祁连山玉石沟橄榄岩岩浆作用的记录和铬铁矿的成因

祁连山玉石沟蛇绿岩型铬铁矿是典型的高铬型铬铁矿床.通过对玉石沟铬铁矿和橄榄岩围岩进行原位定距离采样研究表明:全岩高的MgO和低的SiO2含量指示这些蛇纹石化橄榄岩不是简单的部分熔融残余,而是经历了后期的熔体再富集作用.玉石沟近矿橄榄岩尖晶石(Cr#>65)和铬铁矿尖晶石的矿物包裹体化学特征反映矿体和围岩形成于俯冲带上覆岩石圈地幔.距离矿体由近及远,赋矿围岩尖晶石化学成分(Cr#=43.9~77.2,TiO2=0.06%~0.34%)无规律变化.这说明高铬型铬铁矿体成矿所需铬并非来源于围岩,而是与玻安质熔体渗滤有关.尖晶石含水矿物包裹体仅赋存在玉石沟铬铁矿尖晶石与硅酸盐矿物接触部位附近可能揭示了玉石沟铬铁矿体的成因:成矿铬尖晶石在上侵的含水熔体与周围方辉橄榄岩的反应过程中形成聚集.

The Magmatic Record in the Peridotites from Yushigou,Qilian Orogen and the Petrogenesis of the Ophiolite-Type Chromitites

The origin of ophiolite-type chromites remain poorly understood despite the great effort over the years. We have sampled podiform chromites and its host peridotites at certain intervals in Yushigou ophiolite, which is well-known for its high-Cr chromites in the Early Paleozoic Qilian suture zone. The very high MgO and low SiO2 content of these serpentinized peridotites reflect that they are too depleted to be residues of partial melting. Both the chromites and peridotites may have undergone multi-processes of melt refertilization. Compared with global abyssal peridotites, their spinels have very high Cr# (>65). Furthermore, the uniform chemical characteristics of chromite-hosted silicate mineral inclusions suggest that the orebody may have formed in the suprasubduction zones setting. With irregular changes of spatial distribution in Cr# from 77.2 to 43.9 and TiO2 from 0.34% to 0.06%, the spinels in host peridotites show that formation of the high-Cr chromites have a boninitic melt affinity instead of originating from the host peridotites. The hydrous mineral inclusions only occur near the outer edge of chromian spinels and silicate minerals may provide evidence for our hypothesis: chromite ore formation results from interaction of hydrous melt with harzburgitic ambience during ascent.