内蒙古阿拉善地块中北部地区辉长岩岩体特征、锆石定年及其构造意义

1∶25万区域地质调查、地质剖面测制及研究发现，阿拉善左旗乌力吉地区分布有基性岩，其侵入上石炭统阿木山组中，后又被晚期酸性岩浆侵入，与其接触关系为侵入接触。研究表明，该基性岩主要由角闪辉长岩和辉长闪长岩组成，其中角闪辉长岩为主体岩性。采用LA-ICP-MS技术对其中典型的具基性岩浆特征的锆石进行了U-Th-Pb同位素测定，获得的206Pb/238U年龄加权平均值为248.0±0.9Ma，为早三叠世。结合区域构造和岩性关系，该地区辉长岩的形成年龄明显晚于区域内古亚洲洋板块向华北板块俯冲作用结束的时间，因此其并非是古亚洲洋板块向华北板块于晚古生代俯冲引起的构造岩浆旋回早期阶段的产物，而更可能与大规模的陆内裂解有关，并产生于地幔物质上涌造成的区域性大陆拉张环境中。

Characteristics,zircon dating and tectonic significance of the gabbros along the north-central segments of the Alxa Block,Inner Mongolia

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 mean 206Pb/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.