阿尔泰山南缘中泥盆统苦橄岩低钙橄榄石的成因探讨

A discussion on the low-Ca olivine phenocrysts in Middle Devonian picritic lavas on the southern margin of the Altay Mountains

阿尔泰山南缘中泥盆统北塔山组地层发育世界罕见的总厚度达百米的苦橄质熔岩。苦橄岩的斑晶主要为蛇纹石化的自形一半自形橄榄石。对橄榄石斑晶的电子探针分析结果表明，其成分相当均匀，其Fo含量为79-8％～81-8％，并且具有异常低的CaO含量（0～0．06％），明显比典型岛弧苦橄岩中橄榄石的Fo含量和CaO含量要低的多。根据前人对Ca在橄榄石-硅酸盐熔体中的行为实验成果推测，这种橄榄石可能是高硅、低铁、低碱的玄武质岩浆结晶形成的。

A total of nearly 100 m-thick picritic lava succession rarely seen in the world was recognized in the Middle Devonian Beitashan Formation on the southern margin of the Altay Mountains. Serpentinized euhedralsubhedral olivine phenocrystals are dominant in the picrites. Electron microproble analyses of the olivine phenocrysts reveal their homogeneous composition, with Fo contents ranging from 79.8 % to 81.8 % and CaO contents being extremely low （0--0.06 % ）, much lower than their values in typical arc picrites. Based on previous experiments on Ca partitioning between olivine and silicate melt, it can be inferred that such low-Ca and low-Mg olivine crystals might have been generated by crystallization of high-Si, low-Fe and low alkali basalts.