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Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California

Zeng, Lingsen and Saleeby, Jason B. and Ducea, Mihai (2005) Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California. Contributions to Mineralogy and Petrology, 150 (4). pp. 386-402. ISSN 0010-7999. http://resolver.caltech.edu/CaltechAUTHORS:20130508-153801624

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Abstract

We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial ^(87)Sr/^(86)Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125–0.7221). However, the leucosomes have a much wider range of initial ε_(Nd) values, which range from −6.0 to −11.0, as compared to −8.7 to −11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K_2O contents, Rb/Sr ratios (0.15<Rb/Sr<1.0), and initial ε_(Nd) values. Group B leucosomes have relatively low Rb, Pb, Ba, and K_2O contents, Rb/Sr ratios (<0.15), and initial ε_(Nd) values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H_2O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K_2O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb–Sr and Sm–Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities.


Item Type:Article
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URLURL TypeDescription
http://dx.doi.org/10.1007/s00410-005-0010-2DOIArticle
http://link.springer.com/article/10.1007/s00410-005-0010-2PublisherArticle
Additional Information:© 2005 Springer-Verlag. Received: 6 October 2003. Accepted: 10 March 2005. Published online: 5 October 2005. Communicated by Ian Carmichael. Support for this research was provided by NSF grants EAR-9815024 and EAR-0087347. We thank H.P. Taylor, P. Asimow, J. Eiler, and G. Rossman for critical comments on an early version of this manuscript. We thank A.E. Patiño-Douce and C. Miller who gave thoughtful and constructive reviews and I. Carmichael for handling this manuscript.
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Funding AgencyGrant Number
NSFEAR-9815024
NSFEAR-0087347
Record Number:CaltechAUTHORS:20130508-153801624
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20130508-153801624
Official Citation:Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California Lingsen Zeng, Jason B. Saleeby, Mihai Ducea Pages 386-402
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38368
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:08 May 2013 22:54
Last Modified:08 Jul 2014 02:12

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