大兴安岭西南坡成矿带晚古生代中期未变质岩浆岩的SHRIMP锆石U-Pb年代学

SHRIMP zircon U-Pb ages of the middle Neopaleozoic unmetamorphosed magmatic rocks in the southwestern slope of the Da Hinggan Mountains,Inner Mongolia

大兴安岭西南坡晚古生代中、晚期未变质的岩浆岩广泛分布,它们的侵位时间虽不尽相同,但都晚于中亚-蒙古造山区(带)东部峰期造山的时间,且大多与主缝合带拼合后的拉张构造环境有关,岩石性质也多具有偏碱性或双峰式的特点。本文测定了内蒙古东南部二连浩特-贺根山缝合带与索伦山-西拉木伦缝合带之间的维拉斯托石英闪长岩和闪长岩、拜仁达坝花岗闪长岩以及道伦达坝英安质晶屑凝灰岩的SHRIMP锆石U-Pb年龄,它们分别为310±2、311±2、319±3和300±5Ma,时代均为晚石炭世。这些岩浆岩的形成时间比(峰期造山形成的)锡林郭勒杂岩的变质年龄(337±6Ma)晚约20～40Ma,未受到变形和变质作用的影响,推测其属于晚造山/后造山的岩浆岩。认为西伯利亚板块与华北板块之间主缝合带的闭合时间应在晚石炭世前,主缝合带的位置应在二连浩特-贺根山一线。

Located in the compound part of the EW-trending Paleozoic Central Asia-Mongolia orogenic belt and the eastern margin of the NNE-trending Asian continent orogenic belt of littoral West Pacific,the southwestern slope of the Da Hinggan Mountains in Inner Mongolia is a newly discovered nonferrous metallogenic belt with ideal mineralization potential.The unmetamorphosed magmatic rocks emplaced at the middle-late stage of Neopaleozoic are widely distributed in this area,and the lithologic association includes not only intermediate-acidic to alkaline rocks but also bimodal volcanic rocks.Although there exists great difference in their emplacement ages,they were all formed later than the metamorphic time of the Xilin Gol complex and the peak orogenic stage of the eastern segment of the Central Asia-Mongolia orogenic belt.Spatially,most of these magmatic rocks,with A-type or bimodal properties in geochemistry,are related to the extensional tectonic setting after the closure of the main suture.In this study,the authors acquired zircon SHRIMP U-Pb data of late Variscan magmatic rocks outcropped in southeastern Inner Mongolia,tectonically situated at the area between Erlian-Hegen suture and Solon-Xarmoron suture,with the purpose of making constraint on the closure time of the main suture between the Siberian plate and the Sino-Korean plate.Zircon SHRIMP U-Pb analyses yield emplacement ages of 310±2 and 311±2 Ma for quartz-diorite and diorite from the Weilasituo silver-polymetallic deposit,319±3 Ma for granodiorite from the Bairendaba deposit,and 300±5 Ma for dacitic tuff from the Daolundaba deposit,indicating that they all belong to late Carboniferous.The emplacement ages of these magmatic rocks are about 20～40 Ma later than the metamorphic age （337±6 Ma） of the Xilin Gol complex formed at the peak stage of orogeny.These magmatic rocks were not affected by deformation and metamorphism,implying that they belong to late-/post-orogenic magmatic rocks.The authors hold that,although the Central Asia-Mongolia orogenic belt is characterized by poly-island arc accretions and poly-sutures,the orogenic process must have undergone many times of collision and convergence between palaeo-ocean crust and palaeocontinent,island arc and island arc,island arc and continent,palaeo-ocean crust and micro-continental block,micro-continental block and micro-continental block,and micro-continental block and palaeocontinent,and the main suture between the Siberian plate and the Sino-Korean plate is situated near the Erenhot-Hegenshan belt.