东昆仑莫河下拉银多金属矿床花岗斑岩年代学、地球化学特征及其构造背景

Geochronological Dating,Geochemical Characteristics and Tectonic Setting of the Granite-porphyry in the Mohexiala Silver Polymetallic Deposit,Eastern Kunlun Orogenic Belt

莫河下拉银多金属矿床是东昆仑成矿带近几年新发现的研究程度较低的斑岩型矿床。本文首次采用LA-ICP-MS技术对成矿花岗斑岩进行了锆石U-Pb定年分析,获得其成岩年龄为222±1 Ma,属印支晚期。莫河下拉花岗斑岩SiO2含量为66.62%~70.75%,富碱（（Na2O＋K2O）=7.69%~8.69%）, K2O/Na2O=1.38~2.38, A/CNK=0.77~0.90,属高钾钙碱性至钾玄岩系列的准铝质花岗岩。岩石明显富集Rb、Th、U等大离子亲石元素（LILE）,亏损Nb、Ti、P等高场强元素（HFSE）；稀土总量（ΣREE）为144×10-6~189×10-6,轻重稀土元素分馏明显（（La/Yb）N=9.15~12.1）,并表现出中等负 Eu 异常（δEu=0.43~0.64）。综合区域地质背景及年代学数据认为,莫河下拉花岗斑岩为上地壳变质杂砂岩部分熔融的产物,成岩成矿的动力学背景为印支晚期阿尼玛卿洋闭合以后的后碰撞伸展环境。

The Mohexiala silver polymetallic deposit is a new found porphyry deposit in the eastern Kunlun metallogenic belt, and it is far from well documented. In this paper, we first present zircon LA-ICP-MS U-Pb dating for the ore-bearing granite-porphyry, with a concordia age of 222±1 Ma, i.e., Late Indosinian. The granite-porphyry have SiO2 of 66.62%-70.75%, high alkali content （Na2O＋K2O） of 7.69%-8.69%, K2O/Na2O=1.38-2.38, and A/CNK=0.77-0.90. Chemically, they are metaluminous and belong to high-K calc-alkaline to shoshonite series. Moreover, they are enriched in large ion lithophile elements （LILE; e.g., Rb, Th, and U）, and depleted in high field strength elements （HFSE; e.g., Nb, Ti and P）. The total rare earth element （ΣREE） contents of the granite-porphyry are 144×10-6-189×10-6, and characterized by enrichment of light rare earth elements （LREE）, depletion of heavy rare earth elements （HREE;（La/Yb）N=9.15-12.1）, with moderate negative Eu anomalies （δEu=0.43-0.64）. Combined with the regional geological background and geochronological data, we consider that the granite-porphyry in the Mohexiala silver polymetallic deposit was derived from primary magmas generated by partial melting of the upper crustal metagreywacke, and the magmatism and mineralization took place in a post-collisional extensional environment following the closure of A’nyemaqen Ocean in the Late Indosinian.