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

Abstract：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.