Characteristics, zircon dating and tectonic significance of the gabbros along the north-central segments of the Alxa Block, Inner Mongolia
Characteristics, zircon dating and tectonic significance of the gabbros along the north-central segments of the Alxa Block, Inner Mongolia
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发表时间：
2014
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通讯作者：
Xu Dong-zhu
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作者：
Xu Dong-zhu
通过1∶ 25万野外地质调查，揭示了阿拉善左旗乌力吉地区侵入上石炭统阿木山组并受晚酸性岩浆侵入的镁铁质岩的分布。基性岩由角闪岩辉长岩和辉长闪长岩组成，以角闪岩辉长岩为主。采用LA-ICP-MS技术对斜长角闪岩辉长岩中的锆石进行了U-Th-Pb定年。具有典型镁铁质岩浆特征的锆石的206 Pb/238 U加权平均年龄为248.0±0.9Ma，属早三叠世。测年结果结合区域构造和岩性关系分析表明，辉长岩的形成明显晚于古亚洲洋的闭合。认为辉长岩不是晚古生代古亚洲洋壳向华北板块俯冲环境下造山运动早期岩浆活动的产物，而可能与大规模的大陆裂谷作用有关。本区辉长岩形成于地幔物质上涌引起的区域大陆伸展环境。
Detailed 1∶250000 field geological survey has revealed the distribution of mafic rocks in Wuliji area of Alxa Left Banner,which intruded the Upper Carboniferous Amushan Fomration and was also intruded by late acidic magma. The mafic rocks are composed of amphibolite gabbro and gabbro-diorite, with amphibolite gabbro being dominant. U-Th-Pb dating of zircons from amphibolite gabbro was conducted with LA-ICP-MS technique. The dating of zircons which have the typical characteristics of mafic magma yielded a weighted mean206Pb/238U age of 248.0±0.9Ma, suggesting Early Triassic. The dating result in combination with the regional structure and an analysis of the relationship of lithology characters shows that the gabbro was formed obviously later than the closure of the Paleo-Asian Ocean. It is thus held that the gabbro did not result from early magmatic activity during orogeny in a subduction environment of Paleo-Asian oceanic crust beneath the North China plate in late Paleozoic, but was probably related to large-scale continental rifting. The gabbro of this region was generated in a regional continental extensional environment caused by the upwelling of mantle material.