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

摘要

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, A12O3 and K2O > Na2O, and its A/CNK average was 0. 96. The diorite was enriched in K2O, NaO, Al2O3, MgO, but K2O < Na2O, 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/l77Hf 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/177 Hf 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 collisioa 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 255 Ma.%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A12O3,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同住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.