 wt % silica). In contrast, samples fiom high-silica group have a small
chemical variation (65 wt. % to 69 wt. % Si02). In the plots of Si02 versus K20, Ti02,

C30, and Mn0 there are clear breaks, or “gaps”, between each of the two silica groups.

13

Plots of Si02 versus Mg0, A1203, P205 and Fe0 simply display a break in slope at the
transition between the two groups (Figure 7).

With the exception of K20 and Ti02, the concentrations of most major element
oxides decrease sharply in the high-silica group. In particular, the concentration of P205
drops below the detection limit of the XRF in the high-silica group. Ca0, Mg0, A1203,
Fe203T concentrations in the high-silica group decrease by over 25% (in some cases over
50%) in comparison to concentrations in the most mafic samples of the low-silica group
(Figure 7)-

Pumice chemistry is directly correlated to pumice colors (Figure 6). For example,
the high-silica group consists of white pumice clasts and light gray pumice clasts. In the
highly welded sections of the tufi' there are fiamrnes, which are obsidian fiagments,
representing collapsed white pumice fiagments. The low-silica group consists of black
and dark gray pumice clasts. Within black pumice clasts, as silica content increases,
phenocryst content decreases. Banded pumice fiagments are intermediate in chemistry
between the low and high-silica groups. The thickness of the bands of black glass and

white glass has a direct correlation with the average silica content of the sample.

2.3.4 Trace Elements

Trace element abundances for all pumice clasts are listed in Table 2, and variation
for some elements is illustrated in Figure 8. Most trace elements show a linear
relationship with silica content (Figure 8). The high-silica group is enriched in the large
ion LILs (Ba, Rb) as well as Y, and Ta compared to the low-silica group. However, the

middle rare earth elements (MREE) and heavy rare earth elements (HREE) do not show a

14

linear relationship with silica (e.g. Sm and Yb in Figure 8). The high-silica group is
depleted in the MREE and HREE compared to the low-silica group.

Figure 9 is a spider diagram (normalized to primitive mantle concentrations) of
selected Valle Central Tuff pumice fragments representing the high-silica group, low-
silica group, and the mingled group. The overall composition of the Valle Central Tuff
samples is consistent with subduction zone environments; em'ichment in the large ion
lithophile (LILs) and depletion in the high field strength (HF S) elements (Carr et al.,
1990). The rare earth element (REE) patterns for all groups are similar (Figure 10) and
show a steep pattern for light rare earth elements (LREE) and MREE with a flat HREE
pattern. La/Yb for low-silica group samples ranges fi'om 3.58 to 4.63, while La/Yb ratios

for the high-silica group ranges from 3.19 to 5.19 (Figure 10).

2.3.5 Sr and Nd Isotopes

Pumice samples from the high and low-silica groups have an indistinguishable
isotopic compositions (Figure 11). For the low-silica group, 87Sr/86 Sr values range from
0.70372 to 0.70373. For the high-silica group, ”Sr/“Sr have a slightly larger spread,
ranging fiom 0.70371 to 0.70375. The 143Nd/WNd isotope ratios are equally well
constrained. Values range from 0.512932 :1: 32 to 0.512951 i 9. Figure 11 is a plot
comparing Sr and Nd isotopes for the Valle Central Tufl‘ to isotopes from other volcanics
in the Costa Rica. The Valle Central Tufi samples plot well within the range of isotopic
variation and is consistent with the isotopic composition seen in other volcanic products
(mainly lavas) fiom the Valle Central. However, there is a distinct difference in the

isotopic composition of volcanic products fi'om the Valle Central (including the Valle

15

Central Tufl) with volcanic products from northern Costa Rica, such as lavas and tufl‘s

fi'om Rincon de la Vieja (Figure 11).

2.3.6 Petrography

The pumice types in the Valle Central tufl‘ (black, gray, white, and banded) low-
and high-silica magmas and mingling between the two. As previously outlined in Figure
6A and 6B, black and gray pumice fiagments are basaltic-andesites and trachy-andesites,
and belong to the low-silica group (LeBas et al., 1986).. White and light gray pumice
fi'agments are trachy—dacites and trachytes and belong to the high-silica group (LeBas et
al., 1986). Banded pumice fi'agments represent the mingling between the end-members
and belong to the mingled group (Figure 6A). Mineral chemistry for phenocrysts in each
group are displayed in Table 3.

The black pumice fiagments consist of basaltic andesites, andesites, and dacites.
Sample 132 (Figure 12) is representative of the basaltic andesites (54.83 wt. % Si02).
Black pumice fiagments range fi'om crystal rich to crystal poor. Phenocryst content is
inversely proportional to silica content (Figure 13). Crystal-rich, low-silica black
pumices are 67% glass, 28% plagioclase, 4% pyroxene, and less than 1% olivine and
oxides by volume. Representative point counts are listed in Table 4. Crystal rich
samples have abundant plagioclase, olivine, and orthopyroxene and clinopyroxene
phenocrysts, with minor amounts of magnetite and illrnenite (Figure 12A).

Crystal-poor samples have the same phase assemblages but in significantly
reduced amounts. The most silica-rich black pumice (62 wt. % Si02) has 97% glass, 2%

plagioclase, and 1% pyroxene. Oxides are present in only trace amounts, well below 1%

16

by volume (Table 4; Figure 13). In all low-silica group samples, pyroxene crystals are
euhedral, and no glomerophyric clots are present. Plagioclase crystals have abundant
melt inclusions. Vesicles within the glass of low-silica group pumice clasts are well
rounded (Figure 12). Olivine phenocrysts within the low-silica group are euhedral, and
are often altered to iddingsite.

The high-silica group consists dominantly of crystal poor rhyolites, fiom both the
ash-fall and the ash-flow tufl‘. Figure 14 is a photomicrograph of sample 18-2,
representative of the high-silica group. Most white pumice samples are nearly aphyric,
with 0 to 2% crystals by volume. Phenocrysts that do occur appear within glomopheric
clots (Figure 14A &B). Glomopheric clots include plagioclase, pyroxene (both
clinopyroxene and orthopyroxene), and oxides (dominantly magnetite, with minor
illrnenite). Vesicles within the glass of high-silica group pumice clasts are collapsed and
stretched (Figure 14C).

Banded pumice fiagments are transitional in composition between the low and
high-silica groups, and this is reflected by whole rock chemistry (Figure 7 & 8).
Phenocrysts within the banded pumice clasts include plagioclase, clinopyroxene,
orthopyroxene, abundant oxides, and minor olivine. The banded pumice clasts are
interpreted to be the result of mingling between the high and low-silica magmas.
Viscosity contrasts between the low and high-silica magmas most likely prevented
mixing. Textural evidence includes flow banding of brown and white glass, and broken,

rotated phenocrysts (Figure 15A and 15B).

17

2.3.7 Mineral Chemistry

Table 3 lists electron-microprobe analysis for phenocrysts fi'om pumice samples.

Plagioclase. There is a large variation in the anorthite content of plagioclase
phenocrysts fiom the Valle Central Tufl‘ (Figure 16). The lowest silica black pumice (54
wt. % Si02) contains a xenocryst of plagioclase of A1196. Plagioclase phenocrysts in the
low-silica group fill into two populations. Plagioclase phenocrysts in the low-silica
group fill into two populations. Pumice clasts with a whole rock composition of 54 wt.
% Si02 have plagioclase phenocrysts with an average content of Ann. Pumice clasts
with a whole rock composition of 57 wt. % Si02 have plagioclase phenocrysts with an
average content Anso. Plagioclase phenocrysts in the low-silica group are not strongly
zoned. Variations seen in rim to core are less than 2% An. Plagioclase textures in the
low-silica group vary dramatically. The majority of plagioclase phenocrysts exhibit
“sieve” texture full of melt or glass inclusions (Figure 12B). This texture could represent
crystals that have grown very quickly, or reabsorption textures. Remaining phenocrysts
are euhedral laths of plagioclase.

Sieve texture in plagioclase phenocrysts fiom the high-silica group is not as
predominant, although present in minor amounts. Plagioclase compositions have a
continuous range of compositions, and do not fill into distinct populations (An34 — A1160)
in the high-silica group as in the low-silica group (Figure 16). As with the low-silica
group, plagioclase phenocrysts in the high-silica group are not strongly zoned. Variations

are less than 1.5% All for individual phenocrysts.

18

Alkali Feldspar. Two pumice samples, one banded, and one white pumice clast
(67 wt. % Si02) have alkali feldspar present (sanidine), with a composition of Ab“ 0r43
Ans, In both cases, the alkali feldspars are in glomopheric clots.

Clinopyroxene. All clinopyroxene phenocrysts in low-silica group samples have
a very similar chemistry (Figure 17 and 18A), augites averaging a composition of W047
En.“ F 89. Clinopyroxene phenocrysts are euhedral. For fem-magnesian minerals,
equilibrium can be tested by calculated an observed minerthost lava Fe0/Mg0 partition
coefficient (Kd) and then comparing it to published Kd values. For all equilibrium
calculations, it was assumed that Fe0 represented 0.87 of total ferric iron (Sisson and
Grove, 1993). The K. values used for comparison are 0.23 to 0.26 for augite in a basaltic
andesite (Sisson and Grove, 1993). Kd values for clinopyroxene phenocrysts fiom the
low-silica group pumices average 0.202, which is slightly low, but within the error bars
for augite crystallizing from a basaltic andesite melt.

Clinopyroxene compositions in the high—silica group are remarkably similar to
that of the low-silica group (Figure 18A). This is unusual, because with the large
difference in silica content of the low and high-silica groups, it would be expected that
the pyroxene compositions would record the whole rock variation in silica. Instead, all
the clinopyroxenes fiom the Valle Central Tufi are tightly grouped, with a composition of
W042.“ E04347 F 83.12. F s compositions do not vary with silica content (Figure 18A & B).
The clinopyroxenes in the high-silica group are also out of equih’brium with a rhyolitic
liquid, as compared to calculated Kn values from the literature (Mills et al., 1997).

Orthopyroxene. As expected from the olivine-out/orthopyroxene—in phase

boundary, the high-silica group does contain orthopyroxene (Figure 17). Orthopyroxene

l9

compositions are W034; En69-75 F 5 22.29. As previously stated, in general, phenocrysts in
the banded pumice clasts do not show strong evidence for resorption. However, in one
banded pumice clast (sample 16-1) there is a set of orthopyroxene phenocrysts with clear
evidence for reaction rims. These can be seen using back-scattered electron images
(Figure 19A and B). While the orthopyroxene cores are typical of other orthopyroxenes
within the high-silica group, averaging W034; En69-75 F 3 22-29, the reaction rims have a
higher iron content. The reaction rims have a uniform chemistry, W03 Ens, Fs43.

Olivine. Only low-silica group samples have olivine phenocrysts (F 070.73) present
in trace amounts.

Fe-Ti Oxides. Low-silica group pumices have minor amounts of accessory
magnetite, and ilmenite. In the low-silica group there are no magnetite and ilmenite
phenocrysts that share edges, and we do not have electron microprobe analysis for
phenocrysts of magnetite and ilmenite in the same low-silica group pumice (thin section).
Therefore, geothermometry in the low-silica group is diflicult. Howeveer, in the high-
silica group there are several magnetite/ilmenite pairs for which we have electron
microprobe data. There is little variation in magnetite Mg/Mn ratios, but there is
significant variation in illrnenite Mg/Mn ratios.

Apatite. There are small apatite phenocrysts present in both the low and high-
silica group pumices in trace amounts.

Glass. Glass compositions from black pumice averages 60 wt. % Si02. Vesicles
within black (or brown) glass from black pumice clasts are always rounded and abundant
(Figure 10A). Gray pumice clasts with whole pumice chemistry within the low-silica

group do not have as many vesicles. Glass from high-silica group pumice clasts has 68-

20

70 wt. % Si02, again, richer in silica than the whole pumice chemistry. In contrast to the
low-silica group, the vesicles in the high-silica group are collapsed and stretched (Figure
14B and 14C).

Electron microprobe analyses of the glass within the pumice fi'agments
demonstrates that the high and low-silica groups remained chemically distinct during
magma mingling. The brown glass has a silica content ranging between 59 % and 62 %,
while the white glass has a silica content ranging between 68% and 71%. Even in banded
pumices, the vesicles of brown glass versus white glass retain their original
characteristics. Vesicles in brown glass are well rounded and abundant (Figure 12A and

B) and vesicles within white glass are collapsed and stretched (Figure 14A and B).

2.3.8 Comparison with Central American Volcanic Are

The Valle Central Tufi‘ is distinctive when compared to lavas from the Central
American Volcanic Arc. It has long been recognized that Costa Rican volcanic products
have a distinct chemistry when compared to the rest of the arc, and the Valle Central Tuff
is no exception (Patino, 1997; Feigenson and Carr, 1993). The CENTAM database
represents lavas fiom throughout Central America; these lavas in turn represent
Pleistocene and Quaternary evolution and arc-history fi'om many volcanic edifices. One
of the major distinctions of the Valle Central Tufi‘ is that it has higher potassium contents
than the lavas and tutfs from the entire Central American Volcanic are (Figure 20).

While clearly enriched in silica and potassium, other major and trace element
concentrations of the Valle Central Tufl' samples can also be used to characterize the

chemical variations of the volcanic products in the central valley of Costa Rica. The REE

21

elements are can be used to compare the chemistry of the Valle Central Tufl and other
volcanic rocks from other parts of the volcanic are.

For example, Nicaraguan lavas have nearly flat REE patterns (Walker et al.,
1990), while those of El Salvador and northern Costa Rica are slightly enriched (Carr et
al., 1990). Those of Guatemala and central Costa Rica are strongly LREE em'iched
(Feigenson and Carr, 1993). The samples fiom the Valle Central Tuff (both high and
low-silica groups), are LREE enriched, which is similar to other lavas from the central
valley of Costa Rica. La/Yb ratios are two to three times higher for the Valle Central
Tufl‘ than values for the rest of the are, but similar to La/Yb values from lavas fi'om the
same area.

Another trace element ratio useful in characterizing trace element patterns is

Ba/La. The Valle Central Tufl chemistry is consistent with lavas from the Central Valley
in Costa Rica, which have the lowest ranges in Ba/La values recorded ill Central
America. While the low-silica and high-silica groups of the Valle Central Tufi have a
similar chemical signature compared to other volcanic products within Central Costa
Rica, they both have much higher concentrations of potassium, especially the high-silica
group. Other elements, such as Zr, Rb, La, Ba, Eu, Ce, Pr, Nd, Sm, and Gd are also

em‘iched (Figure 21).

22

DISCUSSION

3.] Fracflortal Crystallization

Within the low-silica group, trends in major element variation are consistent with
magma evolution via crystal fractionation of a magma body (i.e. Gill, 1981); MgO, FeO,
and CaO abundance’s decrease systematically as Si02 content increases, and K20
concentration increases. There is strong negative correlation between A1203 and Si02
(Figure 4), especially in the low-silica group, which my be due to plagioclase
fi'actionation. In contrast, potential fi'actionation trends are not as well defined for the

high-silica group (Figure 4). There are no fractionation trends for the mingled group, as

they represent the physical mingling of the low and high-silica group magmas.

3.1.1 Low-silica Group

The entire range of major and trace element compositions within the low-silica
group can be modeled using batch fiactional crystallization. These models were
completed using multiple linear regression of major element oxides (Bryan et al., 1969;
Wright et al., 1970). Once the amount of each crystallizing phase was determined, using
the partition coeflicients for trace elements listed in Table 5, we were able to determine
the expected concentrations for the daughter.

For our models of crystal fractionation in the low-silica group, we assumed
sample 13-2, with 55 wt. % Si02 and 2 wt. % K20 to be the parental composition.
Sample 13-2 has the lowest silica sample fi'om which we have microprobe data for the

mineral phase assemblages Additional evidence for using sample 13-2 is parent is that

23

this pumice clast includes plagioclase phenocrysts of AD96. Other workers have also
found anorthsitic rocks as xenoltihs, at V. Arenal and V. Peas, in Costa Rica, by Cigolini
et al. (1991), Cigolini (1998) and Sachs and Alvarado (1996).

Table 6 lists selected modeling runs for both major and trace elements, using steps
A and B in Figure 22A. The sum of the squares of the residual in modeling runs ranged
from 0.03 to 0.091, well below accepted of a Zr2 less than 0.5. The Zr2 values are based
on major elenrent oxides only. The fiactionating phases are plagioclase (Ann), olivine,
clinopyroxene (W044 Enn Fso), magnetite, and apatite. Models can reproduce the entire
chemical variation within the low-silica group with 38% crystal fractionation.
Plagioclase feldspar is the dominant phase crystallizing out of the liquid. Batch fiactional
crystallization for major elements reproduced the observed trace elements well
(calculated concentrations averaged 91% of the observed concentrations), as seen in

spider diagrams of calculated and observed parent rock values (Figure 223).

3.1.2 High-silica Group

The evolution of high-silica rocks within a mature island arc environment has
long been a controversy. In most island arc settings, abundant high-silica rmgrms are
rare due to the absence of continental crust. In continental arcs, it is possible to
assimilate continental crust or melt in order to achieve high proportions of silica in the
melt (70 wt. %). However, in Costa Rica, which lacks a continental crust, that is not a
possible mechanism for producing the high-silica group pumice fragments. There are
two generally accepted models in the literature to explain the existence of high-silica

magmas in calc-alkaline island arcs. The first is the fiactional crystallimtion of basalt, or

24

basaltic andesite melts (Sisson and Grove, 1993; Feely and Davidson, 1994; Brophy et
al., 1999). The second model to explain the origin of the high-silica group is the partial
melting of previously emplaced arc-related igneous rocks (Beard and Lofgren; 1991;
Roberts and Clemens, 1993). We will discuss fiactional crystallization models first.

Recently, studies have proposed a combination of convection-driven crystal
fractionation, solidification floats, and liquid segregation to explain compositional gaps
such as the one seen between the low- and high-silica groups in the Valle Central Tufl‘
(Marsh, 1984; Brophy, 1999). In basaltic magrm bodies, convection prevents large-scale
crystal settling, which can drive magma chamber differentiation (Marsh, 1984).
Therefore, other mechanisms occurring within the magma body need to explain magma
difl’erentiation.

Brophy et al. (1999) propose the following model. They state that as fiactional
crystallization occurs, a solidification front descends downward into the magma body.
At roughly 50% crystallization of a basaltic magma, convection above the solidification
ceases, although convection still occurs below the solidification front. Above the
solidification fi'ont, the crystal-liquid becomes rigid, and can be fiactured. If these
fractures occur, they can drain evolved interstitial liquids out fi'om the mush, that migrate
upwards by buoyancy driven liquid/crystal segregation (Brophy et al., 1999). These
evolved liquids represent a high-silica magma that is related to a lower silica magma by
fi'actional crystallization. These evolved liquids can accumulate in a magma chamber,
which can also be fractionated by a similar process.

To test this model, several multiple linear regressions were performed (methods

described above) for both major and trace elements to see if it is possible to simulate the

25

evolution of the low-silica magma into a high-silica melt. It is possible to model the
chemical variation of major and trace elements in the high-silica group fiom the low-
silica group. This was done by using the most evolved low-silica sample, 990710—2 (63
wt % Si02) as the parent melt and fractionating it to form the high-silica group (Figure
22A, steps C and D). Fractionating phases dominantly consist of plagioclase (An42),
followed by clinopyroxene (W044 En45 Fslo), orthopyroxene (W03 En53 Fs44), magnetite,
and illrnenite, in decreasing order. The sum of the squares of residuals ranges fiom 0.008
to 0.085, well below accepted values (Table 5B). It is interesting to note, however, that
there seem to be two possible trends that a liquid can follow when evolving from the low-
silica group to the high-silica group, Steps C and D in Figure 22A. Both steps have
several samples that fill along the liquid line of decent (Figure 22A).

One of the problems of using fi'actional crystallization models to explain the
origin of high-silica magmas is the large amounts of fiactionation required to attain high-
silica melts. Our models are ficed with this same problem, requiring over 55%
crystallization of plagioclase to evolve the melt from the least silicic sample (13-2) to the
most silicic sample (1-7). Results of fiactionation models are listed in Table 5b and
shown in Figure 22C and 22D. Another problem with our fi'actional crystallization
models is that these results can be slightly misleading, especially in the REE plots. The
spread of REE elements within both the high-silica group and the low-silica group is very
small. This is because incompatible elements do not change significantly with crystal
fi'actionation until well over 50% of a phase has crystallized out of the liquid. The

amount of plagioclase fractionation required to evolve the lowest silica magma to the

26

highest silica magma of the Valle Central Tufi‘ is not enough to change the concentrations
of most incompatible elements (with the exception of En, which is discussed below).

As discussed earlier, fiactional crystallization models for producing the high-
silica group fiom the low-silica group require large amounts of plagioclase fiactionation.
In this scenario, as increasing amounts of plagioclase fiactionate from the melt, then
increasing amounts of Eu2+ should be partitioned into plagioclase, resulting in an inverse
relationship of Eu/Eu“ and Si02 (or any other index of fractionation). Eu anomalies can
be evaluated by calculating Eu#, which is the value of Eu calculated from the linear
equation of the line connecting Sm and Tb on a REE plot (Appendix C). Eu/Eu* is a
measure of the Eu anomaly. Logically, then, the high«silica group should have a larger
Eu/Eu* anomaly than the low-silica group. In the Valle Central Tufl‘, this is not the case
(F ism 10)-

The Eu/Eu“ for the high-silica group is much smaller than would be expected if
crystal fiactionation is the dominant process (Figure 10). Other volcanic rhyolites, in
both continental and island arcs, have Eu anomalies that are much more pronounced,
indicating large degrees of plagioclase fiactionation (Brophy et al., 1996). Figure 23A, a
plot of Eu/Eu“ vs. Si02, the high-silica group and the low-silica group fill in two distinct
populations that can not be related simply through fractional crystallization. If the high-
silica group is fiactionating fi'om the low-silica group, one would expect the linear trend
of the low-silica group in this plot to continue without breaks. It does not; instead, the
Eu/Eu" for the lowest high-silica group sample (sample 2-1) is higher (0.83) than that of

the least-mafic low-silica group sample (sample 990713-4c, 0.71; Figure 23A). This is a

27

simple but powerful test to reject pure crystal fiactionation as the mechanism for

producing the high-silica group magma fiom the low-silica group magma.

3.2 Partial Melting

Whereas crystal fiactionation clearly is the dominant mechanism controlling the
chemical variation within the low-silica group, this is not the case for the high-silica
group. A possible origin for the high-silica magma is the partial melting of a previously
emplaced and metamorphosed rock (e.g. amphrholite; Beard and Lofgren,l991; Roberts
and Clemens, 1993).

In many studies, Sr and Nd isotopes are used to determine source for
compositionally different magma batches. For example, if the Sr and Nd isotopes for
each magma are unique, this indicates these magmas have sources of different ages. In
the Valle Central Tufl; the Sr and Nd isotopes for both the high and low-silica magmas
are very similar (Figure 11). This implies that the low and high silica group magmas do
not originate from sources of different ages. The isotopic data rejects the possibility that
the high-silica magma represents a partial melt of an older oceanic crust. If it were, the
8"Sr/“Sr and mNd/MNd isotopes for the low and high-silica group would be very
different. However, the 87Sr/M’Sr and I43Nd/ 144Nd isotopes do not either support nor
reject partial melting as a mechanism that relates the high- or low-silica magmas. The
isotopic data does not reject the possibility that the high-silica magma represents a partial
melt of a young oceanic crust, such as the lower Caribbean crust, where the 87Sr/M’Sr and

143Nd/ 1“Nd isotopes have not had time to evolve.

28

A strong argument in fivor of partial melting is the behavior of the middle rare
earth elements (MREE) within the high-silica magma. The light rare earth elements
(LREE) such as La and Ce behave incompatibly in both the low and high-silica groups
(Figure 23B). The MREE and HREE elements, fi'om Sm through Yb, do not behave the
same way in the low and high-silica groups. In the low-silica groups, the MREE behave
as incompatible elements, increasing with silica content (Figure 23C). This is consistent
with a magma that is undergoing fiactional crystallization. However, MREE in the high-
silica group are depleted, behaving as compatible elements irt terms of liquid/crystal
equilibrium (Figure 23C). We interpret this data to suggest that the origin of the high-
silica group is separate fiom the origin of the low-silica group, and the two groups are not
related through fiactional crystallization.

Another test for the petrogenesis of the high-silica group is to evaluate Rb/Hf
ratios. If there has been any involvement with older, altered crust, then a melt should
have elevated Rb/Hf ratios. Crustal melts generally have higher Rb/Hf ratios than mantle
melts. In Figure 23D, is evident that the Rb/Hf ratios for the low-silica group are nearly
constant, while Rb/Hf ratios for the high-silica group are elevated. This would be
consistent with the low-silica group evolving from a mantle melt via crystal fiactionation
(as discussed earlier) and a high-silica group could represent melts of an altered lower
crust (Geist et al., 1998; Price et al., 1999).

Within this model, the high-silica group can be the result of one of three
processes. First, that the high-silica group represents the partial melt of a young,
subducted slab. Second, that either hornblende or clinopyroxene are residual phases in

the partially melted source, as both clinopyroxene and hornblende are compatible with

29

the MREE. Third, that magma that has fiactionally crystallized and has interacted with a
hydrated crust. We also recognize that both of these processes could work in concert; for
example, liquid from a partially melted source can accumulate, undergo minor amounts
of fi'actional crystallization, as well as interact with hydrated country rock.

The first hypothesis is unlikely. The Y concentrations in the Valle Central Tufl'
are greater than 15 ppm, and the Sr/Y ratios is less than 50 ppm, suggesting that the high-
silica group rhyolites are not the adakitic rhyolites of Defint and Drummand (1990). The
third hypothesis is equally unlikely. There are two reasons to reject this third hypothesis.
The first is based on the lack of large Eu anomalies within the high-silica group. The
spread in Eu/Eu“ ratios is not large. While there appears to be a slight decreasing trend
of Eu/Eu“ with Si02 (or any fiactionation index) fiom the least silicic sample within the
high-silica group (2-1) to the most silicic (3-2), it does not present convincing evidence
that crystal fi‘actionation has occurred. Second, if the high-silica group magma interacted
with a hydrated crust, one would expect hydrous phases in the magma, such as
amphibole. The Valle Central Tufl does not have any of these hydrous phases. It is
possible that this issue can be firrther resolved with additional data, especially 80”

analysis and/or the analysis of melt inclusions.

3.3 Magma Mngling and Mixing

The high and low-silica groups erupted together in El Valle Central Tufl". Textural
evidence from banded pumice clasts indicates that there were two distinct rmgma batches
that did not mix or equih'brate in situ, rather, that mingling occurred during evacuation of

the nmgma chamber and resulted in banded pumice fiagments.

30

The chemical variation of the mingled group can be duplicated by a mixing curve
between the high and low-silica group end-members. Tests for mixing were carried out
by using a simple ratio—ratio plots of four separate elements. For example, Figure 24A is
a plot of Sr/Y versus Ti/P, and if the mixing occurred these data should plot on a
hyperbola with the end-members (low and high-silica groups) plotting on the ends of the
curve. A firrther test is the plot of the denominators of the original ratios pairs ( e.g. P/Y)
plotted against one of the original ratios (e.g. SrfY). If the data from banded pumice
clasts fill on a straight line, with the same two end-members on the previous plot, then
this provides additional support that magma mixing or mingling has occurred.

These plots can also be used to test if the chemical variation of either the low- or
high-silica magmas can be reproduced through homogeneous magma mixing. Figure
248 is a plot of Y/Sm versus Ta/T i and Y/Sm vs. Ti/Sm. In these plots it is clear that
while the mingled pumice fi'agments fill on mixing lines between the two end-members,
the chemical variation in neither the low-silica or high-silica group can be expla’md
using magma mixing. These relationships are consistent when MREE and HREE are
used in the mixing plots, as the behavior of the MREE and HREE in the high-silica
magma are inconsistent with mixing of the low and high-silica magma to produce these
trends.

In addition, temperature differences between these two magma batches could not
have been maintained for any length of time in a magma chamber (Turner and Campbell,
1986; Martin et al., 1987). Instabilities in the thermal regime makes it unlikely that the
low-silica mgma and the high-silica magma co-existed in the magma chamber for a long

period of time.

31

3.4 K20 Trends

There are many hypothesis to explain potassium enrichment in subduction
derived volcanics. Early hypothesis included pressure-dependent variation in partition
coefficients for concentration in slab melts (Marsh and Carmicheal, 1974), variation in
the degree of slab melting with depth (lakes and White, 1972), enrichment of K in fluids
or melts by wall-rock interaction (Best, 1975), and fractional crystallization of
orthopyroxene and not olivine basaltic magma at high pressure (Meen, 1987). As
discussed earlier is this study, it is unlikely that The Valle Central Tufl represents a slab
melt. Other, more recent, hypothesis discuss variations of the thickness of the melting
column in the rmntle wedge (Plank and Langmuir, 1988), variation in the degree of
melting within the mantle wedge (Stern et al., 1993), and the influence of sediment
derived fluids (Le Bel et al., 1985). These fluids are often emiched in “fluid-mobile”
elements such as Ba, Rb, K, and Sr, and consequently influence the character of the
subarc mantle (Tatsumi et al., 1986). However, low values for trace element ratios such
as Ba/La and Ba/Th, make it highly unlikely that slab input have played a large role in the
petrogenesis of The Valle Central Tufl‘ magmas.

An alternative model is that partial melting of amphibolite has played a role in K-
emichments (Price et al., 1999). This would be consistent with the petrogenesis of the
high-silica group through partial melting of amphibolite. Further work with melt
inclusions within phenocrysts is required to investigate these models, but is not within the

scope of this project.

32

4. Origin of the Chemical Zonation in the Valle Central Tufl'

The Valle Central tufl‘ formed from two chemically, mineralogically and
thermally distinct magma batches. Basaltic-andesite and rhyolitic magmas have distinct
rheological and thermal properties. Temperature differences between these two magma
batches could not have been maintained for any length of time in a magma chamber (e. g.
Turner and Campbell, 1986; Martin et al., 1987) and therefore the eruption occurred
almost immediately afier the emplacement of the separate magrm batches in the
chamber.

Figure 25 is a cartoon of our model. We propose that the low-silica magma
represents a mantle melt that has undergone fractional crystallization, creating a
continuous range of silica contents fi'om 53 wt % to 63 wt %. Black pumice fiagments
are both crystal-rich and aphyric. Low-silica, phenocryst rich black pumice fi'agments
represent the early stages of fractional crystallization. The low-silica magma is a zoned
magma, because both pumice types are co-erupted. The higher-silica (aphyric) low-silica
magam has a lower liquidous temperature, thus fewer crystals. Fractional crystallization
occurred through both side-wall fiactional crystallization and a descending solidification
front.

The high-silica nfigma represents a partial melt of an amphibolite. Based on
uniform potassium enrichments, tigllt constraints on the REE values, and the isotope
composition, it is clear that the high-silica magma and the low-silica magma are inter-
related, although difi'erent Rb/Hf ratios suggest that they have a slightly different source.

We suggest that the high-silica group originated fi'om partially melted meta-igneous rock

33

(amphibolite; explaining the different Rb/Hf ratios), but with original chemical

composition very similar to the low-silica group within the Valle Central Tufi‘.

5. CONCLUSIONS

The Valle Central Tufi‘ is one of the most potassic and silicic calc-alkaline
volcanic deposits along the Central American Volcanic Arc. Within the tufl‘ are three
chemical groups; the low-silica group, the high-silica group, and the mingled group. The
low-silica group and the high-silica group represent two distinct magmas, which, based
on their temperature differences, could not have evolved in the same magma chamber.
The mingled group represents the physical and chemical mixing between these two
magmas.

Based on comparisons with other volcanic products along the Central America
Volcanic arc, it is possible to make inferences as to the mantle sources and deep crustal
processes operating beneath the central valley of Costa Rica. Based on trace element
ratios such as La/Yb, the mantle beneath central Costa Rica is an enriched mantle that has
not been largely modified by slab input (fluids from subducted sediments; Patino et al.,
1997). Potassium enrichments in the Valle Central Tufl' can be accounted for by the
interaction of a hydrous mafic melt with wall rocks at the base of the oceanic crust
followed by incongruent melting of wall rock amphibole. This produces potassic liquids
that mix with the magma and any amphibole crystallized earlier in the system is
reabsorbed back into the melt.

We propose that the low-silica magma represents a mantle melt that has

undergone fiactional crystallimtion, creating a continuous range of silica contents from

34

53 wt % to 63 wt %. The high-silica magma represents a partial melt of an amphibolite.
Based on uniform potassium enrichments and tight constraints on the REE values and
REE pattern, it is clear that the high-silica magma and the low-silica magma are inter-
related. We suggest that the high-silica group partially melted from arnphibolite, which
had original chemical characteristics very similar to the low-silica group within the Valle

Central Tuff.

35

APPENDIX A

Tables

36

Table 1. Site Locations In El Valle Central Tuff

 

Site West’ North" fipographlc Site
Quandrangle

1 498.3 218.3 Rio Grande Tajo La Garita

2 527 216.9 Abra Tajo La Roca

3 Abra Tajo Santo Tomas

4 515.6 217.25 Abra Tajo Santa Ana

5 515.85 217.1 Abra Tajo Ramirez-Crexpo

6 499.3 218.5 Rio Grande Tajo Polvorén

7 506 213.4 Rio Grande Tajo del MOPT

8 496.4 222.35 Naranjo Tajo Rio Colorado

9 490 228.5 Naranjo Tajo Buenos Aires

11 496.4 222.35 Naranjo Tajo Rio Colorado

13 521.6 216.8 Abra Tajo Barreal

16 521 216.05 Abra Tajo Electriona

17 521.5 215.9 Abra Tajo del \flrilla

18 497.1 216.05 Rio Grande Tajo Rio Grande

19 504.6 216.05 Rio Grande Corte en Calle Vueltas

990710 498.3 218.3 Rio Grande Tajo La Garita

990710-2 498.3 218.3 Rio Grande Tajo La Garita

990711-3 498.3 218.3 Rio Grande Tajo La Garita

”07124 496.4 222.35 Naranjo Tajo Rio Colorado

990713-2 498.8 218.8 Rio Grande Tajo La Aduana

990713-3 498.8 218.8 Rio Grande Tajo La Aduana

990713-4 498.8 218.8 Rio Grande Tajo La Aduana

‘ Units in Lambert

37

Table 2

Major and Trace Elements

6-1:122197

6-2:122197

6-3z122197

643:1 22197

64:1 221 97

 

 

Color black light gray black white black
Site Polvordn Polvorén Polvorén Polvorén Polvorén
$102 (70) 60.20 65.30 53.80 66.00 54.40
T102 1.08 0.76 0.88 0.71 0.86
A1203 15.80 15.10 19.60 15.20 19.60
F920" 6.17 3.44 7.16 3.07 6.83
MnO 0.16 0.11 0.11 0.11 0.11
M90 1.69 0.58 2.09 0.59 1.69
CaO 3.96 1.46 7.25 1.70 7.17
N920 3.66 3.51 3.02 3.56 2.85
K20 3.28 5.29 2.28 5.42 2.20
P205 0.41 0.14 0.52 0.12 0.51
TOTAL 96.41 95.69 96.71 96.48 96.22
Cr (ppm) 3 9 1 1 1 17

N1 19 20 35 15 20
Cu 8 3 90 1 83
Zn 73 58 84 56 79
Rb 90 128 55 129 63

Sr 623 31 1 867 276 907
Y 38 43 23 44 29

Zr 320 428 204 447 225
Nb 23 30 7 37 1 1

Ba 1258 1619 873 1694 836
La - _ - - -

Ce - - - - -

Pr - - - - -

Nd - - - - -

Sm - - - - -

Eu - - - - -

Gd - - - - -

Tb - - - - -

Dy - - - - -

Ho - - - - -

Er - - - - -

Yb - - - - -

Lu - - - - -

Hf - - - - -

Ta - - - - -

Pb - - - - -

Th

38

Table 2 continued

 

 

6-5:122197 6-6:122197 1 1-1:122397 1 14:122397 114:122397
Color black white black black whole rock
Site Polvordn Polvorén Rio Colorado Rio Colorado Rio Colorado
$102 ('16) 56.70 65.80 63.00 61.30 58.10
Ti02 0.92 0.74 1.05 1.14 1.26
A1203 19.30 15.20 15.90 16.10 17.10
F020” 6.46 3.23 4.87 5.98 8.63
MnO 0.13 0.12 0.13 0.14 0.12
M90 1.78 0.69 0.96 1.34 1.03
CaO 6.43 1.82 2.79 3.55 4.38
Map 3.80 3.54 3.18 3.10 3.90
K20 2.71 5.21 3.84 3.47 2.45
P20, 0.48 0.13 0.27 0.35 0.61
TOTAL 98.71 96.48 95.99 96.47 97.58
Cr (ppm) 17 1 9 6 1 9 24
Ni 28 14 8 7 6
Cu 49 6 35 33 91
Zn 63 60 80 82 67
Rb 72 134 96 81 70
Sr 864 298 448 533 778
Y 31 41 44 41 31
Zr 247 444 374 327 243
Nb 15 38 27 23 16
Ba 1055 1621 1534 1392 1243
La - - - - -
Ce - - - - -
Pr - - - - -
Nd - - - - -
Sm - - - - -
Eu - - - - -
Gd - - - - -
Tb - - - - -
Dy - - - _ -
Ho - - - - -
Er - - - - -
Yb - - - - -
Lu - - - - -
Hf - - - - -
Ta - - - - -
Pb - - - - «-

Th - - - - -

39

Table 2 continued

 

 

11-52122397 164:010298 164:010298 5-1 :122097 7-1:122197
Color banded dark gray banded banded banded
Site Rio Colorado Electriona Electriona Ramirez-Crexpo MOPT
$102 ('6) 63.90 55.10 59.60 57.80 56.80
T102 1.00 0.90 1.19 1.00 0.96
AI203 16.20 18.70 16.30 17.90 20.60
F020” 4.17 7.13 6.41 6.58 6.74
M110 0.12 0.12 0.16 0.14 0.11
M90 1.13 2.14 1.88 2.00 0.58
0110 2.96 6.87 4.16 5.75 5.56
N820 3.80 3.26 4.24 3.98 3.77
K20 4.16 2.47 3.64 3.03 2.58
P205 0.22 0.54 0.44 0.53 0.54
TOTAL 97.66 97.23 98.02 98.71 98.24
Cr (ppm) 4 22 1 160 BD 1
Ni 7 39 106 4 15
Cu 21 171 42 64 91
Zn 74 55 88 79 58
Rb 108 50 96 78 51
Sr 479 837 656 761 777
Y 44 30 34 30 25
Zr 369 234 319 319 270
Nb 32 17 23 19 17
Ba 1611 1079 1345 1178 1165
La - - - 74.5 65.2
C0 - - - 131 .0 1 18.2
Pr - - - 17.1 14.5
Nd - - - 62.7 52.0
Sm - - - 10.99 9.06
Eu - - - 2.48 2.32
Gd - - - 9.71 8.00
Tb - - - 1 .25 0.97
Dy - - - 6.38 4.88
Ho - - - 1 .35 1 .02
Er - - - 3.73 2.83
Yb - - - 3.18 2.38
Lu - - - 0.50 0.36
Hf - - - - -
Ta - - — - -
Pb ~ - - - -

Th - - - - -

40

Table 2 continued

 

 

8-1:122297 18—1:010298 184:010298 18-5:O1OZ98 1-2:121897
Color banded dark gray black black black
Site Rio Colorado Electriona Electriona Electriona La Garita
$102 (96) 63.70 60.90 56.40 55.20 55.60
T102 1.06 0.97 0.96 0.95 1.04
A1203 16.10 17.70 19.40 19.30 19.70
F620“ 5.07 5.66 6.38 7.17 7.94
Mn0 0.13 0.12 0.12 0.13 0.14
M90 1.11 1.70 2.10 2.47 2.18
CaO 2.97 4.96 6.93 7.41 5.58
N820 3.71 3.88 3.89 3.66 2.71
K20 4.16 3.46 2.62 2.46 2.10
P20. 0.30 0.40 0.54 0.52 0.38
TOTAL 98.31 99.75 99.34 99.27 97.37
Cr (ppm) 80 BD 30 BD 0
N1 19 0 7 42 4
Cu 63 169 197 104 123
Zn 87 64 68 72 79
Rb 104 88 66 62 66
Sr 451 680 865 848 758
Y 36 30 27 28 28
Zr 430 273 223 215 206
Nb 32 24 15 14 23
Ba 1 500 1 324 1085 988 950
La 84.9 72.9 66.2 59.6 56.0
Ce 152.3 134.6 121.1 110.3 106.3
Pr 18.5 16.6 14.9 13.6 13.2
Nd 65.7 60.8 55.9 50.7 49.1
Sm 11.65 10.63 9.82 9.42 8.98
Eu 2.46 2.42 2.36 2.16 2.16
Gd 11.20 7.26 7.06 6.68 6.55
Tb 1.42 1.10 1.02 0.97 0.95
Dy 7.76 5.71 5.40 5.11 5.04
Ho 1.66 1.22 1.13 1.12 1.02
Er 4.99 3.18 3.00 2.91 2.96
Yb 4.11 3.38 2.87 2.66 2.55
Lu 0.66 0.48 0.42 0.45 0.43
Hf - - - - 5.73
To - - - - 1.33
Pb - - - - 7.07
Th - - - - 13.12

41

Table 2 continued

14:121897

14:121897

1-5z121897

14:121897

1-7:121897

 

 

Color light gray light gray black light gray white
Site La Garita La Garita La Garita La Garita La Garita
$102 (96) 65.90 65.70 61.50 66.60 66.20
T102 0.75 0.79 1.05 0.74 0.74
Al203 15.60 15.50 16.30 15.40 15.50
F920“ 3.21 3.29 5.70 3.16 3.09
Mn0 0.11 0.11 0.15 0.11 0.11
M90 0.65 0.65 1.65 0.62 0.61
C90 1.76 1.77 3.84 1.74 1.71
N820 4.20 3.95 4.40 4.40 4.43
K20 5.25 5.34 3.77 5.24 5.32
P20. 0.13 0.13 0.42 0.13 0.13
TOTAL 97.56 97.23 98.78 98.14 97.84
Cr (ppm) BD 80 BD 80 30
Ni 80 BD 30 BD 11
Cu 19 10 21 8 19
Zn 66 68 84 68 67
Rb 137 136 101 132 135
Sr 294 302 595 283 273
Y 41 42 36 42 42

Zr 406 405 309 403 412
Nb 49 55 38 53 55
Ba 1818 1833 1465 1807 1766
La 78.4 78.7 86.1 76.8 77.0
C0 191.5 186.3 158.1 187.1 186.3
Pr 19.2 18.1 19.7 18.0 18.3
Nd 62.1 57.8 71.2 58.2 60.2
Sm 10.68 10.43 12.27 10.20 11.07
Eu 2.34 2.24 2.65 2.23 2.22
Gd 8.41 7.93 9.28 7.56 8.15
Tb 1.01 1.00 1.28 1.03 1.08
Dy 6.00 5.44 6.90 5.55 5.50
Ho 1.28 1.17 1.48 1.21 1.25
Er 3.35 3.27 3.88 3.31 3.07
Yb 3.38 3.40 3.69 3.25 3.32
Lu 0.52 0.51 0.56 0.52 0.51
Hf 8.12 8.95 9.1 8.88 9.29
Ta 2.54 2.87 2.22 2.69 2.89
Pb 18.76 14.73 11.33 23.83 25.58
Th 20.21 22.31 22.13 21.41 22.38

Table 2 continued

 

 

1-8:121897 1-9:121897 18-1:010498 18-1b:010498 18-2:010498
Color white white banded banded white
Slto La Garita La Garita Rio Grande Rio Grande Rio Grande
8102 ('16) 64.40 66.00 61.40 64.10 66.70
T102 0.74 0.77 1.14 0.97 0.74
AI203 15.60 15.40 16.20 16.00 15.40
Mn0 0.12 0.11 0.15 0.13 0.11
M90 0.64 0.65 1.57 0.96 0.58
080 1.71 1.75 3.57 2.38 1.56
N920 4.52 4.44 4.36 4.04 3.85
K20 5.34 5.15 3.89 4.76 5.49
P205 0.13 0.13 0.39 0.19 0.13
TOTAL 96.64 97.63 98.13 97.85 97.77
Cr (ppm) 745 80 BD 30 BD
Ni 79 BD 80 BD 1
Cu 21 7 18 28 18
Zn 70 68 87 76 62
Rb 140 132 98 121 140
St 280 280 580 413 284
Y 42 41 36 39 41
Zr 413 404 316 369 404
Nb 55 56 42 50 53
Ba 1819 1784 1489 1617 1878
La 78.2 78.3 89.8 85.8 80.2
Ce 189.1 185.5 167.1 172.1 195.8
Pr 18.7 18.5 20.1 19.2 18.9
Nd 59.9 60.8 72.5 66.4 59.6
Sm 10.17 11.01 12.57 11.08 10.97
Eu 2.18 2.21 2.77 2.54 2.11
Gd 7.78 8.27 9.05 8.55 7.91
Tb 1.03 1.08 1.34 1.20 1.02
Dy 5.89 5.34 7.23 6.51 5.89
Ho 1.29 1.23 1.55 1.35 1.30
Er 3.33 3.22 4.01 3.64 3.30
Yb 3.41 3.11 3.85 3.62 3.99
Lu 0.56 0.54 0.58 0.56 0.41
Hf 8.95 8.43 8.72 9.04 9.26
Ta 2.78 2.8 2.26 2.69 2.93
Pb 24.87 24.54 12.01 15.4 12.84
Th 22.43 21.34 21.02 23.02 22.45

43

Table 2 continued

 

 

184:010498 184:010498 18-5:010498 13—1z122997 13-2z122997
Color banded collapsed whole rock black black
Site Rio Grande Rlo Grande Rio Grande Barreal Barreal
$102 (7.) 66.10 64.50 61.30 56.70 54.80
T102 0.77 0.88 1.01 0.95 0.93
A1203 15.60 16.00 17.50 19.10 19.70
Mn0 0.10 0.12 0.13 0.13 0.13
M90 0.59 0.82 1.12 2.08 2.37
CaO 1.61 2.19 3.79 6.97 7.61
N820 3.87 3.84 4.04 3.77 3.56
K20 5.45 4.99 3.67 2.56 2.38
P20. 0.14 0.17 0.29 0.53 0.52
TOTAL 97.59 97.43 98.33 99.45 99.00
Cr (ppm) BD BD 30 BD BD
Ni 80 6 BD 10 2
Cu 11 26 38 297 91
Zn 53 75 80 80 76
Rb 142 127 97 66 61
Sr 313 382 589 873 918
Y 38 39 32 26 26
Zr 400 377 31 2 288 261
Nb 54 55 41 13 9
Ba 1895 1758 1500 1111 1010
La 79.0 74.8 82.7 71.7 65.0
Co 186.4 180.0 151.0 127.4 113.3
Pr 17.9 17.6 17.7 14.4 13.1
Nd 57.4 58.2 62.3 55.2 49.8
Sm 9.85 10.10 10.78 9.86 8.95
Eu 2.07 2.35 2.42 2.32 2.20
Gd 7.37 7.53 7.89 8.36 7.88
Tb 0.98 1.07 1.10 1.11 1.00
Dy 5.11 4.93 6.31 5.07 5.14
Ho 1.15 1.11 1.38 1.04 0.99
Er 3.08 3.09 3.59 3.14 2.83
Yb 3.69 3.06 3.30 2.63 2.49
Lu 0.45 0.53 0.53 0.43 0.40
Hf 8.82 8.33 8.52 - -
Ta 2.96 2.63 2.15 - -
Pb 10.16 23.46 11.97 - -
Th 23.06 21.86 20.54 - -

Table 2 continued

 

 

13-3:122997 134:122997 9-1:122397 9-2:122397 17-1:010298
Color black black banded banded dark gray
Site Barreal Barreal Buenos Aires Buenos Aires Virilla
8102 (“M 55.90 55.70 60.10 60.90 56.49
Ti02 0.94 0.94 1.07 1.11 0.87
A1203 19.30 19.30 17.50 16.50 19.35
F920" 7.00 6.73 6.10 6.08 6.84
MnO 0.13 0.13 0.14 0.15 0.14
7490 2.29 2.22 1.52 1.71 2.08
CaO 7.15 7.13 4.13 4.05 7.16
Map 3.47 3.66 3.67 4.09 3.63
K20 2.44 2.58 3.43 3.63 2.42
P205 0.48 0.53 0.38 0.44 0.49
TOTAL 99.10 98.92 98.04 98.66 99.47
Cr (ppm) 30 80 BD 30 80
Ni BD BD BD BD BD
Cu 56 63 31 23 95
Zn 78 76 82 85 78
Rb 63 68 88 91 60
Sr 868 876 629 616 783
Y 26 27 36 36 29
Zr 240 252 375 379 225
Nb 10 12 21 22 15
Ba 1046 1090 1426 1392 956
La 60.6 64.3 88.0 86.4 57.1
Ce 112.1 117.9 151.3 150.4 104.2
Pt 14.2 15.0 19.4 19.0 12.7
Nd 51.1 53.5 69.0 69.0 48.2
Sm 8.86 9.45 12.07 11.92 8.64
Eu 2.21 2.33 2.74 2.69 2.11
Gd 8.01 8.19 11.09 10.93 7.50
Tb 1.02 1.05 1.41 1.44 0.94
Dy 5.13 5.25 7.34 7.44 4.80
Ho 1.02 1.11 1.55 1.59 0.96
Er 3.03 3.12 4.52 4.68 2.93
Yb 2.66 2.76 3.91 3.69 2.58
Lu 0.40 0.42 0.62 0.60 0.39
l-If - - - - -
Ta - - - - -
Pb - - - - -

Th

45

Table 2 continued

 

 

17-2:010298 174:010298 19-1:010498 194:010498 194:010498
Color banded gray black black whole rock
Site Vm’lla Virilla Calla Vueltas Calle Vueltas Calle Vueltas
810; (‘79) 58.39 58.07 58.87 58.66 59.95
T102 0.89 1.00 1.05 1.01 1.03
N203 18.27 18.26 16.76 18.43 17.58
F020“ 6.27 6.18 6.07 6.74 5.96
"110 0.14 0.14 0.15 0.14 0.14
M90 1.74 1.76 1.46 1.90 1.66
CaO 6.36 4.79 3.44 5.67 4.18
Mac 3.91 4.03 4.46 3.69 4.26
K20 2.83 2.96 3.60 2.86 3.33
P205 0.46 0.49 0.41 0.45 0.43
TOTAL 99.26 97.68 96.27 99.55 98.52
Cr (ppm) 80 BD 1 109 80 BD
Ni 30 BO 110 BD 80
Cu 54 26 35 64 32
Zn 74 76 79 74 75
Rb 75 82 103 78 91
Sr 744 648 570 685 589
Y 32 33 35 28 32
Zr 271 311 372 281 332
Nb 17 19 20 16 21
Ba 1111 1250 1461 1155 1314
La 66.0 72.3 81.9 67.1 74.9
Cc 118.0 134.4 156.4 124.8 140.4
Pr 14.1 15.6 18.6 15.8 16.7
Nd 54.0 58.2 64.0 55.5 58.3
Sm 9.79 10.22 10.93 9.62 9.93
Eu 2.30 2.37 2.43 2.21 2.31
Gd 8.08 8.71 10.42 8.58 9.57
Tb 1.05 1.16 1.27 1.08 1.19
Dy 5.25 5.46 6.65 5.45 5.77
Ho 1.03 1.12 1.41 1.13 1.25
Er 3.07 3.41 4.02 3.17 3.55
Yb 2.79 3.01 3.51 2.85 3.16
Lu 0.45 0.47 0.56 0.44 0.50
Hf - - - - -
Ta - - - - -
Pb - - - - -

Th

46

Table 2 continued

 

 

3-1:121997 3-2z121997 3-3z129197 443122097 4—2:122097
Color whole rock collapsed collapsed ash fail ash fall
Site Santo Tomas Santo Tomas Santo Tomas Santa Ana Santa Ana
Si02 (96) 65.98 67.19 68.88 63.42 66.78
T102 0.84 0.80 0.77 1.06 0.86
N203 16.03 15.76 15.69 15.86 15.38
F020;" 4.29 3.89 3.49 4.58 3.70
"no 0.12 0.11 0.11 0.14 0.12
M90 0.92 0.79 0.68 1.18 0.79
CaO 2.68 2.16 1.93 2.85 2.05
N820 4.42 4.53 4.48 3.78 3.40
K20 4.35 4.71 4.94 4.19 4.84
P205 0.17 0.14 0.13 0.16 0.14
TOTAL 99.80 100.08 101.10 97.22 98.06
Cr (ppm) 80 BD 80 BD 30
NI BD 80 80 BD 80
Cu 47 21 39 4 33
Zn 70 70 58 80 79
Rb 115 127 131 115 131
Sr 402 330 317 487 317
Y 39 41 42 39 42
Zr 426 424 436 347 393
Nb 32 34 36 50 54
Be 161 1 1697 1772 1664 1708
La 83.6 84.0 84.6 80.6 70.6
Co 162.4 175.3 184.6 164.0 174.6
Pr 16.6 16.6 16.5 18.1 16.6
Nd 57.9 56.2 56.5 61.1 54.7
Sm 10.09 10.05 9.93 10.59 9.80
Eu 2.15 2.16 2.08 2.46 2.14
Gd 8.82 8.47 8.27 7.56 7.30
Tb 1.16 1.15 1.15 1.09 1.00
Dy 5.62 5.48 5.19 5.48 5.16
Ho 1.13 1.13 1.13 1.27 1.27
Er 3.57 3.65 3.53 3.34 3.22
Yb 3.30 2.75 3.46 3.40 3.01
Lu 0.58 0.55 0.53 0.55 0.48
Hf - - - 7.5 8.04
To - - - 2.45 2.53
Pb - - - 18.86 23.67
Th - - - 20.14 19.64

47

Table 2 continued

 

 

44:1 22097 4-4:1 22097 99071 0-‘l 99071 0-2 99071 1 -3a1
Color ash fail ash fall black white white
Site Santa Ana Santa Ana La Garita La Garita La Garita
810; (“M 65.94 67.30 67.29 62.37 67.28
NO; 0.89 0.79 0.72 1.15 0.74
Ale3 15.71 15.49 15.46 16.15 15.49
F920" 3.93 3.43 3.12 6.13 3.13
MnO 0.12 0.11 0.11 0.15 0.11
M90 0.92 0.68 0.63 1.91 0.64
CaO 2.46 1.75 1.76 4.19 1.81
NazO 3.45 3.38 4.54 4.43 4.22
K20 4.53 5.31 5.22 3.54 5.23
P30; 0.15 0.12 0.12 0.44 0.12
TOTAL 98.10 98.36 98.97 100.46 98.77
Cr (ppm) BD BD BD BD BD
Nl BO 80 BD 80 3
Cu 4 46 11 12 80
Zn 73 79 67 86 69
Rb 120 144 131 88 130
Sr 400 272 264 602 277
Y 41 44 42 35 40
Zr 373 407 423 295 417
Nb 47 53 57 37 59
Ba 1725 1787 1803 1462 1818
La 73.1 80.9 74.6 75.0 72.9
Ce 169.8 188.2 168.9 136.1 165.6
Pr 17.0 18.3 17.1 17.2 16.7
Nd 55.9 61.9 54.2 62.5 53.3
Sm 9.86 10.82 10.40 11.58 9.45
Eu 2.27 2.24 2.06 2.59 1.98
Gd 6.93 7.82 9.62 10.72 8.74
Tb 1.07 1.09 1.07 1.24 0.97
Dy 5.52 5.75 5.38 6.58 5.65
Ho 1.18 1.27 1.16 1.23 1.05
Er 3.24 3.45 3.17 3.50 2.90
Yb 3.84 3.68 3.39 3.40 3.17
Lu 0.50 0.55 0.54 0.54 0.50
l-lf 8.95 8.58 8.45 7.8 8.13
To 2.53 2.74 2.84 1.97 2.81
Pb 19.68 28.95 20.64 8.23 20.2
Th 21.63 21.93 23.73 20.36 23.07

Table 2 continued

 

 

990711-3112 990711-3b1 990711.332 seam-ac 990711.39
Color white white black banded banded
Slto La Garita La Garita La Garita La Garita La Garita
SiO, ('6) 64.49 66.95 67.38 56.36 66.49
TIC; 0.75 0.77 0.73 0.87 0.86
AIZO, 15.40 15.56 15.55 19.19 15.67
F920,, 3.66 3.36 3.22 6.62 3.73
MnO 0.12 0.11 0.11 0.12 0.12
M90 0.68 0.67 0.64 1.70 0.81
CaO 1.77 1.91 1.83 6.86 2.10
Nazo 4.22 4.27 4.27 3.25 4.46
K20 4.96 5.25 5.32 2.43 4.89
P30. 0.13 0.13 0.13 0.45 0.16
TOTAL 96.18 98.98 99.18 97.85 99.29
Cr (ppm) BD 3 BD 3 80
Ni 6 10 8 3 4
Cu 27 29 18 85 3
Zn 76 71 69 68 74
Rb 128 130 130 66 120
Sr 275 296 279 832 328
Y 41 40 42 25 41
Zr 414 409 417 220 399
Nb 39 58 53 25 56
Ba 1851 1834 1780 1094 1798
La 58.1 71.5 74.9 54.5 70.7
Ce - 165.0 163.5 104.9 160.4
Pr - 16.1 16.4 12.8 15.6
Nd - 52.8 53.7 46.5 51.3
Sm - 9.32 9.86 8.74 9.75
Eu - 2.01 1.94 2.04 2.14
Gd - 9.12 8.79 7.42 9.05
Tb - 0.98 1.01 0.92 0.91
Dy - 4.98 5.47 4.87 5.56
Ho - 1.00 1.13 0.93 1.13
Er - 2.82 2.99 2.49 2.90
Yb - 3.22 3.39 2.46 3.22
Lu - 0.48 0.54 0.38 0.47
l-lf - 8.28 8.92 5.55 8.22
Ta - 2.71 2.71 1.29 2.66
Pb - 19.94 19.43 5.76 18.74
Th - 21.73 23.37 14.33 21.76

49

Table 2 continued

 

 

99071 1 -31 99071 2-4c 99071 3-2b 99071 3-2d 99071 3-2f
Color banded gray gray light gray banded
Site La Garita Rio Colorado La Aduana La Aduana La Aduana
Si02 (‘70) 66.69 60.89 64.85 66.41 61.88
"O; 0.81 1.24 0.84 0.77 1.16
Ale; 15.61 16.30 15.81 15.25 15.99
MnO 0.12 0.15 0.11 0.11 0.15
"90 0.70 1.70 0.76 0.54 1.52
C80 1.90 5.00 2.11 1.42 3.45
N820 4.23 3.71 3.87 3.85 4.05
K20 5.18 2.75 4.84 5.34 3.90
P205 0.13 0.63 0.20 0.13 0.34
TOTAL 98.81 100.52 97.42 97.13 97.74
Cr (ppm) 80 311 BD BD BD
Ni 6 44 1 5 80
Cu 38 89 17 5 23
Zn 68 86 58 57 85
Rb 128 71 118 129 97
Sr 289 676 368 281 541
Y 37 29 41 40 37
Zr 413 244 379 416 321
Nb 57 27 56 63 42
Ba 1800 1239 1773 1886 1552
La 68.1 61.0 68.0 73.4 73.4
Ce 164.0 111.5 158.3 179.9 143.3
Pr 15.6 14.8 15.3 17.3 17.2
Nd 49.8 56.1 49.0 51.4 61.3
Sm 9.36 10.55 8.73 9.68 11.30
Eu 2.04 2.49 1.99 2.00 2.45
Gd 8.25 9.22 8.17 8.93 9.56
Tb 0.96 1.15 0.90 1.03 1.11
Dy 5.17 6.30 4.89 5.30 6.04
l-lo 1.03 1.19 0.98 1.02 1.18
Er 2.88 3.19 2.74 2.69 3.15
Yb 3.23 3.32 3.16 3.04 3.23
Lu 0.47 0.49 0.45 0.51 0.50
Hf 7.93 6.51 7.31 8.11 7.58
Ta 2.77 1.54 2.54 2.98 2.17
Pb 19.72 4.56 11.17 10.67 9.51
Th 21.41 15.35 20.32 21.56 20.4

50

Table 2 continued

 

 

990713-2ll 990713-3b 9907134. 990713-4b 99071 34c
Color light gray banded banded black black
Site La Aduana La Aduana La Aduana La Aduana La Aduana
$102 (96) 63.12 63.29 64.30 56.77 62.62
TiOz 1.10 1.03 1.05 0.86 1.08
AI203 16.15 16.11 15.94 18.39 15.67
MnO 0.14 0.13 0.13 0.12 0.15
7590 1.35 1.27 1.27 1.76 1.88
C80 3.08 3.02 3.05 6.47 4.38
NazO 4.05 3.94 3.99 3.19 3.80
K20 4.03 4.04 4.10 2.69 3.39
P305 0.33 0.26 0.27 0.43 0.47
TOTAL 98.66 97.93 98.93 96.95 99.90
Cr (ppm) 30 BD 80 BD 1479
Ni 1 4 0 ED 166
Cu 25 13 8 68 30
Zn 73 75 78 70 94
Rb 100 104 106 72 90
Sr 548 448 455 770 637
Y 37 38 38 28 34
Zr 316 346 346 237 296
Nb 42 43 45 27 36
Ba 1573 1580 1619 1 138 1455
La 75.2 71.3 73.3 56.5 71.4
Ce 141.9 139.9 150.0 105.9 139.3
Pr 17.2 16.7 17.0 13.3 17.8
Nd 60.7 57.4 59.1 48.5 64.7
Sm 11.00 9.98 10.87 8.90 11.92
Eu 2.36 2.26 2.40 2.07 2.70
Gd 9.44 9.19 9.47 7.80 10.05
Tb 1.18 1.07 1.13 0.96 1.21
Dy 6.39 6.11 5.98 5.05 6.44
Ho 1.20 1.16 1.10 0.97 1.24
Er 3.22 3.25 3.17 2.57 3.40
Yb 3.33 3.29 3.25 2.65 3.44
Lu 0.49 0.52 0.49 0.41 0.51
Hf 8.07 8.33 7.82 5.95 7.35
Ta 2.23 2.33 2.27 1.43 1.87
Pb 8.09 12.12 12.82 8.26 8.54
Th 21.05 21.9 21.31 14.91 18.75

Table 2 continued

 

 

99071341
Color white
Site La Aduana
Si02 (°/o) 65.09
NO; 0.81
A120, 15.90
FOzOg‘r 4.23
MnO 0.1 3
M90 1 .05
Geo 2.76
NaZO 3.53
K30 4.66
P20. 0.16
TOTAL 98.32
Cr (ppm) BD
Ni 3
Cu 24
Zn 72
Rb 1 17
Sr 363
Y 39
Zr 372
Nb 34
Ba 1 172
La 63.3
Ce -

Pr -
Nd -
Sm -
Eu -
Gd -
Tb -
Dy -
Ho -
Er -
Yb -
Lu -
Hf -
Ta -
Pb -

Th

52

 

 

Table 3 Phenocryst Commitions
Clinopyroxene $102 7102 211,0, Feo' MnO M90 080 NazO
13-2.1-core-cpx 50.71 0.60 2.07 10.44 0.39 15725 19.71 0.42
13.2.1-rim-cpx 50.40 0.51 2.38 8.48 0.34 15.48 20.22 0.40
13-2.1-int-cpx 51.34 0.61 1.96 10.89 0.33 15.42 20.06 0.41
13-2.4-1-rim-cpx 50.13 0.58 2.46 7.09 0.28 15.50 20.92 0.38
13-2.41-int-cpx 50.08 0.56 2.39 8.79 0.30 15.57 20.83 0.36
13-2.4-1-rim-cpx 50.96 0.70 2.26 10.37 0.44 15.57 20.47 0.43
13-2.4-2-rim-cpx 50.39 0.56 2.34 8.09 0.33 15.51 21.03 0.38
13-2.4-2-core-cpx 50.90 0.55 2.12 9.65 0.29 15.71 20.92 0.38
11-5.1-cpx 51.29 0.48 1.42 9.42 0.71 15.23 20.96 0.43
1-3.3-rim—cpx 52.10 0.46 1.32 9.57 0.72 15.03 20.63 0.45
1-3.3-int-cpx 52.17 0.41 1.09 8.70 0.64 15.57 20.08 0.43
1-3.3-core-cpx 51.24 0.68 1.83 9.52 0.67 14.83 20.53 0.47
1-3.3-rim-cpx 51.44 0.45 1.23 10.02 0.72 15.05 20.34 0.40
1-3.3-rim-cpx 51.84 0.61 1.52 10.01 0.64 15.10 20.26 0.47
16-3.3-core-cpx 50.32 0.62 2.14 8.68 0.30 15.33 20.08 0.39
16-3.3-int-cpx 50.85 0.60 2.35 10.33 0.30 15.29 20.18 0.41
16-3.3-rim-cpx 50.81 0.63 2.66 9.20 0.37 15.39 20.40 0.40
16—6.3—core-cpx 51.61 0.66 1.86 10.72 0.78 15.08 20.39 0.42
16-6.3-rim-cpx 52.69 0.26 0.83 10.67 1.03 14.31 20.90 0.47
16-6.4-int-cpx 52.88 0.27 0.91 9.68 0.90 14.54 21.37 0.42
16-6.4-int2-cpx 52.43 0.31 0.85 10.09 0.88 14.13 21.32 0.49
16-6.4-oore-cpx 52.93 0.37 1.26 9.63 0.68 15.01 21.02 0.49
16-6.4-core2-cpx 52.68 0.45 1.21 9.14 0.77 14.91 21.22 0.43
1-7.1-rim-cpx 54.51 0.19 0.51 19.75 1.27 24.82 1.49 0.16
1-7.1-rim2-cpx 53.93 0.23 0.60 16.76 1.40 25.30 1.45 0.12
1-7.2-rim-cpx 51.61 0.46 1.37 8.20 0.74 15.09 20.80 0.44
1-7.2-core—cpx 51.12 0.54 2.13 10.32 0.75 14.76 20.30 0.46
1-7.3-rim-cpx 51.10 0.54 1.44 8.87 0.71 15.01 20.89 0.46
1-7.3-int-cpx 51.89 0.42 1.06 10.35 0.84 14.71 20.81 0.45
1-7.3-core-cpx 51.72 0.44 1.45 8.18 0.72 15.15 21.02 0.46
18-2.4-1rim-cpx 51.35 0.41 1.25 9.41 0.72 15.13 20.86 0.46
18-2.4-1core-cpx 51.31 0.48 1.19 9.94 0.71 15.10 20.63 0.41
18-2.4-2rim-cpx 50.56 0.48 1.31 7.80 0.71 15.16 20.95 0.41
18-2.4-200re-cpx 51.32 0.43 1.19 10.64 0.78 14.93 20.76 0.41
18-2.4-2rimrpt1-cpx 49.96 0.58 1.53 8.26 0.77 14.60 20.58 0.49
18«2.4—2rimrpt2-cpx 51.08 0.44 1.12 11.06 0.75 14.92 20.71 0.41
18-2.4-2int—cpx 50.84 0.41 1.33 8.80 0.68 14.99 20.82 0.40
18-2.4-3int-cpx 50.39 0.50 1.29 8.84 0.81 14.81 20.62 0.47
18-2.3-core-cpx 57.16 0.04 25.56 4.52 0.02 0.01 7.97 6.12
18-2.3-rim-cpx 56.17 0.02 25.47 0.47 0.00 0.00 8.36 5.92
6-3B.2—inplrim-cpx 51.23 0.59 1.68 5.27 0.75 15.32 20.65 0.45

53

 

 

Table 3 Continued Phenocryst Compositions
Orthopryoxene Si02 TiO, Alzo3 FeO' MnO _Mjg_O CaO Nazo
1-3.1-rim-opx 52.94 0.26 0.54 18.61 1.26 24.84 1.41 0.12
1-3.1-core-opx 52.71 0.36 0.99 18.94 1.37 24.36 1.52 0.17
18-2.1-rim-opx 53.57 0.25 0.68 16.18 1.34 24.90 1.42 0.12
18-2.1-oore-opx 53.12 0.29 0.62 17.22 1.33 24.98 1.32 0.14
18-2.1-othrim-opx 53.50 0.22 0.57 18.95 1.19 25.00 1.42 0.12
6-38.1-rim-opx 52.35 0.36 1.00 16.43 1.22 24.88 1.96 0.13
6-3B.1-oore-opx 52.03 0.21 0.67 18.15 1.33 25.02 1.38 0.17
6-3B.1-200reoopx 52.74 0.26 0.71 19.00 1.18 25.26 1.43 0.11
6-3B.1-2rim-opx 52.10 0.35 1.03 18.25 1.24 24.78 1.48 0.13
11-5.1-opx 53.85 0.32 0.94 19.28 0.61 24.95 1.87 0.12
11-5.1-opx 52.85 0.31 0.89 16.26 0.55 24.64 2.92 0.14
16-1.4-1opx-a-opx 52.01 0.16 0.32 27.30 1.04 18.32 1.48 0.11
16-1.4-iopx-b-opx 50.90 0.13 0.34 27.31 1.07 18.35 1.41 0.10
16-1.4-1opx-c—opx 52.88 0.32 1.14 16.48 0.54 26.20 1.82 0.14
16-1.4-1opx-d—opx 53.33 0.31 1.18 16.09 0.56 26.19 1.73 0.08
16—1.4-1opx-e—opx 53.14 0.30 1.15 16.65 0.63 26.29 1.68 0.10
16-1.4-20px-a-opx 51.54 0.10 0.21 27.13 0.97 18.83 1.28 0.10
16-1.4-20px-b—opx 51.93 0.09 0.20 27.92 0.76 18.49 1.38 0.15
16-1.4-20px-c-opx 53.39 0.27 1.22 13.25 0.58 26.57 1.74 0.15
16-1.4-20px-c-opx 54.15 0.23 0.90 17.20 0.57 26.63 1.74 0.12

 

 

Table 3 Continued Phenocryst Compositions
Plagioclase Sioz Ale, FeO' CaO NaZO K20
13-2.2-rim-pl 49.61 31.20 0.71 14.89 2.90 0.21
13-2.2-int-pl 49.29 31.76 0.71 15.05 2.86 0.19
13-2.2-int/core-pl 49.07 31.99 0.62 15.20 2.61 0.15
13-2.2-core—pl 48.06 32.35 0.85 15.84 2.43 0.08
13-2.3-rim-pl 49.34 2.53 8.20 20.95 0.44 0.02
11-5.1-pl 52.99 29.32 0.76 12.36 4.13 0.46
11-5.1-pl 54.27 29.09 0.75 11.70 4.54 0.49
11-5.1-pl 63.80 16.60 1.55 1.19 5.26 4.70
11-5.3-pl 58.01 26.13 0.42 8.01 6.23 0.78
11-5.3-pl 57.63 26.10 0.56 8.16 6.26 0.79
11-5.3-pl 56.91 27.05 0.45 9.30 5.88 0.61
1-3.1-rim-pl 59.44 25.50 0.51 7.21 6.66 0.97
1-3.1-core-pl 58.37 25.59 0.32 7.93 6.51 0.86
1 -3.3-rim-pl 57.83 26.24 0.52 8.15 6.19 0.81
1-3.3—core-pl 58.68 25.70 0.42 7.78 6.43 0.93
16-1.3-rim-pl 49.77 32.22 0.73 15.10 3.01 0.17
16-1.3-int-pl 48.76 32.31 0.55 15.53 2.73 0.15
16-1.3-core-pl 48.02 32.37 0.67 16.04 2.50 0.17
16-1.3-coreZ-pl 48.11 32.22 0.59 15.99 2.37 0.16
16-1.3-core3-pl 47.68 32.36 0.72 16.25 2.44 0.11
16-1.3-int-p| 48.16 31.92 0.63 16.01 2.51 0.10
16-1.4-rima-pl 60.49 23.72 0.39 6.29 6.87 1.38
16-1.4-rimb-pl 60.99 23.65 0.42 6.22 6.76 1.70
16-1.4-corec-pl 48.35 32.40 0.68 15.83 2.46 0.17
16—1.4-cored-p| 49.31 31.24 0.71 15.10 2.85 0.18
16—3.2-rim-pl 57.96 26.46 0.51 8.77 6.18 0.65
16-3.2-int-pl 55.39 28.04 0.65 10.61 5.31 0.49
16—3.2-core-pl 54.81 28.42 0.31 10.75 5.11 0.46
16-3.2-otrim-pl 55.20 27.99 0.53 10.41 5.31 0.46
16-6.1-a-pl 58.58 26.13 0.47 7.74 6.46 0.86
16-6.1-b-p| 59.17 25.23 0.52 7.33 6.65 0.89
16-6.1-c-pl 57.68 26.44 0.49 8.40 6.05 0.72
16.6.1-d-pl 57.84 26.47 0.55 8.68 6.00 0.72
16-6.1-e-p| 58.42 26.32 0.64 8.52 6.26 0.76
16-6.1-f-pl 54.94 28.78 0.52 11.04 5.13 0.51
16-6.1-g-pl 54.88 28.49 0.52 10.71 5.03 0.45
16-6.1-h-pl 54.83 28.93 0.38 10.73 5.19 0.49
16-6.1-l-pl 54.80 28.84 0.41 11.26 4.84 0.40
16~6.1-i2-pl 54.65 28.15 0.35 10.87 5.04 0.41
16—6.1-k-pl 59.42 25.92 0.43 7.63 6.47 0.93
16~6.1-L-pl 59.56 25.60 0.40 7.34 6.52 0.85
16—6.1-m-pl 58.28 26.66 0.37 8.07 6.29 0.83
16-6.1-n-pl 57.37 26.92 0.54 9.12 5.96 0.66
16-6.1-o—pl 57.99 27.32 0.42 8.90 6.00 0.69
16-6.1-p-pl 58.93 26.20 0.39 8.05 6.30 0.76
16—6.1-q-pl 57.19 26.39 0.49 8.64 6.09 0.68
16—6.2-rim-pl 57.77 26.33 0.44 8.60 6.17 0.86
16-6.2-int-pl 57.69 26.34 0.50 8.20 6.17 0.77
16—6.2-core-pl 58.51 25.76 0.48 7.86 6.39 0.78
1-6.1-cor8-pl 53.28 29.34 0.49 12.11 4.46 0.37
1-6.1-int-pl 54.92 28.17 0.42 10.91 5.02 0.51
1-6.1-rim-pl 58.20 25.29 0.42 7.74 6.36 0.94

55

Table 3 Continued

Phenocryst Compositions

 

 

Plagioclase $102 7102 Al203 FeO' CaO Nazo K20
1-6.2-rim-pl 57.95 0.00 25.78 0.49 8.49 6.15 0.71
1-7.1-rim-pl 58.59 0.00 25.74 0.45 7.78 6.43 0.87
1-7.1-intb-pl 50.09 0.00 24.71 0.33 6.40 6.94 1.25
1-7.1-intc-pl 57.73 0.00 26.09 0.30 8.27 6.24 0.79
1-7.1-intc—pl 57.80 0.00 26.37 0.45 8.32 6.14 0.74
1-7.1-core-pl 58.77 0.00 25.92 0.50 7.69 6.55 0.83
1-7.2-rim-pi 59.22 0.00 24.29 0.41 6.45 6.83 1.04
1-7.2-rim2-pl 59.16 0.00 24.37 0.48 6.69 6.90 1.05
1-7.2-rim3-pl 58.06 0.00 25.13 0.43 7.79 6.45 0.85
1-7.2-core-p| 58.93 0.00 25.12 0.46 7.44 6.49 1.01
1-7.2-othrim-pl 59.23 0.00 25.86 0.55 7.70 6.42 0.79
18-2.4-1rim-p| 57.18 0.00 25.64 0.36 8.04 6.39 0.80
18-2.4-1rimrpt-p| 59.98 0.00 24.71 0.38 6.83 6.94 1.11
18-2.4-1int-pl 58.08 0.00 26.09 0.45 7.94 6.26 0.81
18-2.4-1int2-pl 57.55 0.00 26.17 0.46 8.37 6.03 0.75
18-2.4-1core-pl 57.79 0.00 26.85 0.50 8.89 6.05 0.73
18-2.4-1othrim-pl 57.19 0.00 26.73 0.37 8.88 6.08 0.61
18-2.4-2rim-pl 58.53 0.00 24.90 0.33 7.34 6.41 1.00
18-2.4-2int-p| 56.91 0.00 26.07 0.38 8.51 6.07 0.72
18-2.4—200re—pl 56.49 0.00 26.43 0.40 9.00 5.85 0.58
18-2.4-p| 57.00 0.00 26.46 0.47 8.68 6.06 0.70
18-2.3-rim-pl 56.95 0.00 25.11 0.46 7.78 6.45 0.85
18—2.2-rim2-pl 56.60 0.00 27.47 0.57 9.34 5.56 0.56
18-2.2-int-p| 56.99 0.00 26.79 0.61 8.82 6.00 0.67
18-2.2-othrim-pl 67.12 0.00 14.96 2.13 1.24 4.20 5.49
18-2.2-0thrim2-pl 67.54 0.00 14.99 2.12 1.24 4.19 5.61
18-2.2-rimd-p| 57.35 0.00 26.34 0.52 8.98 6.03 0.78
18-2.1-rim-pl 57.98 0.00 25.56 0.31 7.66 6.52 0.90
18-2.1-core-pl 58.44 0.00 25.62 0.37 7.67 6.53 0.87
18-2.1-int-pl 58.93 0.00 25.13 0.44 7.13 6.65 0.95
18-2.1-othrim-pl 58.76 0.00 25.61 0.50 7.77 6.43 0.95
6-3B.1-rim-pl 61.30 0.00 21.12 0.97 5.38 5.61 2.34
6-3B.1-rimrpt-pl 56.91 0.00 26.38 0.43 8.53 6.09 0.76
6-3B.1-int-pl 57.18 0.00 26.24 0.44 8.43 6.25 0.76
6-3B.1-core-pl 56.85 0.00 26.71 0.46 9.01 5.94 0.73

Table 3 Continued Phenocryst Compositions

 

 

K-Feldspar $102 7102 AI203 FeO' CaO Nazo K20
6-3B.2-rim-fld 58.67 0.00 25.77 0.39 7.83 6.29 0.77
6-3B.2-int-fld 56.51 0.00 26.46 0.57 8.88 6.02 0.62

6-3B.2-core-fld 57.36 0.00 25.86 0.55 8.34 6.14 0.71
6-3B.2-corerpt-fld 57.11 0.00 25.94 0.72 8.31 6.14 0.80

57

Table 3 Continued

Phenocryst Compositions

 

 

Olivine $10, 7102 Al203 FeO' MnO MgO CaO
13—2.3b-rim-0l 37.09 0.01 0.00 25.30 0.49 36.69 0.14
13—2.3b—int—0I 36.82 0.01 0.04 25.68 0.45 36.94 0.13
13-2.3b-core-o| 37.06 0.05 0.03 25.59 0.57 36.87 0.19
13-2.4-1-rim-o| 37.03 0.02 0.02 25.45 0.53 36.74 0.18
13-2.4-1-int-0l 37.19 0.01 0.01 25.26 0.50 36.77 0.16
13-2.4-1-core-0| 36.99 0.00 0.00 25.20 0.48 36.43 0.16
13-2.4-1-rim-0I 36.98 0.02 0.05 25.51 0.46 37.04 0.15
13-2.4-2-rim-0l 36.97 0.01 0.00 25.48 0.56 36.75 0.14
13-2.4-2-int-0l 36.90 0.00 0.05 25.22 0.49 37.00 0.21
13—2.4-2-core-ol 37.04 0.02 0.03 25.13 0.46 36.87 0.23
13—2.4-2-rim-0| 37.15 0.00 0.03 25.11 0.47 37.19 0.20
16-1.2-rim-ol 37.94 0.04 0.03 25.18 0.54 36.61 0.14
16-1.2-int—ol 37.35 0.03 0.01 25.71 0.53 36.45 0.15
16-1.2-c0re-0l 37.37 0.05 0.02 24.94 0.53 36.69 0.13
16-1.1-c0re-0l 37.36 0.02 0.05 25.81 0.45 36.66 0.15
16-1.1-rim-0l 37.28 0.02 0.00 25.21 0.54 36.77 0.19
16-1.4-rimonopx-0I 51.07 0.17 0.34 27.30 0.97 18.55 1.34
16-1.4-rim-0| 36.91 0.00 0.04 26.51 0.63 35.40 0.14
16-3.4-rim-0l 36.81 0.01 0.02 24.75 0.46 37.01 0.14
16-3.4-int-ol 36.73 0.00 0.02 24.19 0.45 37.40 0.16
16-3.4-int2-ol 36.49 0.02 0.02 24.68 0.54 37.31 0.17
16-3.4-core-0l 37.31 0.00 0.01 24.51 0.45 37.29 0.13
16-3.4-rim2-0l 36.80 0.00 0.02 24.65 0.49 37.65 0.16

58

Table 3 Continued

Phenocryst Compositions

 

 

 

 

 

 

 

 

 

Magnetite- 8102 7102 Al203 FeO' MnO MgO CaO
lllmenite Pairs

1-3.1il 0.02 45.53 0.25 49.84 1.08 3.95 0.04
1-3.1-mt 0.08 12.16 2.14 82.55 0.94 2.66 0.02
16-3.1-il 0.03 43.42 0.22 48.78 1.11 3.38 0.02
16-3.1-mt 0.08 10.581.84 86.11 0.50 0.88 0.04
16-6.3-a-il 0.00 53.55 0.02 42.21 0.51 3.76 0.00
16-6.3-mt 0.04 12.29 2.19 83.19 0.97 2.64 0.05
16.6.36." 0.05 45.53 0.25 50.38 1.19 3.39 0.02
16—6.3-b-mt 0.20 12.52 2.09 80.48 0.99 2.34 0.10
18-2.1-il 0.03 44.94 0.24 51.47 1.18 3.41 0.01
18—2.1-a-mt 0.07 12.28 2.24 82.32 0.94 2.56 0.01
18-2.1-b—il 0.01 44.85 0.26 50.78 1.04 3.27 0.09
18-2.1-b-mt 0.07 12.06 2.26 82.32 0.95 2.51 0.02
6-3B.1-core—il 0.04 44.88 0.26 50.67 1.10 3.88 0.04
6-3B.1-core-mt 0.06 12.36 2.23 82.76 0.87 2.71 0.01
6-3B.1-rim-il 0.00 44.83 0.28 50.41 1.05 3.98 0.04
6-3B.1-rim-mt 0.08 12.12 2.23 82.37 0.92 2.69 0.06

 

59

 

Table 3 Continued Phenocryst Compositions
Other Magneite Sio2 rio2 AIZO3 FeO' MnO MgO CaO

 

and Illmenites

16-3.1-dmt 0.09 10.651.74 86.24 0.42 0.88 0.04
16-3.1-cmt 0.09 10,581.63 87.61 0.38 0.89 0.01
6-3B.1-corerpt-mt 0.05 12.25 2.31 82.91 0.85 2.70 0.00
6-3B.1-corerpt2-mt 0.04 12.16 2.30 82.51 1.00 2.65 0.00

16-6.3-c-i| 0.00 45.73 0.23 50.46 1.00 3.54 0.05
1-7.3—a-i| 0.02 45.49 0.22 50.23 1.04 3.98 0.18
1-7.3-b-il 0.03 45.39 0.27 49.34 1.13 3.93 0.17

1-7.3-2-i| 0.00 45.56 0.22 49.40 1.18 3.91 0.03

60

Table 4. Point Counting: By volumgflienmryst content

 

 

54 wt.% 58 wt.% 62 wt. °/o
Counts %Volume Counts %Volume Counts % Volume
matrix 382 67 610 80 540 97
plagioclase 162 28 116 15 5 2
pyroxene 21 4 23 3 0 1
olivine 4 0.70 1 0.13 0 0
oxides 1 0.18 81.06 1 0.18
total 570 100 758 100 546 100

Values based on an average of 600 point counts; 8% error.

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Figure 7. Major element oxide variation plots versus SiO2 for El Valle TuffD =Low-
Silica Group. 9 =High-Silica Group.A =Mingled-Group.

73

 

 

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Figure 8. Trace element variation plots versus Si02 for El Valle TuffD =Low-Silica
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phenocryst from the same sample; the pock-marked texture of the crystal is referred to
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nearly aphyric samples of 62 wt. % Si02. In all photomicrographs, the bar scale

represents 1 mm. UXP= Uncrossed polars, or plane-polarized light. XP= View using
crossed polars.

78

 

       
 

 

 

 
   
  
   

    

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pumice. Dashed lines in vertical axis indicates a change in scale.

79

 

 

 

 

Figure 14. A. Photomicrograph of a glomopheric clot from a white pumice clast from
the high silica group. B. Photomicrograph of glass from an aphyric, white pumice clast
from the high silica group. Note the stretched and collapsed vesicules within the glass.
In all photomicrographs, the bar scale represents 1 mm. UXP= Uncrossed polars, or
plane-polarized light. XP= View using crossed polars.

80

 

Figure 15. A. Photomicrograph of banded pumice fragments with abundant rotated
and broken phenocrysts. B. Photomicropgraph of remnant glomopheric clot
surrounded by white glass, typical of white pumice of the high-silica group,
surrounded by brown glass typical of the low silica group C. Photomicrograph of
glass in banded pumice fragments. Note that the brown glass (59—60 wt % Si02)
retains well rounded vesicules while the white glass (69-70 wt% Si02) has stretched
and collapsed vesicules. In all views, the scale bar represents 1 mm. UXP=
Uncrossed polars, or plane-polarized light. XP= View using crossed polars.

81

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85

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enriched in K compared to other volcanic products from along the Central

American Volcanic arc.

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Time 3: Shallow
crustal level

 

 

heated maflc crust
with minor
fractional
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High Silica Group

Time 2: Lower to
middle crustal
level

Fractional
Crystallization: Low
Silica Group

Time 1:
Subcrustal
level

Emplacement of
mafic magma

 

 

Figure 25. Schematic model of the evolution of El Valle Central Tuff.

91

 

APPENDIX C

Equations

92

Eu anomalies can be evaluated by calculating Eu#, which is the value of Eu calculated
from the linear equation of the line connecting Sm and Tb on a REE plot (Appendix C).
Eu/Eu“ is a measure of the Eu anomaly. To calculate the Eu/Eu“ anomaly, it is necessary
to use the chondrite normalized values of Sm, Eu, and Th. Then, given equations 1, 2,

and 3, it is possible to determine the :1 (eq. 4) and b (eq. 5) for the linear equation of Sm,

 

Eu, and Tb on an REE plot.

loglo (Tb) = a*65 + b (I)
loglo (Eu*) = a*63 + b (2)

lo Sm = a*62 + b
810 ( ) (3)
a _ loglo (Sm)- 10810 (Tb) (4)

(62-65)

b =10810 (Sm)- (3*62) (5)

Through reorganization and substitution, it is possible to calculate the value of Eu/Eu*, or

the measure of the Eu anomaly ( eq. 6).

Eu Eu
— = ,1 (6)
Eu* log (a*63 + b)

 

93

 

BIBLIOGRAPHY

94

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