藏东昌都地块东南缘贡觉地区中深变质岩系的岩石学和年代学特征

Petrological and chronological characteristics of medium-high grade metamorphic rocks in Gonjo area,southeastern margin of Qamdo block,eastern Tibet

青藏高原东缘的昌都地块是连接青藏高原腹地和藏东南三江-印支地区的关键区域,发育有较完整的晚古生界,以三叠系、侏罗系-白垩系广泛分布为特色;但其前寒武纪基底岩系出露较少,主要以宁多岩群(Pt_(1-2)Nd.)和草曲群(Pt_(3)C)为代表。本文针对昌都地块东南缘贡觉地区出露的一套被前人称为“宁多岩群”的中深变质岩系,开展了系统的野外地质调查、显微薄片分析、锆石U-Pb年代学等研究工作,认为该套中深变质岩系属于一套变质杂岩,其岩石组合既包括了原岩沉积时代不早于奥陶纪的片麻岩、变粒岩和大理岩等副变质岩,又包含了原岩时代分别为中二叠世的斜长角闪岩和中三叠世花岗质片麻岩等正变质岩。其原岩时代特征与昌都地块北缘的宁多岩群有较大差别,不应当归属为昌都地块的前寒武纪结晶基底。结合片麻岩的变质新生云母类矿物Ar-Ar测年数据和变质后期侵入的闪长玢岩脉锆石U-Pb年龄,认为贡觉地区出露的中深变质岩的变质时代为ca.260~244Ma(P_(2)-T_(2))。该期发生在昌都地块东南缘的角闪岩相变质作用,是金沙江古特提斯洋向西俯冲-碰撞的构造响应。

The Qamdo block in the eastern margin of the Tibetan Plateau is a key area connecting the central region of the Tibetan Plateau and the Sanjiang-Indochina region.The Qamdo block has a relatively complete Late Paleozoic and is characterized by the widely distributed Triassic and Jurassic-Cretaceous.However,the Precambrian basement rock series are less exposed,mainly represented by Ningduo metamorphic group and Caoqu metamorphic group.In this paper,the systematic field geological survey,microanalysis and zircon U-Pb chronology are carried out for the medium-high grade metamorphic rocks known as“Ningduo metamorphic group”in Gonjo area,southeastern margin of Qamdo block.It is considered that the medium-high grade metamorphic rocks belong to a set of metamorphic complex,which includes not only parametamorphic rocks such as gneiss,granulite and marble whose protolith sedimentary age is not earlier than Ordovician,but also orthometamorphic rocks such as amphibolite of Middle Permian and granitic gneiss of Middle Triassic.Its protolith age is quite different from that of Ningduo metamorphic group in the northern margin of Qamdo block,so it should not belong to the Precambrian basement of this block.Combined with the Ar-Ar dating of mica minerals of gneiss and the zircon U-Pb age of diorite vein intruded after the metamorphic period,it is considered that the metamorphic age of the medium-high grade metamorphic rocks in Gonjo area is ca.260~244Ma(P_(2)-T_(2)).This amphibolite facies metamorphism in the southeastern margin of the Qamdo block is a tectonic response to the westward subduction collision of the Jinshajiang Paleo-Tethys Ocean.