阿尔泰南缘冲乎尔盆地酸性火山岩地球化学及年代学研究

阿尔泰南缘冲乎尔盆地发育一套浅变质长英质岩系,火山岩原岩为酸性凝灰质-流纹质岩。盆地岩石化学分析显示,该套火山岩为钙碱性系列,微量元素显示出与大洋板块俯冲有关的陆缘岛弧构造环境。晶屑凝灰岩的SHRIMP锆石U-Pb年代学研究表明,该套岩石的喷发年龄为436±4Ma,与盆地北缘花岗质岩石侵入时代相近(413±4Ma),属早古生代志留纪喷发而成,说明冲乎尔盆地是一个早古生代火山沉积盆地。盆地火山岩地球化学和年代学揭示,志留纪区域构造性质已经从被动大陆边缘转变为活动大陆边缘,古亚洲洋在志留纪(甚至奥陶纪)逐渐俯冲、消减,形成早古生代弧盆体系和晚奥陶世-志留纪磨拉石建造,即加里东增生陆壳(陈毓川等,2003),这种俯冲可能一直持续到泥盆纪,甚至石炭纪。从冲乎尔盆地酸性火山岩岩石学、构造环境和火山活动时代可以推断其成矿效应,盆地内产出的矿床(点)可能属于加里东期与变质-岩浆热液作用有关的有色、稀有金属-白云母成矿系列。

Geochemical Characteristics of Acid Volcanic Rocks from Chonghuer Basin,Altai,Xinjiang and Their Geochronological Implication

There are a series of epimetamorphosed felsic igneous rocks in the Chonghuer basin, south margin of the Altai Mountiains, North Xinjiang, petrochemical analysis shows they are the calc-alkaline tuffaceous-rhyolite suite. SHRIMP U-Pb zircon age for crystal tuff shows the rock suite were formed in 436 ±4Ma, which is similar to that of granitoids intrusion in the basin. Hence the basin should be classified as an early Paleozoic volcano-sedimentary basin. The synthetic study on magmatite ages, structural style and strata-intrusions suggests the geotectonic setting of the basin probably is a volcanic arc of an active continental margin. Therefore its structural attribute leads to metal-logenesis in the basin is different from that in the late Palaeozoic basins in south margin of the Altai Mountiains. During the Ordovican and Silurian, the paleo-Asian plate subducted beneath the Siberia plate, the Altai region transferred from the passive continental margin to active continental one, local extension following the compressional background resulted in the Chonghuer basin was formed. The volcanic rocks in the Chonghuer basin are distinguish geologically from the Devonian those, the metallogenesis is probable classified as the Caledonian nonferrous, rare metal-muscovite series related to magmatic hydrothermal solution-metamorphism.