拉萨地体东南缘始新世早期变质作用及其构造意义

本文对位于青藏高原拉萨地体东南缘林芝杂岩中的片麻岩进行了岩石学和锆石U-Pb年代学研究.所研究的样品包括正片麻岩和副片麻岩,它们经历了中压角闪岩相变质作用.岩石地球化学分析结果表明,所研究的正片麻岩的原岩具有钙碱性岛弧岩浆岩的特征.锆石U-Pb年代学分析结果表明,副片麻岩中的碎屑锆石核部为岩浆成因,它们给出的206Pb/238U年龄范围为3012～ 522Ma,其锆石的增生边给出了～51Ma的变质年龄.在正片麻岩中,黑云母片麻岩给出了～67Ma的原岩结晶年龄和～ 55 Ma的变质年龄；石榴石角闪黑云斜长片麻岩给出了～58Ma的原岩结晶年龄和～54Ma的变质年龄.因此,所研究的林芝杂岩并不能代表拉萨地体中的前寒武纪变质基底,而是古生代的沉积岩和晚白垩纪至早新生代的岩浆岩在始新世早期变质而成.这一时期,表壳岩和侵入岩一起经历的中压角闪岩相变质作用很可能跟新特提斯洋俯冲导致的地壳增生、加厚有关.

The Paleocene metamorphism of the southeastern margin of Lhasa terrane and its tectonic significance

The Nyingchi Complex is located in the southeastern margin of Lhasa terrane, Tibetan Plateau. Its petrology and geochronology are reported in this paper. The studied samples consist of orthogneiss and paragneiss, which were metamorphosed under a medium-pressure amphibolite-facies condition. Petrogeochemistry studies show that the protoliths of orthogneisses have the characteristic of calc-alkaline island-arc volcanic rocks. Detrital zircons from the paragneiss are typical of magmatic origin, which give a ²⁰⁶Pb/²³⁸U age range of 3012~522Ma. The overgrowth rims of the detrital zircons yielded metamorphic ages of ~51Ma. For the orthogneisses, magmatic cores and metamorphic rims of zircons from biotite gneiss and garnet-hornblende-biotite-plagioclase gneiss yield protolith ages of ~67Ma and ~58Ma, and metamorphic ages of ~55Ma and ~54Ma, respectively. The studied Nyingchi Complex does not represent Precambrian basement of the Lhasa terrane, but it is composed of Paleozoic sedimentary and Late Cretaceous to Early Cenozoic Gangdese igneous rocks. The ca. 55~50Ma medium-pressure amphibolite-facies metamorphisms of these supracrustal rock and intrusive rocks would be related to the crustal accretion and thickening resulted from the subduction of the Neo-Tethys.