甘肃北山南部早古生代古亚洲洋俯冲作用:来自明舒井埃达克质侵入体锆石U-Pb年龄及地球化学的证据

Early Paleozoic Paleo-Asian Ocean subduction in the southern Beishan,Gansu:Evidence of zircon U-Pb age and geochemical records from Mingshujing adakitic pluton

甘肃北山南部具有重要的构造位置与复杂的构造演化历史,制约着北山造山带大地构造归属研究。北山明舒井埃达克质侵入体位于南部辉铜山以西地区,岩性主要为二长花岗岩、正长花岗岩、花岗闪长岩、石英闪长岩等。对明舒井侵入体中的二长花岗岩、正长花岗岩和花岗闪长岩进行LA-ICP-MS锆石U-Pb测年,定年结果分别为423.4±3.4 Ma、425.4±2.3 Ma和437.8±3.5 Ma。岩石SiO2含量多数大于56%(55.53%~64.42%),Al2O3大于15%(15.1%~17.79%),MgO多数小于3%,全部小于6%(2.07%~3.5%),Y和Yb含量均较低,同时轻稀土元素富集,Eu异常不明显,Sr含量较高,具有与埃达克岩相似的特征。综合区域地质背景,认为北山南部志留纪埃达克质侵入岩形成于活动大陆边缘火山弧环境,可能是志留纪古亚洲洋向北消减俯冲背景下岩浆作用的产物,同时发生了大规模的地壳增生。明舒井埃达克质侵入岩体的识别为北山造山带早古生代构造演化提供了重要依据。

The southern Beishan has an important tectonic position and a complex tectonic evolution history,which restricts the research on the tectonic attribution of the Beishan orogenic belt.The Mingshujing adakitic pluton is located in the south of Beishan,Gansu,to the west of Huitong Mountain,and the lithology is mainly adamellite,syenogranite,granodiorite,quartz diorite,etc.The LA-ICP-MS ages of the adamellite,syenogranite,granodiorite zircons of the Mingshujing adakitic pluton are 423.4±3.4 Ma,425.4±2.3 Ma and 437.8±3.5 Ma,respectively,which represent the crystallization age of the pluton.The content of SiO2 is mostly higher than 56%(55.53%~64.42%),Al2O3 is larger than 15%(15.1%~17.79%),MgO is mostly less than 3%,and all are less than 6%(2.07%~3.5%).The content of Y and Yb is relatively low.At the same time,no Eu anomaly or only weak negative Eu anomaly can be observed,REE distribution patterns show strong REE segregation and belong to the type of enriched LREE,and there exist very prominent negative Nb anomaly and positive Sr anomaly in the trace element spidergram.The petrogeochemistry has similar characteristics to that of adakite.Based on the regional geological background,it is believed that the Silurian adakitic pluton in the southern Beishan were formed in the volcanic arc environment of the active continent margin,and which may be the products of the northward subducting magmatism of the Silurian paleo-Asian oceanic crust in Beishan Mountains,at the same time,there was a massive continental accretion.The identification of adakitic pluton in the Mingshujing provides an important basis for the early Paleozoic tectonic evolution of the Beishan Orogenic Belt.