摘要
距今约260 Ma的白马岩体位于上扬子板块西缘的攀西裂谷中,是一个大型的含钒钛磁铁矿镁铁质-超镁铁质杂岩体,是峨眉山大火成岩省的重要组成部分。含矿岩体主要由磁铁橄长岩和橄榄辉长岩组成,主要工业矿体赋存在下部的橄长岩岩相带中。显微镜下显示橄榄石和角闪石均存在2种不同的结构状态,岩浆具有多次脉动的侵位特点。矿物结构特点及磁铁矿、钛铁矿、橄榄石、角闪石及斜长石等矿物电子探针成分测定显示,矿物的结晶顺序大致为斜长石+橄榄石+辉石→角闪石+磁铁矿+钛铁矿→角闪石。根据角闪石和斜长石成分计算角闪石最低结晶温度为1090℃,斜长石的最高结晶温度是1120℃,推测磁铁矿的结晶温度介于1090~1120℃之间。橄榄石的Fo值由下部的磁铁橄长岩向上部的橄榄辉长岩呈逐渐降低的变化趋势,表明随着岩浆的结晶分异进程,系统的氧逸度是逐步变化的,暗示整个结晶分异过程系统处于封闭状态。磁铁矿中w(V2O3)变化于0.72%~1.37%之间,可近似看成是岩浆演化过程氧逸度较低的量化标志(<FMQ+0.5),这种低氧逸度条件下硅酸盐矿物的结晶,会导致粒间熔体氧逸度逐步升高且成分向着富Fe的方向演化。岩浆的这种成分演化特点,是晚期形成不混溶熔浆及富Fe-Ti矿浆的主要原因。
A about 260 Ma aged Baima layered intrusion,located in the Panxi rift within the western margin of the Yangtze plate,is an important part of the Emeishan large igneous province( ELIP). It is a large mafic-ultramafic complex hosting a giant Fe-Ti-V oxide deposit. The ore-bearing intrusion is mainly composed of magnetite troctolite and olivine grabbro. The major industrial orebodies are hosted in the troctolite phase in lower part of the complex. and.Microscopic study of the mineral structure reveals that olivine and amphibole both have two different structural states,indicating the emplacement characteristics of multiple pulses of magma. The mineral textural features and EPMA analytical results of magnetite,ilmenite,olivine,amphibole,and plagioclase show that the inferred crystallization sequence is plagioclase + olivine + clinopyroxene → amphibole + magnetite + ilmenite → amphibole. According to compositions of amphibole and plagioclase,the calculated lowest crystallization temperature of amphibole is 1090 ℃,and the calculated highest crystallization temperature of plagioclase is 1120 ℃. It is inferred that the crystallization temperature of magnetite is from 1090 ℃ to 1120 ℃. Electron microprobe data indicate that forsterite percentages of olivine( Fo) are gradually decreased from the olivine in magnetite troctolite in the lower part to the olivine in olivine grabbro in the upper part of the complex. This indicates that the oxygen fugacity of the system varies gradually with the magmatic crystallization differentiation process,inferring the crystallization differentiation process of the magmaoccurred in a closed system. The V2 O3 contents of the magnetite vary from 0. 72% to 1. 37%,indicating a relative low oxygen fugacity( <FMQ + 0. 5) for the magma during its evolution process. The crystallization of large amounts of silicate minerals under such a low oxygen fugacity would results in the gradual increase of the oxygen fugacity and Fe contents in the residual melt. This could be the main reason why the immiscible melt and the Fe-Ti-rich were formed at the late stage of the magma.
出处
《矿物学报》
CAS
CSCD
北大核心
2018年第4期449-461,共13页
Acta Mineralogica Sinica
基金
国家自然科学基金项目(批准号:41272095)
关键词
钒钛磁铁矿
成矿
白马岩体
四川攀西地区
峨眉山大火成岩省
Vanadium-titanium magnetite
Metallogenesis
the Baima intrusion
the Panxi area in Sichuan
Emeishan large igneous province
