内蒙古北山地区哈日阿玛岛弧花岗岩地球化学、年代学特征及其地质意义

GEOCHEMISTRY AND GEOCHRONOLOGY OF HARIAMA ISLAND ARC GRANITOIDS IN BEISHAN AREA OF INNER MONGOLIA:Geological Implication

哈日阿玛岛弧花岗岩出露于北山造山带公婆泉岛弧带内,与奥陶纪—志留纪公婆泉组弧火山岩伴生,包含闪长岩、石英闪长岩、斜长花岗岩、英云闪长岩等.LA-ICP-MS锆石U-Pb测年表明闪长岩、英云闪长岩分别就位于430.9±4.7 Ma和452.5±3.2 Ma,为晚奥陶世—早志留世.岩石地球化学分析结果显示,这些花岗岩属钙碱性、准铝质-弱过铝质Ⅰ型花岗岩.岩石样品Mg^(#)值界于44.58~51.85,Rb/Sr比值0.025~0.282,Nb/Ta比值0.068~14.01,Zr/Hf比值13.05~34.00.稀土元素总量(包含Y元素)为59.87×10^(-6)~118.56×10^(-6),球粒陨石标准化稀土配分曲线均呈现明显的右倾型,轻稀土富集,(La/Yb)_(N)为3.11~23.86,Eu异常不明显.在微量元素原始地幔标准化图中,富集大离子亲石元素Rb、Ba、Th、U、K以及中等不相容元素Ce、La、Hf,亏损高场强元素Nb、Ta、P、Ti,具岛弧花岗岩特征,显示来自地壳的岩浆与来自深部的亏损地幔岩浆混溶.结合其他地质、地球化学证据,认为哈日阿玛花岗岩形成于岛弧环境,为红柳河-牛圈子-洗肠井古洋盆向北俯冲的结果,并最晚在晚奥陶世晚期已经开始俯冲,该洋盆闭合时限应晚于早志留世.

The Hariama island arc granitoids are outcropped in Gongpoquan island arc belt of Beishan orogenic belt,including diorite,quartz diorite,plagiogranite,tonalite and so on,associated with the arc volcanic rocks of Ordovician-Silurian Gongpoquan Formation.The LA-ICP-MS zircon U-Pb dating results indicate that the diorite(430.9±4.7 Ma)and tonalite(452.5±3.2 Ma)were formed in the Late Ordovician-Early Silurian.The petrogeochemical analysis results show that the granitoids belong to calc-alkaline,quasialuminous-weakly peraluminous Ⅰ-type,with the Mg^(#)value of 44.58-51.85,Rb/Sr ratio of 0.025-0.282,Nb/Ta ratio of 0.068-14.01 and Zr/Hf ratio of 13.05-34.00.TheΣREE+Y is 59.87×10^(-6)-118.56×10^(-6).The right-dipping chondrite-normalized REE distribution curves reflect the LREE is enriched,with(La/Yb)_(N) of 3.11-23.86 and unobvious Eu anomaly.The primitive mantle-normalized trace element spidergram reveals the enrichment of LILEs(Rb,Ba,Th,U and K)and medium incompatible elements(Ce,La and Hf),and depletion of HFSEs(Nb,Ta,P and Ti),with the characteristics of island arc granite,indicating the crust magma is mixed with magma from deep depleted mantle.Combined with other geological and geochemical evidences,it is considered that Hariama granitoids were formed in island arc setting,which is the result of northward subduction of Hongliuhe-Niujuanzi-Xichangjing paleo-ocean basin.The subduction began in the late Late Ordovician at the latest,and the ocean basin should be closed later than the Early Silurian.