东天山香山镁铁-超镁铁岩中富CH_4流体包裹体的特征及其意义

Characteristics and significance of CH_4 -rich fluid inclusions from the mafic-ultramafic complex at the Xiangshan, eastern Tianshan Mountains, Xinjiang of China

香山岩体是东天山镁铁-超镁铁杂岩带的一个典型岩体,相带发育。各相带都含有韭闪石质普通角闪石巨晶,后者环绕橄榄石和拉长石生长,构成包橄结构和包含结构,表明母岩浆富水,本研究的样品采自于香山岩体中岩体。拉长石和橄榄石中的流体包裹体孤立分布,或者成群分布,大小不等,无明显方向性,或者成串分布,平行于寄主矿物的颗粒边界和生长晶面流体包裹体主要为原生,一部分为次生或假次生,是在以角闪石结晶为标志的岩浆流体大量出溶期间捕获的,因而代表了岩浆源区的流体运用显微激光拉曼光谱仪,在100-4000cm-1全波段范围内对拉长石和橄榄石中的50多个流体包裹体的气泡进行了分析。结果表明,流体包裹体的气体成分分为4类:a)富CH4气体;b)富H2O气体;c)H2O+CH4混合气体;d)多组分(C2H6+N2+CH4,C2H6+CH4+N2+C4H6)混合气体。因而,香山岩体所携带的流体是一种富CH4等还原性挥发份和富H2O的流体。由于上地幔处于相对氧化状态,上地幔的挥发份以CO2为主。因而,香山岩体的母岩浆和流体应该来自于地幔过渡带或者软流圈,这同时也表明新疆北部晚古生代的地幔过渡带或者软流圈可能是未被氧化的、含C-H系列流体的圈层。俯冲板片的再循环,为地幔深处的H2O提供了来源。富CH4还原性C-H流体和富H2O流体,为地幔深处的熔融发挥了重要作用,进而产生了新疆北部造山后广泛分布的镁铁-超镁铁岩浆以及铜镍硫化物矿床。

Post-collisional Cu-Ni-bearing mafic-ultramafic intrusive complexes densely occur in the eastern Tianshan Mountains, Xinjiang of China. Compositions of rock and, especially, fluid in their source region remain to be resolved. Xiangshan pluton is typical of the mafic-ultramafic complexes in the eastern Tianshan Mts. It comprises pendotite, gabbro-norite, diorite and gabbro-diabase. Megacrysts of poikilitic hornblende that is abundant in all the rock phases have grown around olivine and labrador, indicating that the parent magma is rich in water. Studied samples were collected from the fresh gabbro-norite in the central body of the Xiangshan pluton. Labrador- and olivine-hosted fluid inclusions occur in isolation or in group without obvious orientation. Fluid inclusion trails were observed occurring parallel to the grain boundary or growth plane of the host mineral. Fluid inclusions are primary or secondary and were trapped during growth of the host mineral or extensive exsolution of fluid from the magma as marked by the crystallization of hornblende. Hence, these fluid inclusions represent fluids derived from the source region. Available stable sulfur and lead isotopic data show meteoric affinity and normal common lead characteristics, respectively. Therefore, crustal contribution for rock and fluid via assimilation or infiltration can be excluded. Bubbles of more than 50 fluid inclusions were analyzed using micro-laser Raman spectrometer. It is shown by the analysis that CH4 and H2O are the dominant gas species, and hydrocarbons C2H2 , C2H6, and C4H6, and N2 are the subordinate gas species in the vapor phase of these fluid inclusions. Four types of vapor compositions are distinguished that include a) CH4-rich vapor,b) H2O-rich vapor, c) H2O + CH4 mixing vapor, and d) multi-species (C2H6 + N2 + CH4 , C2H6 + CH4 + N2 +C4H6) mixing vapor. Therefore, fluids that are brought by the Xiangshan pluton from the mantle source region are rich in H2O and reductive volatiles CH4 etc. Because CO2 fluid instead of CH4 fluid is abundant in the upper mantle whose redox state is near to fayalite-magnetite-quartz buffer, it is likely that the parent magma and inclusion fluids of the Xiangshan pluton derive from the transition zone of the mantle or asthenosphere underneath the Paleozoic orogens of the northern Xinjiang. Subducted slab brought water to the un-oxidized deep mantle during the subduction period before the end of the Early Carboniferous. Mixing of water with the reducing CH4-rich C-H fluids played an important role in the melting of the deep mantle, thereby resulting in the production of widespread post-collisional mafic-ultramafic complexes and the related Cu-Ni sulfide ore deposits.