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Non-modal partial melting of metasedimentary pendants in the southern Sierra Nevada and implications for the deep origin of within-pluton isotopic heterogeneity

Citation

Zeng, Lingsen (2004) Non-modal partial melting of metasedimentary pendants in the southern Sierra Nevada and implications for the deep origin of within-pluton isotopic heterogeneity. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechETD:etd-10272003-131444

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 metasedimentderived 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 into Group A (high 87Sr/86Sr(T) and low eNd(T)), and Group B (low 87Sr/86Sr(T)) and high eNd(T)); (5) H2O-fluxed melting of quartz + plagioclase with minor involvement of muscovite melting 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 source results in melts following two paths in eNd-87Sr/86Sr ratio space. Path 1 represents those partial melting reactions that favor muscovite/biotite dehydration and apatite but not monazite dissolution, leading to melts with elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd 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 Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd values than their sources.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:isotopic heterogeneity; metasedimentary pendants; non-modal partial melting; Sierra Nevada
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • Saleeby, Jason B.
Thesis Committee:
  • Taylor, Hugh P. (chair)
  • Rossman, George Robert
  • Saleeby, Jason B.
  • Eiler, John M.
  • Asimow, Paul David
Defense Date:15 September 2003
Author Email:lzeng (AT) gps.caltech.edu
Record Number:CaltechETD:etd-10272003-131444
Persistent URL:http://resolver.caltech.edu/CaltechETD:etd-10272003-131444
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:4275
Collection:CaltechTHESIS
Deposited By: Imported from ETD-db
Deposited On:28 Oct 2003
Last Modified:26 Dec 2012 03:07

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