西藏罗布莎蛇绿岩豆荚状铬铁矿石中的合金成分  

Chemical Compositions of Alloys from Podiform Chromitites in the Luobusa Ophiolite, Tibet

作　　者：白文吉[1] 杨经绥[1] 方青松[1] 颜秉刚[1] 张仲明[1] 

机构地区：[1]国土资源部大陆动力学重点实验室

出　　处：《地质学报》2004年第5期675-682,i007-i008,共10页Acta Geologica Sinica

基　　金：国家自然科学基金项目 (编号 49972 0 73 )资助成果

摘　　要：从西藏雅鲁藏布江蛇绿岩带的罗布莎豆荚状铬铁矿床中 ,揭示出包含 70～ 80种矿物的一个地幔矿物群 ,其中特别引人注意的是含有多种合金。本文报道了已发现的合金类型和它们的化学成分。这些合金矿物主要通过人工重砂选矿提取的 ,少数合金在矿石光片中可以见到。本文报道的部分合金系有 :Ni(Fe) - C- Cr系 ,W-Cr- Co系 ,Al- Fe- L a系 ,Fe- Si- Ti系 ,Ag- Sn- Si系 ,Ni- Ir- Fe系 ,Fe- Pd- Pt系 ,Fe- Ni- C系。这些碳化物、金属硅以及铁合金等表明它们形成于还原环境 ,然而主岩铬铁矿石则形成于氧化环境 ,认为罗布莎铬铁矿是从玻安质岩浆中结晶的。这样合金矿物可能是外来晶体 ;或者它们形成于地核被后来上升的地幔柱带到浅部 ,包在铬铁矿中 ;或者是滞留在地幔中的成核物质后来被铬铁矿捕获。A group of mantle minerals containing about 70～80 subtypes of minerals have been discovered from podiform chromitites in the Luobusa ophiolite, Tibet, China, in which the alloys attract the attention of geologists. This paper reports the types and compositions of the alloys. The alloys were mostly handpicked from heavy mineral separates of the chromitites, and a few of them can be found in thin sections of chromitites. PGE minerals associated with the alloys in the Luobusa chromitites have been reported by Bai et al. (2000). Some alloy systems are reported in this paper, including the Ni (Fe)-C-Cr, W-Cr-Co; Al-Fe-La; Fe-Si-Ti; Ag-Sn-Si; Ni-Ir-Fe; Fe-Pd-Pt and Fe-Ni-C systems. The alloys in the Luobusa chromitites clearly indicate that they were formed under reducing conditions, but we argue that the chromites may have been formed from boninitic melts and the alloys are xenoliths of crystals to be held up in the mantle during the core formation, or have been transported from the core to lower mantle by a plume, and then incorporated in the chromitites during its crystallization.

关 键 词：西藏罗布莎 金矿物 地幔 蛇绿岩 矿石 铬铁矿 氧化环境 合金 结晶 成核 

分 类 号：P618.31[天文地球—矿床学]