柴达木和欧龙布鲁克陆块基底的组成和变质作用及中国中西部古大陆演化关系初探

中国西部的柴达木陆块和欧龙布鲁克陆块的基底岩系虽然在地球化学上与扬子陆块具有亲缘性,但它们之间的组成和变质作用历史却显著不同。欧龙布鲁克陆块下部基底德令哈杂岩和达肯大坂岩群于~1.95Ga发生了角闪岩相-麻粒岩相区域变质作用并克拉通化,响应了全球Columbia超大陆汇聚事件;随后又与中元古代万洞沟岩群一道于~1.0Ga发生绿片岩相变质作用,共同响应了全球Rodinia超大陆汇聚事件;新元古代中晚期裂解后于中奥陶纪受原特提斯洋关闭影响而隆起。柴达木陆块基底主体由中元古代金水口岩群白沙河岩组(柴南缘)和沙柳河岩群乌龙滩岩组(柴北缘)组成,以S-型花岗岩的侵入活动和相应的变质作用响应了全球Rodinia超大陆汇聚事件。晚泛非期(520~480Ma)柴达木陆块与冈瓦纳主大陆俯冲碰撞,发生中压角闪岩相-麻粒岩相和高压超高压变质作用,经短暂拼贴后很快进入到原特提斯洋域,随460~420Ma的大洋关闭而发生变质叠加。区域对比表明,在基底组成和地壳演化史上,欧龙布鲁克陆块与阿拉善陆块和塔里木陆块(包括扬子陆块)相似,柴达木陆块则与北秦岭陆块以及祁连陆块相似,因而分属两个陆块群。塔—欧陆块群记录的~500Ma热事件与塔—欧和柴—秦陆块群共同记录的~450Ma热事件是两个性质不同的构造热事件。

Components and metamorphism of the basements of the Qaidam and Oulongbuluke micro-continental blocks, and a tentative interpretation of paleocontinental evolution in NW-Central China

The Qaidam and Oulongbuluke micro-continental blocks are geochemically similar to the Yangtze Block, but their metamorphic history are strikingly different from each other. The Delingha Complex and Dakendaban Group in the lower basement of the Oulongbuluke Block underwent upper amphibolite-facies to granulite-facies regional metamorphism and cratonization at ~1.95 Ga and were part of the assembly Columbia supercontinent. The greenschist-facies metamorphism along with the Mesoproterozoic Wandongguo Group at ~1.0 Ga corresponds to the assembly event of the Rodinia supercontinent. This was followed by continental breakup in the late Neoproterozoic and uplift in the Ordovician during the opening and closing of the Proto-Tethys Ocean. The basement of the Qaidam Block is mainly composed of the Mesoproterozoic Baishahe Formation of the Jinshuikou Group in the southern margin, and the Wulongtan Formation of the Shaliuhe Group on the northern margin. These formations were part of the assembly event of the Rodinia supercontinent, and were associated with intrusion of S-type granitoids. During the late Pan-African tectonothemal event (520-480 Ma), the Qaidam Block was subducted and collided with Gondwanaland terranes and was subjected to medium-pressure amphibolite-to granulite-facies metamorphism and HP-UHP metamorphism. Following these metamorphic events, the Block broke up and moved into the Proto-Tethys Ocean, and was subjected to metamorphism during 460-420 Ma due to closing of the Proto-Tethys Ocean. Regional comparisons show that the Oulongbuluke Block has similarities with the Arlashan Block and the Tarim Block as well as Yangtze Block in the Paleoproterozoic basements in principal part, the late Paleoproterozoic cratonization, the assembly of parts of the Rodinia supercontinent and the ~450 Ma tectonothermal event. However, the Qaidam Block has similarity with the North Qinling Block as well as the Qilian Block in the ~500 Ma tectonothemal event in addition to the Mesoproterozoic basement components, responding to assembly event of Rodinia supercontinent and the ~450 Ma tectonothermal event. Accordingly, these blocks can be separately grouped into Tarim-Ouloungbuluke blocks and Qaidam-Qinling blocks. The ~500 Ma and ~450 Ma tectonothermal events that recorded in the Qaidam-Qinling blocks are of different origins.