距今约276 Ma的皮羌层状岩体位于塔里木板块西缘,是二叠纪塔里木大火成岩省的组成部分.岩体主要由辉长岩组成并赋存大型钒钛磁铁矿矿床.矿物结构和磁铁矿、钛铁矿和单斜辉石成分变化特征表明,矿物结晶顺序为单斜辉石十斜长石→磁铁矿十钛铁矿.磁铁矿中V2O3含量变化于0.49%~0.97%,说明岩浆演化过程氧逸度较低(<FMQ+0.5),并抑制了磁铁矿的较早结晶.在低氧逸度的条件下,大量硅酸盐矿物的结晶导致残余岩浆中Fe-Ti含量增加,造成不混熔的富Fe-Ti熔体从岩浆中分离,最后堆积形成块状磁铁矿矿体.磁铁矿-钛铁矿矿物对平衡温度为490℃~670℃,远低于岩浆磁铁矿的结晶温度,表明岩体的形成是一个缓慢冷却的过程.%The ~276 Ma Piqiang layered intrusion,located at the western margin of the Tarim Block,is a part of the Permian Tarim large igneous province.The intrusion mainly consists of gabbro and hosts a giant Fe-TiV oxide deposit.The inferred crystallization sequence based on textures and mineral compositions is clinopyroxene+plagioclase → magnetite+ilmenite.Magnetite contains 0.49%-0.97% V2O3,consistent with a relative low oxygen fugacity (<FMQ+0.5).The low oxygen fugacity is the main factor that restrained the early crystallization of magnetite from magma.Crystallization of large amounts of silicate minerals under such a low oxygen fugacity would increase Fe and Ti in the residual magma,from which an immiscible Fe-Ti-rich melt could be separated.Large amounts of Fe-Ti oxides formed from the immiscible Fe-Ti-rich melt made up the massive ore body of the intrusion.Calculated equilibrium temperatures based on coexisting magnetiteilmenite pairs range from 490 to 670 ℃,indicating a slow cooling process during magma solidification.
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