西藏恰功铁矿二长花岗斑岩锆石的U-Pb年代学与地球化学特征及意义

恰功矽卡岩型Fe(Cu)-PbZn(Ag)矿床的形成与二长花岗斑岩关系密切。该矿床与查藏措、斯弄多、加多捕勒、青都、那扎等矿床已初步在冈底斯中段勾勒出1条矽卡岩成矿带,但目前对这些矿床的研究还较少。本文对发育于西藏恰功矿区南部的2种斑岩的锆石采用CL和LA-ICP-MS进行了成因矿物学和微区微量元素及U-Pb年代学研究,获得石英斑岩的侵位时代为66.83±0.72Ma (MSWD=2.4,n=9),与成矿关系最为密切的二长花岗斑岩侵位时代为67.42±0.80Ma (MSWD=3.8,n=15)。2种斑岩锆石的U/Yb-Hf及U/Yb-Y在微量元素图解中均显示结晶于陆壳环境。二长花岗斑岩中锆石的矿物结构和Zr/Hf、Th/U、Nb/Ta、Nd/Yb值等微量元素特征和根据Ti含量估算的T_Tiz显示,其是源于上地幔基性岩浆脉动上涌过程中同化、混合地壳物质后形成的岩浆熔体,伴随围岩压力降低在上侵过程中结晶、分异,最终于近地表冷却形成的。这些结果暗示恰功矿床的形成与印亚陆陆初始碰撞时回卷的新特提斯洋壳撕裂诱发的壳幔混源岩浆活动有关,代表了一种尚未充分认识的陆陆初始碰撞阶段壳幔混源岩浆活动有关的成矿作用。在冈底斯中段中北部针对该时代矿床的勘查工作有一定的找矿潜力。

U-Pb age and geochemical characteristics of zircon in monzogranite porphyry from Qiagong deposit, Tibet, and geological implication

Qiagong Fe (Cu)-PbZn (Ag) skarn is genetically related to a monzogranite porphyry, and it together with Chazangcuo, Sinongduo, Jiaduobule, Qingdu and Nazha, all of which are also rarely studied, comprises a skarn-belt in the middle Gangdese metallogentic belt. In this paper, minerageny, micro-situ trace element and U-Pb age of zircons from two types of porphyry in southern part of Qiagong skarn deposit are studied using cathodeluminescence and LA-ICP-MS. The age of quartz porphyry is 66.83±0.72Ma (MSWD=2.4, n=9), and monzogranite porphyry associated with mineralization is 67.42±0.80Ma (MSWD=3.8, n=15). In U/Yb verse Hf and U/Yb verse Y discriminant diagrams, all the samples plot in continental zircon zone. Mineral structure, Zr/Hf, Th/U, Nb/Ta, Nd/Yb rate and T_Tiz estimated by Ti concertration of zircon from monzogranite porphyry show that the magma was sourced from upper mantle upwelling basite mixing or assimilating crustal components. It emplaced with crystallization differentiation when wall rock decompressed. These results imply that Qiagong deposit formed at the onset of collision of India to Asia continent, and is related to magma derived from both mantle and continental crust. The mantle source of the monzogranite porphyry was via upwelling through the tearing window of the subducted Neo-Tethys slab. The tearing window was caused by the different subducted angle induced by the roll-back of slab. The formation of Qiagong deposit represents a type of metallogenesis still needs further understanding in collision zone and additional ore potential in the northern part of middle-Gangdese.