甘肃金川早古生代正长花岗岩锆石SHRIMP U-Pb年代学、岩石学、地球化学特征及其构造意义

Characteristics and tectonic significance of the early Paleozoic syenite granite from Jinchuan,Gansu Province

报道了甘肃金川正长花岗岩体的岩石学、锆石U-Pb年代学和全岩地球化学资料,以确定岩体的形成时代、岩石成因及其构造属性。正长花岗岩样品中岩浆锆石SHRIMP U-Pb加权平均年龄为(425.7±2.5)Ma(MSWD=1.07),属晚志留世,与金川地区出露的辉绿岩脉年龄相近。正长花岗岩低硅(SiO 266.71%~70.05%),富碱(Na 2 O+K 2 O 10.00%~10.7%),K 2 O/Na 2 O比值为1.88~2.41,高全铁(FeO T 1.79%~2.62%),低钛(TiO 20.34%~0.52%),A/CNK=0.82~1.05。岩石学及化学成分显示其属于准铝质碱性系列A1型花岗岩,具有较高的锆石饱和温度(T Zr=878~1015℃);稀土元素含量高,ΣREE含量为374.4×10^(-6)~529.4×10^(-6)(平均428.7×10^(-6)),稀土元素配分曲线呈现轻稀土元素相对富集、重稀土相对亏损的右倾分布特征,(La/Yb)N=28.07~41.28,具Eu负异常(δEu=0.53~0.66)。原始地幔标准化微量元素蛛网图显示Nb、Ta、Ti和P的亏损。岩石具有较高的Rb/Sr值(介于0.81~1.27之间,平均值为1)和Rb/Nb值(介于4.72~9.75之间,平均值为7.15),显示壳源岩浆的成分特征。结合区域地质背景及构造判别认为,金川正长花岗岩形成于上地壳长英质物质在高温条件下的部分熔融,同时代出露的辉绿岩脉与其组成的双峰式侵入岩形成于伸展的板内裂谷环境,与约426 Ma阿拉善地块和祁连柴达木地块导致的俯冲板片断裂、折返以及软流圈地幔的上涌有关,金川地区“I6隐伏矿体”形成以后所经历的构造活动及热液成矿作用的叠加可能与该双峰式岩浆活动有关。

In this paper,the petrology,zircon SHRIMP U-Pb age and whole rock geochemical data of the Jinchuan syenite granite in Gansu Province were examined in order to determine the formation age,petrogenesis and tectonic environment of the pluton.The SHRIMP U-Pb weighted average age of magmatic zircons from syenite granite samples is(425.7±2.5)Ma(MSWD=1.07),belonging to the early Paleozoic Silurian and similar to the age of the exposed dolerite dikes in the Jinchuan area.The syenite granite is low in silicon(SiO 2,66.71 to 70.05%),rich in alkalis((Na 2 O+K 2 O),10.00 to 10.70%;K 2 O/Na 2 O ratio,1.882.41),high in total iron(FeO T,1.79 to 2.62%),low in titanium(TiO 2,0.34 to 0.52%),and metaluminous(A/CNK ratio,0.821.05).Its petrology and chemical composition show that it belongs to the quasi-aluminous alkaline series A1 type granite,with a relatively high zircon saturation temperature(T Zr,8781015℃).It has high contents of rare earth elements(REE)(ΣREE,(374.4529.4)×10^(-6),averaging 428.7×10^(-6)).The REE distribution curve shows relative enrichment of light REE and depletion of heavy REE by the right-inclined distribution;(La/Yb)N ranges between 28.07 and 41.28,with negative Eu anomaly(δEu,0.530.66).The standardized primitive mantle spider diagram shows depletions of Nb,Ta,Ti and P.The rocks have relatively high Rb/Sr(0.811.27,averaging 1)and Rb/Nb(4.729.75,averaging 7.15)ratios,showing the compositional characteristics of crust-derived magma.Combined with regional tectonic evolution and tectonic discrimination,it is considered that the Jinchuan syenite granite is formed from partial melting of felsic material in the upper crust at high temperature,and the bimodal intrusive rocks,composed of dolerite dikes of the same age,are formed in an extensional intraplate rift environment and related to the subduction plate break-up and exhumation and asthenosphere mantle upwelling caused by the~426 Ma Alashan block and the Qilian-Qaidam block.The Jinchuan syenite granite is formed by partial melting of felsic material in the upper crust at high temperature,and the bimodal intrusive rocks is formed in the extensional intraplate rift environment.The structural activity and hydrothermal mineralization of the Jinchuan Cu,Pt-rich orebody may be related to this magmatic activity.