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' - m 9.5:- 4! u .- « i315? 4‘ 0! mai— ineam ICHIGAN STATE MR Illlllllllllllllllllllilllllllllllll‘llll ll 3 1293 01022 556 This is to certify that the thesis entitled The Role of Volume Loss in the Development of Deformation Fabrics in Proterozoic Metadiabase Dikes in the Marquette- Republic Region of Northern Michigan presented by Thomas Lynn Weaver has been accepted towards fulfillment of the requirements for Masters degree in Geological Sciences /7 Major professor Date / f 4 0-7639 MS U is an Afimatiw Action/Equal Opportunity Institution LIBRARY M'Chigan State University me: u RETURN aox to remove mi. checkout train your record. TO AVOID FINES Mum on or bdoro dd. duo. DATE DUE DATE DUE DATE DUE THE ROLE 0? VOLUME LOSS IN THE DEVELOPMENT OF DEFORMATION FABRICS IN PROTEROZOIC METADIABASE DIKES IN THE MARQUETTE- REPUBLIC REGION OF NORTHERN MICHIGAN BY Thomas Lynn Weaver A THESIS submitted to Michigan state University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geological Sciences 1994 ABSTRACT THE ROLE OF VOLUME LOSS IN THE DEVELOPMENT OF DEFORMATION FABRICS IN PROTEROZOIC METADIABASE DIKES IN THE MARQUETTE- REPUBLIC REGION OF NORTHERN MICHIGAN BY Thomas L. Weaver Foliated Lower-Proterozoic ‘metadiabase dikes intrude Archean granite-gneiss and greenstone terranes, and lower- Proterozic metasediments in Marquette County. These dikes are cut by non-foliated Keweenawan diabase dikes, suggesting that foliation is related to Penokean orogenic events. Intensity of foliation increases and the angle between dike wall and margin decreases, from dike interior to dike margin, indicating substantially higher shear strain at the margins. Shear strain may result in volume loss (solution transfer) in addition to bulk-rock material transport. Similar chemical trends from dike interior to margin are observed in most dikes, decreasing CaO, Nazo, and A1203. Wall rock contamination appears negligible, with possible exception of K20 in two dikes. Based on chemical and petrographic data, it appears that strain-induced volume loss occurred, primarily as a result of plagioclase dissolution, although it was probably not significant enough to account for pervasive foliation observed in the dikes. ACKNOWLEDGEMENTS I wish to express deep gratitude to Dr. Bill Cambray, chairman of my thesis committee, whose knowledge and continuous input made this thesis and my career in geology a reality. Special thanks to the other committee members, Dr. Tom Vogel, whose courses and critical guidance provided me the knowledge of igneous and metamorphic systems necessary to complete this study, and Dr. Duncan Sibley for helpful suggestions throughout the study and critical review of the final manuscript. Thanks also to Dr. David Westjohn, whose friendship, guidance, knowledge of the Marquette-Republic region, and knowledge of field geology, are greatly appreciated. My field work was funded through Geological Society of America Grant #4830-91 and is gratefully acknowledged at this time. The Officer-in-Charge at the United States Coast Guard facility' at Lighthouse Point. graciously' gave permission to remove the samples necessary to complete the study, even though requests to sample at this site are normally denied. My fellow students at the Geology Club helped make MSU iii very enjoyable, particularly Tom Sabin and Chris Christensen. Special thanks to Kris Huysken, whose unfailing support, advice, encouragement, and friendship made the completion of this project possible. The Geology Department staff was a tremendous help during my stay, thanks to Loretta, Cathy, Jackie, Diane, and Bob. Thanks also to Steve Smith, a very great friend, who has twice encouraged me to leave the security of an excellent job at the railroad to return to academia. My deepest gratitude is to my family. They have given me the skills necessary to succeed in my many endeavors, with special encouragement throughout my educational career. My wife, Dawn, has never quit believing in me and I owe my success to her undying support. I wish that my father, Robert Weaver, had survived to see me graduate and to fly fish, I know that he would be very proud. iv TABLE OF CONTENTS LIST OF TABLESOOOOOOOOO......OOOOOOOOOOO... ..... 00...... vii LISTOF FIGURESOO00.0.00...0.0..0.........OOOOOOOOOOOOOOViii INTRODUCTIONOOOOOOOOO......OOOOOOOOOOOOOOOOOO00.0.0.0... REGIONALGEomGYCOOOOOOOO......OOOOOOOOOOOOOO THE ROLE OFVOLUME 11088000000000.0000ooeeoooo APPLICABILITY OF VOLUME LOSS TECHNIQUES IN THE STUDY OF SHEARED METADIABASE DIKES....... SAMPLINGANDMETHODSOOOOOOOOOOOOOOO00......O. Loss on Ignition Analysis... ......... ... Petrographic Analysis........... ..... . ........... .. Shear Strain Analysis................... X-Ray Fluorescence............. ...... ... Methods for Identifying Wall Rock Contamination....... ..... . .................... COMPARISON OF LIGHT VERSUS DARK MINERALS..... RESULTS OF LOSS ON IGNITION ANALYSIS......... RESULTS OF PETROGRAPHIC ANALYSIS............. ANALYSIS OF SHEAR STRAIN..................... RESULTS OF X-RAY FLUORESCENCE ANALYSIS....... THE EFFECT OF SHEAR STRAIN ON BULK-ROCK CHEMISTRY. DISCUSSIONCOOOOO......OOOOOOOOOOOOOOO0.0.0.0000... CONCLUSIONS. OOOOOOOOOOOOOOOOOOO ......OOOOOOOOOO REFERENCESOOOO ..... ......OOOOOOOOOOOOOOOO ........ 00.... APPENDIX 1. Results of XRF analysis of samples from Republic area dike 1.... 1 6 13 18 21 26 26 26 27 28 31 32 34 60 7O 98 106 111 113 117 APPENDIX APPENDIX APPENDIX APPENDIX APPENDIX APPENDIX TABLE or CONTENTS (continued) Results samples Results samples Results samples Results of XRF analysis of from Republic area dike 2 ......... of XRF analysis of from Republic area dike 3......... of XRF analysis of from Republic area dike 4......... of XRF analysis of samples from Republic area dike 5......... Results of XRF analysis of samples from east-west dike at Lighthouse Point............... ........ Results of XRF analysis of samples from north-south dike at Lighthouse Point....................... vi 118 119 120 121 122 123 Table Table Table Table Table Table Table Table Table Table 10. LIST OF TABLES Comparison of light and dark minerals from samples of Republic area dikes 1, 3, and 4.. ....................... ..... ....... Results of loss-on-ignition procedure for samples from Republic area dikes 1, 3’ and 4...... Modal analysis of Republic area dike 1 ....... MOdal dike 2000.... analysis of Republic area Modal analysis of Republic area dike Modal analysis of Republic area dike Modal analysis of Republic area dike Modal analysis of Lighthouse Point east-west dike.................... ........... Modal analysis of Lighthouse Point north-south dike ..... ... ...... ..... .......... Comparison of shear strain values for Republic area dikes 1, 2, and 5. Angle 9' is the angle measured from dike margin to line of maximum extension (foliation) ....... ..... ............ vii 31 33 41 54 58 65 Figure Figure Figure Figure Figure Figure Figure Figure Figure LIST OF FIGURES Geologic sketch map of the western Upper Peninsula of Michigan. (Modified from Gair and Thaden, 1968)......OOOOOOOOOOOOOC0......O ...... .00. Photograph of Republic dike 5, illustrating sigmoidal foliation (highlighted in red).. ........ . ............ Map of the Marquette-Republic study area showing Republic area and Lighthouse Point sampling locations ........ Geologic map of Lighthouse Point, Marquette, Michigan, showing sampling locations. (Modified from Gair and Thaden, 1968) ................ Sketch of Republic area dikes 1 and 2, which outcrop approximately 12 m apart, with an opposite sense of ShearOOOOOCOOOOOO.......OOOOOOOO...... ..... Photograph of portion of outcrop illustrated in Figure 8, showing Republic area dike 2 and granite-gneiss country rock ................ Photomicrograph of amphibole from dike center sample, displaying curved cleavage traces, bent crystal boundaries, and radial extinction .......... Photomicrograph of amphibole from dike interior sample, showing alteration to chlorite and biotite.............. ...... .......... ...... Photomicrograph showing coarse, twinned, sericitic plagioclase, which may be a remnant of original igneous texture, dike center sample......................... viii 22 23 25 25 36 36 37 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 10. 11. 12. 13. 14a. 14b. 15. 16. 17a. 17b. 18a. 18b. LIST OF FIGURES (continued) Photomicrograph showing coarser, euhedral amphibole and biotite surrounded by fine-grained groundmass, dike center sample............. Photomicrograph of elongate fibrous mat of very fine mice, and coarser chlorite and biotite, dike center sample ..... . ........... .. ...... . ........... Photomicrograph of fabric in dike margin sample, euhedral biotites displaying primary foliation (horizontal) and superimposed crenulation cleavage (semi-vertical, sigmoidal bands)................... ....... . Photomicrograph of altered pyroxene, with significant alteration to opaque minerals. Surrounding grains display remnant igneous textures .......... . ........ Diagram illustrating the geometry of a typical shear zone in heterogeneous rookSOO0..........OOOCOOOOOOOOOOOOOO. ...... Diagram illustrating the geometry of a typical shear zone in homogeneous rookSOOI.OOOOOOOOOOOOOOOIOOIOOOOO ......... Diagram illustrating the relation of the strain ellipse to shear in the sample shear system .................... Diagram illustrating a graphical approximation of total displacement (g units) for dike 2 by finding the area under the strain/distance curve. ...... Plot of 3102 vs. distance from dike margin for Republic area dikes............. Plot of 8102 vs. distance from dike margin for Lighthouse Point dikes.......... Plot of A1203 vs. distance from dike margin for Republic area dikes............. Plot of A1203 vs. distance from dike margin for Lighthouse Point dikes.......... ix 4O 45 48 56 62 62 67 67 73 73 74 74 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 19a. 19b. 20a. 20b. 21a. 21b. 22a. 22b. 23a. 23b. 24a. 24b. 25a. 25b. 26a. 26b. 27a. LIST or FIGURES (continued) Plot of Feo vs. distance from dike margin for Republic area dikes............. Plot of Fee vs. distance from dike margin for Lighthouse Point dikes.......... Plot of Mgo vs. distance from dike margin for Republic area dikes...... ....... Plot of MgO vs. distance from dike margin for Lighthouse Point dikes... ...... . Plot of CaO vs. distance from dike margin for Republic area dikes ..... . ....... Plot of CaO vs. distance from dike margin for Lighthouse Point dikes.......... Plot of Nazo vs. distance from dike margin for Republic area dikes....... ...... Plot of Nazo vs. distance from dike margin for Lighthouse Point dikes.......... Plot of K20 vs. distance from dike margin for Republic area dikes............. Plot of K20 vs. distance from dike margin for Lighthouse Point dikes.......... Plot of Tioz vs. distance from dike margin for Republic area dikes....... ...... Plot of T102 vs. distance from dike margin for Lighthouse Point dikes.......... Plot of P205 vs. distance from dike margin for Republic area dikes............. Plot of P205 vs. distance from dike margin for Lighthouse Point dikes... ....... Plot of Mno vs. distance from dike margin for Republic area dikes.. ........... Plot of MnO vs. distance from dike margin for Lighthouse Point dikes... ....... Plot of Cr vs. distance from dike margin for Republic area dikes..... 75 75 76 76 78 78 79 79 81 81 82 82 83 83 84 84 86 Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure 27b. 28a. 28b. 29. 30a. 30b. 31a. 31b. 326. 32b. 33a. 33b. 34a. 34b. 35a. 35b. 36a. LIST OF FIGURES (continued) Plot of Cr margin for Plot of Ni margin for Plot of Ni margin for Plot of Cu margin for Plot of Zn margin for Plot of Zn margin for Plot of Rb margin for Plot of Rb margin for Plot of Sr margin for Plot of Sr margin for Plot of Y vs. vs. distance from dike Lighthouse Point dikes.. ........ vs. distance from dike Republic area dikes..... vs. distance from dike Lighthouse Point dikes..... ..... vs. distance from dike Lighthouse Point dikes. ...... ... vs. distance from dike Republic area dikes ............. vs. distance from dike Lighthouse Point dikes.......... vs. distance from dike Republic area dikes..... vs. distance from dike Lighthouse Point dikes ......... . vs. distance from dike Republic area dikes..... vs. distance from dike Lighthouse Point dikes.. distance from dike margin for Republic area dikes.. .......... Plot of Y vs. margin for Plot of Zr margin for Plot of Zr margin for Plot of Nb margin for Plot of Nb margin for Plot of Ba margin for distance from dike Lighthouse Point dikes.. vs. distance from dike Republic area dikes ..... vs. distance from dike Lighthouse Point dikes.. vs. distance from dike Republic area dikes..... vs. distance from dike Lighthouse Point dikes.. vs. distance from dike Republic area dikes.... xi 86 87 87 89 9O 90 91 91 92 92 93 93 94 94 96 96 97 Figure Figure Figure Figure 36b. 37. 38. 39. LIST OF FIGURES (continued) Plot of Ba vs. distance from dike margin for Lighthouse Point dikes......... 97 Plot of major-oxides vs. shear strain, Republic dike 1.0.0.0000000000000.0.0.0... 101 Plot of major-oxides vs. shear strain, Republic dike 2........................... 102 Plot of major-oxides vs. shear strain, Republic dike 5...................... ..... 105 xii INTRODUCTION Strain can be described in terms of translation, rotation, distortion, and dilation (also known as dilatation), collectively known as kinematics. For structural geologists, the most difficult of the four movements to prove is dilation, which is a net change in volume. Historically, most workers have interpreted strain indicators such as slaty cleavage, to be the result of a distortion-type strain, such as flattening. These studies frequently noted little or no change in volume. The apparent lack of a known mechanism to remove the material, lack of a place to dispose of large amounts of material, and absence of applicable finite strain gauges are all reasons volume loss has been largely' neglected. in. past studies (Wright and Platt, 1982). The Marquette-Republic region of Michigan's Upper Peninsula (Figure 1) is primarily comprised of two major Archean basement-rock units known as the Southern and Northern complexes (Van Rise and Bayley, 1897). Metadiabase dikes and sills of six ages intrude basement rocks throughout the region (Baxter and Bornhorst, 1988). Many of the dikes are Post-Archean (Lower Proterozoic) in age, as evidenced by relatively linear boundaries against the E Figure 1.--Geologic sketch map of the western Upper Peninsula of Michigan. (Modified from Gair and Thaden, 1968). Figure 2.--Photograph of Republic area dike 5, illustrating sigmoidal foliation (highlighted in red). surrounding country rock. Some dikes intrude the lower part of the Lower Proterozoic Marquette Range Supergroup (Cannon and Gair, 1970) metasediments, which were deformed in the Penokean Orogeny. Original igneous textures are commonly absent in foliated Lower Proterozoic dikes, although they are common in younger, non-foliated Keweenawan (Upper Proterozoic) dikes (Baxter and Bornhorst, 1988). The foliation prevalent in many Lower Proterozoic dikes is attributed to the Penokean Orogeny. In other studies of sheared dikes, the presence of oblique foliation has been attributed to layer-parallel shear along the dike margin (Miller, 1945; Berger, 1971; Talbot, 1982; and. others). During the Penokean Orogeny, the dikes of the Marquette- Republic region probably acted as ductile-shear zones, while the surrounding country rock remained fairly rigid. As evidence of this, dike foliation is non-parallel to country rock foliation, with the exception of an area several centimeters wide at the dike margin (Myers, 1984). Most foliation of the Southern complex granite-gneiss is attributed to Archean deformation. The foliation of Lower Proterozoic dikes in the Republic area is typically visible in outcrop, as a sigmoidal pattern (Figure 2). Differential mineral weathering has accentuated the foliation on this outcrop. The dihedral angle measured between the dike margin and line of maximum extension (trend of the foliation) within the shear zone is small at the dike margin, indicative of very- high shear strain, and large in the dike interior, indicating much lower shear strain. Ramsay and Graham (1970) studied shear zones in limestone, metagabbro, and felsic rock, demonstrating that schistosity first appeared as planar structures oriented at approximately 45 degrees to the margins of the shear zone, becoming more pervasive toward the center of the shear zone. As the degree of schistosity increased, the angle of the schistosity with the shear zone margins (dihedral angle) decreased. These observations would be analogous to changes in foliation noted in the Republic area dikes. Ramsay and Huber (1983) noted that mineral alignment in shear zones is parallel to the line of maximum extension, with minimal shape change in the areas of least strain, and maximal shape change in areas of highest strain. It may be expected that Penokean-age deformation of the Lower Proterozoic diabase dikes was accompanied by deformation and recrystallization of minerals in low-strain regimes. In higher-strain regimes, deformation may have resulted in several episodes of recrystallization and solution transfer (volume change) of some materials, e.g. sioz, Cao, and the alkalis into/out of the dikes. The objective of the study is the investigation of potential volume loss in the dikes. The study compares textural, mineralogical, and chemical data from lowbstrain regimes (dike interiors) with data from high-strain regimes (dike margins). A complete suite of samples was analyzed petrographically and chemically by X-ray fluorescence. The study also compared foliated dikes from the Republic Trough area with non-foliated, but highly-altered dikes from Lighthouse Point in Marquette. The Republic area dikes intrude the granite-gneiss of the Southern Complex, while the Lighthouse Point dikes intrude the mafic Mona Schist, allowing a direct comparison of mafic and felsic wall-rock types. Samples from several dikes were analyzed for water content, to compare mineral hydration of high and low-strain regimes. REGIONAL GEOLOGY The Marquette-Republic region of Michigan is comprised of rock units ranging in age from 3.2 Ga to approximately 1.9 Ga (Sims, 1976; Van Schmus, 1976; Sims and Peterman, 1983). A tonalite-gneiss unit in Watersmeet, which is south of the study area, has been dated 3.4 Ga (Peterman and others, 1980). This unit is similar to gneiss found throughout the study area. The surface appearance of much of the region is typical of the southern Canadian Shield, with infrequent bedrock, outcrops, and a thin ‘veneer of glacial debris. The region was thrust into national prominence in 1844 with the discovery of large quantities of high-grade iron ore, at the present day location of the city of Negaunee. Sporadic discoveries of copper, lead, silver, and gold in the region continued throughout the late 1800’s. As a consequence of the mineral wealth of the region, mudh of the geology has been understood since the turn of the century, primarily through the early work of Van Rise and Bayley (1897) and Van Rise and Leith (1911). Since that time, large areas of the region have been investigated and mapped in detail by the U.S. and Michigan Geological Surveys. The Marquette Range Supergroup (Cannon and Gair, 1970) sedimentary package, which contains the economic deposits of’ banded iron formation, is particularly' well documented. Cambray (1984) includes a comprehensive list of references which led to the present understanding of the geology of the region. The geology of the region is divided by the east-west trending Marquette Trough. The area south of the trough, which is known as the Southern Complex, is comprised of a migmatite gneiss unit, and a younger gneiss unit that also includes coarse-grained granites. Various ages have been assigned to the Southern Complex, ranging from the currently accepted 3.5-2.8 Ga (Sims and Peterman, 1983) to >2.6 Ga (Cannon and Simmons, 1973; Van Schmus and Woolsey, 1975). Radiometric dating in 'the Southern ‘Complex is difficult because of widespread disturbance in the isotopic systems (Sims and Peterman, 1983). The area north of the Marquette trough, which is known as the Northern Complex, is comprised of eight meta-igneous units and two metasedimentary units dated 2.75-2.6 Ga (Peterman, 1979). A large portion of the Northern Complex, immediately north of the Marquette Trough, is a greenstone terrane. The prolific pillow basalts and interflow sediments of this part of the complex are the result of underwater vulcanism. The Northern Complex Gneiss is a tonalite and quartz monzonite unit that includes migmatitic gneiss similar to migmatite gneiss found in the Southern Complex. The Archean basement is unconformably overlain by the Lower Proterozoic Marquette Range Supergroup (Marquette Range Supergroup is hereafter referred to as MRSG), which was deposited between 2.5-1.9 Ga (Sims, 1976; Van Schmus, 1976). The MRSG, which has been correlated with Animikie Group in Minnesota and Ontario (Morey, 1973), is comprised of three groups, Chocolay, Menominee, and Baraga, and several individual formations, consisting' of‘ clastic and carbonate sediments and mafic and felsic volcanics. Within the study area, the MRSG is primarily confined to the steep- sided, fault-bounded Marquette and Republic Troughs. Each of the three MRSG groups consist of a basal conglomerate and quartzite unit overlain by a transgressive sequence, which Cambray (1984) and others have interpreted as multiple episodes of crustal instability. The MRSG thickens to the south, which Cambray (1978) attributed to rifting along the southern margin of the craton. The Chocolay Group seems to represent widespread progressive subsidence and near-shore sedimentation, while the Menominee Group appears to represent sedimentation in a more localized environment, confined primarily to the fault-bounded troughs. The sedimentation rate appears to have kept pace with the rate of subsidence in the Chocolay and Menominee Groups, while the Baraga group, with the exception of the basal Goodrich Quartzite, represents a period of regional subsidence, with a thick sequence of deeper-water turbidites (Cambray, 1984). 9 All rock units in the study area are metamorphosed, with metamorphic grade varying from widespread lower- greenschist facies to amphibolite facies at a metamorphic node centered near Republic (James, 1955) . The source of the metamorphism is interpreted as two orogenic events; the Algoman Orogeny at approximately 2.7 Ga (Sims and others, 1980), which resulted in remobilization of the Archean basement; and the Penokean Orogeny, having an approximate age of 1.89-1.82 Ga (Hoffman, 1988), which deformed the MRSG sediments as well as the dikes described in this study. It is possible that a third metamorphic event impacted the region. The Keweenawan rifting (1.1-1.0 Ga) resulted in the eruption of at least 300,000 km3 of flood basalts into the Lake Superior Basin from a area as close as 100 km to the Marquette-Republic region (BVTP, 1981). It is probable that this event thermally overprinted previous metamorphic signatures in the region. The tectonic forces associated with the initial rifting and consequent thrust-reactivation of the Keweenaw Fault may’ have impacted. the Marquette- Republic region as well. Mafic dikes, sills, and other tabular intrusive bodies outcrop in many locations within the study area. These intrusives range in size from <0.5 m to hundreds of meters in width, such as the sill that forms the north wall of the Republic Iron Mine. Although the intrusives of this study are limited to Lower-Proterozoic diabase dikes, a brief review of the findings of Baxter and Bornhorst (1988) is 10 included to simplify the complex relations of the various intrusions. Most studies have assigned three broad ages to these intrusives which are sufficient for most discussions; Archean, Lower Proterozoic, and Keweenawan. Baxter and Bornhorst (1988) utilized petrographic differences and cross-cutting relationships to suggest a minimum of six intrusive events. The Northern Complex greenstone belt contains the oldest mafic intrusions. These are cut by plutons ranging in composition from gneissic tonalite to granodiorite, which in turn are cut by amphibolitic units in the vicinity of Wetmore's Landing. The two oldest groups of intrusives pre-date the culmination of Archean orogenic events. The next intrusive activity post-dates the Archean Orogeny and pre-dates the deposition of the MRSG. Baxter and Bornhorst (1988) and other workers interpret these dikes as equivalents to the Archean Matachewan Dike swarm in Ontario, which have an age of approximately 2.6 Ga. The next group of intrusives are the Lower Proterozoic dikes which are the subject of this study. These intrusives cut the Chocolay, Menominee, and the lower part of the Baraga Group. While the intrusives of the other five groups appear to have a preferential orientation, the orientation of these dikes is highly variable. The intrusives of this group are variably deformed depending on orientation with respect to Penokean-age stress directions. The fifth and sixth groups of intrusives are Keweenawan in age. The older-Keweenawan 11 intrusives are fine-grained, trend approximately north- south, and are typically less than 30 m in width. The younger-Keweenawan intrusives are medium to coarse-grained, trend approximately east-west, and can be >200 m in width. There are a number of interpretations of the regional effects of the Penokean Orogeny. Brief reviews of several differing interpretations are included as Ibackground material. For details, the reader is advised to consult one of the recent rigorous discussions on the subject. James’s studies of the region from the 1950's provided a interpretation of the depositional environment that has not been significantly altered by subsequent studies. He suggested that compression-controlled basement faulting controlled the deformation of the MRSG, albeit without a plate tectonic driving force. Cannon (1973) proposed deformation was the result of vertical mobilization of the Archean basement, rather than horizontal tectonics. Cannon suggested that gravity sliding on a northward inclined surface was followed by vertical displacement of fault bounded blocks of Archean basement, giving rise to major basins such as the Marquette and Republic Troughs. Cambray (1978) proposed a continental collision model. In ‘this ‘model, a southern. continent is ‘thought to Ihave overridden the Superior Province resulting 511:3 southward- dipping subduction zone. The Niagara fault zone is proposed to be the suture, and the Menominee River, which follows the 12 fault, forms the geographic border of Michigan and Wisconsin. Cambray (1978) suggests that the absence of arc- type rocks north of the Niagara fault zone, supports a southward-subduction model. He proposed that subduction was terminated by the collision with the southern continent. The subduction and collision would have produced horizontal compression in the Archean basement, causing ductile shear. The horizontal compression may have reactivated the fault- bounded Marquette Trough (interpreted as the Archean-age suture of the Northern and Southern Complexes), and the Republic trough, deforming the MRSG sediments. More recently, Hoffman (1988) proposed that Penokean deformation was the result of attempted subduction of a passive continental margin during a collision with oceanic crust. This theory interprets the area as a foredeep, resulting from thrust loading of the continental crust, with sedimentation necessarily moving into the basin created by crustal loading and subsidence. THE ROLE OF VOLUME LOSS One of the significant tasks facing the structural geologist is the interpretation of data from strain analyses. Specifically, were the observed strain indicators deformed by a constant-volume process, or a volume-reduction process, such as solution transfer? The role of volume loss in the formation of foliation, cleavage, and other deformation fabrics has been the subject of disagreement within the scientific community since the mid 1800’s. Sorby (1853) was the first worker to propose a substantial volume loss. In his study of slates and slaty cleavage, Sorby suggested that volume losses as great as 50% were possible. Several workers, including Fisher (1884) and Becker (1904), were critical of volume reduction, suggesting it was improbable, if not impossible. Perhaps Sorby was influenced. by’ these criticisms and. his final conclusion (1908) suggests a volume reduction of only 11%. Some workers including Cloos (1947) and Westjohn (1989) continue to regard volume loss with some skepticism, suggesting that the process is of minimal importance. Until recently, most volume reduction studies have utilized two or more strain gauges, typically deformed fossils, mineral veins, ferruginous reduction spots, or some 13 14 other geometric object common in the outcrop being examined (Cloos, 1947; Wright and Platt, 1982; Westjohn, 1989; Wright and Henderson, 1992). The amount of volume loss suggested in these studies is variable, ranging from none (Cloos, 1947; oolites) to 50% (Wright and Platt, 1982; graptolites), and 40%-60% (Wright and Henderson, 1992; worm burrows). In each of these studies, the undeformed dimensions of the strain gauges are assumed. This method is limited to rock units containing quantities of suitable strain gauges, which typically excludes igneous and metamorphic rock units. Ramsay and Wood (1973) made 990 determinations of strain ellipsoids in slates from slate belts located in Wales, Scotland, Great Britain, Norway, and Vermont. All of the ellipsoids measured were of oblate form, plotting within the flattening field of the deformation plot. The mean ellipsoid was situated on the line separating the fields of true constriction and true flattening. This plot, which suggested a 60% volume change, was rejected as unrealistic. Noting that a sufficient number of points plotted beneath the 20% volume change line, Ramsay and Wood then suggested volume loss of 20%. Subsequent volume loss studies such as Glazner and Bartley’s study of Mojave mylonite zones (1991) have shown volume losses as large as 70%. Wright and Platt (1982) investigated deformation and cleavage development in the Martinsburg Shale formation of the Eastern Appalachians. The Martinsburg shale is thought to be deformed as a result of the Taconic and younger 15 orogenic events, which resulted in the formation being folded, faulted, and cleaved. Graptolite fossils with a known undeformed size and shape were used as finite strain gauges, in addition to bivalves, brachiopods, and calcite veins. The graptolites measured were assumed to have been flattened parallel to bedding as a result of compaction, prior to cleavage development. This study demonstrated that maximum shortening occurred when bedding was at high angles to cleavage, with shortening ranging from near zero at small bedding-cleavage angles to 70% at large bedding-cleavage angles. Wright and Platt (1982) interpreted the shortening to be the result of volume reductions as great as 50%, due primarily to pressure dissolution of quartz, calcite, and phyllosilicate minerals, further suggesting that cleavage formed as a result of mineral dissolution. The studies of O'Hara (1990) and Glazner and Bartley (1991) approached volume reduction from a geochemical perspective, permitting the analysis of any deformed rock. Each study compared compositional and petrographic differences of deformed and non-deformed rock of the same unit. Both studies showed that increased strain is accompanied by large changes in bulk-rock chemistry, thought to be the result of different solubilities, diffusion rates, and other chemical properties of the mineral constituents. O’Hara (1990) examined mylonite zones in Grenvillian composition gneisses in two thrust areas of the Blue Ridge, in Virginia and South Carolina. He suggested that these 16 mylonite zones may have originated as solution zones which were weakened sufficiently to shear during regional thrusting. Utilizing mineral fabric and whole-rock chemical and modal analyses, his study suggested that mylonites deformed, by inhomogeneous shortening normal to the trend of the foliation, and shearing, parallel to the trend of the foliation. The study suggested a >60% bulk volume loss, as a result of incongruent dissolution of feldspar (producing mica) and crystal-plastic dislocation creep and flattening of quartz, showing a three-fold enrichment of the immobile trace elements Ti, Zr, Y, P, V, and the REE. Glazner and Bartley (1991) examined mylonite zones in gneiss and granodiorite in the footwall of the Waterman Hills detachment fault in California. The mylonite zones in this area are thought to have formed during Miocene crustal extension as a result of normal shearing. This study showed a two- to six-fold enrichment of the immobile trace elements Ti, Zr, P, and Cr and large depletions in mobile Si, K, and Rh, interpreted as evidence of a 20-70% bulk volume loss. The study estimated the fluid/rock volume ratio for the system assuming no dissolved silica initially and at least one immobile element. They suggest the large fluid/rock ratios necessary to remove up to 70% of the rock (at estimated temperatures of 300-400°C) imply that the mylonite served as a fluid channel. This also supports O'Hara’s (1990) suggestion that the mylonite zones in his study originated as solution zones, prior to deformation. 17 O’Hara (1990) suggested the rocks in his study were deformed by pure shear, but heterogeneous flattening could also have been produced by simple shear; Glazner and Bartley (1991) noted only a poorly-developed foliation in their study and suggest that pervasive lineation and lack of foliation indicate that strain was constrictional. Their interpretation is ‘unusual because of 'the combination of large volume loss and constrictional strain. APPLICABILITY OF VOLUME LOSS TECHNIQUES IN THE STUDY OF SHEARED METADIABASE DIRES There are similarities between the metadiabase dikes of this study and the mylonites studied by O’Hara (1990) and Glazner and Bartley (1991). In each case, the unit being studied is a discrete highly-deformed zone, surrounded by less-deformed material. To insure the acceptability of the findings of the study, there are several problems that need to be addressed. Gresens (1967) suggests that the interpretation of changes in elemental concentration is not unique, unless the absolute loss or gain of an element(s) can be determined independently, using knowledge of the relationship between composition changes and volume change that accompanied the process. Gresens also suggests that change in elemental patterns could be interpreted to reflect a constant-volume alteration, should an equal volume of new material be introduced into the system as a replacement for material removed during metasomatism. O'Hara (1990) and Glazner and Bartley (1991) addressed this concern by comparing their samples with known elemental compatibilities of hydrothermal systems. The concentration of conserved elements and depletion of non-conserved elements in their studies were interpreted as the result of bulk-rock volume reduction. 18 19 Electron microprobe analyses were not utilized in this initial study, and the absolute loss or gain of an element(s) is unknown. Because of this, results of the study are qualitative. If volume loss occurred during deformation of the dikes, bulk-rock chemistry should vary from non-deformed to deformed rock. Unlike quartz-rich mylonites of previous studies, the metadiabase dikes of this study are silica-poor (<52%) and quartz-poor (typically <5 modal percent). However, the dikes of the study do contain large quantities of Ca-rich plagioclase, and removal of calcium and alkali’s from this system could have been accomplished by passage of hydrothermal fluids through the shear zone, exploiting weaknesses in the crystal structure of the plagioclase, such as twin—planes. Two common criticisms of volume reduction studies are lack of a mechanism to remove material and a lack of a place to dispose of the material removed (Wright and Platt, 1982). Fluid migration through or along the rock units is typically thought to be the material removal mechanism, but opponents of volume loss argue that fluid migration, capable of removing even 20% volume, may be viewed with skepticism. By comparison, the dikes of this study are small (<4 meters in width), discrete bodies, and the fluid and disposal requirements are proportionately less than those necessary for a larger volume of rock. The discrete contact between the dikes and country rock may have acted as a conduit for 20 transporting the quantities of fluid necessary to dissolve and remove significant quantities of material from the system. Fluids that may have migrated upward along discrete shear zones (diabase intrusions) in the basement rock could have intercepted additional conduits such as faults within the overlying MRSG sediments, but removal of much of the MRSG package by erosion makes this suggestion speculative. SAMPLING AND METHODS For the purposes of this study, only those metadiabase dikes that post-date Archean deformation and pre-date the Penokean Orogeny were sampled. Seven Lower Proterozoic dikes in the Marquette-Republic region were sampled for this study (Figure 3). The five Republic area dikes, which are located either side of the Republic Trough, intrude Southern Complex granite-gneiss. These dikes were sampled from road cuts along M-95 to ensure fresh samples with minimal exposure to weathering, as well as ease of sampling. The two dikes at Lighthouse Point in Marquette (Figure 4) intrude the Lighthouse Point member of the upper Mona Schist, a unit of the Northern Complex greenstone belt. Dikes of 'varying' structural orientations and.‘widths were chosen to provide a representative sampling of lower Proterozoic dikes in the region. Myers (1984) structural analysis of dikes in the region showed Penokean-age maximum principle stress directions (01) of 18/070 at the Republic Trough, and 03/192 just north of the Marquette Trough at Marquette, and these stress directions were utilized for this study. The strike of the Republic area dikes ranges from 235° to 357°, dipping at angles ranging from 65’ to 89°. The 21 22 .mcofiumooH m:«HQEMm ucfiom omsonusmfiq ocm mmum chaosmmm mcfisosm mono xosum ofiaosmmm-muumsvnmz_mcu uo ans--.m musmflm 38¢ 38¢ 35 Ba»— 38“ - 3:3— 8...:— C n d . t O a d u q < a 20:. 5(8 29:. . =68: E§¢5 .5; 3.m as 2.5% 2.283 .25. 303.58 .592 885st mo .235 352--.” 2.3 55 finer grains (<0.5 mm) are more equant. Twinned and sericitic grains are typical in both samples, although sericitized grains are more common in the interior sample. Twin-plane controlled dissolution is noted in grains from the margin sample. Poikiloblastic plagioclase grains enclose fine amphibole, chlorite, and opaque minerals. Zoned plagioclase grains occur in the dike interior sample. Many amphiboles in the margin sample appear to be altering to chlorite, and several amphiboles with bent cleavage traces and sweeping' extinction similar to Figure: 7 are noted. Amphiboles are pleochroic from green to blue-green to brown. IMost pyroxenes appear to ‘be altering (some pseudomorphic) to magnetite, chlorite, and amphibole. Typically, grain interiors are. comprised. of Ibirefringent pyroxene, surrounded by a reaction rim of opaque minerals and amphibole (Figure 13). Several relict pyroxenes display sweeping extinction. Pyroxenes are pleochroic from brown- green to brown, suggesting that. most grains are augite (Philpotts, 1989). Myrmekitic intergrowths of quartz and feldspar are observed in the dike interior sample. The interior sample contains trace amounts of rutile (needles in quartz grains), and. calcite and. epidote/clinozoisite (singular grains and fracture fill). The presence of calcite and epidote/clinozoisite is consistent with the breakdown of Ca-plagioclase (Philpotts, 1989), although no modal decrease at the margin is noted. 56 Figure 13.--Photomicrograph of altered pyroxene, with significant alteration to opaque minerals. Surrounding grains display remnant igneous textures. 57 North-South Dike Dike Petrographic Analysis An interior and a margin sample of this dike, and the surrounding wall rock were analyzed. Mode, grain size (the margin sample is primarily fine- grained), and metamorphism and replacement of minerals varies from interior to margin in this dike. Although some coarse-grained igneous texture is observed, most has been replaced by a much finer-grained mineral suite. Absent from this dike are the relict pyroxenes and opaque minerals common in the east-west trending LHP dike. No preferred mineral orientation was noted in any samples. Modal analysis of the samples is summarized in Table 9. Plagioclase grains are commonly twinned and heavily sericitic, with twin-plane controlled dissolution, and epidote/clinozoisite filled cavities. Several grains poikiloblastically enclose fine quartz and biotite. There is some alteration of amphibole to chlorite in the interior sample, with some complete replacement in the margin sample. Amphiboles are pleochroic from green to blue-green. Quartz occurs as inclusions in the interior sample, and interstitial grains in the margin sample. The fine grain size precludes an optical determination of many suspected quartz grains and the mode reflects the inclusion of these 58 E -de 8a.: - .. 28: 2E: samba no.2 .c 2 - - 88 £28 - - use: - .. one: 8ng 838950 33.2 23v n .22 85v 2 \ 28.8 .335. 8.33 mm fimém :1 .c w 382° .3. :v 8 3.3 3v n was. 3-2 .o +n samba 3-2 .c n 23.. - - use: - - 28: 22828.3 £6.92 32 .o n 3252 3.2 .o 2 $2832.— €m€< 3-2 .o n .3QO 3.— .o a 28298 €25 3.5 as: 580 8a 580 3 so: as: 550 0% 596 g 082 E05: 3995 3885 mi .52 mi .5.— 295m vacuum .er £8.-.an .53 385.2»: to ”is: 3oz--.a 2.5 59 unknowns, which may actually be feldspar. Biotite is pleochroic from yellow-brown to red. The presence of calcite and decrease in modal plagioclase and epidote/clinozoisite in the margin sample is consistent with breakdown of Ca-plagioclase (Philpotts, 1989) and removal of calcium from the system. Although this dike was not sheared, the mineralogy of the interior and margin samples are markedly different. ANALYSIS OF SHEAR STRAIN Crystalline basement terranes deformed under metamorphic conditions frequently contain localized, narrow, and sub-parallel shear zones. Although brittle and combination brittle-ductile shear zones are common at higher crustal levels, ductile shear zones may be their deep-level counterparts. Ductile shear zones appear to be the dominant deformation mode at depth, under medium to high grades of deformation (Ramsay, 1980). The foliated Lower Proterozoic dikes near the Republic Trough are thought to have been deformed and displaced as a result of ductile flow (Myers, 1984) . Although Myers and others have studied the bulk- strain analyses of these dikes, this section will characterize the shear strain of individual Republic area dikes. Ramsay (1980) suggests that the basic component in nearly all shear zones is heterogeneous simple shear. He also suggests that unstrained walls in a shear zone indicate that volume change may have accompanied the shearing. The variation in intensity of foliation and changes in mineralogy from dike center to dike margin is an indication of increasing shear strain over that interval and is interpreted as the result“ of heterogeneous simple shear. 6O 61 The dike margins of the Republic area dikes appear largely unstrained, as evidenced by the linear contacts and lack of dike margin-parallel foliation within the wall rock. These features indicate that the wall rocks remained largely rigid, while the dikes deformed as a result of heterogeneous simple shear during deformation. The Republic area dikes have a single low-strain zone running down the approximate center of the dikes, and two high-strain zones sub-parallel and immediately adjacent to the dike margins (Figure 14a), while classic shear zones typically contain two low-strain zones along the margins and a single high-strain zone in the approximate center of the shear zone (Figure 14b). These differences are probably the result of shearing that was concentrated in discrete mafic bodies (diabase dikes) intruded into felsic country rocks, while most classical shear-zone studies have examined shear zones in either felsic Q; mafic rocks. A deformed rectangular shape is shown in each figure. The line of maximum extension is increasingly parallel to margins of the shear zone in Figure 14a and the center of the shear zone in Figure 14b. The geometry and methods of analyzing shear zones is well documented by Ramsay (1967); Ramsay and Graham (1970); Ramsay (1980); and Ramsay and Huber (1983), and the methods used to analyze the Republic area dikes are largely excerpted from these previous studies. Two boundary 62 +— Indicates shearing direction Figure l4a.--Diagram illustrating the geometry of a typical shear zone in heterogeneous rocks. «<(§-- ‘- ... a ’ EXPLANATION 1 P _ ..vo... . LB? E-W dike 'l-ll - ~ "- 4 LHP s—w wall rock 0 "f “ LHP N-s dike 7" 9 I— ““ __‘ . ““ ‘ ‘~ 4 D7 [LAULLAAALILAAIIAAAAIA AAAl‘AALl‘JJLlLLAllAAIAl “ 'stanoe Variable ° 0-5 Distance from Dike Margin (m) from dike margin for Lighthouse Point dikes. 76 Plot of Major-Oxide vs. Distance from Margin YVWY'VV Y'YY Ifirfifi T—YfifiIYf I r Tfi rTj Y fT I V V rTY Y Y I V .., “, WALL ROCK DIKE 1“ ___-4 ————— _“2 i e 10 ’_~ --....‘tll‘=-OIII.IOIIO.I.II... \0 - __ 0. \- _a s - - “+1 O -.6 §’ ‘ «4 1 ~ 2 - 0 1 111...... a. anrLLIALaaanmallnnmJ Distance 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Distance from Dike Margin (m) EXPLANATION Republic dike l _ Republic dike 4II-Il Republic dike 2 ——- Republic dike 5"; Republic dike 3 - - - - Figure 20a.--Plot of Mgo vs. distance Republic area dikes. from dike margin for Plot of Major-Oxide vs. Distance from Margin 1‘ ” WALL ROCK DIKE j ........‘ .4 mnmmnmnm — LHP R-W dike II-II LHP s-w wall rock 0 LHP N-S dike 7" 5:;0 Isles-eelassesses-......_ . 11”," . '1’ F I....I....1....I....L ........ I...-I....1....1....1‘ 8W 0 o 5 Distance from Dike Margin (rn) Figure 20b.--Plot of MgO vs. distance from dike margin for Lighthouse Point dikes. 77 in Mgo could also be interpreted as a relative gain in dikes 3 and 4 due to a loss of other element(s). As anticipated, the Republic dikes, except dike 5, show depletions in CaO at the dike margins (Figure 21a). Although CaO in dike 5 was enriched approximately two weight percent at the margin, a modal decrease in plagioclase was noted at the margin. The depletions of Cao represent the largest changes of a major oxide noted in these samples, approximately 50 percent of the total Cao for Republic dikes 3 and 4. The trends of the LHP dikes oppose one another with a change of approximately one weight-percent (Figure 21b) . The depletion of Cao toward the margins of the Republic dikes parallels petrographic observations of decreased modal amounts of plagioclase and plagioclase alteration products in some margin samples. Variation in Cao is much greater in the wall rock surrounding the north- south trending LHP dike, than in the dike itself. Republic dikes 2, 3 and 4, and both LHP dikes show depletions of Nazo ranging from 0.1 to 0.5 weight percent Nazo, at the dike margins (Figures 22a,b). Nazo trends are nearly flat for Republic dikes 1 and 2, while dike 5 is enriched at the margin. Nazo and CaO trends of dikes 2, 3, 4, and 5 are similar, and may be evidence of volume change in these dikes. Variation in Nazo is much greater in the wall rock of several Republic dikes, than in the dikes. Republic dikes 1, 3, and 4 show enrichments of approximately two weight-percent K20, while dikes 2 and 5 78 Plot 01 Major-Oxide vs. Distance from Margin 12 1V’Yr'T'rVTVl1'VIr‘ffiY'T'T’T‘T wwvv‘vvvafivy'vvvv‘ 1 Tr'v—r-rT—1‘2 WALL ROCK DIKE . J 10 - 10 I’- 1? 8 / 8 o\ g 6 6 <3 8 4 4 2 .e-e-e-e-e-o-. 1 Ittgg'MM 0 - 0 -MWLJJAJ,1,I‘IIIJ_J_‘_I LLALIAALLLLLAAAALAAIAALAILJILILAAJI Distance 0 0.1 0.2 0.3 0.4 0.5 0.0 0.7 Vuilbi. ° - - DIstance from DIke MargIn (In) EXPLANATION Republic dike l — Republic dike 4II-II Republic dike 2 -—- Republic dike 5'... Republic dike 3 - - — - Figure 21a.--Plot of Cao vs. distance from dike margin for Republic area dikes. Plot of Major-Oxide vs. Distance from Margin 5? lmnuuuunou g we s-w dike III-I- O LHP s-w wall rock 0 8 LR? u-s dike r.- Distanoe from Dike Margin (m) Figure 21b.——Plot of Cao vs. distance from dike margin for Lighthouse Point dikes. 79 r hhflOflNL%» Plot of Major-Oxide vs. Distance from Margin 'fIY'UV‘VTjIVIVVTYr‘YYT A L A A A A L L A A A 1 A A l 0.1 Distance from Dike Margin (m) EXPLANATION Republic dike l _ Republic dike 4II-II Republic dike 2 ——- Republic dike 5'... Republic dike 3 - — - - Figure 22a.—-Plot of Nazo vs. distance from dike margin for Republic area dikes. Plot of Major-Oxide vs. Distance from Margin WALL ROCK 4- NdNDONLfiQ 0.5 Distance from Dike Margin (In) Figure 22b.--Plot of Nazo vs. distance Lighthouse Point dikes. EXPLANATION LHP s-w dike LHP E-W wall rock LHP N-s dike from dike margin for 80 are depleted approximately one weight percent at the dike margins (Figure 23a). K20 in the N-S LHP dike is depleted slightly, and nearly flat in the E-W dike (Figure 23b). Some potassium enrichment at the dike margins was anticipated , as a result of de f ormation-induced contamination from surrounding wall rock. Wall rock surrounding dikes 1 and 4 shows some depletion at the margins, while wall rock surrounding dike 3 shows a significant enrichment at the margin. Most potassium in these dikes is contained in biotite, and changes in modal concentrations of biotite primarily parallel the chemical trends. Republic dikes 1 and 2 show small enrichments of T102 and P205, while dikes 3, 4, and 5 show small depletions (Figures 24a, 25a), at the margins. In the LHP dikes, T102 is slightly depleted and P205 is slightly enriched (Figures 24b, 25b). Studies of O’Hara (1990) and Glazner and Bartley (1991) showed many-fold enrichments of Ti and P which they interpreted as evidence of bulk—volume loss, since these elements are normally conserved. The behavior of T102 and P205 in these dikes fails to support any significant volume change. MnO in Republic dikes 1 and 5, and both LHP dikes is enriched at the dike margins, and depleted in dike 4, while the trends of the remaining dikes are nearly-flat (Figures 26a,b). 81 Plot 01 Major-Oxide vs. Distance from Margin VYV‘I ‘V—ViYY VIVIVTYY‘WTYV'VIYY‘fVYV‘ WALL ROCK DIKE ICKDONL9Q Dim 0 0.1 0.2 0.3 ”m Distance trom Dike Margin (m) summon Republic dike 1 _ Republic dike 4Il-II Republic dike 2 ——- Republic dike 5"4 Republic dike 3 - - - ' Figure 23a.--Plot of K20 vs. distance from dike margin for Republic area dikes. Plot of Major-Oxide vs. Distance from Margin EXPLANATION LNP E-N dike “-IU LHP R-W well rock 0 LHP N-S dike r... Distance from Dike Margin (m) Figure 23b.-—Plot of K20 vs. distance from dike margin for Lighthouse Point dikes. 82 Plot 01 Major-Oxide vs. Distance from Margin ‘TY‘T T V—r‘Y-Y‘T'T'Tfi f—T fir IYv—fTrvvfi‘rYfivv‘frvv[rrvvyvyvfi' 2 2.5 l —-_—__——-+- 1 - - —__——-' "’ - ’— ’ 2 1 ‘3 o\. a 1.5 S - 1 i— . _-“..l-e-.‘ - 0.5 1 0 an. Aaaa an a a alaULlaaaalJLAIALALIJLIAIL411 Distance 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 “'"b" Distance from Dike Margin (m) EXPLANATION Republic dike l _ Republic dike «HI-II Republic dike 2 ——- Republic dike 5'47. Republic dike 3 - — - - Figure 24a.--Plot of T102 vs. distance from dike margin for Republic area dikes. Plot of Major-Oxide vs. Distance from Margin rTWY—rYYYTY" YTTV—rvvt YYYYYYYY "Y'VT YYYYYYYYYYYY ‘f WALL ROCK a? : summaries é 2” “ LNP E-W dik. "-ll 8 ; ‘ LRP R-W wall rock 0 I: I LHP N-S dike "‘ ......4'." < Distance from Dike Margin (m) Figure 24b.--Plot of T102 vs. distance from dike margin for Lighthouse Point dikes. 83 Plot 01 Major-Oxide vs. Distance irorn Margin r—T-T—f—T—YT'T-T—T—fIf‘rva' V'VVIYVYVI r1 yvvvfyvvvrj °-° i WALL ROCK DIKE i °-° 0.25 ~ 0.2 '- O.15 " P205 (Wt. %) 0.1 0% 01m 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Varlabie Distance from Dike Margin (m) armament Republic dike l — Republic dike 4II-II Republic dike 2 ——- Republic dike 5'... Republic dike 3 - - - - Figure 25a.--Plot of P205 vs. distance from dike margin for Republic area dikes. Plot of Major-Oxide vs. Distance from Margin VV' TTV“ VII INTI VTV vvvvvvvvvvvvvvv 1' WALLRocK DIKE j J O.8- _. E oa— - sxpmarxos ’. -O-.—' . ' - "----.d Lap 3411 dike III-II 3 0.4- _— .3 —‘« Lap :41 wall rock 0 8f LHP N-S dike 7" 02'- Distance from Dike Margin (In) Figure 25b.--Plot of P205 vs. distance from dike margin for Lighthouse Point dikes. 84 Plot of Major-Oxide vs. Distance from Margin fiY—T—IT‘TFT"‘YV‘W“Y-T—1‘T‘W VV'VIYVVVIWYIVVYVIflVVIT—fVYIVVVVI 0.3 t' “ 0.3 » WALL ROCK DIKE 1 0.25 - \\ 5' 0.25 { \‘. « O a? 02" --------- -—-—-—""’-'.- “0.2 O\ ' - — ...-e—an- 4 C .’ s » « 0.15 *- '* 0.15 C) I: , . 2 0.1 t- “ 0.1 0.“ "' -1 0.05 0 1:21! -0-0-0 e-efi -:0 mm 0 0.1 0.2 0.3 0.4 0.5 0.0 0.7 "W Distance from Dike Margin (In) EXPLANATION Republic dike l — Republic dike 4Il-II Republic dike 2 ——- Republic dike 5’... Republic dike 3 - — - - Figure 26a.--Plot of Mno vs. distance from dike margin for Republic area dikes. Plot 01 Major-Oxide vs. Distance from Margin , ............ 0°: WALLROCK DIKE ’ 1 0.4- -: SE » < smarter . 0.3— 2 g 'P--..... """-'-------o~ Lap s-w dike II-II _ 1 Lap s-w wall rock 0 g 02 — j LHP N-s dike 7'. 0.1 - _ 051....1 1 at Air ....1....l.“.l..“l.un a Distance from Dike Margin (In) Figure 26b.--Plot of Mno vs. distance from dike margin for Lighthouse Point dikes. 85 Trace Elements Several trace elements are shown to be conserved in metamorphic systems. Although trace elements comprise a only small portion of the whole rock, their trends are useful in volume loss studies. Large enrichments in normally-conserved elements could be interpreted as evidence of volume loss. Flat trends for normally-conserved elements could be interpreted as evidence of little or no volume loss, or an indication that the element was non-conserved. Republic dike 2 has an approximately six-fold enrichment in Cr, while trends of the other dikes are nearly flat (Figures 27a,b). The very large enrichment of Cr in dike 2 may indicate volume loss, although dike 2 is highly- mafic, and concentrations of chrome may be related to the original chemistry of the dike. Glazner and Bartley (1991) also showed large Cr enrichments in their study. Republic dikes 2 and 4 show a large enrichment in Ni, while trends for the other Republic dikes are nearly flat to slightly-enriched, and. the LHP’ dikes. show’ depletions as large as two-fold (Figure 28a,b). Enrichment in Ni in dikes 2 and 4 may indicate volume loss, particularly when accompanied by enrichments in other trace elements. Cu falls below the acceptable detection limits of 50 ppm in the Republic dikes and is not considered for this analysis. Both LHP dikes show enrichments in Cu (Figure 8 6 Plot of Trace-Element vs. Distance from Margin i - . T . if * . 1 ' y Y I , E 4800' i4800 ‘ WALL ROCK LDIKE i 4200 ! 14000 3600 3200 E 3000 Q Q. 2400 2400 v L- o 1800 1600 1200 8CD 6w 0 -....l....1..-.1....L... 11-11....1-..11....1.LJ.L...1_. 0 Distance 0 0.1 0.2 0.3 0.4 0.5 0.6 07 WW Distance from Dike Margin (m) EXPLANATION Republic dike l _ Republic dike 4II-Il Republic dike 2 ——- Republic dike 5'... Republic dike 3 - - - '- Figure 27a.--Plot of Cr vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin I VIVVV'IV‘VV V 7" IV'V'I V'VYV'I 00° - WALL ROCK DIKE ‘i I ,,Itt""” ; 50°: € > 1 400'- j ’5 : ; smarter: . 1 3 son'— 1 we s-w dike u—u 5 + 1 LHP R-W wall rock 0 ""--.. ~ we u-s dike er" 100 — ~ ~ . -s-a-s-e-e-s-e‘ ‘ b 1 0 ~ 5 lLLJJlALA l_1_LALLAAAll All“). 111111111411 AJLI Variable ° 05 Distance from Dike Margin (m) Figure 27b.--Plot of Cr vs. distance from dike margin for Lighthouse Point dikes. 540 480 420 240 Ni (PPM) 100 120 60 0 87 Plot of Trace Element vs. Distance from Margin It! Distance Variable 154° -' 400 420 » sac -« 300 ' 240 130 -« 120 3 60 4.1.1 1-1.1.11: L..I I 1 11-1.-. 1.44-1 L W_J_l 1 ill 1.1.1.1..I_1i1-1..a 1_L-L__aLI-L-l a 1-4.1.1.. 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Distance from Dike Margin (m) EXPLANATION Republic dike l — Republic dike 4II-II Republic dike 2 ——- Republic dike 5"; Republic dike 3 - — - - Figure 28a.--Plot of Ni vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin Ni (ppm) EXPLANATION LHP R-W dike 'l-li LHP E-W wall rock 0 LHP N-S dike 7" Distance from Dike Margin (m) Figure 28b.--Plot of Ni vs. distance from dike margin for Lighthouse Point dikes. 88 29). The N-S trending dike shows enrichment of dike and wall rock, at the margin. Republic dikes 3 and 4 show an enrichment in Zn, while the remaining dikes show depletions (Figures 30a,b). Enrichment in Zn may indicate volume loss, particularly when it accompanies enrichments in other trace elements. As anticipated, the trends for Rb are nearly identical to those of K20. Republic dikes 1, 3, and 4 show enrichments, while the remaining dikes are depleted (Figures 31a,b). Calcium and Sr substitute for one another, and their trends should be nearly identical. For unknown reasons, Republic dike 2 and the E-W LHP dike have opposing Ca and Sr trends. Republic dikes 1, 3, and 4, and both LHP dikes show depletions in Sr as large as five-fold, while dikes 2 and 5 show slight enrichments (Figure 32a,b). As anticipated, Sr trends complement those of Rb. Republic dikes 2 and 4, and the E-W LHP dike show enrichments in Y, while dike 1 shows a depletion, and the remaining dikes have approximately flat trends (Figures 33a,b). O’Hara (1990) suggested that a three-fold enrichment in Y in his study was evidence for volume loss, and dike 2 has a two-fold enrichment, but enrichment in dike 4 and the E-W LHP dike are of a much smaller magnitude. Republic dikes 1, 2, and 4 show slight enrichments in Zr, while remaining dikes have slight depletions (Figures 34a,b). O’Hara (1990) suggested that large enrichments in 89 Plot of Trace-Element vs. Distance from Margin 120L « 100 Cu (ppm) 3' r 20 J L 0 ‘1 . [ALLALLUJMJAJAJAAI AAAAAAAA IAAAAIAALAIAIALlLLAAI DIstance Variable ° 0'5 Distance from Dike Margin (m) EXPLANATION LHP E-W dike lI-II LHP 34! wall rock O LHP N-S dike 7'. Figure 29.--Plot of Cu vs. distance from dike margin for Lighthouse Point dikes. 90 Plot of Trace- Element vs. Distance irorn Margin jfi T V 7 If 'V 77‘ Distance 0 0.1 0.2 0.3 0.4 0.5 0.8 0.7 V idbl . . . " ' DIstance from DIke MargIn (In) EIPLANATI ON Republic dike l — Republic dike 4II-II Republic dike 2 ——- Republic dike 5 ... Republic dike 3 - - - - Figure 30a.--Plot of Zn vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin 150 140 EXPLANATION e m. E LHP E-W dik. "-'| V , LHP l-W wall rock 0 IS 120 LNP N-S dike 7" 110 ’ ....II'...‘ . 100 L - use$LAAJAALLlJLAAJJLAAIAJAl AAALILAAAIAAALIIJJJLLLAAI Variable ° 0-5 Distance from Dike Margin (m) Figure 30b.--Plot of Zn vs. distance from dike margin for Lighthouse Point dikes. 9 1 Plot of Trace-Element vs. Distance from Margin I'll rloV'r Y"Y'l""'l""1" 'Ik' 1' 400iI i""|"" WALLROCK 350 3(1) 250 200 Rb (ppm) ‘— -m-e-e —e-e-e- ......— J . .... A-J-J_A_.L_L1-L . Distance Varlabie 0 A_.L ..L—LLJ 1.11;] 1, 0 0.1 0.2 0.3 Distance from Dike Margin (m) J .l l I t l—L1-L..L_L_l 0.4 EXPLANATIOI Republic dike l — Republic dike All-II Republic dike 2 ——- Republic dike 5"; Republic dike 3 - - — - I l i I j... 350 3C!) 250 2C!) Figure 31a.--Plot of Rh vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin 5°: WALL ROCK DIKE *0 40.— I manner: ’E °°” & I 1 Lap s-w dike uI—u : C j LHP a-w wall rock 0 r: 20* ‘ LHP N-S dike r.- ’ 1’ . P It > I, -4 L I, 4 ‘0h" -4 0,-1.1 ihi 111 .1. 11. A AlidiiiiliiiilritfltttilM Distance Variable ° °°5 Distance from Dike Margin (m) Figure 31b.--Plot of Rb vs. distance from dike margin for Lighthouse Point dikes. 92 Plot of Trace-Element vs. Distance from Margin lvlvvlv I'Olylyylll Irrvr|rrr;v'r 1rrv‘rflrrvavl 280 2” WALL ROCK DIKE 240 " 240 ..- zoo ""'--—.-._. I 200 E ~. /._ I I a. a, 160 160 v (7) 120 120 30 80 U - ‘0 1.14.; L-l_L.J.J A 1 A A A 1.L.A._L.4.4J..1-A_l.a .1_1 l.L.L_l I l-..L. L._J_J__A.J-L-.LA_J-J._.L_1_J A..A._LJ ‘0 Distance 0 0.1 0.2 0.3 0.4 0.5 0.3 0.7 Variable Distance from Dike Margin (m) MIDI Republic dike 1 _ Republic dike «HI-II Republic dike 2 ——- Republic dike S'Ic Republic dike 3 - - - - Figure 32a.--Plot of Sr vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin fiV V V l V V V Frv V V V I V V V V I YYYYYYY ' V r171 I V V V V l V V V V I V VfiT 700 - .. _ WALL ROCK DIKE r ’- EXPLANATION \ 1 L11_1 3 LHP a-w dike II-n ; LHP a-w wall rock 0 ‘ LHP N-s dike 7" Sr (ppm) Distance from Dike Margin (m) Figure 32b.--Plot of Sr vs. distance from dike margin for Lighthouse Point dikes. 9 3 Plot of Trace-Element vs. Distance from Margin lnrrr’v |""I""i l"rr”711‘rrv‘rvrirrvrrr‘v'r-rrrry—v-r-v’rivy'v-T-i7o 70 -i WALL ROCK DIKE 60 ‘ 60 50 50 E 40 40 a. 9: >- 3° . 3° " , 20 ~ ‘ \ '-'~=‘.'u an \ e e ...-.-.-. 10 V‘ ‘0 ‘ 10 \~‘. 0 o .-l.L-A_J L. Li. I 44.1 4.4.: 1-1.44 14—J_L_a A I $~A—£—J—l..A_J.J—J.J_LJ A._J J_.L 1.414-44-..L I-L.J—A J_l_l-a_.all.1.-.4 Distance 0 0.1 0.2 0.3 0.4 0.5 — 0.8 0.7 Variable Distance from Dike Margin (m) EXPLANATION Republic dike l — Republic dike Ul-II Republic dike 2 ——- Republic dike 5". Republic dike 3 - - - - Figure 33a.--Plot of Y vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance irorn Margin WALL nook DIKE A IXPLANAIION E g Lil? B-fl dike In." >_ L8? a-w wall rock 0 LHP N-s dike 7.. Vadeble 0 0.5 Distance from Dike Margin (m) Figure 33b.--Plot of Y vs. distance from dike margin for Lighthouse Point dikes. 94 Plot of Trace- Element vs. Distance from Margin I'll [VII V [l‘I'xl“"VT‘YYW'y‘VVVIYVV'V' V 210i WALL Rook DiKE 210 180 i 180 150 150 E 120 .. ——-—--" "' "- 120 O. 3 a h 90 90 N 60 a--e-e-e‘ 60 30 < 30 O 0L lJ .L.‘ -A.J-J-A _L_L. 1.4L A_J_1 kLt-Ll A—t.‘ | I | LLA.‘ l-‘ ..L AJll I J—A..L-lfll J ‘l L. l-l L471 ‘ Distance 0 0.1 0.2 0.3 0.4 0.5 0.8 0.7 Variable Distance from Dike Margin (m) EXPLANATION Republic dike l — Republic dike «tn—u Republic dike 2 ——- Republic dike 5"; Republic dike 3 - - - - Figure 34a.--Plot of Zr vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin 240 ‘ WALL ROCK DIKE -e-e-e-e-e-e-eQ . 4 210 - ‘ 1w - IXPLANATION LHP l-W dik. ll-ll LHP s-w wall rock 0 LHP u-s dike 7" 160.. Zr (ppm) Distance from Dike Margin (m) Figure 34b.--Plot of Zr vs. distance from dike margin for Lighthouse Point dikes. 95 Zr were evidence for volume loss, but the enrichments in the dikes of this study are quite small. Republic dikes 2 and 3 show slight enrichments in Nb, while dikes 1, 4, 5, and the E-W LHP dike show depletions (Figures 35a,b). Nb was not present in the samples from the N-S LHP dike. Ba falls below the acceptable detection limits of 250 ppm in portions of every Republic dike, and analyses must be used with caution. Republic dikes 1, 3, and 4 show enrichments in Ba, while the remaining dikes show depletions (Figures 36a,b). The enrichments in the Republic dikes range up to six-fold (dike 3). Depletions in the Republic dikes are as large as two-fold, while depletion in the LHP dikes is of a much smaller magnitude. F'fi Hrfi‘fl 9 6 Plot of Trace-Element vs. Distance from Margin IQTY'Y‘FYV'T—Vrl’VTfi’Y‘T—T’YTT-Y‘Y‘l'r—f—VfiIvvrferTvlvvvv' fi—v‘lvvvv'vvrv 12° WALL ROCK DIKE ? "° 1“) I w _ A E D. e co .0 Z 40 L 20 ~ 0 Lfie-e-e-e-e-e-e-e-e-e-e ’ — o waw—A—A—LJ—L—L—L AAAJILJ—ALIAAAAJAA-AllAAAJA4-LA144AA1 Distance 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Vwidaie Distance from Dike Margin (m) 8mm)! Republic dike l _ Republic dike ell-II Republic dike 2 -—- Republic dike 5'... Republic dike 3 - - - - Figure 35a.--Plot of Nb vs. distance from dike margin for Republic area dikes. Plot of Trace-Element vs. Distance from Margin ,....,....,....,....,.... 3°: WALLROCK DIKE 25~ 2°.” summon 2:: ~- 8: 15L LHP a-w dike ll-Il I i LHP n-w wall rock 0 Z , LHP u-s dike 7'4- 10- 5’. .... . ...... . 0 P ~ .........‘_, l... l .l.. l l m Vendale ° °5 Distance from Dike Margin (m) Figure 35b.--Plot of Nb vs. distance from dike margin for Lighthouse Point dikes. P’ 97 Plot of Trace- Element vs. Distance from Margin ‘ ' V I V ‘ V V V 'w V V V T V'T‘l P‘T’T V'I V Vfi" I V V 17' V V V.‘ Y'Y‘T‘Y‘T‘fi‘Y‘v fi' h r Vfifl 1200 WALL nook DIKEW 1200 i 1000 1m 800 - m E a. D. an ‘ 6m v 1 4m 4m 2m ‘ 2m 0 ' 0 L—1_L_L_L_.A.LJ_‘L_L.J_.L-J._L_L.JJ_L_L_LLJ_LL_A_L. WLWMLJ _-L._.L L..l.i-I ‘44-. Dim 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 “'"b" Distance from Dike Margin (m) summon Republic dike l — Republic dike 4II-II Republic dike 2 -—- Republic dike 5'..- Republic dike 3 - - - ' Figure 36a.--Plot of Ba vs. distance from dike margin for Republic area dikes. Plot of Trace- Element vs. Distance from Margin 'V VIY'VV “fl VVVrV VVVVVVVVVV ITVVrI VVVVVVVV 10‘” ’ WALL ROCK DIKE ‘ am! . * XXPLANATION A _. E 'm’ 3'. : LHP s—w dike n-u " ~° LHP s-w wall rock 0 68 ‘°°* ' .-.—0""; LHP N-s dike 7'. e—e—'—.— I , I 1” _ .” Obi . l l .41 l .4 .JJHLL l l l gauges; o 05 Distance from Dike Margin (m) Figure 36b.--Plot of Ba vs. distance from dike margin for Lighthouse Point dikes. EPPECT OP SHEAR STRAIN ON BULK-ROCK CHEMISTRY The mineralogy of foliated Republic area dikes varies from dike interior to dike margin. In margin samples, there is typically a higher modal percentage of mafic phases, such as biotite, chlorite, and amphibole, and a lower' modal percentage of felsic phases, primarily plagioclase. .As noted in the section on petrography, modal variations are accompanied by other changes including degree of recrystallization and formation of foliation. As noted in the section on shear strain analysis, some very large displacements occurred as a result of Penokean shearing. This section, which compares bulk-rock chemistry to shear strain for Republic area dikes 1, 2, and 5, illustrates that little change in bulk-rock chemistry accompanies some very large shear strains. The condition of the dikes in outcrop determined the usefulness of a particular dike for shear strain measurements. Republic dikes 1, 2, and 5 were chosen for this analysis because each dike had a well-developed foliation which could be easily measured and interpreted. The foliation of Republic dikes 3 and 4 was poorly exposed, and neither dike was utilized for this portion of the study. 98 Was IT‘— 99 This was unfortunate, as both dikes had large variations in mineralogy and bulk-rock chemistry. Major oxides used in this section were chosen after reviewing XRF analyses and plots of bulk-rock chemistry vs. distance from the dike margin. Major oxides (in weight percent) are plotted as a function of shear strain (gamma units). In other volume loss studies, Ti and P have been considered conserved (0' Hara, 1990; Glazner and Bartley, 1991), with enrichments as large as three-fold noted. There are only small enrichments in Ti and P for dikes 1 and 2, and Mno comprises only a small fraction ( ,v 2 11 l— "‘ " i L ’,I l r .I‘ ‘ . "¢v 1 10*- *1 i I . . 1 1 . . 1 i . A .4 . . . l . . . I . . . 1 J 0 2 4 8 8 1O 12 Shear Strain (gamma units) Republic Dike #2 1o_i ‘ Y ' fTfi ‘ T ’ Yfi T ' ‘ ' i ' ' ' T T 7 ‘ i_1| 4 l l ._ ._ J, a . - i E. a; .. exam-non m :8 i 1 Cao II..- x ‘ K20 lI-II 2 4* 1 Nazo - — —. .0 r .-l-. . .1 g . - -e-I-l-.-.‘.-. ; 2L -e-e-.-" .1 I ~ .——————————————— . i 0%- . 1 +1 1 l 4 l l . . L 1 O 2 4 8 8 1O 12 Shear Strain (gamma units) Figure 38.--Plot of major-oxides vs. shear strain, Republic dike 2. 103 dike center. The dike sampling sites roughly correspond with strain measurement locations on this dike. Figure 38 shows very different major-oxide trends from those of dike 1. A comparison of low- and high-strain regimes shows a greater than two weight—percent enrichment in Fee, and one to less than one weight-percent depletion's of A1203, Ugo, Cao, and K20. The trend of Nazo is nearly flat. Other Republic dikes have larger variations in some oxides, particularly Cao, but the consistency of depletion of A1203, Mgo, Cao, and K20 in this dike is notable. Coinciding with depletions in major oxides, are enrichments in Ti, P, Cr, Ni, Y, Zr, and Nb, noted in the previous section. Total strain in this dike was 13.8 gamma units, which translated to a 1400% extension along the principal strain axis. These strain measurements are for the entire width of this dike, and are somewhat higher than those of dikes 1 and 5 where only a portion of the dike was measured. Analysis of select major-oxides from this section will be combined with other observations in the discussion, but these data indicate that distinct chemical changes accompany increasing strain in this dike. 104 W A generalized trend of select major-oxides as a function of shear-strain is shown in Figure 39. Data is from two sampling locations, dike margin and approximate dike center. The sampling sites for this dike directly correspond with strain measurements locations on this dike. Figure 39 shows some interesting trends for the six oxides. A comparison of low- and high-strain regimes shows a greater than two ‘weight-percent enrichment in Cao, a slight enrichment in Nazo, and one to less than one weight- percent depletions of A1203, Feo, Mgo, and K20. The trends of the major-oxides are opposite those of other Republic dikes. Total strain from margin to center of the dike was 4.6 gamma units, which translated to a 500% extension along the principal strain axis. Analysis of select major-oxides from this section will be combined with other observations in the discussion, but these data indicate that distinct chemical changes accompany increasing strain in this dike. The width of this dike and orientation to Penokean stress directions may account for the unusual trends, although the extent of influence is unknown. 105 Republic dike #5 , ..., ..., ...j e.., ...7 ..:; 15r — ‘ .——e g? me ~ 5. ._._._._._. EXPLANATION F; . 1 A1203 — '8 11 _ fl FeO lI-Il '2 . ‘ Mgo ...... 2 .O 3' :2 e— _ ’ 1 C. \ ... 4 7 _ 1 I i . . . l . L . l . A . l . . . l #1 . l . . . l o 2 4 e a 10 12 Shear Strain (gamma units) Republic Dike #5 I ' I w I ' ' T I ’fi ' I ' ' ' I ' ' . I I 7 I 1 12r — ’ 'l . l 1o~ - A r 1 a? C 1 V L EXPLAMTION m 8 e e 7 CaO — '5? ... K20 lI-Il 2 L ~'~.~.~ : N320 I - - . .2. 4 r N. _ fl * . 5 : 2e _ : _ — -——-O o— a 1 . 4 l . 4 . L . . . l L . . l . . . l . . 1 l o 2 4 e a 10 12 Shear Strain (gamma units) Figure 39.--Plot of major-oxides vs. shear strain, Republic dike 5. ' DISCUSSION Chemical and mineralogical changes accompanied the Penokean deformation of Lower Proterozoic diabase dikes in the Marquette-Republic region. Solution transfer of some chemical constituents appears to have accompanied Penokean shearing in several of the dikes studied. The largest contribution to this process appears to be calcium, although sodium depletion was also noted, and the mineral most affected by this process is plagioclase. The orientation of the dikes with respect to Penokean maximum principal stress (01) appears to have controlled the extent of these changes (Myers, 1984). Recent studies of shear zones by O'Hara (1990) and Glazner and Bartley (1991) have demonstrated the applicability of using bulk-rock chemical analyses to show volume loss in deformed zones, and provided the model used in this study. It was anticipated that some or all of the foliated dikes sampled and measured near the Republic Trough would display chemical and mineralogical characteristics indicative of volume loss. The analyses of the Republic dikes, which intrude a felsic terrane, were combined with analyses of surrounding wall rock, and two dikes intruding 106 107 an igneous terrane at Lighthouse Point. The wall rock and LHP dikes were analyzed primarily for comparison purposes. The Republic area dikes were targeted as probable sites for analyzing deformation-induced volume loss, primarily on the basis of prominent sigmoidal foliation, and straight country rock contacts, indicating that the dikes were exposed to a single major episode of deformation, the Penokean tectonic event dated 1.89-1.82 Ga. (Hoffman, 1988). Dike samples were analyzed petrographically. Original ophitic texture is nearly absent in all Republic dikes and it is assumed that all minerals present in the samples have been recrystallized. The Republic dikes sampled are all amphibolites, and the most abundant minerals are amphibole, biotite, and plagioclase. Deformation and additional recrystallization of individual grains is increasingly prevalent toward the dike margins (high-strain regimes). Dike margin samples are commonly comprised of needle-like mafic minerals oriented sub-parallel to foliation, with fresh felsic :minerals, contained interstitiallyu Dike interior samples are commonly comprised of irregularly- shaped, non-oriented grains. Margin samples typically contain a higher modal percentage of mafic minerals (biotite, chlorite, amphibole, and opaques) than dike interior samples. The modal increase in mafic phases is often accompanied by a modal decrease in plagioclase and plagioclase alteration.:minerals, such. as epidote, and a 108 mineralogical change from highly-sericitic, twinned grains to untwinned grains with little alteration. Bulk-rock chemistry was analyzed using X-ray fluorescence techniques. Major oxides and trace elements were analyzed and the results plotted against distance from dike margin to document chemical changes over the range of strain regimes. Depletion of major oxides and enrichment in conserved trace elements would indicate the possibility of volume loss. CaO depletion of 1-5 weight percent was noted in all dikes, except dike 5. Cao comprises a significant portion of the total bulk rock in these samples, and depletions of 1-5 weight percent are the largest noted among the major oxides, suggesting solution transfer of calcium from these dikes was probably the single largest contributor to volume change. Many-fold enrichments of conserved trace elements such as Ti and P, used as evidence of volume loss in other studies, are conspicuously absent in these dikes. Shear strain measurements allowed an estimate of total displacement for Republic dikes l, 2, and 5. Following methods of Ramsay (1967, 1980), shear strains were calculated for individual sampling locations, as well as the entire shear zone. The total shear strain is high, ranging from 4.6-13.9 1 units. Total extension measured along the principal strain axes (A1) ranges from SOD-14001;. These strains {are large, although Wood (1974) noted similar extensions in slate belts. Nonetheless, extensions of this 109 magnitude may be justifiable however, as foliation is nearly asymptotic at the dike margins. Select major oxides were also plotted against total shear strain. The results of these analyses were mixed. Dike 2 was the only dike with major-oxide depletion and trace-element enrichment trends similar to those noted in other volume loss studies. Cao was depleted in dike 1, but little variation in the other oxides accompanied increased strain. The chemical trends of dike 5 differ from those of other Republic area dikes, with large variations of select major oxides accompanying increasing strain, included enrichment in Cao. No strain data were available for dikes 3 and 4, although depletion of major oxides, particularly Cao and Nazo, is shown in bulk-rock chemistry. The lighthOuse Point dikes are oriented approximately orthogonal to Penokean principal stress (Myers, 1984), and were not sheared and no fabric is noted in either dike. LHP dikes have remnant igneous textures, and pyroxene comprises a significant modal percentage of the east-west trending dike. The LHP and Republic dikes are assumed to have been texturally similar before the Republic dikes were sheared. Depletion of less than one weight percent is noted for several major oxides, although little complementary enrichment is noted in the immobile trace elements. This study did not investigate heterogeneity within the dikes, although there is some evidence to suggest that the dikes are at least locally heterogeneous, and any future 110 study should consider this possibility. One Republic dike contains a large (approximately 20 cm) xenolith of granitic rock (Figure 6), and more contamination of this type is likely. Several specific locations were resampled, and chemical variations beyond the margin of error were noted for several of the major oxides. Additional sampling should include several samples from each strain regime, particularly the dike margins, which should be rigorously sampled along the length of the exposure. In conclusion, chemical and mineralogical changes evident in several of the dikes can be interpreted as the result of volume loss, primarily the result of removal of Ca and Na from the system by alteration of Ca-rich plagioclase. The magnitude of the volume loss is unknown, although it may have been substantial in dikes 3 and 4. Analyses of the Lighthouse Point dikes indicate that little or no volume loss accompanied deformation of these dikes. Chemical and petrographic analyses for Dike 5 are puzzling, and could be interpreted as the result of positive volume change, rather than loss. The magnitude of strain measured in several of the dikes is great enough that volume change cannot fully account for the deformation observed in these dikes. CONCLUSIONS It is concluded that Penokean deformation of some foliated Lower Proteorzoic metadiabase dikes in the Marquette-Republic region of Upper Michigan was accompanied by volume loss, although much of the deformation appears to have occurred as a result of bulk-rock material transport. Other foliated dikes of similar age appear to have deformed without accompanying volume loss, as do dikes of similar age that are non-foliated because of their orientation to Penokean-age principal stress. Volume loss appears to have occurred when calcium and sodium were removed from the system at some point during the alteration or recrystallization of Ca-rich plagioclase. Much of the plagioclase in high-strain regimes (dike margins) is Na-rich, untwinned, and interstitial, while most plagioclase in low-strain regimes (dike interiors) is Ca- rich, twinned, and highly sericitic. Although highly- altered, these interior grains are often subhedral-euhedral in shape, and some may be remnants of the original igneous textures. Similar chemical trends are observed in most foliated dikes, small to moderate depletions of CaO, NaZO, and A1203 in high-strain regimes, with little enrichment of typically 111 112 conserved elements. Previous volume loss studies have noted much larger depletions of major elements, and enrichments in conserved major and trace elements which are conspicuously absent in these samples. Contamination of the dikes from KZO-rich wall rock appears negligible, with possible exception of two Republic area dikes. sigmoidal foliation. of ‘the Republic tarea dikes 'was highly-visible in the field, with maximum shear strain at each dike margin, decreasing away from the margins. Shear strain measured in several Republic area dikes was very high, and the maximum extension in the measured dikes exceeds 1000%. Much greater volume losses would be required to explain the strain measured in these dikes, than those measured during in this study. {.mam’mw REFERENCES Barker, A. J ., 1989. Introduction to Metamorphic Textures and Microstructures. Chapman and Hall, New York. 162p. Baxter, D. A. and Bornhorst, T. J., 1988. 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Soc. Lond. A. v.280, p.605-628. Van Schmus, R. W. and Woolsey, L. L., 1975. Rb-Sr Geochronolgy o f the Republ ic area , Marquette County Michigan, Can. J. Earth Sci., v.12, p.1723-1733. Westjohn, D. B., 1989. Regional finite strain patterns in proterozoic slates and quartzites: implications for heterogeneous strain related to flexural slip fo1ding in the Marquette synclinorium. Ph.D. Dissertation, Michigan State University, 182p. Wood, D. S., 1974. Current views on the development of slaty cleavage. Annual Review of Earth and Plan. Sci., v.2, p.368-401. Wright, T. 0. and Henderson, J. R., 1992. Volume loss during cleavage formation in the Meguma Group, Nova Scotia, Canada. Journal of Structural Geology, v.14, p.281-290. Wright, T. 0. and Platt, L. B., 1982. Pressure and cleavage in the Martinsburg shale. American Journal of Science, v.282, p.122-135. APPENDICES 117 APPENDIX 1.--Results of XRF analysis of samples from Republic area dike 1. OXIDE 8102 A1203 Feo mg) Ca0 Na20 K20 Ti02 P205 Mno Total TRACE ELEMENT Cr Ni Cu Zn St Zr Nb La Ba Repl-4 Dike Center 49.94 14.60 11.69 7.42 9.27 0.58 2.63 1.20 0.13 0.18 97.64 198.1 113.9 68.7 129.9 181.2 147.0 28.9 95.2 8.3 21.0 143.5 Rep1-3 0.27m from Margin 49.20 14.62 11.72 7.44 8.43 0.61 4.00 1.22 0.13 0.19 97.56 234.7 169.4 54.2 126.4 201.1 148.7 22.7 101.7 14.1 22.5 213.1 Repl-2 0.13m from Margin 50.87 14.24 11.78 7.45 7.88 0.49 3.40 1.26 0.13 0.19 97.69 244.3 125.2 6.4 104.5 262.1 113.0 20.3 103. 13.4 7.8 383.3 Repl-l Dike Margin 49.62 14.77 11.78 7.44 7.76 0.46 3.46 1.26 0.12 0.20 96.87 172.7 125.0 5.2 107.4 317.9 94.4 20.2 97.3 7.9 14.5 285.0 Repl Wall- Rock Margin 79.13 11.79 1.01 0.55 0.90 2.05 4.79 0.09 0.04 0.01 100.36 11.1 19.3 14.8 137.3 129.1 19.4 67.7 4.0 91.1 378.5 Repl Wall- Rock Interior 75.88 13.87 1.87 0.62 0.24 2.26 6.45 0.26 0.05 0.02 100.52 118 APPENDIX 2.--Resu1ts of XRF analysis of samples from Republic area dike 2. Rep2 Rep2 Rep2 Rep2 Dike Dike Wall Rock Wall Rock OXIDE Center Margin Margin Interior Si02 46.67 46.72 72.81 75.88 A1203 13.81 13.56 13.78 13.87 Feo 10.29 12.23 2.10 1.87 M90 12.44 11.91 1.53 0.62 Cao 7.93 7.26 0.92 0.24 Na20 0.43 0.33 1.50 2.26 K20 3.32 2.13 6.55 6.45 Ti02 0.75 0.84 0.28 0.26 P205 0.11 0.13 0 0.05 Mn0 0.20 0.20 0.03 0.02 Total 95.95 95.31 99.50 100.52 TRACE ELEMENT Cr 820.4 4507.3 0 7.1 Ni 281.2 510.3 16.9 4.8 Cu 24.6 39.9 39.1 12.6 Zn 156.2 153.5 27.1 24.7 Rb 242.2 180.5 236.0 162.8 Sr 87.7 101.4 145.4 64.1 Y 11.1 21.1 14.3 35.2 Zr 58.8 65.0 202. 170.7 Nb 13.6 20.3 14.3 12.3 La 85.2 77.3 151.9. 113.9 Ba 511.9 91.2 498.2 641.5 119 APPENDIX 3.--Results of XRF analysis of samples from Republic area dike 3. Rep3-3 Rep3-2 Rep3-1 Rep3 Rep3 2.02m 0.66m Dike Wall Rock Wall Rock OXIDE from from Margin Margin Margin Margin Margin Si02 47.82 46.69 46.75 78.36 78.51 A1203 14.81 15.53 14.64 13.48 13.75 FeO 13.06 12.51 14.06 0.48 0.33 M90 7.46 7.80 9.75 0.36 0.10 CaO 9.01 9.03 3.92 1.60 0.37 Na20 1.55 1.72 1.05 3.86 7.19 K20 1.78 1.53 4.85 2.33 0.73 T102 2.01 2.11 1.92 0.03 0 P205 0.25 0.27 0.25 0 0.01 MnO 0.20 0.20 0.18 0.01 0 Total 97.95 97.39 97.37 100.51 100.99 TRACE ELEMENT Cr 0 0 0 0 0 Ni 3.5 12.9 32.1 4.2 0 Cu 54. 16.7 6.7 28.4 12.2 Zn 127.8 139.4 201.6 8.7 14.3 Rb 135.2 103.0 390.0 52.6 7.6 Sr 161.7 190.5 56.6 252.3 41.5 Y 26.6 21.1 25.3 4.8 18.8 Zr 113.1 123.4 104.4 81.3 97.5 Nb 25.4 16.8 26.3 17.1 105.9 La 46.5 75.6 13.9 35.3 77.6 Ba 343.2 230. 1190. 31.7 8.2 120 APPENDIX 4.--Resu1ts of XRF analysis of samples from Republic area dike 4. Rep4-5 Rep4-3 Rep4-2 Rep4-1 Rep4 Rep4 0.50m 0.30m 0.15m Dike Wall- Wall- OXIDE from from from Margin Rock Rock Margin Margin Margin Margin Interior Si02 52.71 52.24 52.42 53.06 84.69 75.81 A1203 12.57 12.88 12.55 12.31 10.61 15.29 FeO 10.02 10.29 10.07 10.19 0.34 1.05 M90 9.26 9.36 9.30 10.21 0.24 0.73 CaO 9.96 9.50 10.07 5.35 1.23 1.56 Na20 2.00 1.98 2.07 1.66 3.61 5.58 K20 0.84 1.28 0.82 3.20 1.55 1.86 Ti02 0.72 0.75 0.69 0.65 0.02 0.01 P205 0.07 0.07 0.06 0.06 0.03 0.02 MnO 0.18 0.18 0.18 0.15 0 0.01 Total 98.33 98.53 98.23 96.84 102.29 101.92 TRACE ELEMENT Cr 106.5 631.9 657.5 591.3 0 0 Ni 88.6 144.8 145.9 182.3 2.8 19.0 Cu 27.0 46.0 33.3 26.2 23.7 16.6 Zn 122.2 125.1 133.5 144.6 13.4 11. Rb 55.1 83.4 45.0 243. 25.9 30.3 Sr 139.2 165.0 139.9 102.7 186.9 209.8 Y 14.7 12.7 15.0 17.6 0 62.0 Zr 60.4 60.0 49.7 62.6 35.3 56.0 Nb 5.6 9.6 11.8 0 0 0 La 39.8 2.1 20.0 45.1 60.5 76.9 Ba 21.0 149.2 26.5 488.0 124.1 249.7 121 APPENDIX 5.--Resu1ts of XRF analysis of samples from Republic area dike 5. Rep5 Rep 5 Reps Dike Reps Dike Wall Rock Wall Rock OXIDE Center Margin Margin Interior Si02 48.26 48.53 75.03 75.75 A1203 14.67 14.50 13.93 12.50 FeO 12.38 12.28 1.38 1.21 M90 7.75 7.24 0.56 0.37 CaO 6.61 8.59 1.55 0.69 Na20 0.51 0.72 3.11 3.12 K20 4.31 3.30 4.44 5.14 Ti02 2.04 1.97 0.17 0.17 P205 0.19 0.19 0.05 0.04 MnO 0.22 0.27 0.02 0.01 Total 96.94 97.59 100.24 99.00 TRACE ELEMENT Cr 294.5 282.7 0 0 Ni 120.1 128.3 9.3 9.9 Cu 18.4 15.6 25.6 21.5 Zn 245.7 229.7 18.4 31.1 Rb 395.3 271.3 157.5 203. Sr 79.3 81.9 154.8 73.5 Y 23.0 23.3 28.7 16.8 Zr 131.0 129.9 147.4 136. Nb 32.2 27.6 5.1 11.7 La 28.2 62.2 117.9 117.0 Ba 545.1 366.8 643.1 576.2 122 APPENDIX 6.--Resu1ts of XRF analysis of samples from east- west dike at Lighthouse Point. LHP.E-W LHP E-W LHP E-W Dike Dike Wall Rock OXIDE Center Margin Interior Si02 47.12 46.78 46.19 A1203 14.02 13.21 13.34 FeO 15.50 16.41 16.25 Mgo 5.07 5.20 5.19 CaO 7.82 8.84 8.20 Na20 2.53 1.98 2.11 K20 0.76 0.79 1.32 Ti02 3.55 3.49 3.54 P205 0.48 0.55 0.53 MnO A 0.26 0.28 0.27 Total 97.11 97.53 96.94 TRACE ELEMENT Cr 72.7 78.9 69.4 Ni 282.3 59.4 82.4 Cu 46.2 64.9 74.1 Zn 138.8 104.8 122.9 Rb 28.3 16.2 41.8 Sr 231.7 191.2 180.4 Y 36.2 47.4 39.9 Zr 236.0 231.5 229.1 Nb 25.3 11.2 18.3 La 89.0 59.3 78.8 Ba 375.6 302.3 461.5 123 APPENDIX 7.--Results of XRF analysis of samples from north-south dike at Lighthouse Point. LHP N-S LHP N-S LHP N-S LHP N-S Dike Dike Wall Rock Wall Rock OXIDE Center Margin Margin Interior Si02 47.77 46.46 52.09 46.63 A1203 13.34 13.75 15.01 17.01 FeO 7.67 9.41 11.48 12.15 MgO 10.04 10.06 4.05 3.54 CaO 9.19 8.25 8.82 10.65 Na20 3.28 2.99 2.59 2.64 K20 1.54 1.38 0.76 0.40 Ti02 0.60 0.61 1.10 1.34 P205 0.42 0.44 0.11 0.12 MnO 0.15 0.17 0.26 0.27 Total 94.00 93.52 96.27 94.75 TRACE ELEMENT Cr 573.5 532.7 213.7 237.2 Ni 448.8 346.8 139.0 123.8 Cu 61.9 92.9 119.7 96.5 Zn 104.4 103.8 148.8 124.2 Rb 39.2 28.7 19.1 9.7 Sr 674.9 402.7 178.4 139. Y 13.0 11.5 19.8 26.1 Zr 146. 141.2 77.6 86.5 Nb 0 0 0 3.0 La 122.7 128.9 16.7 40.9 Ba 942.0 889.5 303.2 133.6 "‘1111111117'11111115