皮羌盆地新生代基性岩浆活动的年代学及其地质意义

塔里木盆地西北部卷入了西南天山新生代板内造山活动,以发育一系列北东向展布的断层和断层相关褶皱为特征,但该区新生代构造变形的原因尚存在争议。文中以皮羌盆地为例报道了新生代火成岩的地质特征和测年结果,认为构造变形是对幔源岩浆活动的浅部响应,与印度 亚洲大陆碰撞没有必然的联系。皮羌盆地出露有大量的新生代基性岩墙和4个古火山颈,岩墙群整体走向NNW和NNE,产状近于直立,侵入于新生界湖相沉积地层中。火山颈相玄武岩锆石SHRIMP U Pb定年结果为46 Ma,全岩K Ar等时线封闭年龄为(455±613) Ma,表明岩浆活动发生在始新世中期。岩浆活动时间比逆冲推覆构造启动时间(约24 Ma)早约22 Ma。按热传导模式估算,在此期间来自深部热源的热传递距离约为25 km。假定深部热源为底侵或内侵幔源岩浆,新生代岩浆活动可能是塔里木盆地西北部大规模构造变形的触发机制。

The chronological and geological implications of the Cenozoic basic magmatic activities in Piqiang Basin,Xinjiang, China

Abstract: Involved in the Cenozoic intraplate orogeny of southwestern Tianshan, the northwestern Tarim Basin had developed a series of NE trending faults and fault related folds. The cause of the Cenozoic structural deformation in this region, however, remains currently in controversy. This paper aims to report the geology of the Cenozoic igneous rocks and their chronology, proposing that the structural deformation was the response of shallow crust to the mantle derived magmatism rather than the direct result of Indo Asia collision. There are many Cenozoic mafic dikes with NNW and NNE trend and nearly 90 dipping angle and four volcanic necks in the Piqiang basin. The dikes have intruded the Cenozoic lacustrine layers. Our SHRIMP U Pb zircon ages determined for these igneous rocks for the first time indicate that the mafic dykes and necks were formed at about 46 Ma, while the whole rocks K Ar isochron date yielded an age of (45.5±6.13) Ma. The results suggested that the magmatism appeared in the middle Eocene, almost 22 Ma earlier than the nappe structures (~24 Ma). In terms of the heat conduction model, we estimated that the distance of heat transfer from deep heat source was about 25 km. Considering that the underplating or intraplating mantle derived magmas may serve as the deep source of heat, we presume that the Cenozoic deformation of the northwest Tarim Basin may have been triggered by the Cenozoic magmatism.