江达-维西火山岩浆弧中段德钦岩体年代学、地球化学及岩石成因

Chronology,geochemistry and petrogenesis of Deqin granodiorite body in the middle section of Jiangda-Weixi arc

本文对江达-维西火山岩浆弧德钦岩体的寄主岩石——花岗闪长岩及其镁铁质微粒包体(MME)——闪长岩进行了详细研究。二者LA-ICPMS锆石U-Pb年龄分别为254.6±1.8Ma和253.5±1.6Ma,二者形成时代一致。地球化学研究结果表明,花岗闪长岩富K2O和Na2O,且K2O>Na2O,富Al2O3,A/CNK平均为0.96;闪长岩富K2O和Na2O,但K2O<Na2O,富Al2O3和MgO,A/CNK平均为0.72;花岗闪长岩的稀土总量低于闪长岩,二者轻重稀土分馏明显,配分曲线为右倾的轻稀土富集型;二者均富集大离子亲石元素而亏损高场强元素Nb、Ta等;二者均具有相对较高的Mg#(58.8～65.8),并具有相对较高的相容元素Cr、Ni含量(花岗闪长岩平均值分别为115×10-6和31.6×10-6,闪长岩平均值分别为398×10-6和98.2×10-6;花岗闪长岩和闪长岩176Hf/177Hf的平均值分别为0.282383和0.282287,二者εHf(t)平均值分别为-8.3和-11.8,反映了二者属于I型花岗岩,可能为岩浆混合作用的产物。地球化学特征及Hf同位素组成一致显示岩浆来源于地壳的部分熔融,伴有不同比例的地幔物质加入,形成于弧陆碰撞-后碰撞的构造背景,暗示金沙江结合带在～255Ma已经进入了弧陆碰撞-后碰撞的地质时期。

This paper studied in detail the host rock of granodiorite body（granodiorite） and its mafic microgranular enclave（diorite）.The dating results of granodiorite and diorite were separately 254.6±1.8Ma and 253.5±1.6Ma.The geochemical research of granodiorite and diorite indicated the granodiorite was enriched in K2O,Na2O,Al2O3 and K2ONa2O,and its A/CNK average was 0.96.The diorite was enriched in K2O,Na2O,Al2O3,MgO,but K2ONa2O,and its A/CNK average was 0.72.The REE gross of granodiorite was lower than diorite,and the LREE and HREE fractionationg of granodiorite and diorite was distinct.The partition curve was right-deviation and LREE-enriched,and they were enriched in LILEs and depleted in HFS elements such as Nb,Ta etc.Their Mg#（58.8~65.8） values were comparatively high.Zircon laser ablation LA-ICP-MS U-Pb analyses indicated the granodiorite was emplaced at 254.6±1.8Ma with average 176Hf/177Hf values of 0.282383 and negative εHf（t） values of-8.28,and the diorite was emplaced at 253.5±1.6Ma with average 176Hf/177Hf values of 0.282287 and negative εHf（t） values of-11.79.The granodiorite（host rock） and diorite（MME） were I-type granites and originate from the crust magama,and formed in the geological background of spread after the arc-continent collision.According to the evolvement history of regional tectonic and the forming age of magmatic rocks,we considered that Jinshajiang junction zone had been jointing after arc-continent collision about 255Ma.