西秦岭双朋西矽卡岩型金铜矿床花岗闪长岩LA-ICP-MS锆石U-Pb定年、岩石成因和构造意义

摘要

The Shuangpengxi skarn gold-copper deposit is located at the Tongren-Zeku retroarc foreland basin of the West Qinling orogenic belt.The regional strata are mainly composed by the Lower Permian Ganjia Formation and the Upper Triassic Longwuhe Group.The intrusive rocks in the mining area are mainly granodiorite,and the orebodies are located in skarns occurring in the contact zone between granodiorite and marbles/dolomites of the Ganjia Formation.Zircons from the granodiorite samples yield a U-Pb age of 233.9 ±2.8Ma.Whole rock geochemistry analysises show that the granodiorite characterized by high SiO2 and K2O contents,low A12 O3 and MgO contents,with Mg# values of 33.6 to 35.2.The samples are enriched in LREEs ((La/Sm) N =3.14 ～ 11.6) and LILEs,and have relatively flat HREE pattern ((Gd/Yb) N =1.17 ～ 1.63).The granodiorite has Isr values ranging from 0.708299 to 0.708748,εNd (t) from-12.6 to-9.47,206 Pb/204 Pb from 18.623 to 18.965,207 pb/204 Pb from 15.610 to 15.652 and 208 pb/204 Pb from 39.124 to 40.732.Ratios of Nb/Ta (5.5 ～ 12.9) and Ce/Pb (3.0 ～ 11.6) are similar to magmas derived from the melting of crustal material.The fabric of marie microgranular enclaves in the granodiorite show characteristics of magmas mixing.The Shuangpengxi granodiorite was derived by partial melting of high-K basaltic rocks of the lower erust,with crust-mantle magma mixing.The tectonic setting of Shuangpengxi granodiorite indicates that the West Qinling orogenic belt was at the transition stage from oceanic crust subduction to continental collision during the middle Triassic.Exhumation and break-off of the subduction slab caused asthenosphere upwelling,which provided heat and triggered partial melting of the overlying lower crust beneath West Qinling,and generated primitive felsic magmas which formed many granitoid rocks.%青海双朋西矽卡岩型金-铜矿床位于西秦岭造山带同仁-泽库弧后前陆盆地,区域出露地层主要为下二叠统甘家组、下三叠统隆务河群的沉积岩系.矿区侵入岩主要为花岗闪长岩,金矿体赋存于花岗闪长岩体与甘家组大理岩、白云岩接触带的矽卡岩中.本文对双朋西花岗闪长岩体开展了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究,结果表明,双朋西花岗闪长岩的结晶年龄为233.9±2.8Ma,具有高SiO2、高K2O,低A12O3和MgO含量的特征,Mg#值为33.6～35.2,富集轻稀土元素((La/Sm)N =3.14～ 11.6)与大离子亲石元素,重稀土元素分布平坦((Gd/Yb)N=1.17～ 1.63),具有中等的Eu负异常.具有较高的Sr初始值(Isr=0.708299～ 0.708748)和相对较低的εNd(t)值(-12.6 ～-9.47),206 pb/204 Pb、207 pb/204 Pb和208 Pb/204 Pb比值分别为18.623～18.965、15.610 ～15.652和39.124～40.732.Nb/Ta(5.5～12.9)、Ce/Pb比值(3.0～11.6)均与地壳组份一致,岩体中的暗色微粒包体具有岩浆混合的组构特征.因此,双朋西花岗闪长岩主要起源于下地壳高钾性玄武岩的部分熔融,并发生壳幔岩浆混合作用.对双朋西花岗闪长岩体及区内一系列侵入岩构造环境研究表明,西秦岭构造带在中三叠世处于由洋壳俯冲向陆陆碰撞转换阶段,俯冲板片的折返和断离引发地幔软流圈上涌,加热上覆下地壳发生部分熔融,产生长英质岩浆,形成大量中酸性岩体.