青藏高原东北缘早古生代造山系中前寒武纪微陆块的再认识——兼谈原特提斯洋的起源

在青藏高原东北缘的祁连-阿尔金-昆仑早古生代造山系中,夹杂有一些前寒武纪大陆块体,这些地块的组成、性质和演化既蕴含有超大陆聚散的重要信息,也对原特提斯体系的洋陆格局、造山类型和造山机制有重要启示意义。本文综合近年来这些前寒武纪微陆块的研究进展,结合我们所获得的新的研究资料,梳理了这些前寒武纪微陆块变质基底的岩石组成、构造热事件及年代格架,得出以下主要认识:(1)这些前寒武纪微陆块普遍遭受早古生代造山事件的改造并发生再活化。它们或者作为早古生代原特提斯洋的活动大陆边缘,被洋壳俯冲有关的弧岩浆和变质作用改造,以早古生代大陆弧的形式存在;或者被早古生代碰撞造山过程中的陆内变形、增厚地壳及相关的区域变质作用、深熔作用和碰撞型花岗岩所改造。(2)在这些前寒武纪微陆块中,仅仅欧龙布鲁克地块保存有早前寒武纪的变质基底,具有克拉通性质。中元古代以前,欧龙布鲁克地块的变质基底与华北克拉通(特别是阿拉善地块)和塔里木克拉通具有相似的岩石组成和年代格架;而晚中元古代到新元古代,所有的前寒武纪微陆块与华南陆块和塔里木陆块的亲缘性更强。(3)青藏高原北缘早古生代造山系中的大部分前寒武纪微陆块可能在罗迪尼亚超大陆解体时已从冈瓦纳大陆北部分离,而柴达木地块记录了泛非期造山作用的构造热事件,可能在泛非造山期(530Ma)以后才从冈瓦纳大陆分开;在青藏高原东北部,晚新元古代-早古生代并不存在统一的原特提斯洋,原特提斯洋的打开是穿时的。

Revisiting the Precambrian micro-continental blocks within the Early Paleozoic orogenic system of the northeastern Qinghai-Tibet Plateau: Insight into the origin of Proto-Tethyan Ocean

In the Early Paleozoic Qilian-Altun-Kunlun orogenic system of the northeastern Qinghai-Tibet Plateau, there are some Precambrian micro-continental blocks. The composition, nature and evolution of these blocks provide not only important information for breakup and assembly of supercontinent, and also have important insight into the ocean-continent configuration, orogenic type and orogenic mechanism of the Proto-Tethys tectonic system. In this contribution, based on the published data and our new data, the synthesis of the geological and chronological studies of these Precambrian micro-continent blocks has been carried out. We reinterpret their reworking, tectonic affinity and insight into the evolution of the Proto-Tethys Ocean as follows:(1) These Precambrian micro-continental blocks commonly underwent reworking and reactivation during Early Paleozoic orogeny. They are either reactivated as active continental margins of the Early Paleozoic Proto-Tethys Ocean by arc magma and metamorphism associated with oceanic crust subduction; or by intracontinental deformation, crust thickening and associated regional metamorphism, anatexis and collisional magmatism during the Early Paleozoic collision orogeny. (2) In these Precambrian micro-continental blocks, only the Oulongbuluke block has an Early Precambrian metamorphic basement and show a craton property. Before the Mesoproterozoic, the metamorphic basement of the Oulongbuluke block was similar to that of the North China craton (especially the Alxa block) and the Tarim craton, while from the Late Mesoproterozoic to the Neoproterozoic, all micro-continental blocks have more strongly affinitive relation to the South China block and the Tarim block. (3) Most of the micro-continental blocks may have separated by Proto-Tethys Ocean from Gondwana during berakup of the Rodinia. However, the Qaidam block records a Pan-African UHT metamorphism related to the final amalgamation of Gondwana and may split from Gondwana after the Pan-African orogeny (ca. 530Ma). In the northeastern Qinghai-Tibet Plateau, there is no a single Proto-Tethys ocean in the Late Neoproterozoic-Early Paleozoic, and the Proto-Tethys ocean might opean diachronically.