西藏罗布莎蛇绿岩的Os-Ir-Ru合金及其中玻安岩质包体的研究  

A Study on Os-Ir-Ru Alloy and Boninitic Inclusion in the Luobusa Ophiolite,Tibet

作　　者：白文吉[1] 杨经绥[1] 方青松[1] 任玉峰[1] 颜秉刚[1] 戎合[1] 

机构地区：[1]中国地质科学院地质研究所

出　　处：《地质学报》2005年第6期814-822,T0005,共10页Acta Geologica Sinica

基　　金："973"项目(编号2003CB716500);中国地质调查局项目(编号2001CCB00900);自然科学基金项目(编号40399140)资助成果。

摘　　要：笔者最近在西藏罗布莎蛇绿岩块豆荚状铬铁矿的Os-Ir-Ru合金中,发现自形晶的玻安岩质的包体群。按它们的主要化学成分,其相当于高Ca玻安岩。18个包体的平均化学成分为:MgO=20.15%,Al2O3=8.99%,SiO2=54.42%,CaO=11.15%,FeO=2.81%,Na2O=1.00%,并含有少量NiO和Cr2O3。由于该包体粒径较小(<20μm),用Raman激光光谱仪测定,它们具有尖晶石、磁铁矿和辉石等Raman谱,由此确定存在玻安岩质尖晶石。结合已发现的共生矿物,如硅金红石、八面体Mg-Fe硅酸盐等高压矿物,有理由推断玻安岩质尖晶石形成于400～670km深的过渡带。玻安岩质辉石是尖晶石降压相变产物。根据罗布莎铬铁矿高Cr特点,罗布莎铬铁矿的形成与该玻安岩质熔体有关。玻安岩质尖晶石、铬铁矿和Os-Ir-Ru合金三者统一形成于过渡带或地幔深部,由地幔柱快速上升搬运这些矿物到地幔浅部。An euhedral crystal inclusion of silicate in Os-Ir-Ru alloy from podiform chromitite occurs at the Luobusa ophiolite, Tibet. These inclusions have boninitic chemical compositions with average contents of MgO （20. 15%）, Al2O3（8. 99%）, SlO2（54.42%）, CaO （11.15%）, FeO （2.81%）, Na2O （1.00%） and minor NiO and Cr203. The size of the inclusion is about 5-20 gm. Raman spectrum analysis shows Raman spectra of spinel, franklinite and pyroxene. Raman spectra indicate that the boninitic inclusions have similar chemical composition to that of boninite, but have different crystal structures, and formed under different p-t conditions. The spinel structures including franklinite structure indicate that a part of boninitic silicate inclusions come from the mantle-crust transition zone or the mantle of the Earth. Radial microcracks around boninitic inclusions within the host Os-Ir-Ru alloy suggest that the silicate inclusions swelled when pressure decreased as a result of the uplifting of mantle plume from the transition zone. The spinel structure in the transition zone transformed into pyroxene structure in the upper mantle. The boninitic magma may be formed in the transition zone. A genetic relation clearly exists between chromitites and boninitic silicate in the Luobusa ophiolite, and chromitite, boninitic inclusion and spinel structure altogether came from or formed in the transition zone.

关 键 词：Os—Ir—Ru合金 玻安岩 包裹体 蛇绿岩 西藏 

分 类 号：P597[天文地球—地球化学]