Petrogenesis and tectonic significance of the Early Paleozoic granitic gneisses from the Yadong area, southern Tibet

Early Paleozoic granitic gneisses occur in the Higher Himalayan Crystallization Sequence of Yadong area, southern Tibet. Petrological study shows that the granitic gneisses are mainly composed of quartz, plagioclase, K-feldspar and biotite, and show distinct gneissic structure. They have variable SiO2 contents ranging from 68. 04% to 76. 55%, and alumina saturation index (A/CNK) of 1. 04 similar to 1. 14, indicative of calcic to calc-alkaline and weakly peraluminous to peraluminous. They show distinct fractionated REE patterns with negative Eu anomalies, and are characterized by the enrichment of LILEs (such as Rb) and the depletion of HFSEs (such as Nb, Ta, P and Ti). LA-ICP-MS U-Pb dating of zircons from the three granitic gneisses indicates that their protoliths were formed at 491Ma, 501Ma and 512Ma, respectively. The zircon epsilon(HF)(t) values range from -8. 1 to +0. 1 -5.9 to -2. 1 and -4.8 to +6.7, with corresponding HI two-stage model ages ranging from 1440Ma to 1959Ma, 1585Ma to 1826Ma and 1028Ma to 1762Ma, respectively. This study shows that the Early Paleozoic granites of Yadong area include I-type and S-type granitoids. The I-type granitoids are characterized by K2O/Na2O (%) > 1, weakly peraluminous, positive zircon epsilon(Hf) (t) values or negative but narrow epsilon(Hf)(t) values range, and show typical geochemical features of magmatic arcs. In contrast, minor S-type granites are characterized by K2O/Na2O (%) <1, peraluminous, negative and variable zircon epsilon(Hf) (t) values. The coexistence of I-type and S-type granitoids in the Himalayas indicates that the northern margin of the Gondwana supercontinent experienced an Andean-type orogeny during the Early Paleozoic.