青海岗察岩体SHRIMP锆石U-Pb年龄及其地质意义

青海岗察岩体位于西秦岭造山带的西段,由闪长岩、花岗闪长岩、斑状花岗闪长岩体组成,各岩体虽然地质特征和岩性有所差异,但成因和形成的时间基本一致,是不同时期同一成因的产物,由早向晚由中性向酸性演化,属"I"型花岗岩系列。尽管前人对其中个别阶段岩体进行过年龄测定,但没有对多期岩体进行过系统的年代学精测。本文在运用阴极发光技术对岩体中锆石进行细致的内部结构分析的基础上,利用SHRIMP锆石U-Pb原位定年方法,系统测定了岩体中单颗粒锆石的206Pb/238U年龄,结果显示岩体经历了243.8~242Ma、238~234Ma、215Ma三个侵入阶段,是同源、不同期次岩浆脉动形成的产物。岗察岩体周边的铜、金、钨、钼、铁矿床成矿与岩体关系密切,成矿流体、成矿物质主要来源于岩浆,成矿作用与岩体的形成时间一致,形成统一的成岩成矿地质事件,与区域西秦岭-东昆仑三叠纪构造岩浆作用和成矿时代一致,构成西秦岭北缘斑岩-矽卡岩型铜-钼-金成矿系统的一部分。通过构造环境分析,得出岗察岩体产于后碰撞(晚碰撞)构造环境,由挤压向伸展转换的地球动力学背景,为西秦岭造山带与祁连造山带的碰撞拼合提供了依据。

SHRIMP zircon U-Pb dating of Gangcha intrusions in Qinghai and its geological significance

The Gangcha intrusions of the Qinghai Province are located in the western section of the West Qinling orogenic belt, including diorites, granodiorites and porphyritic granites rocks. Although the differents of the geological and lithological characteristics, these rocks have similar petrogenesis and formation, and are derived from the same source ain multi-stage. The Gangcha intrusions belong to I-type granite, evoluting from intermediate to felsic. Despite age dating has been carried out in several stages of rocks, the precise geochronological dating are still absent for the whole rocks. In this paper, analyzing the interior structures of zircons by the CL technique, we report new sensitive high-resolution ion microprobe (SHRIMP) single grain zircon ²⁰⁶Pb/²³⁸U isotopic ages of the Gangcha intrusions. The results show that the Gangcha intrusive rocks have undergone three intrusive stages, which are 243.8~242Ma, 238~234Ma and 215Ma respectively. Therefore, they are cognate and the products of the multi-stage of magma pulses. The copper, gold, tungsten, molybdenum and iron deposits along the margin of Gangcha intrusions have close relationships with the Gangcha intrusions. The ore-forming fluid and material originated from magma. Moreover, metallogenesis and the formation of Gangcha intrusions are contemporaneous in the same diagenetic-metallogenic geological event. Regionally, they are coeval with the Triassic tectonic-magmatic activity and mineralization in the West Qinling and East Kunlun, and made up a part of the porphyry-skarn copper-molybdenum-gold systems in the northern margin of the West Qinling. Tectonic discriminations indicate that the Gangcha intrusions are formed in the post-collisional setting when extension environment substitute compression conditions, which provide insight into the collision between the West Qinling orogenic belt and the Qilianshan orogenic belt.