甘肃青山堡中牌细粒花岗岩成因与构造环境:锆石U-Pb年龄和全岩元素组成制约

Petrogenesis and Tectonic Implications of the Zhongpai Fine-grained Granite in Qingshanbao,Gansu:Constraints from Geochemistry and Zircon U-Pb Chronology

龙首山铀成矿带位于北祁连造山带与阿拉善地块的结合部位,中志留世—早泥盆世受控于祁连造山带的构造演化而处于后碰撞伸展环境,然而关于龙首山铀成矿带在后碰撞伸展环境的结束时限尚缺乏精确的年代学约束。为此本文以该成矿带青山堡中牌细粒花岗岩为研究对象,开展了岩相学、锆石U-Pb年代学和全岩主微量元素组成分析,据此探讨了岩石类型及成因、源区特征及构造环境。结果显示,中牌细粒花岗岩成岩年龄为(413.7±3.0)Ma,为早泥盆世岩浆活动的产物,形成于碰撞后伸展环境。岩石具有高Si、富碱及低CaO、MgO和P_(2)O_(5)的特征,A/CNK=1.01~1.04;富集轻稀土元素,重稀土元素相对亏损,具有明显的负Eu异常;相对富集Th、U、Nd、K,亏损Ba、Sr、P、Ti等元素。岩石的地球化学特征指示,中牌细粒花岗岩应属于弱过铝质高分异I型花岗岩。中牌细粒花岗岩形成于碰撞后的伸展环境,岩浆来源于下地壳,岩浆源区主要残留相为角闪石,在岩浆演化过程中经历了显著的结晶分异作用。结合前人研究成果,提出龙首山地区早古生代形成的岩浆活动按照时间可划分为晚奥陶世—早志留世(452~438 Ma)和中志留世—早泥盆世(430~412 Ma)两个阶段,其分别对应于构造演化的俯冲阶段和后碰撞阶段。

The Longshoushan uranium mineralization belt is located at the junction of North Qilian Orogenic Belt and Alxa Block.This mineralization belt is controlled by the tectonic activities of Qilian Orogenic Belt during Middle Silurian-Early Devonian and formed in a post-collision extensional environment.However,the ending of the Longshoushan uranium mineralization still lacks constraints from a precise geochronologic method.In this paper,we studied the whole-rock geochemistry,petrography,and zircon U-Pb chronology of the Zhongpai fine-grained granites in Qingshanbao,and discussed their rock types and genesis,and tectonic background.The results show that the diagenetic age of the Zhongpai fine-grained granites is(413.7±3.0)Ma,indicating it was formed from the Early Devonian magmatism in a post-collision extensional environment.The rocks are characterized by a high Si content,alkali,and low contents of CaO,MgO and P_(2)O_(5),and with a A/CNK ratio of~1.01-1.04.These rocks are enriched in light rare earth elements and Th,U,Nd,K,and depleted in heavy rare earth elements and Ba,Sr,P,Ti,and with a distinct negative Eu anomaly.These comprehensive geochemical data indicate that the Zhongpai fine-grained granites belong to the weakly peraluminous and highly differentiated I-type granite.The result also shows that the magma originated from the lower crust.The main residual mineral facies in the magma is amphibole,and the magma had undergone significant crystal fractionation and differentiation during its evolution.Combined with the previous studies,the magmatism occurred in Early Paleozoic in Longshoushan can be divided into two stages:the Late Ordovician-Early Silurian(~452-438 Ma)and Middle Silurian-Early Devonian(~430-412 Ma),which corresponds to the subduction stage and post-collision stage,respectively.