新疆东天山金滩金矿区安山岩岩石学、地球化学及地质意义

Petrology, geochemistry and geological significance of andesites of Jintan gold orefield in Eastern Tianshan, Xinjiang

新疆金滩金矿发育一套与成矿密切相关的安山岩,对该安山岩元素质量分数和相关性进行分析。研究结果表明:安山岩中锆石结构以及Th和U的质量分数之比ω(Th)/ω(U)(0.30~1.02)均显示其为岩浆成因,锆石LA-ICP-MS U-Pb年龄为(303.65±4.8)Ma,代表安山岩形成时代;安山岩ω(Si O_2)为54.29%~69.05%,ω(Al_2O_3)为11.27%~19.46%,全碱ω(K_2O+Na_2O)为2.46%~6.81%,ω(Mg O)为3.60%~10.45%,Mg指数为46~66。安山岩轻稀土元素(LREE)富集、轻重稀土元素分异作用明显,绝大多数样品具中等Eu负异常(δω(Eu)=0.44~0.77)和轻微Ce负异常(δω(Ce)=0.93~0.98),富集K,Rb,Ba,Th和U等大离子亲石元素,相对亏损Nb和Ti等高场强元素;岩浆经历了分离结晶作用,且发生部分熔融;安山岩可能是康古尔塔格洋向南俯冲,洋壳脱水进入地幔楔,被俯冲流体交代的地幔楔部分熔融,同时产生结晶分异作用而形成,而且遭受明显的陆壳物质混染;东天山地区在晚石炭世依旧存在康古尔洋的俯冲消减作用。

A set of andesites closely related to the Au mineralization was developed from Jintan Gold Deposit.Element concentration of the andesites and geochemical correlation analysis were made.According to the results,both the zircons structure of the andesites andω(Th)/ω(U)(0.30?1.02,ωis element mass fraction)indicate that the zircons are the origin of magma,and the zircon U-Pb age is(303.65±4.8)Ma,which stands for the formation age of andesites.Theω(SiO2)of andesites is from54.29%to69.05%,ω(Al2O3)is from11.27%to19.46%andω(K2O+Na2O)is from2.46%to6.81%,ω(MgO)is from3.60%to10.45%and Mg index changes from46to66.The light rare earth elements(LREE)of andesites is enriched,and the differentiation between LREE and HREE is obvious.Eu of the vast majority of the sample shows moderate negative anomaly(δω(Eu)=0.44?0.77),while Ce shows slight negative anomalies(δω(Ce)=0.93~0.98).It is enriched in large ion lithophile elements(such as K,Rb,Ba,Th and U),and depleted in high field strength elements(such as Nb and Ti).The magma undergones the fractional crystallization and partial melting.The andesites are probably formed by partial melting of mantle wedge,which is metasomatized by subducting fluids when the Kanggurtag ocean southward subducts,oceanic crust dehydrates and subducts into the mantle wedge.Meanwhile the magma undergones fractional crystallization,and suffers significant continental crust material contamination.Based on comprehensive analysis,subduction of the Kangguer ocean is still underway in the East Tianshan mountain region at the time of Late Carboniferous.