新疆西天山备战铁矿流纹岩的形成时代及其地质意义

The formation epoch of the host wall rock of the Beizhan iron deposit in West Tianshan Mountains of Xinjiang and its geological significance

大哈拉军山组火山岩在西天山分布范围较大,不同区域沉积建造具有不同的岩石组合特征和相对较长的时间跨度(约40Ma)。从西天山西段巩留—特克斯地区的晚泥盆世(>363Ma)、中部新源城南沿那拉提北坡地区的早石炭世(约354Ma),一直到东段拉尔敦达坂一带的晚石炭世(约313Ma),整体表现出从西到东逐渐变年轻的趋势。近年在西天山地区发现有较多的金、铁、铜矿床,赋矿围岩均为大哈拉军山组火山岩,总体呈北部富金,中部富铁铜的特征。备战铁矿是西天山中部阿吾拉勒矿集区一个重要的大型铁矿床,铁矿赋存在石炭纪大哈拉军山组火山岩中。对备战铁矿赋矿的英安岩及侵入于英安岩中的花岗岩进行单颗粒锆石LA-ICPMS U-Pb同位素测年。英安岩获得2组同位素年龄数据,分别为329.1Ma±1.0Ma和296.7Ma±2.0Ma。花岗岩的定年结果为307Ma±1.2Ma。测试结果表明,备战铁矿区英安岩的形成时代为329.1Ma±1.0Ma,属于早石炭世中期的岩浆活动事件;花岗岩体的形成时代是307Ma±1.2Ma,代表了晚石炭世的岩浆侵入事件;英安岩中296.7Ma±2.0Ma(晚石炭世—早二叠世)的年龄代表英安岩中的锆石封闭系统受到后期花岗岩侵入热事件的影响,发生改变重新形成的年龄。

Volcanic rocks of Dahalajunshan Formation in West Tianshan Mountains are distributed extensively, and different regions have different sedimentary associations characterized by relatively long time span （about 40 Ma）. From late Devonian （〉 363 Ma） of Gongliu-Tekesi regions in western Tianshan through early Carboniferous （- 354 Ma） of central part （south of Xinyuan, along the north slope of Nalati） to late Carboniferous （- 313 Ma） of Laerdundaban area, the strata tend to become younger overall from west to east. In recent years, lots of gold, iron and copper deposits have been found in West Tianshan area, and the ore-hosting rocks are Dahalajunshan Formation volcanic rocks. On the whole, the northern part is rich in gold while the central part is rich in iron and copper. The Beizhan ore deposit is a large and important iron ore deposit in the Awulale ore concentration area, whose iron occurs in Car- boniferous Dahalajunshan volcanic rocks. The authors conducted LA-ICP-MS zircon U-Pb isotopic dating of the ore-bearing rock dacite and the granite that intruded into dacite, and the result shows that the dacite has two sets of data： 329.1Ma±1.0Ma and 296.7Ma±2.0Ma, whereas the age of granite is 307Ma＋l.2Ma, suaggesting an Early Carboniferous magmatic activity event. It is thus held that the age of dacite in the Beizhan iron deposit is 329.1Ma±1.0Ma, belonging to the Early Carboniferous magmatic activity event, that the granite pluton age is 307Ma± 1.2Ma, suggesting the late Carboniferous magmatic intrusion, and that the other dacite age of 296.7Ma ±2.0Ma （Late Carboniferous - Early Permian） stands for the intrusion of the granite. This plutonic intrusion event changed the zircon closed system and also changed the age of zircon.