阿拉善地块性质和归属的再认识

摘要

阿拉善地块被认为是一个前寒武纪微陆块,其归属和演化历史长期存在争议.一部分学者认为其为华北克拉通的组成部分,是华北克拉通的西延;一部分学者认为与扬子或塔里木克拉通具有亲缘性;还有一些学者认为具有独立的地质演化历史.本文综合近年来阿拉善地块的研究进展,结合我们新的研究资料,梳理了阿拉善地块自新太古代到显生宙的岩石组成、构造热事件及年代格架,得出以下主要认识:1)阿拉善地块的早前寒武纪变质基底记录了2.7 ～ 3.0Ga陆壳生长、～2.5Ga TTG岩浆-变质事件,2.0～2.3Ga岩浆事件,以及1.9 ～ 1.95Ga和1.80～1.85Ga两期重要的变质事件,具有与华北克拉通相似的岩石组成和地壳演化历史;2)阿拉善地块发育与华北克拉通相似的中-新元古代沉积岩系,其碎屑锆石具有来源华北克拉通本身的物源区特征,反映出阿拉善地块直到中元古代-新元古代早期仍可能是华北克拉通的一部分;3)阿拉善地块显生宙的多期岩浆-构造-变质事件可能主要是中亚造山带早古生代以来造山作用的响应,而不是阿拉善地块与华北克拉通碰撞的产物.从奥陶纪到早二叠世,阿拉善地块可能处在古亚洲洋向南俯冲的活动大陆边缘环境;4)河西走廊-贺兰山南部早古生代沉积岩系(大黄山群和香山群)不属于阿拉善地块,而是祁连造山带的组成部分,其碎屑锆石组成和代表的物源特征不能作为阿拉善地块与华北克拉通在早古生代彼此分离的证据.%The Alxa Block was traditionally considered to be one of the Precambrian micro-continental blocks.However,its affiliation and evolutionary history remains a highly controversial issue.Some authors considered the Alxa Block as the western extension of the North China Craton (NCC).The other authors suggested that the Alxa Block had an affinity with the South China Craton (SCC) or Tarim Craton (TC).The remaining authors suggested that the Alxa Block is a continental block with independent evolution history.In this contribution,based on the published data and our new data,the synthesis of the geological,petrological,chronological studies of the Alxa Block has been carried out.We reinterpret the tectonic affinity and evolution of the Alxa Block as follows:1) The Alxa Block experienced 3.0 ～2.7Ga crustal growth events,a ca.2.5Ga TTG magmatic-metamorphic event,2.3 ～ 2.0Ga multiphase magmatic events,and 1.95 ～ 1.90Ga and ca.1.85Ga high-grade regional metamorphic events.It exhibits a highly similar sequence of events to those of the other part of the NCC.2) The Alxa block contains Meso-Neoproterozoic strata with detrital zircon age populations reflecting the sources from the NCC,suggesting it was still an integrated part of the NCC during MesoNeoproterozoic era.3) The Alxa Block experienced multiple rnagmatic-tectonic-metamorphic events since Early Paleozoic.The Phanerozoic tectono-thermal events in the Alxa Block respond to the orogeny related to the Central Asian Orogenic Belt,rather than the collision between the Alxa Block and NCC.The Alxa Block was located in an active continental margin setting related to the southward subduction of the Paleo-Asian Ocean during Ordovician to Early Permian.4) Early Paleozoic strata (Dahuangshan Group and Xiangshan Group) in the Hexi Corridor and Xiangshan area do not belong to the Alxa Block,but is the part of Qilian orogenic belt.Their detrital zircon age populations can not considered as evidences which the Alxa Block was separated from NCC each other during Paleozoic era.