满归地区早侏罗世岩浆作用及其地质意义

Magmatic activity and its geological significance in Early Jurassic in Mangui area of Inner Mongolia

通过对满归地区主体岩浆岩锆石U-Pb同位素年代学和地球化学研究,探讨了其形成时代、构造背景及地质意义。研究区存在大量早侏罗世侵入岩和少量早侏罗世中酸性火山岩,LA-ICP-MS锆石U-Pb同位素测年结果为(199±1)^(184±1)Ma,类型为细中粒石英闪长岩、中粒花岗闪长岩、细中粒二长花岗岩、中细粒似斑状二长花岗岩、流纹岩、英安岩和安山岩,不是前人划分的新元古代—古生代,揭示了研究区有强烈的中生代早期构造-岩浆作用。岩石地球化学反映岩石为准铝质—过铝质高钾钙碱性Ⅰ型岩浆岩;轻重稀土元素分馏明显,(La/Yb)N=3.42~32.96,Eu元素亏损程度不遵从岩石从基性到酸性增强的演化规律;大离子亲石元素Ba相对富集,Rb、Sr相对亏损,高场强元素Th、U相对富集,Nb、Ti、Y相对亏损。岩浆来源与构造分析表明,石英闪长岩与中细粒似斑状二长花岗岩来源于壳幔混合岩浆,中酸性火山岩、花岗闪长岩和细中粒二长花岗岩来源于地壳物质部分熔融作用,其形成与蒙古—鄂霍茨克缝合带的演化有关。地质和地球化学研究表明蒙古—鄂霍茨克洋壳中段可能在中三叠世末开始俯冲,至早侏罗世封闭,碰撞高峰时期在早侏罗世,晚于以往认为的晚三叠世。漠河逆冲推覆构造可能形成于蒙古—鄂霍茨克洋东段闭合过程中自北向南挤压的远程效应。蒙古—鄂霍茨克洋中段、东段闭合时限对探讨东北地区中侏罗世—晚白垩世盆岭构造形成具有重要的参考作用。

By investigating the U-Pb zircon isotope geochronology and petrogochemistry of the major magmatic rocks in Mangui area,the authors in this paper discussed the forming era,tectonic background and geological significance.A large amount of intrusive rocks and a small amount of medium-acid volcanic rocks in Early Jurassic were found in this area.U-Pb dating by LA-ICP-MS method shows that the ages are from(199±1)Ma to(184±1)Ma and the rock types can be divided into fine-medium-grained quartz diorite,medium-grained granodiorite,fine-medium-grained monzogranite,medium-fine-grained macrophenocryst monzogranite,rhyolite,dacite and andesite,which didn’t ouur in Neoprotezoic-Paleozoic period as previous researchers thought,revealing the tectonic and magmatic activities during the Early Mesozoic period.The geochemical results show that the rocks areⅠ-type magmatic rocks of subluminous-peraluminous high-K calc-alkaline series.The fractionation between light and heavy rare-erath elements((La/Yb)N=3.42~32.96)and the Eu depletion degree is not complied with the evaluation from basic to acidic.The large ion lithophile element Ba is relatively rich and Rb,Sr are relatively delicient.The high field strength elements Th and U are relatively rich and Nb,Ti,Y are relatively deficient.The magma origin and tectonic setting show that quartz diorite and medium-fine-grained macrophenocryst monzogranite come from crust-mantle mixed magma,while the medium-acid volcanic rocks,granodiorite and fine-medium-grained monzogranite are from the partial melting of crustal materials,whose formation is connected with the evolution of Mongol-Okhotsk Suture Zone.The geology and geochemistry of the Early Mesozoic magmatic rocks indicate that the middle part of Mongol-Okhotsk Ocean might begin subduction at the end of the middle Triassic and close the Early Jurassic.The peak collision might take place at the Early Jurassic,not in the Late Triassic as previous thought.The Mohe over-thrust nappe system might form in the remote effect of southward extrusion during the closing process of the eastem part of Mongol-Okhotsk Ocean.The middle and eastern Mongol-Okhotsk Ocean closing age is of great importance to reveal the basin-range tectonic formation during the Middle Jurassic to the Late Cretaceous in Northeast China.