东天山觉罗塔格构造带火山岩的铅同位素组成及意义

Lead isotopic composition of volcanic rocks in the Qoltag tectonic belt,East Tianshan,and its significance

笔者报道了东天山觉罗塔格构造带中企鹅山群、雅满苏组火山岩和土屋铜矿的斜长花岗斑岩及铜矿石的铅同位素组成,进一步讨论了该成矿带的构造演化。企鹅山群火山岩的铅同位素位于地球演化线的右侧,与北半球铅同位素演化线相比,具有弱富集的特征。斜长花岗斑岩及铜矿石的铅同位素分布在企鹅山群范围之内,与之相似。雅满苏组比企鹅山群富放射成因铅,也具有弱富集的特征。在铅构造模式图解中,企鹅山群位于地幔演化线上,雅满苏组位于造山带演化线附近。雅满苏组的铅同位素具有线性排列特征,位于企鹅山群和中天山花岗岩之间,指示两端员混合的趋势。企鹅山群和雅满苏组火山岩都随着流体交代作用的加强,越来越贫放射成因铅,指示交代地幔楔的流体应主要来自蚀变洋壳,而没有泥质沉积物的参与。雅满苏组比企鹅山群富放射成因铅主要是由于其陆壳基底厚度较大,较强的陆壳混染造成的。

This paper reports the lead isotopic composition of volcanic rocks of the Qi＇eshan Group and Yamansu Formation in the Qoltag ore belt and plagiogranite porphyry and copper ore of the Tuwu copper deposit and further discusses the tectonic evolution of this ore belt. The lead isotopic composition of volcanic rocks of the Qi＇eshan Group plots to the right of the geochron line, and has more radiogenic lead than the northern hemisphere reference line （NHRL）, indicating the weak enrichment character. The lead composition of plagiogranite porphyry and copper ore of the Tuwu ore deposit lies in the field of the Qi＇eshan Group and has a similar character. The Yamansu Formation has more radiogenic lead than the Qi＇eshan Group and also shows the weak enrichment character. In the plumbotectonics diagram, the Qi＇eshan Group follows the mantle evolution curve, but the Yamansu Formation plots near the orogenic evolution line, indicating more continental crust involvement. The lead isotope composition of the Yamansu Formation samples shows the features of the linear array and falls between the Qi＇eshan Group and Central Tianshan granites, indicating the trend of mixing of the two end-members （Qi＇eshan Group and Central Tianshan granites）. With enhancing fluid metasomatism, the Qi＇eshan Group and Yamansu Formation become more and more depleted in radiogenic lead, suggesting that the metasomatic agent was mainly derived from altered oceanic crust, without the involvement of pelitic sediments. The presence of more radiogenic lead in the Yamansu Formation than in the Qi＇eshan Group is mainly because its continental crustal basement was thicker so that it was subjected to stronger contamination with continental crust.