Petrogenesis of the early Eocene I-type granites in west Yingjiang (SW Yunnan) and its implication for the eastern extension of the Gangdese batholiths

摘要

This study reports new zircon U-Pb and Hf isotopes and whole-rock elemental and Sr-Nd isotopic data for the gneissic granite and leucogranite from the Nabang metamorphic zone, Yingjiang area (West Yunnan, SW China). The metamorphosed granitoids crystallized during the early Eocene (~55-50Ma) with zircons showing ε_(Hf)(t) values from +11 to -5.3 and crustal model ages of 1.5 to 0.42Ga, comparable to those of coeval I-type granitoids from the Gangdese batholith, southern Lhasa. The rocks are characterized by metaluminous and weakly peraluminous hornblende-bearing gneissic granites with A/CNK=0.95-1.09, Na_2O>K_2O, coupled with low initial Sr isotopic values of 0.7049-0.7070 and high ε_(Nd)(t) values from +1.1 to -7.1. The rocks were derived from crustal materials involving ancient upper crust/sedimentary and juvenile mantle-derived rocks. Together with available data from nearby regions, it is proposed that the early Eocene granitoids in the Nabang and Tengliang area can be correlated to the Gangdese granitoids and represent the southeastward continuation of the magmatic arc resulting from the Neotethyan subduction in southern Tibet. The petrogenesis of early Eocene granitoids in western Yunnan was probably related to the rollback of the subducting Neotethyan slab that caused the remelting of the crustal materials newly modified by the underplated basaltic magma.