Nd isotope disequilibrium during crustal anatexis: A record from the Goat Ranch migmatite complex, southern Sierra Nevada batholith, California

Abstract

Geological and geochemical studies of a pelitic migmatite complex within the Isabella pendant of the southern Sierra Nevada batholith, California, suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent Goat Ranch pluton ca. 100 Ma. The leucosomes preserve a record of large-magnitude Nd isotope disequilibrium with respect to their pelitic source. The leucosomes have a wide range of ε_(Nd(100 Ma)) from −6.0 to −11.0, as compared to −8.7 to −11.3 for the source. They can be subdivided into two groups based on their major elements and Sr and Nd isotope geochemistry. Group I leucosomes have higher P_2O_5 contents and ε_(Nd(100 Ma)) values than those of group II. The ε_(Nd(100 Ma)) values of group I leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units, suggesting that group I leucosomes are in Nd isotope disequilibrium with their sources. Correlations among P_2O_5 contents, ε_(Nd(100 Ma)) values, and Sm/Nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating Sm from Nd and generating Nd isotope disequilibrium. Dissolution of apatite or monazite might play a critical role in regulating the behavior of the Sm-Nd isotope systems and thus the Nd isotope compositions of melts generated during crustal anatexis, especially in metasedimentary protoliths.