喜马拉雅地体的泛非-早古生代造山事件年龄记录

Pan-African and Early Paleozoic orogenic events in the Himalaya terrane: Inference from SHRIMP U-Pb zircon ages

喜马拉雅地体是55±10Ma以来印度陆块与欧亚大陆碰撞而形成的增生地体,位于其中的高喜马拉雅与特提斯-喜马拉雅构造单元的变质基底主要由角闪岩相的富铝变质沉积岩和花岗质片麻岩组成。对两类岩石中锆石的SHRIMPU-Pb测年结果表明,除了记录了20Ma以来的构造事件年龄外,主要保存了529-457Ma的变形和变质事件记录,另外还保存了更早期(>835Ma)的年龄信息。根据20Ma以来崛起的喜马拉雅挤出岩片中包含早期强烈褶皱和向南的斜向逆冲构造以及伴随的角闪岩相变质作用记录,结合岩石测年所获得的大量泛非-早古生代年龄和奥陶纪底砾岩的发现,说明曾位于南半球印度陆块北部的变质基底岩石经历过泛非-早古生代造山事件,同位素年代学数据表明:(1)原始喜马拉雅山是泛非-早古生代造山事件的产物;(2)印度陆块早-中元古代变质基底的再活化在原始喜马拉雅山形成中起重要的作用;(3)现在的喜马拉雅山是在泛非-早古生代造山事件基础上再造山的结果。

The Himalayan terrane is an accretional terrane formed during the collision between the India Plate and the Eurasian Plate since 55 +/- 10 Ma. SHRIMP U-Pb dating has been performed on zircons from the amphibolite facies aluminum-rich metasediments and granitic gneisses from the Greater Himalaya and the Tethyan-Himalayan units of the Himalayan terrane. The dating results indicate an widespread 529 similar to 457Ma deformation and metamorphism event, and greater than 835 Ma metamorphic basement formation age, except for the latest tectonic event at 12 similar to 20 Ma. Field observations on the Himalayan slice which has been exhumed since 20 Ma have documented earlier than Cenozoic tectonic events represented by the early intense folding, oblique southward thrusting and concurrent amphibolite facies metamorphism and the Ordovician basal conglomerate. Recent studies also yielded a number of Pan-African and Early Paleozoic ages for the metasedimentary and metamorphic basement rocks by Ion Probe U-Pb zircon dating. These lines of evidence together with our new dating results lead us to propose that the Early middle Proterozoic metamorphic basement of the India block once located in the southern hemisphere has undergone the Pan-African - Early Paleozoic orogenic events. The geochronological data suggest that: (1) the Proto-Himalaya formed as a result of the Pan-African and the Early Paleozoic orogenic events; (2) reactivation of the Early-middle Proterozoic metamorphic basement of the India block has played a key role in the formation of the Proto-Himalayan Mountains; (3) the building of the present-day Himalaya Mountains may have been initiated as early as the Pan-African and the Early Paleozoic.