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California State University, Northridge Geochemistry

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California State University, Northridge Geochemistry CALIFORNIA STATE UNIVERSITY, NORTHRIDGE GEOCHEMISTRY AND PETROLOGY OF TRIASSIC GRANITOIDS OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA AND NEVADA A thesis submitted in partial satisfaction of the requirements for the degree of Master of Scienc~ in Geology by Mark Eugene Unruh J/\NUARY 1985 9 ' The Thesis of Mark Eugene Unruh is approved: Dt/ Peter W. \~ei gand Dr. George C. Dunne {Chair) California State Universityt Northridge ; i TABLE OF CONTENTS PAGE List of Plates v List of Figures v List of Tables vii Acknowledgements viii Abstract ix Introduction 1 Background and Purpose of Investigation 1 Geographic Setting 3 Geologic Setting 5 Previous Investigations 7 Rock Units and Their Ages 9 Field and Laboratory Techniques 9 Petrology 11 General Statement 11 Wheeler Crest Quartz Monzonite 11 Benton Range Granodiorite 17 The Granodiorite of Mono Dome 20 Tungsten Hills Quartz Monzonite 23 Lee Vining Quartz Monzonite 25 The Granite of Baldwin Canyon 28 The Granodiorite of Lobdell Summit 30 The Biotite Quartz Monzonite of Strosnider Ranch 33 The Unnamed Granophyric Quartz Monzonite Near Lake Topaz 33 Summary of Petrology 34 Geochemistry 38 iii TABLE OF CONTENTS (Continued) PAGE Chemical Trends 49 Factor Analysis of Geochemistry 55 Computer Assisted Analysis 56 Results of Q-Mode Factor Analysis 63 Discussion and Conclusions 75 References Cited 81 Appendix A 86 Appendix B 93 iv LIST OF PLATES Plate 1. Location of samples in pocket LIST OF FIGURES FIGURE PAGE 1. Location of Triassic granitoids. 4 2. Streckeisen diagram showing modal composition range of 12 Triassic Scheelite sequence granitoids. 3. Modal analyses from Wheeler Crest Quartz t'1onzoni te. 15 4. Modal analyses from Benton Range Granodiorite. 18 5. Modal analyses from the Granodiorite of Mono Dome. 21 6. Modal analyses from the Tungsten Hills Quartz Monzonite. 26 7. Modal analyses from the Lee Vining Quartz Monzonite. 29 B. t~Joda 1 analyses from Nevada granitoids. 31 9. Modal feldspar divided by quartz plus feldspar versus 35 latitude. 10. Average modal analyses of plutons from entire Sierra 37 Nevada batholith. 11. Streckeisen diagram showing normative plots from 39 Scheelite sequence. 12. K2o vs Si02 - Scheelite sequence. 41 13. Peacock diagram of Scheelite sequence. 42 14. Mol A/CNK vs Si02 - Scheelite sequence. 43 15. Alkalis vs Si02 - Scheelite sequence. 45 16. Comparision between Triassic, Jurassic, and Cretaceous 46 granitoids on a Streckeisen diagram. 17. Comparison between Triassic, Jurassic, and Cretaceous 47 granitoids on a plot of K20 vs s;o2. 18. Comparison between Triassic, Jurassic, and Cretaceous 48 granitoids on a plot of Alkalis vs s;o2• 19. Variation in average chemistry vs longitude across the 50 batholith. v LIST OF FIGURES (Continued) FIGURE PAGE 20. Si02 plotted against latitude along a north-south line. 51 21. K20 plotted against latitude along a north-south 51 1 Tne. 22. Potassium index plotted against latitude along a 53 north-south line. 23. Na 2o plotted against latitude along a north-south line. 53 24. CaO plotted against latitude along a north-south line. 54 25. Factor variance diagram from Q-modal analysis of 59 Sierra Nevada batholith. 26. Factor variance diagram from Q-modal analysis of all 64 Triassic samples. 27. Factor variance diagram Q-modal analysis of the south­ 65 ern portion of the Triassic granitoids. 28. Factor variance diagram Q-modal analysis of the north­ 66 ern portion of the Triassic granitoids. 29. Factor variance diagram Q-modal analysis of Wheeler 67 Crest Quartz Monzonite and Benton Range Granodiorite. 30. Factor variance diagram Q-modal analysis of Jurassic 69 granitoids. 31. Factor variance diagram Q-modal analysis of Cretaceous 70 granitoids. 32. Stereogram showing three-dimensional vector system from 72 the northern Triassic granitoids. vi LIST OF TABLES TABLES PAGE 1. Modal analyses - Wheeler Crest Quartz Monzonite 14 2. Modal analyses - Benton Range Monzonite 19 3. Modal analyses - The Granodiorite of Mono Dome 22 4. Modal analyses - Tungsten Hills Quartz Monzonite 27 5. Modal analyses - Nevada Granitoids 32 vii Q • ACKNOWLEDGEMENTS I would like to express my appreciation to Drs. George Dunne, Peter Weigand, Robert Stull and to tljark E. Bryant for their discussions and critical comments. Dr. Herbert Adams• help with the statistical rou­ tines is appreciated. Thanks are due to Dr. Alfred Meisch who provided copies of his Q-mode factor analysis programs used during this study, and to Robert Griffis for his assistance in running those programs and his discussions about their applications. Further thanks are due to Converse Consultants, Inc. for financial and secretarial aid, especially the typing by Beverly Lindy. I am indebted to Franklin Hall and Colleen Unruh for their assistance in field sampling and drafting. Special thanks are due to Gene Unruh for guidance and inspiration. viii 0 . ABSTRACT GEOCHHHSTRY AND PETROLOGY OF TRIASSIC GRANITOIDS OF THE SIERRA NEVADA BATHOLITH, CALIFORNIA AND NEVADA By Mark Eugene Unruh Master of Science in Geology The Triassic Scheelite intrusive sequence is studie.d here in relation to geochemical and petrologic constraints found in earlier studies. The Triassic granitoids show chemical composition trends in the north-to-south direction that are simi 1ar to those found across the entire batholith in the west-to-east direction in earlier studies. An analysis of 32 samples from the Triassic granitoids shows that potassium increases and sodium decreases from north to south. The parent magma for the Triassic granitoids is shown to be an andesitic meltt and the existing intrusions were evolved through fractionation of plagioclase and amphibole mineral assemblages. The northern portion of the Triassic sequence is shown to be simpler in chemical composition than the southern portion. The Triassic granitoids are shown to be different from the Jurassic granitoids but similar to those of the Cretaceous. The genetic models of batholithic rocks which best fit these data are those which allow for varying source materials. ix INTRODUCTION Background and Purpose of Investigation The granitoids of the Sierra Nevada batholith were intruded as hundreds of individual plutons in distinct pulses from the Triassic to the Cretaceous periods. Evernden and Kistler (1970) indicate that there were five pulses of intrusion, one in the Middle and Late Tri­ assic 210 to 195 Ma, and four others in the Early to Middle Jurassic, the Late Jurassic, the Early Cretaceous, and the Late Cretaceous. Stern and others (1981) agree with the distinct pulses in the Triassic and Early to Middle Jurassic; however, they show continuous intrusion in the Cretaceous. Both studies show the Triassic intrusive sequence as a distinct pulse. This pulse, known as the Scheelite intrusive sequence (Stern and others, 1981), is studied herein. The granitoids intruded during these pulses form belts along a general north-south to northwest-southeast trend. The source regions and other petrogenetic aspects of these granitoids have been debated by many workers, but no generally accepted model has yet been developed. Six major models of magma generation have been proposed. They are: 1) partial melting of a thick prism of crustal rocks marginal to the continent {Bateman and others, 1963); 2) partial melting of lower crust and upper mantle with assimilation of upper-crustal rocks (Hurley and others, 1965); 3) partial melting of a subducted slab of oceanic basalt and trapped sediments (Hamilton, 1969; Dickinson, 1970); 4) mixtures of melts from chemically depleted mantle identical to the source of island arcs and old continental crust (Kistler and Peterman, 1973); 5) generation of magma in a zone of melting that laterally transects the upper mantle 1 2 and the lower crust; and 6) partial melting of lower crust by intrusions from the lower mantle (Hildreth, 1981). The Sierra Nevada is one of the world•s most studied batholiths. The large amounts of geochemical, petrologic and age data developed in this region have placed the following constraints on any model attempt­ ; ng an exp 1anati on of the origin of the batho 1i th: 1) the batho 1i th must be emplaced in pulses, either by the same mechanism, which is somehow turned on and off, or by a different mechanism for each pulse; 2) geochemical and petrologic trends which are distinctive in the east-west direction must be accounted for. These trends include 87 86 eastward increases of K2o, U, Th, (Bateman and Dodge, 1970), Sr ;sr (Kistler and Peterman, 1973), and a decrease in CaO (Bateman and Dodge, 1981). A corresponding increase in K-feldspar (K-feldspar +quartz) is reported by Bateman (1981). This study was undertaken to examine some of the constraints developed over the batholith as a whole utilizing petrologic and major element compositional data for the Triassic pulse of intrusion. The Triassic intrusive sequence was chosen because it is the oldest and most limited in terms of time span. Studying this age sequence in relation to the trends found in the batholith as a whole and in compar­ ison to the other pulses of intrusion might provide further constraints for genetic models applicable to the entire batholith. The specific goals of this investigation are fourfold: 1) Discover any geochemical or petrologic trends or lack of trends in the Scheelite sequence of Triassic plutonic rocks of the Sierra Nevada batholith. The Triassic rocks are not 3 widely spaced in the east-west direction, but any north-south trends may help explain the origin of the granitoids; 2) Describe the chemistry and petrology of plutons and . portions of plutons previously undescribed. Some granitoids of the batholith have only recently been dated as Triassic, and for these there are few geochemical or petrologic data in the published literature. These additional data are needed for a complete evaluation of the Triassic Scheelite intrusive sequence. 3) Mathematically evaluate the geochemical structure and the genesis of the Triassic granitoids.
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