新疆西天山阿拉套山东部孔吾萨依A型花岗岩成岩年代、地球化学特征及成因

Geochronology,geochemistry and petrogenesis of the Kongwusayi A-type granites in the eastern Alataw Mountain,West Tianshan,Xinjiang

孔吾萨依花岗岩体位于新疆西天山阿拉套山南坡东侧,岩体主要由碱长花岗岩、粗粒钾长花岗岩、细粒钾长花岗岩组成。碱长花岗岩的^(206)Pb/^(238)U加权平均年龄值为293.1±3.6Ma,细粒钾长花岗岩的年龄为293±3.7Ma。三类花岗岩具相似的地球化学特征,属准铝质—弱过铝质的高钾钙碱性系列花岗岩。从碱长花岗岩到粗粒钾长花岗岩再到细粒钾长花岗岩,其Eu负异常程度越来越高,元素Ba、Sr、P、Ti等的亏损及Rb、Th、U等的富集程度逐步增高,显示三者的岩浆之间具有结晶分异趋势。碱长花岗岩、粗粒钾长花岗岩及细粒钾长花岗岩均富Ga(10~4Ga/Al比值2.6～3.5)、高Fe~*值(=FeO^T/(FeO^T+MgO),0.88～0.97)和具较高的TiO_2/MgO比值(0.51～1.25),显示出典型A型花岗岩的特征。研究区A型花岗岩具有高的锆石饱和温度(853.86～931.56℃)和低的锆石Ce(Ⅳ)/Ce(Ⅲ)比值(1.9～77),暗示该A型花岗岩岩浆形成于高温和低氧逸度条件下。三类花岗岩具较高的Y含量(43.3×10^(-6)～106×10^(-6))、较高的ε_(Nd)(t)(+3.0～+5.2)和较低的(^(87)Sr/^(86)Sr)_i值(0.7027～0.7069),表明孔吾萨依A型花岗岩可能源于玄武质地壳的部分熔融。但与西天山石炭纪玄武岩相比,孔吾萨依花岗岩的ε_(Nd)(t)值较低,而(^(87)Sr/^(86)Sr)_i值较高,因此孔吾萨依花岗岩的岩浆源区应有古老地壳物质的加入。

The Kongwusayi granitic pluton is located in the south slope and east side of the Alataw Mountain,West Tianshan of Xinjiang.The pluton is composed mainly of alkali-feldspar granite,coarse grained Kfeldspar granite and fine grained K-feldspar granite.The weighted average U-Pb age of zircons in the alkali-feldspar granite and the fine grained K-feldspar granite are 293.1±3.6 Ma and 293±3.7 Ma,respectively.The three types of granites belong to aluminum-weakly peraluminous and high potassium calcium alkaline series.Eu negative anomalies increase quickly from the alkali feldspar granite to the coarse grained K-feldspar granite and the fine grained K-feldspar granite,and depletion degree of Ba,Sr,P,Ti and the enrichment degree of Rb,Th,U are gradually increasing,indicating a tendency of crystallization differentiation between the three magma types.These granites are enriched in Ga with high 104 Ga/Al(2.6~3.5),Fe*(0.88~0.97)and TiO2/MgO(0.51~1.25),suggesting that they are characteristic of A-type granites.A-type granites in the study area are characterzied by the high saturation temperature of zircons(853.86~931.56℃)and low Ce(Ⅳ)/Ce(Ⅲ)(1.9~77),implying that the A-type granites were formed in higher temperature and lower oxygen fugacity conditions.The Kongwusayi granites have higher Y content(43.3×10^-6~106×10^-6),higherεNd(t)(+3.0^+5.2)and lower(87Sr/86Sr)i(0.7027~0.7069),suggesting that the Kongwusayi A-type granites may formed by the partial melting of the basaltic crust.By comparing with the carboniferous basalt in West Tianshan,the Kongwusayi A-type granites display lowerεNd(t)and higher(87Sr/86Sr)i,indicating the magma source of the kongwusayi A-type granites may include the ancient crustal materials.