新疆北山坡一基性-超基性岩体铬铁矿矿物学特征及其指示意义

坡一含铜镍基性-超基性岩体是新疆北山西段早二叠世幔源岩浆活动产物之一。本文主要对该岩体中铬铁矿及内部固体包裹物进行了电子探针分析。坡一岩体铬铁矿主要成分呈线性变化并具低Zn、Ti和Fe3+的特征。铬铁矿内部固体包裹物由橄榄石、辉石等无水单矿物相包裹物,角闪石、金云母等含水多矿物相包裹物以及硫化物三类组成,硅酸盐包裹物化学成分上表现出富镁高铬的特点,其中云母具有富碱高镁的特征。估算得到铬铁矿结晶温度在1340~1411℃之间,结晶压力在2.56~3.33 GPa之间,坡一岩体原始岩浆Al2O3含量约为14.13%,Ti O2含量约为1.57%。结合已有研究成果,认为坡一岩体原始岩浆早期属于低钛苦橄质岩浆,可能是软流圈地幔环境下经历了15.5%~18.9%程度的部分熔融产物。原始岩浆早期存在含水富挥发份流(熔)体相且达到了硫饱和,铬铁矿结晶可能是导致硫饱和的重要原因。

Mineralogical Characteristics of Chromite from the Poyi Mafic-ultramafic Intrusion in Beishan,Xinjiang, and its Geological Significance

The Poyi mafic-ultramafic intrusion is the product of the Early Permian mantle-derived magmatic activity in the western Beishan, Xinjiang. Electron microprobe analysis of the chromites and solid inclusions therein showed that the major components of chromite in the Poyi intrusion exhibit linear covariation, and characterized by low ZnO, TiO2 and Fe3+contents. The compositions of the chromite are stable in the core and variable at the edge that was possibly altered by volatile-rich melt. The solid inclusions within chromite grains contain anhydrous phase such as olivine and pyroxene, hydrous multi-phases of amphibole and biotite, as well as sulfides. The silicate inclusions have relatively high magnesium and chromium contents, and high alkali in amphibole and biotite. The crystallization temperature of chromite is estimated to be from 1340 ℃ to 1411 ℃ and the forming pressure is 2.56 GPa to 3.33 GPa. The Al2O3and TiO2contents of the parent magma are 14.13% and 1.57% respectively. Combined with the previous researches, we conclude that the Poyi intrusion was derived from a low-Ti picritic magma resulted from 15.5% to 18.9% partial melting of asthenospheric mantle. The parent magma was likely a volatile-rich melt in early stage and the sulfur saturation and sulfide segregation were possibly caused by the crystallization of spinel.