中甸浪都铜矿区二长斑岩中锆石的微量元素组成、U-Pb年龄及地质意义

Trace Element Composition and U-Pb Chronology of Zircons in Monzonite Porphyry from the Langdu Copper Deposit in Zhongdian and Their Geological Significance

中甸浪都铜矿位于印支期义敦岛弧南段的中甸岛弧,浪都二长斑岩体出露于矿区中北部,呈岩株状侵入于曲嘎寺组砂岩、灰岩中。本文应用LA-ICP-MS分析了二长斑岩中锆石的微量元素组成和U-Pb同位素年龄。获得该二长斑岩成岩年龄为223±1 Ma,与近几年报道的中甸岛弧晚三叠世中酸性侵入岩的Rb-Sr、K-Ar、锆石U-Pb年龄基本一致,属同一构造期的产物。该二长斑岩中锆石的ΣREE值为1168.55×10-6～2125.76×10-6,较集中。结合岩石主量、微量元素组成,我们认为其岩浆为地幔和部分地壳混熔作用形成,是甘孜-理塘洋在晚三叠世向西俯冲消减过程中形成的岛弧型花岗质岩石。该二长斑岩中锆石的REE球粒陨石标准化配分模型具有强烈的Ce正异常和中度的Eu负异常,指示其成岩岩浆具有较高的氧逸度,结合浪都矿区铜矿化与该岩体密切的时空关系和成因联系,表明岩浆的较高氧逸度对该矿区矽卡岩-斑岩型铜矿化具有重要意义。

The Langdu copper deposit is located in the south part of the Indosinian Yidun island arc which is often referred to as the Zhongdian island arc. The Langdu monzonite porphyry occurs in the central and north part of the mine,intruded the Qugasi sandstone and limestone. LA-ICP-MS zircon U-Pb dating yields an age of 223±1 Ma,which is in consistent with the reported Rb-Sr,K-Ar and zircon U-Pb ages of the Late Triassic granitoids in the Zhongdian island arc. It showed they were generated under the same setting. For zircons from the monzonite porphyry,ΣREE values vary mainly in the range of 1168.55 × 10-6～2125. 76×10-6,relatively enriched in HREE.The major and trace element compositions of the monzonite porphyry suggest that the magma was possibly derived from a mixed source of mantle and crustal material due to the western subduction of the Ganzi-Litang ocean plate in Late Triassic. The strong positive Ce anomalies and medium negative Eu anomalies of zircons from the monzonite porphyry showed that the magma had high oxygen fugacity. The temporal-spatial-genetic relationship between the copper mineralization in the Langdu deposit and the monzonite porphyry indicates that high oxygen fugacity was critical for the porphyry-type mineralization.