塔里木大火成岩省皮羌层状岩体的矿物结晶顺序和钒钛磁铁矿矿石成因探讨

Crystallization sequence and origin of the Fe-Ti-V oxide ores from the Piqiang layered intrusion in the Tarim large igneous province,NW China

距今约276Ma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+斜长石→磁铁矿+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-Ti-V 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（＆lt;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 magnetite-ilmenite pairs range from 490 to 670 ℃,indicating a slow cooling process during magma solidification.