内蒙古中部朱日和地区中奥陶世洋壳俯冲作用:来自奥长花岗岩年龄及地球化学的制约

Oceanic crust subduction in the Zhurihe area,central Inner Mongolia during Middle Ordovician:Constrains from age and geochemistry of trondhjemites

系统研究了内蒙古中部朱日和地区最新发现的奥长花岗岩锆石U-Pb同位素年龄、Hf同位素组成及岩石地球化学特征,对该区中奥陶世洋壳俯冲作用给出制约。结果显示,奥长花岗岩形成于中奥陶世(472±3 Ma和466±5 Ma),富Si、富Al、富Na、贫K,高Sr、低Y和Yb,具正Eu异常、正Sr异常,轻、重稀土元素分馏强烈,富集大离子亲石元素,亏损高场强元素,表现为典型的俯冲洋壳型埃达克岩地球化学特征。奥长花岗岩的锆石ε_(Hf)(t)值为+8.8~+15.1,一阶段模式年龄(T_(DM1))为475~728 Ma,二阶段模式年龄(T_(DM2))为476~885 Ma。最新区域地质调查资料显示,朱日和地区存在一个早古生代洋壳俯冲带,表明至少在中奥陶世,朱日和日地区开始出现古亚洲洋向华北板块的洋壳俯冲作用,而具有典型埃达克岩特征的奥长花岗岩所代表的岩浆侵位结晶活动正是对该洋壳俯冲事件的响应。

This paper reports zircon U-Pb ages and Hf isotope data as well as geochemical data of trondhjemites which are latest discovered from the Zhurihe area in central Inner Mongolia,with the aim of constraining oceanic crust subduction during Middle Ordovician.The dating results show that the trondhjemites formed in Middle Ordovician(472±3 Ma and 466±5 Ma).The trondhjemites manifest as the geochemical characteristics of typical subducted-oceanic-crust-type adakites being rich in Si,Al,Na and poor in K,with high values of Sr and low values of Y and Yb,with Eu and Sr positive anomalies,with intense fractionation between light rare earth elements and heavy rare earth elements,being rich in large ion lithophile elements(LILEs)and loss of high field strength elements(HFSEs).The zircon ε_(Hf)(t)values of trondhjemites are between+8.8 and+15.1,so are T_(DM1) values between 475 Ma and 728 Ma as well as T_(DM2) values between 476 Ma and 885 Ma.According to the latest regional geological survey,there exist an Early Paleozoic oceanic subduction zone in the Zhurihe area what indicates at least during Middle Ordovician the Paleo-Asian Ocean began to subduct towards the North China plate in the Zhurihe area.The magmatic emplacement-crystallization activities which are represented by the trondhjemites with the characteristics of typical adakites are response to the oceanic crust subduction.