西藏东南部察隅地区德玛拉岩群的同位素年代学研究及其意义

Study on geochronology and its geological significance of the Demala Group complex in Chayu area,Southeast Tibetan Plateau

青藏高原冈底斯地块东南部的德玛拉岩群为一套角闪岩相变质岩系,一直被认为是前寒武纪变质基底,但并没有可靠的年代学证据。论文对采自其中的黑云角闪片岩和黑云母石英片岩进行了锆石LA-ICP-MSU-Pb定年和黑云母39Ar-40Ar定年,测试表明,黑云角闪片岩原岩锆石U-Pb年龄为217.1Ma,由黑云母39Ar-40Ar获得的变质年龄为22.3Ma,黑云母石英片岩中碎屑锆石主要为岩浆成因,年龄范围主要集中在520～600Ma和900～1100Ma,黑云母39Ar-40Ar变质年龄为16.3Ma和22.3Ma。上述结果虽不能完全否定西藏东南部察隅地区前寒武纪基底变质岩系的存在,但至少说明现今的德玛拉岩群中还包含有遭受中生代岩浆侵入的古生代沉积岩,它们在新生代经历了变质和岩浆作用的再造,是一套变质杂岩。

The Demala Group complex,located in Southeast Gangdise block of Tibetan Plateau,is a suite of amphibolite facies metamorphic rocks.Conventionally it is known as Precambrian metamorphic basement,but without credible geochronology evidences.In this paper,we focus on the samples biotite-hornblende schist and biotite-quartz schist we recently obtain in this area.Results of zircon U-Pb study display that the U-Pb age of magmatic zircons from biotite-hornblende schist is 217.1Ma and detrital zircons from biotite-quartz schist is mainly 520~600Ma and 900~1100Ma.The biotite 40Ar-39Ar age of the biotite-hornblende schist is 22.3Ma and the biotite-quartz schist is 16.3Ma and 22.3Ma,representing the age of their later metamorphism.These accurate geochronology evidences above at least suggests that the Demala Group complex is a suite of metamorphic complex which consists of Paleozoic sedimentary stratum and Mesozoic intrusive rocks modified by the Cenozoic magmatism and metamorphism,although we can not completely negate the existence of Precambrian metamorphic basement in Chayu area,Southeast Tibetan Plateau.