吉林二密铜矿区含矿次火山岩年代学及其形成环境探讨

吉林通化二密铜矿区次火山岩为其成矿的主要围岩。该次火山岩成岩时代未定,对其成岩及成矿机理等问题仍有很大的争论。本文即通过利用LA-MC-ICP-MS锆石U-Pb分析测定了该次火山岩的形成时代:二长闪长玢岩(JEM02)、石英二长闪长岩(JEM03-1)以及石英二长岩(JEM03-2)锆石的206Pb/238U加权平均年龄分别为(95.71±0.33)Ma(MSWD=1.6)、(97.18±0.46)Ma(MSWD=1.19)和(96.10±0.50)Ma(MSWD=2.0),与其相应的206Pb/238U-207Pb/235U谐和年龄一致。松顶山次火山岩侵位的先后顺序为:石英二长闪长岩、石英二长岩、二长闪长玢岩。通过主、微量元素分析,本文认为该次火山岩为碰撞后到晚造山或非造山环境下形成的花岗岩-A型花岗岩的系列。该次火山岩形成的过程为:在中国东部岩石圈减薄的过程中,地幔物质上涌与地壳发生强烈的相互作用形成富碱的钙碱性岩浆。该岩浆在岩浆房中进一步分异形成二长-闪长岩浆和花岗斑岩岩浆。两种岩浆先后沿火山喷发通道浅成侵入形成次火山岩。

A Discussion on the Chronology and Formation Environment of the Ore-bearing Sub-volcanic Rocks in the Ermi Copper Deposit, Jilin Province

The sub-volcanic rocks in the Ermi copper deposit constitute the host rocks of the copper orebodies. As their ages remain unclear, many basic geological problems, such as the mechanism of their intrusion and the mineralization of the deposit, remain controversial. In this paper, the technique of LA-MC-ICP-MS was applied to determining the age of the sub-volcanic rocks. The results show that the weighted 206Pb/238U ages of the zircons from monzodiorite porphyrite (JEM02), monzodiorite (JEM03-1) and quartz monzonite (JEM03-2), are (95.71±0.33) Ma (MSWD=1.6), (97.18±0.46) Ma (MSWD=1.19), and (96.10±0.50) Ma (MSWD=2.0), respectively, which are consistent with their corresponding concordant ages, and therefore the intrusion sequence of the sub-volcanics should be quartz monzodiorite, quartz monzonite, and monzodiorite porphyrite. According to the investigation of the composition of the sub-volcanics, the sub-volcanics should be island-arc granitoid-A type granitoid formed in the environment from the post-collision to late-orogenic or anorogenic stage. Combined with the ages of these sub-volcanics, the formation process of the sub-volcanics are considered to be as follows: the lithospheric thinning of eastern China triggered the interaction between the enriched mantle and the crust, and formed the calc-alkaline magma with alkali enrichment. The calc-alkaline magma evolved into two types of magma (monzodiorite-quartz monzonite magma and monzodiorite porphyrite-granite porphyry magma) in the magma chamber in the course of the magma rising, and formed the two kinds of sub-volcanics.