Eocene magmatic processes and crustal thickening in southern Tibet: Insights from strongly fractionated ca. 43 Ma granites in the western Gangdese Batholith

Abstract

This study reports zircon U–Pb age and Hf isotope, whole-rock major and trace element, and Sr–Nd–Pb–Hf isotope data for the Dajia pluton, western Gangdese Batholith, in southern Tibet. These data indicate that the pluton consists of moderately (Group 1) and strongly (Group 2) fractionated granites that were emplaced synchronously at ca. 43Ma. Group 1 samples have SiO2 contents of 69–72wt.% and vary in terms of the differentiation index (DI=84–93). These rocks are depleted in Ba, Nb, Sr, P, and Ti, with moderate negative Eu anomalies, and display low heavy rare earth elements (HREEs) and Y abundances. Group 2 samples are characterized by high SiO2 (75–78wt.%) and DI (95–97); significantly negative Eu anomalies; marked concave-upward middle REE (Gd–Ho) patterns; and Ba, Sr, P, and Ti anomalies that are significantly more negative than those of Group 1 samples. Group 1 samples have whole-rock εNd(t) (−5.9 to −6.0), εHf(t) (−4.0 to −4.5), and zircon εHf(t) (−6.0 to +5.8) values identical to those of Group 2 samples [εNd(t)=−5.7 to −6.7, εHf(t)=−3.5 to −2.9, and zircon εHf(t)=−2.0 to +4.2], as well as similar initial Pb isotopic compositions. These data indicate that the two groups were derived from a common source region with garnet as a residual mineral phase. Group 1 samples were most likely derived from partial melting of garnet-bearing amphibolite (rather than eclogite) within the juvenile southern Lhasa crust and mixed with the enriched components from the subducting ancient Indian continental crust and/or the ancient central Lhasa basement. Group 2 samples are interpreted as the products of extensive fractional crystallization (plagioclase, K-feldspar, biotite, apatite, allanite, titanite, monazite, and ilmenite) of the melts represented by Group 1 samples. Low HREEs and Y abundances of the Dajia pluton, together with the presence of strongly fractionated granites (Group 2) identified for the first time in the Gangdese Batholith, indicate that the crust beneath the Dajia region had already been thickened by ca. 43Ma. High whole-rock zircon saturation temperatures (815°C–869°C) of Group 1 samples and the other ca. 43Ma coeval magmatism documented both in the Gangdese Batholith and in the Tethyan Himalaya can be best interpreted as the final consequences of the magmatic responses to the Neo-Tethyan oceanic slab breakoff.