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Geochronology and geochemistry of the c. 80 Ma Rutog granitic pluton, northwestern Tibet: implications for the tectonic evolution of the Lhasa Terrane

TAI-PING ZHAO^*,, MEI-FU ZHOU, JUN-HONG ZHAO, KAI-JUN ZHANG^* and WEI CHEN^*

^* Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, P.R. China
Department of Earth Sciences, The University of Hong Kong, Hong Kong, P.R. China

Author for correspondence: tpzhao{at}gig.ac.cn

The Rutog granitic pluton lies in the Gangdese magmatic arc in the westernmost part of the Lhasa Terrane, NW Tibet, and has SHRIMP zircon U–Pb ages of c. 80 Ma. The pluton consists of granodiorite and monzogranite with SiO₂ ranging from 62 to 72 wt % and Al₂ O₃ from 15 to 17 wt %. The rocks contain 2.33–4.93 wt % K₂O and 3.42–5.52 wt % Na₂O and have Na₂O/K₂O ratios of 0.74–2.00. Their chondrite-normalized rare earth element (REE) patterns are enriched in LREE ((La/Yb)n = 15 to 26) and do not show significant Eu anomalies (Eu = 0.68–1.15). On a primitive mantle-normalized trace element diagram, the rocks are rich in large ion lithophile elements (LILE) and poor in high field strength elements (HFSE), HREE and Y. Their Sr/Y ratios range from 15 to 78 with an average of 30. The rocks have constant initial ⁸⁷Sr/⁸⁶Sr ratios (0.7045 to 0.7049) and slightly positive _Nd(t) values (+0.1 to +2.3), similar to I-type granites generated in an arc setting. The geochemistry of the Rutog pluton is best explained by partial melting of a thickened continental crust, triggered by underplating of basaltic magmas in a mantle wedge. The formation of the Rutog pluton suggests flat subduction of the Neo-Tethyan oceanic lithosphere from the south. Crustal thickening may have occurred in the Late Cretaceous prior to the India–Asia collision.

Key Words: Late Cretaceous • I-type granite • Rutog • Gangdese magmatic arc • Lhasa Terrane