摘要:
西藏雅鲁藏布江缝合带西段普兰纯橄岩中橄榄石存在透辉石和磁铁矿共生出溶现象。这类纯橄岩非常新鲜,主要由橄榄石、尖晶石和透辉石等矿物组成。橄榄石的 Fo 值是90.1~90.7,铬尖晶石的 Cr#为19.8~20.8,远小于蛇绿岩地幔橄榄岩中常见纯橄岩中铬尖晶石的铬值(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、铝、钙熔体发生交代作用,从而形成橄榄石的固溶体存在 Ti4+、Al3+、Ca2+、Fe3+,而 Cr3+进入铬尖晶石结晶。后期由于岩体在抬升过程中降温,橄榄石中出溶形成单斜辉石和磁铁矿,而铬尖晶石结晶被保留。依据铬尖晶石-橄榄石的矿物化学成分,识别出低铬纯橄岩具有最低的部分熔融程度为~8%, Cr#最高的纯橄岩壳其部分熔融程度达到~50%,表明不同岩相间其形成背景存在明显差异。因此,笔者认为普兰蛇绿岩具有多阶段的过程。首先是在洋中脊环境下经历富Ca、Al和Na等熔体交代和部分熔融作用,后受到俯冲带玻安质熔体改造。
Abstract:
Diopside and magnetite exsolutions occur as oriented intergrowths within olivine of the lower Cr# dunite in the Purang ophiolite, Tibet. The fresh lower Cr# dunite has a mineral assemblage of olivine, spinel and diopside. The Fo content of its olivine is 90.1~90.7, whereas the Cr# of spinel is very lower (about 19.8~20.7), much less than Cr# of the spinel in common dunite from ophiolite mantle (Cr#>60). It is thus held that the formerly depleted mantle harzburgite reacted with the melt containing Ti, Al and Ca, and produced an olivine solid solution with the addition of Ti4+, Al3+, Ca2+, Fe3+, Cr3+, which 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 have been preserved. The mineral chemical composition of picotite-olivine indicates<8% partial melting in exsolutions dunite. However, the higher Cr# dunite without exsolutions has highly partial melting (>50%), suggesting reaction with boninitic melt. The authors consider that the Purang ophiolite experienced multi - stage processes. Firstly, UHP minerals were trapped in these magnesiochromite grains. When oceanic crustal slabs were trapped in Mid Ocean Ridge, they were modified by tholeiitic magmas or partial melting, which caused interaction or metasomatism and subsequent reaction with boninitic magma in suprasubduction zones (SSZ).
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