Non-modal Partial Melting of Metasedimentary Pendants in the Southern Sierra Nevada and Implications for the Deep Origin of Within-pluton Isotopic Heterogeneity Thesis by Lingsen Zeng In Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy California Institute of Technology Pasadena, California Defended on September 15, 2003 © 2004 Lingsen Zeng All Rights Reserved - ii - Acknowledgment Every rock speaks for itself, and every geomorphic feature speaks for itself too. The best achievement is to learn, appreciate, and enjoy the beauty and essence of geology for the past five years. Pursuing a PhD degree at Caltech is proved to be a great personal journey. It is great to get to know those great scientists and personalities, but also learn from fellow students. I would like to thank my advisor, Jason Saleeby. To me, he is more a friend than an advisor. His door always is open to me and he listens to my research plans and half- baked ideas. I greatly appreciate his support, friendship, advice, and patience. I want to thank Hugh Taylor for serving as my academic advisor and for his encouragement. Thanks to James Chen for his timely help whenever I had questions or troubles when I was working in the radiogenic isotope lab. Thanks to Brian Wernicke and Kenneth Farley for their support of my PhD qualification research. Thanks also to John Eiler, George Rossman, Peter Wyllie, Paul Asimow, Leon Silver, Gerald Wasserburg, Kerry Sieh, Ed Stolper, and other faculty members in the GPS Division for encouragement and sharing ideas. Thanks to Mihai Ducea and Cin-Ty Lee for their friendship. I am grateful to Zhiqing Xu, Haibing Li and Jingsui Yang who are at the Institute of Geology, Chinese Academy of Geological Sciences, for continuously to share the progresse made in China. Thanks to Jim O’Donnel and Susan Leising who make the library a lovely working place. Thanks to Lindsey M. Hedges for her assistance in sample and photo preparation. Thanks to Francis Alberade and Janne Blichert-Toft for their sharing ideas with regard to the mantle and crustal geochemistry, when they were at Caltech as visiting scholars. Thanks Terry Gennaro for his friendship and assistance that greatly facilitated every field trip I have taken. Thanks to all the secretaries working in the GPS Division for their efforts to make things work smoothly. - iii - Thanks to all fellow graduate students and postdocs in the GPS Division who make seemingly dull graduate study enjoyable. Thanks especially to Zhengrong Wang, Edwin Schauble, Ji Chen, Sidao Ni, Xianglei Huang, Kaiwen Xia, Liz Johnson, Julie O’Leary, Er Tan, Bruce Hsu, Yongqin Jiao, Zhimei Yan, Shing-Lin Wang, Ying Tan, The-Ru Song, Danny Natawidjaja, Chi Ma, Hiroto Kimura, Heping Liu, Joe Akins for their friendship. I would like to thanks my wife, Jing Liu, for her love, her push, and her encouragement, and my daughter, Carisa Zeng, for her enrichment of my daily and family life. - iv - Abstract Results from field mapping, structural analysis, major and trace element geochemistry, and radiogenic isotopic data from the Goat Ranch migmatite complex on the south shore of Lake Isabella, southern Sierra Nevada, California, are presented to (1) determine the major and trace element, Sr and Nd isotopic compositions of anatectic melts from pelitic sources; (2) investigate the structural and metamorphic responses of the Isabella pendant to the emplacement of the Goat Ranch pluton at ~100 Ma; and (3) evaluate the magnitude of assimilation of metasediment- derived melts by the Goat Ranch pluton at a mid-crustal level. Nd and Sr isotopic compositions were measured on the non-migmatitic distal wallrocks, leucosomes, migmatites, traverses into the Goat Ranch pluton, leucogranite dikes, and samples from the Rabbit Island, Heal Peak, and Bob Rabbit plutons. Major and trace element analyses were performed on selected samples of the leucosomes, migmatites and metapelites. Major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products from partial melting of the pelitic protolith host. These data show that (1) contamination of Goat Ranch intrusion is restricted to the immediate proximity to migmatitic wallrocks. Isotopic heterogeneity of the Goat Ranch intrusion is not related to assimilation at or near the level of exposure, but from a deeper source; (2) as compared to the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. Sr and Nd isotope compositions of leucosomes, migmatites and metapelites suggesting a disequilibrium partial melting of the metapelite protolith; (3) based on their Sr, Nd and other trace element characteristics, two groups of leucosomes have been identified. Group A leucosomes show distinct positive Eu anomalies, relatively high Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios. Group B leucosomes have negative Eu anomalies, relatively low Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios as well; (4) the leucogranite dikes also can be subdivided 87 86 87 86 into Group A (high Sr/ Sr(T) and low εNd(T)), and Group B (low Sr/ Sr(T) and high εNd(T)); (5) H2O-fluxed melting of quartz + plagioclase with minor involvement of muscovite melting - v - dominated the leucosome production; (6) Group A leucogranite dikes resulted from partial melting of the lower pelite, and Group B dikes from partial melting of the upper pelite; and (7) the Bob Rabbit pluton represents an extreme end-member case that was derived completely from melting of the upper pelite or its equivalent in depth in the I-SCR (strongly contaminated and reduced I-type pluton) zone. Strain analysis shows that progressive partial melting resulted in the loading framework transition in the upper pelite unit from LBF structure (the stronger phase forms a load-bearing framework) to IWL structure (the weaker phase forms an interconnected weak matrix) with proximity to the pluton. The presence of melts has greatly affected the strain partitioning within the migmatite zone. By incorporating accessory phase dissolution kinetics into non-modal partial melting of metasedimentary sources, theoretical modeling shows that non-modal partial melting of a pelitic 87 86 source results in melts following two paths in εNd- Sr/ Sr ratio space. Path 1 represents those partial melting reactions that favor muscovite/biotite dehydration and apatite but not monazite 87 86 dissolution, leading to melts with elevated Rb/Sr, Sr/ Sr, Sm/Nd, and εNd values. In contrast, Path 2 represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role, and favor monazite over apatite dissolution, and lead to melts with lower 87 86 Rb/Sr, Sr/ Sr, Sm/Nd, and εNd values than their sources. - vi - Table of Contents Title page………………………………………………………………………………….i Copyright page…………………………………………………………………………..ii Acknowledgement………………………………………………………………………iii Abstract…………………………………………………………………………………..v Table of Contents……………………………………………………………………...vii List of Tables……………………………………………………………………………iv List of Figures……………………………………………………………………………x Chapter 1. Granitic batholith, crustal anatexis, and migmatite: Introduction and thesis overview…………………………………………………………………………..1 Chapter 2. The origin of within-pluton isotopic heterogeneities in the southern Sierra Nevada batholith: Constraints from Sr and Nd isotopes…………………..13 Chapter 3. Non-modal crustal anatexis during the formation of migmatite at the southern Sierra Nevada, California………………………………………………….42 Chapter 4. Progressive melting of metasedimentary wallrocks and its effects on the strain partitioning: An example from the southern Sierra Nevada batholith, California………………………………………………………………………………..94 Chapter 5. Partial melting of pelitic sources and the origin of leucogranitic dikes in the Isabella pendant, southern Sierra Nevada, California…………………….131 Chapter 6. Sr and Nd isotope systematics of anatectic melts: exploring the role of accessory phases and non-modal partial melting………………………………...164 References……………………………………………………………………………204 Appendix 1 Samples for petrography, geochemical and radiogenic isotopic analysis………………………………………………………………………………. 216 - vii - Appendix 2 Major and trace element geochemistry data of the leucosomes, migmatites, and the upper pelite unit………………………………………………218 Appendix 3 Plate 1 Geologic map of the Goat Ranch area, south Lake Isabella area, California……………………………………………………………………….219 - viii - List of Tables Chapter 2 Table 1 Whole rock Rb-Sr and Sm-Nd isotopic compositions of granodiorites, mafic enclaves, pelitic metapelite, psamitic metapelite, migmatites and leucosomes from Lake Isabella area, southern Sierra Nevada……………………………………………………41 Chapter 3 Table 1 Major and trace element analysis results for selected samples of metapelite, migmatite and leucosomes……………………………………………………………….92 Table 2 Nd and Sr isotope compositions of metapelite protoliths, migmatites and leucosomes……………………………………………………………………………….93 Chapter 5 Table 1. Leucogranite dike sample descriptions..............................................................151 Table 2. Rb-Sr and Sm-Nd isotopic compositions of leucogranite dikes, the Rabbit Island, Rob Rabbit and Heald Peak plutons, lower and upper pelite units, and leucosomes from the Lake Isabella area, southern Sierra Nevada, California…………………………….153 Chapter 6 Table 1 Parameters for Rb, Sr, Sm and Nd trace element modeling…………………...202 - ix - List of Figures Chapter 1 Figure 1 Generalized geologic map of the Lake Isabella area, southern Sierra Nevada,