西藏班公湖岛弧带含硫化镍超基性岩的源区性质与基底背景

Provenance nature and basement background of nickel-sulfide-bearing ultrabasic rocks in Bangong Lake island arc zone,Tibet

青藏高原班公湖-怒江成矿带上的超基性岩型硫化镍矿化是近年来在西藏地区发现的矿化新类型。文章根据成矿带西段班公湖地区含镍超基性岩体的岩石地球化学和Sr、Pb同位素分析结果,论述了含镍超基性岩浆的源区性质及生成条件,并根据锆石U-PbLA-ICP-MS年龄测定结果,探讨了藏西北地区的基底背景。研究发现,班公湖地区的含镍超基性岩体以富集大离子不相容元素Rb、Th、U、Sr、Pb,亏损Ba、K为特点,高场强元素亏损Nb、Ti,富集Ta;稀士元素相对球粒陨石亏损强烈,但轻稀土元素相对富集。这些特点一致反映出含矿岩浆产生于受俯冲沉积物熔体交代的富集型岩石圈地幔源区。岩浆的生成深度较浅,为尖晶石二辉橄榄岩相,源区部分熔融程度较低,大体在10%左右。岩石中集中出现一批24.79亿年的残留锆石年龄,标志着当时班公湖中特斯洋盆沉积物的物源区较为单一,主要来自太古代末—元古代初的古老基底,并且推测太古代与元古代之交(25亿年)有可能是藏西北的一个古陆壳快速生长期。

The nickel sulfide mineralization of ultrabasic rocks in Bangong Lake-Nujiang metallogenic belt of western Qinghai-Tibet Plateau constitute a new type of ore deposit discovered in Tibet in recent years. Based on litho-geochemistry and a Sr-Pb isotopic analysis of the nickel-bearing ultrabasic rocks in the Bangong Lake within the west segment of the metallogenic belt, this paper describes the characteristics and formation conditions of magma sources of these rocks and, according to the zircon U-Pb LA-ICP-MS dating data, deals with the basement background of northwest Tibet. The results indicate that the Ni-bearing ultrabasic rocks are enriched in large-ion incompatible elements Rb, Th, U, Sr, Pb and high strength field element Ta but depleted in Ba, K（LILE） and Nb, Ti （HFSE） in the primitive mantle-normalized trace element patterns, exhibiting a strong REE depletion but a slight LREE enrichment in the chondrite-normalized REE patterns. All these features indicate consistently that the ore-bearing magma was derived from an enriched lithospheric mantle source which had been metasomatized by the subducted sedimentary melt,and that the magma originated from a source with low extent of partial melting （about 10%） at a shallow depth of the spinel lherzolite facies. In addition, a group of residual zircon ages of 2.48 Ga in the rocks indicates a relative simple source of sediments in the mid-Tethys ocean of Bangong Lake area at that time, which probably came mainly from the Late Archean-Early Proterozoic basement. It is thus inferred that the changing period from Archean to Proterozoic （ ca. 2.5 Ga） might have been a rapid growing period of ancient continental crust in northwestern Tibet.