Abstract

U-Pb systematics in small fractions of zircon, monazite, and sphene (5 to 20 grains each) have been analyzed in order to decipher the age and origin of magmatism in the Lhasa block and to date the tectono-metamorphic events. Additionally, we analyzed a few minerals for Rb-Sr. The results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust, the magma source regions of which consisted of Palaeozoic to Precambrian materials up to 2 b.y. in age. The following episodes can be distinguished: (1) an early Tertiary plutonism-volcanism in the southern Lhasa block (Yangbajing region), strongly related to intracontinental tectono-metamorphism, (2) a late Jurassic to early Cretaceous plutonism in the central and northern Lhasa block, and (3) a middle Jurassic low-grade metamorphism, affecting the basement at the northern margin 171 ± 6 m.y. ago. The early Tertiary magmatism and metamorphism in the south are probably a direct result of Tethys subduction (Indian plate)...

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