阿尔金中段大通沟花岗岩体年代学、地球化学特征及其成因

Geochronology, geochemistry and petrogenesis of the Datonggou granitic plutons in the middle section of Altun Mountains

出露于阿尔金中段的大通沟花岗岩体,岩性主要为二长花岗岩,LA-ICP-MS锆石U-Pb年龄为(353.7±1.1)Ma。SiO_2为74.5%~76.2%,Al_2O_3为12.62%~14.15%,(Na_2O+K_2O)为7.71%~8.29%,K_2O/Na_2O为0.9~1.22,A/CNK介于1.0~1.19之间,属高钾钙碱性系列的过铝质-强过铝质花岗岩。该岩石富集大离子亲石元素(LILE),亏损素Ba、Nb、Sr、P、Ti等高场强元素(HFSE);∑REE变化范围大(∑REE=85.7×10^(-6)~153.2×10^(-6),平均为111×10^(-6)),具有强烈的负Eu异常(δEu=0.03~0.59,平均0.33),LREE/HREE=3.52~11.9,轻稀土相对富集,轻重稀土分馏明显。微量元素地球化学特征表明,其源区物质主要来源于上地壳的变杂砂岩和变泥质岩类。锆石饱和温度计演算结果显示锆石的结晶温度为722~745℃(平均735℃),估算二长花岗岩源区的压力为0.8~1.6GPa。综合区域地质资料,阿尔金中段大通沟花岗岩体的形成与造山带根部块体的拆沉作用有关。

The Datonggou granitic plutons,outcropped in the middle part of Altun Mountains,are principally composed of monzonite granite.The LA-ICP-MS zircon U-Pb isotopic dating indicates that the Datonggou monzonite granite was emplaced at(353.7±1.1)Ma.They have high-K cal-alkaline and strong peraluminous features(SiO2=74.5%-76.2%,Al2O3=12.62%-14.15%,(Na2O+K2O)=7.71%-8.29%,K2O/Na2O=0.9-1.22,A/CNK=1.0-1.19).The Datonggou granitic plutons are rich in large ion lithophile elements(LILE)and depleted in high field strength elements(HFSE)with a clear negative Ba,Nb,Sr,P,Ti anomaly.REE distribution patterns show strong negative anomaly of Eu(δEu=0.03-0.59)and distinct enrichment of LREE relative to HREE.The geochemical characteristics of trace elements indicate that the rocks were formed by partial melting of variable hybrid sandstone and variable mudstone from the upper crust.The partially melting temperature calculated by whole rock Zr thermometer is 722-745°C and pressure is 0.8-1.6 GPa.Combined with the data from the Altun Mountains,it is held that the Datonggou granitic plutons in middle Altun Mountains might be related to the delamination of deep blocks under the Altun orogen.