攀枝花岩体钛铁矿成分特征及其成因意义

Composition and genetic significance of the ilmenite of the Panzhihua intrusion

峨眉大火成岩省是全球最大的钒钛磁铁矿床聚集区,攀枝花岩体是其中的典型代表。根据岩性特点,攀枝花岩体主体可划分为上、中、下三个岩相带,其中中部岩相带和下部岩相带岩性旋回非常发育,每个旋回从下向上铁钛氧化物和暗色硅酸盐矿物逐渐减少,块状铁钛氧化物矿石或磁铁矿辉长岩都出现在每个旋回的底部和下部。然而,尽管钛铁矿固相线以下固溶体出溶远弱于磁铁矿,从而能更好地保留成因信息,但其成分变化的成因意义没有受到足够重视。本次研究发现作为主要金属氧化物之一的钛铁矿的成分不仅在不同岩性中有明显差异,同时,中、下部岩相带的各岩性旋回中钛铁矿成分也具有周期性变化。例如,块状矿石中钛铁矿具有最高的MgO和TiO2及最低的FeO、Fe2O3和MnO,而辉长岩中钛铁矿则具有相反的成分特征。同时,钛铁矿的MgO含量与磁铁矿的MgO含量及橄榄石的Fo牌号具有显著的正相关关系。这种规律性变化说明每个旋回可以代表一次比较明显的岩浆补充,每次新岩浆补充后,钛铁矿和磁铁矿及橄榄石都是结晶较早的矿物。与Skaergaard岩体相比,攀枝花岩体钛铁矿的MgO含量较高,表明攀枝花岩体分离结晶过程中铁钛氧化物结晶较早;与挪威Tellnes斜长岩套铁钛矿床中的钛铁矿相比,攀枝花岩体的钛铁矿不仅具有较高的MgO和FeO,还具有极高的TiO2和MnO,但Fe2O3却很低,说明地幔柱背景下形成的钛铁矿与斜长岩套中钛铁矿的成分有显著的区别。

The Panzhihua intrusion is the layered intrusion hosting Fe-Ti oxide deposit mined earliest in the Emeishan large igneous province, which is the largest ore-concentrated area of V-Ti magnetite deposits over the world. Therefore, the petrogenesis of the Panzhihua intrusion is significant for understanding of the formation of the other ore-bearing intrusions in the area. The Panzhihua intrusion can be divided into Upper, Middle and Lower zones. The lithologic cycles, shown by periodically decreases of the Fe-Ti oxides and dark silicate minerals from the bottom upwards, are well developed in the Middle and Lower zones, in which massive Fe-Ti oxide ore and magnetite gabbro occur in the bottom and lower part of each cyclic unit. However, although ilmenite can remain its original compositional feature due to weak sub-solidus exsolution relative to magnetite, the significances of the compositional variation of the ilmenite have not been well addressed yet. We find that the ilmenite compositions are distinguishable in different types of rocks, and periodically variable in each cyclic unit of the Middle and Lower zones. For example, the ilmenite of the massive ores is the highest in MgO and TiOa and the lowest in FeO, Fe2O3 and MnO, whereas the ilmenitc of the gabbro has opposite compositional characteristics. MgO content of the ilmenite and magnetite has a significant positive correlation with forsterite content （Fo） of olivine. Such regular variations in each cyclic unit represent obvious magma replenishment and the ilmenite and magnetite as well as olivine is the minerals crystallized early in each pulse of magma. This study also reveals that, MgO contents in the Panzhihua ilmenite are higher than those of the Skaergaard intrusion, indicating that the Fe-Ti oxides in the Panzhihua intrusion crystallize earlier during magma evolution. Compared with ilmenite of the Tellnes Fe-Ti oxide deposit in Norway, the ilmenite in the Panzhihua intrusion is high in MgO and FeO, very high in TiO2 and MnO, but very low in Fe2O3, suggesting that the ilmenite formed in mantle plume environment is significantly different from that in the anorthosite massif.