摘要:基性岩年代学及地球化学精细研究有助于判别蛇绿岩套的类型及构造背景，在追溯和重建古洋盆的构造演化过程中发挥着重要作用。本文报道了新近在阿尔山西北部发现的迈罕特乌拉蛇绿混杂岩中基性单元的初步定年工作。对选自3件枕状玄武岩以及1件变质辉长岩进行了LA-ICP-MS U-Pb定年。结果表明，它们的年龄和锆石内部结构复杂多样，包含少量的热液锆石和大量捕获锆石，基性特征锆石次之并被定为玄武岩和变质辉长岩的自生锆石，形成时代为晚二叠世—早三叠世。枕状玄武岩地球化学特征具有LREE富集，类似E-MORB的稀土配分模式，而相对于E-MORB又富集大离子亲石元素，轻度亏损Ta、Zr等高场强元素。结合枕状玄武岩及变质辉长岩中锆石普遍来自陆壳成因的特点，我们推测迈罕特乌拉蛇绿混杂岩产自古亚洲洋封闭后，由于岩石圈拆沉导致的伸展环境，可能为时空规模都有限的新生陆内小洋盆代表。
关键词: 大兴安岭/
迈罕特乌拉蛇绿混杂岩/
古亚洲洋/
锆石/
LA-ICP-MS
Abstract:The detailed analysis of chronology and geochemistry analysis of mafic rocks into an orogenic system can not only key distinguish the nature and type of ophiolite, but also can provide important insights into the evolution of involved tectonics. Recent found pillow lava basalts and metagabbros in the Maihantewu ophiolitic mélange, the Northwest Arxan, and its preliminary dating has been reported here. 73 zircons of three samples from the pillow lava basalts and 64 zircons of one sample from the metagabbro have been dated using LA-ICP-MS. The distribution of ages and internal structures of these zircons are complicated, including the basic magmatic zircon, most capturing zircons, and a few metamorphic zircons. One of the basic magmatic zircons have been believed as the autogenetic zircons of mafic rocks, the ages of these zircons range from the end of Permian to the Early Triassic. The chondrite?normalized REE patterns of the mafic rocks of the pillow lava basalts are of characteristics of LREE enrichment, similar to E-MORB, while the spider diagram shows the enrichment of the large iron lithosphite elements and weak depletion of high field strength elements. Referring to the most zircons of continental crust in the pillow basalt and metagabbros, we speculate that the Maihantewula ophiolitic mélange were likely formed in the extensional tectonics resulting from the delamination of the lithosphere after the closure of the Paleoasian Ocean, and related to the representing a tectonic setting of limited intra-continental ocean basin.
Key words:Great Xing'an Range/
Maihantewula ophiolitic mélange/
Paleo-Asian Ocean/
Zircon, LA-ICP-MS/
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