ND ISOTOPIC DISEQUILIBRIUM DURING CRUSTAL ANATEXIS: A RECORD FROM THE GOAT RANCH MIGMATITE COMPLEX, SOUTHERN SIERRA NEVADA, CALIFORNIA

Migmatite zones occur in several of the deeper level pendants of the Triassic-Jurassic Kings sequence metasedimentary framework rocks of the Sierra Nevada batholith. They commonly formed as a result of partial melting of the fertile pelite units within the proximity of granitic plutons. Therefore, geochemical observations on the leucosomes may yield key information to evaluate factors affecting the elemental and isotopic characteristics of anatectic melts. Geological and geochemical studies of a migmatite complex within the upper pelite unit of the Isabella pendant suggest that partial melting was driven by the emplacement of Goat Ranch pluton at ~ 100 Ma. This process was complex and resulted in Nd isotopic disequilibrium between the leucosomes and their protolith. The leucosomes have a much wider range of initial e_Nd(T) (T=100 Ma) values which range from -6.0 to -11.0, as compared to -8.7 to -11.3 for the metapelites. They can be subdivided into two groups based on their major element and Nd isotope geochemistry. Group A leucosomes have higher P₂O₅ contents, Sm/Nd ratios and e_Nd(T) values than those of Group B. The e_Nd(T) values of Group A leucosomes are significantly higher than those of metapelites and migmatites by 2-4 epsilon units, suggesting that Group A leucosomes are in Nd isotopic disequilibrium within their sources. There are correlations among P₂O₅ contents, e_Nd(T) values, and Sm/Nd ratios in the leucosomes. In metasedimentary rocks, apatite and monazite are the major carriers of phosphorous, Sm and Nd. In contrast with monazite, apatite has high Sm/Nd and time-integrated ¹⁴³Nd/¹⁴⁴Nd ratios. Increasing dissolution of apatite into the melts is likely to elevate their P₂O₅ contents, e_Nd(T) values, and Sm/Nd ratios simultaneously which are the characteristics of the Group A leucosomes. The occurrence of apatites along the grain boundaries among quartz, K-feldspar, and plagioclase in some of the leucosomes also support this interpretation. This observation suggests that apatite might play a dominant role in regulating the behavior of the Sm-Nd isotope system during crustal anatexis, especially of a metasedimentary source.