甘肃北山怪石山铜镍矿化基性-超基性杂岩体锆石SHRIMP U-Pb同位素定年及其意义

SHRIMP Zircon U-Pb Age and its Signification of Guaishishan Mafic-ultramafic Complex in Beishan Mountains, Gansu Province

近年来于甘肃北山地区新发现的怪石山铜镍矿化基性–超基性杂岩体,位于塔里木板块东北缘北山裂谷带中西段,大山头–岗流脑子–平头山南深大断裂北侧。岩体侵位于蓟县系平头山组薄层大理岩中,出露面积仅0.01 km^2,平面形态呈长轴北西走向且较规则的椭圆状,岩相分异明显,由伟晶状角闪辉长岩、细粒辉长岩、中粒辉长岩、橄榄辉石岩、辉石橄榄岩和二辉橄榄岩等岩相组成,矿化体主要赋存于二辉橄榄岩中。岩石学及地球化学特征表明,怪石山杂岩体各类岩石是拉斑玄武岩系列同源岩浆分异的产物,以高CaO、MgO、Fe Ot,且SiO_2、Al_2O_3变化大,低Ti O_2、m/f、ΣREE及不相容元素,具明显的Nb、Th负异常为特征。采用锆石离子探针微区原位(SHRIMP)法获得辉长岩中锆石U-Pb年龄为358.6±3.9 Ma,表明该杂岩体的侵位及矿化作用发生于晚泥盆世–早石炭世。这一年龄数据与四顶黑山和黑山铜镍矿化岩体成岩成矿年龄近于一致,表明晚泥盆世–早石炭世是甘肃北山地区一个重要的铜镍成矿作用时期,它与新疆北山和东天山地区二叠纪大规模铜镍矿化作用发生时期明显不同。

Cu-Ni mineralization was recently discovered in the Guaishishan mafic-ultramafic complex in the Beishan area, which is located in the northeastern margin of the Tarim plate. The Guaishishan mafic-ultramafic complex is situated in the west part of the Beishan rift zone, and north of Dashantou-Gangliunaozi-Pingtoushan south deep fault. It was emplaced in the Pingtoushan Formation thin-layered marble of the Jixianian System. The complex has an outcropping area of only 0.01 km2, consisting of the pegmatitic hornblende gabbro, gabbro, fine grained gabbro, medium grained gabbro, olivine pyroxenite, pyroxene peridotite and lherzolite. Cu-Ni mineralization mainly occurs in the lherzolite. Petrological and geochemical characteristics of the complex show that the rocks of the Guaishishan complex are of comagmatic. The rocks of the complex are characterized by high CaO, MgO, Fe Ot and variable SiO2 and Al2O3, low TiO2, m/f, REE and incompatible elements, remarkable negative Nb, Th anomalies. SHRIMP U-Pb dating of zircon from the ore-bearing gabbro yields an age of 358.6±3.9 Ma, which suggests that the intrusion of the ore-bearing complex and the mineralization both occurred in Late Devonian-Early Carboniferous. This age is coincident with those of the Sidingheishan and Heishan copper-nickel mineralized intrusions, which shows that the Late Devonian-Early Carboniferous is an important period of the Cu-Ni mineralization in the Beishan area of Gansu province. It is significantly different from the Permian large-scale Cu-Ni mineralization period in the northeastern margin of the Tarim plate and the eastern Tianshan area.