Metamorphism, anatexis, zircon ages and tectonic evolution of the Gongshan block in the northern Indochina continent-An eastern extension of the Lhasa Block

The Gongshan block near the Eastern Himalayan Syntax's is a fault bounded block at the northern tip of the triangle-shaped Indochina continent (NIC) Exposed in this block are late Paleozoic (Carboniferous to Permian) strata and a north-south belt of intermediate to felsic batholiths (le Gaoligongshan magmatic belt) The contact between the Gaoligongshan batholiths and Carboniferous/Permian strata is characterized by a series of high-grade metamorphic gneisses with leucosome granite veins e the so-called Gaoligong Group) U-Pb SHRIMP and LA-ICP-MS dating of zircons indicate that these gneisses are actually metamorphosed Paleogene sediments containing inherited Archean to Cretaceous detrital zircons (from 2690 to 64 Ma) and have undergone medium- to high-pressure granulite-facies metamorphism at similar to 22 Ma Leucosome and S-type granite of 22-53 Ma by anatexis are ubiquitous within high-grade metamorphic rocks in the southern part of the Gongshan block An Early Paleozoic gneissic granite and granitoid intrusions of Jurassic Cretaceous and Oligocene-Miocene ages are also recognized in NIC blocks These ages suggest that the NIC differs distinctly from the Indian continent the Greater and Lesser Himalaya zones and the Yangtze Craton but resembles the Lhasa Block in terms of Paleozoic to Mesozoic magmatism and detrital zircon ages This offers an entirely new perspective on the tectonic evolution of the Gongshan block in particular and of the history of the Lhasa Block in the context of the India Asia continental collision in general Furthermore the high grade metamorphism in the NIC indicates a strong crustal thickening (vs strike-slip shearing) event during much of the Eocene to the Oligocene (similar to 53-22 Ma) that has brought the Paleogene sediments to depths of greater than 25 km Continuous northward convergence/compression of the Indian Plate at the Eastern Himalayan Syntax's may have led to the clockwise rotation southeastward extrusion and extension of the southeastern part of the Indochina continent (C) 2010 Elsevier B V All rights reserved