内蒙古1017高地银多金属矿区二长花岗岩锆石SHRIMP U-Pb年龄及其地球化学特征

ZIRCON SHRIMP U-Pb GEOCHRONOLOGY AND GEOCHEMICAL CHARACTERISTICS OF MONZONITIC GRANITE IN THE1017 GAODI Ag-POLYMETALLIC MINING FIELD , INNER MONGOLIA , CHINA

内蒙古1017高地银多金属矿是与花岗岩类有关的矿床,矿区发育了大量的晚古生代花岗岩,主要岩石类型为中细粒二长花岗岩、中粗粒二长花岗岩和似斑状二长花岗岩。SHRIMP锆石U-Pb同位素定年结果表明,二长花岗岩的成岩年龄296.8 Ma±4.1Ma,时代为晚石炭世晚期。岩石具有富硅,w(SiO2)为68.56%～78.79%,过铝质,w(Al2O3)为12.12%～15.452%之间,A/CNK比值为0.93～1.64,碱质含量较高,w(K2O+Na2O)为5.57%～9.26%,相对富钾,K2O/Na2O比值为0.93～2.64,属高钾钙碱性系列。富集Rb,Th,U等元素和轻稀土元素(LREE),相对亏损Sr,Ba,Ti等元素,稀土元素总量为16.31×10-6～186.69×10-6,中等Eu负异常(δEu=0.16～0.78),稀土元素配分曲线呈现出略微右倾型,轻稀土较陡,重稀土较缓,具有向A型花岗岩过渡的后碰撞高钾花岗岩特征;岩石具有相对较低的87Sr/86Sr初始比值(0.673 401～0.704 236)和正的εNd(t)值(0.4～4.6),反映其物质来源可能是主要来源于以幔源岩浆底侵为主的新生大陆地壳。基于上述分析研究和构造环境判别,结合区域对比,推测1017高地二长花岗岩应当为后碰撞早期伸展阶段的产物,与华北板块和西伯利亚板块后碰撞作用有关,这一认识为东乌珠穆沁旗地区晚古生代构造岩浆演化及古亚洲洋闭合的时限提供了新的约束。

The 1017 Gaodi Ag-polymetallic deposit is associated with late Paleozoic intrusive rocks consisting mainly of medium-fine,medium-coarse grained monzonitic granites and porphyraceous monzonitic granite.SHRIMP U-Pb geochronology of zircons indicates that monzonitic granites was formed in 296.8Ma±4.1Ma,namely in the late stage of Late Carboniferous epoch.Main chemical element analysis reveals that these rocks are relatively rich in w（SiO2）（68.56% ～ 78.79%）,w（Al2O3）varies from 12.12% to 15.452%,A/CNK ratio is in the range of 0.93～ 1.64;alkaline content w（K2O＋Na2O）ranges from 5.57%to 9.26%,K2O/Na2O ratio is in the range of 0.93～2.64.Therefore,they belong to high-potassium calc-alkaline igneous rock series.Rare element analysis indicates that these rocks are rich in Rb,Th,U and LREE,relatively poor in Sr,Ba,Ti,ΣREE varies from 16.31×10-6 to 186.69×10-6,medium Eu negative anomaly（δEu =0.16～0.78）.Primitive mantle-normalized spidergrams display slightly-right-tendency-type and the curves of LREE and HREE in the diagram is steep,demonstrating the characteristics of postcollisional high-potassium transition to A-type granite.With relatively lower initial ratio（87Sr/86Sr,0.673 401～0.704 236）and positive values ofεNd（t）（0.4～4.6）,the material of the rock may mainly derive from juvenile continental crust resulted from underplating of mantlesourced magma.Based on above analysis,study and tectonic setting differentiation,combined with regional correlation,it is speculated that monzonitic granite in 1017Gaodi is the product in the extensional regime in early stage of post-collision,ascribed to collision between Siberia block and Northern China block.This recognition provides new constraint about the time limit of late Paleozoic era magma-tectonic evolution in Dongwuzhumuqinqi area and the closure of paleo-Asia oceans.