阿尔巴尼亚布尔其泽纯橄岩壳中橄榄石的包裹体研究

Inclusions in Olivine and Implications—Based on Mineral Research of Dunite of Bulqiza Ophiolite,Albania

阿尔巴尼亚布尔其泽纯橄岩壳非常新鲜,主要由橄榄石、尖晶石和单斜辉石等矿物组成.其中橄榄石存在单斜辉石和铬尖晶石(磁铁矿)共生包裹体现象,包裹体矿物粒度在1~10μm,有些甚至为纳米级200~500nm.纯橄岩橄榄石的Fo值为94.7~96.0,铬尖晶石的Cr#为76.5~82.4,远高于蛇绿岩地幔橄榄岩中常见纯橄岩的铬值(Cr#>60).基于前人研究结果,提出这种现象是由于亏损方辉橄榄岩与含钛、铬、铁熔体发生交代作用,从而形成橄榄石的固溶体并存在Ti 4+、Al 3+、Ca2+、Fe3+,而部分Cr3+进入铬尖晶石结晶.后期由于岩体在抬升过程中降温,橄榄石中混溶的组分析出包裹体形成磁铁矿和铬尖晶石.并且依据铬尖晶石-橄榄石的矿物化学成分,识别出岩体内方辉橄榄岩相对较低的部分熔融程度约为30%~40%,纯橄岩部分熔融程度约为40%,表明不同岩相间其形成背景存在明显差异.因此,认为布尔奇泽蛇绿岩具有多阶段的过程,首先是在洋中脊环境下经历部分熔融作用形成了方辉橄榄岩,后受到俯冲环境(SSZ)的岩石-熔体反应生成更富Mg、Si和Cr等的熔体,致使地幔橄榄岩高度部分熔融,形成此类纯橄岩.

Cinopyroxene and spinel（magnetite）occur as oriented intergrowths within olivine of the dunite in the Bulqiza ophiolite,Albania.The size of the inclusion minerals is 1-10μm,and some of them are nanoscale in 200-500 nm.The fresh dunite has a mineral assemblage of olivine,spinel and cinopyroxene.The Fo content of its olivine is 94.7-96.0,and the Cr#of spinel is about 76.5-82.4,higher than that in the spinel in common dunite from ophiolite mantle（Cr#60）.Thus it is proposed that previously depleted mantle harzburgite reacted with the melt containing Ti,Cr,Fe,and produced an olivine solid solution added with Ti 4＋,Al 3＋,Ca2＋,Fe3＋,and some of Cr3＋entered interstitial chromite.Due to the fast cooling rate of the rock or rapid tectonic emplacement,the exsolution textures in olivine and compositional zones of chromite are preserved.Based on the mineral compositions of chromian spinel-olivine,it is found that the relatively low partial melting degree of the harzburgite is30%-40%,and the degree of partial melting of dunite is about 40%,indicating a significant difference of tectonic setting.It is suggested that the Bulqiza ophiolite had multi-stage evolution processes.When oceanic crustal slabs were trapped in mid oceanridge,they were modified by tholeiitic magmas or partial melting,which occurred interaction or metasomatism,then later reaction with boninitic magma in suprasubduction zones（SSZ）generated more Mg,Si and Cr melt,resulting in high degree of partial melting for the mantle peridotite and dunite.