柴达木盆地北缘新元古代蛇绿岩的厘定——罗迪尼亚大陆裂解的证据?

Recognition of Neoproterozoic ophiolite on the northern margin of the Qaidam basin: evidence of the breakup of Rodinia?

柴达木盆地北缘鱼卡河及绿梁山一带“奥陶纪”滩涧山群中分出一套“新元古代”的蛇绿岩组合,主要由玄武质(枕状)熔岩、岩墙群、辉长岩等古洋壳岩石组成,其中以MORB和岛弧玄武岩为主,岩石变质程度达角闪岩相。蛇绿岩洋壳的Rb-Sr同位素等时线年龄值为768Ma±39Ma,Sm-Nd同位素等时线年龄值780Ma±22Ma。这套古洋壳岩石与典型的奥陶纪(约500Ma)火山岩不同,后者主要为中基性岛弧火山岩,岩石仅发生轻微绿片岩相变质;而前者与榴辉岩的成分可以对比,并且与榴辉岩原岩的时代(750～800Ma)相同。推断绿梁山的海底玄武质岩类可能就是榴辉岩的原岩,祁连地区新元古代蛇绿岩洋盆是在罗迪尼亚大陆大约800～750Ma发生裂解的基础上形成的。

A Neoproterozoic ophiolite was recognized in the Tanjianshan Group in the vicinity of the Iqa River and Lüliang Mountain on the northern margin of the Qaidam basin. The ophiolite consists of ancient oceanic crustal rocks such as basaltic (pillow) lavas, a sheeted dike swarm and gabbros, with MORB and island-arc basalt predominating. The metamorphism is generally of amphibolite facies. It has a Rb-Sr isochron age of 768±39 Ma and a Sm-Nd isochron age of 780± 22 Ma. The rocks are different from typical island arc volcanic rocks of the Ordovician Tanjianshan Group (～500 Ma BP). The latter mainly contains intermediate-basic lavas and tuffs and only underwent weak greenschist facies metamorphism, while the former is comparable in composition to the North Qaidam eclogite and has the same age (750-800 Ma) as the protolith of eclogite. Therefore the Lüliang Mountain submarine basaltic rock is probably the protolith of eclogite. The Neoproterozoic ophiolite oceanic basin in the Qilian area formed on the basis of a breakup of Rodinia at ～800-750 Ma BP.