Timing of Ti-magnetite crystallisation and silicate disequilibrium in the Panzhihua mafic layered intrusion: Implications for ore-forming processes
Timing of Ti-magnetite crystallisation and silicate disequilibrium in the Panzhihua mafic layered intrusion: Implications for ore-forming processes
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DOI：
10.1016/j.lithos.2013.02.020
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发表时间：
2013-06
期刊：
影响因子：
3.5
通讯作者：
G. Howarth;S. Prevec;Mei‐Fu Zhou
中科院分区：
文献类型：
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作者：
G. Howarth;S. Prevec;Mei‐Fu Zhou
攀枝花基性层状岩体是与峨眉山大火成岩省（ELIP）有关的一套岩体之一。攀枝花侵入体底部赋存较大（±60m厚）FeTi氧化矿体。本研究利用地球化学、岩石学、母岩浆结晶与fo2和H2O含量变化的模拟等方法，为攀枝花岩体氧化钛矿层的成因提供了新的约束条件。全岩主元素地球化学趋势受铁钛氧化物（钛磁铁矿和钛铁矿）丰度的控制。岩体下部±270m明显以钛磁铁矿的富集趋势为主，而岩体上部的地球化学变化与钛磁铁矿和钛铁矿的富集趋势一致。这表明，相对于±270m以上的侵入物，±270m以下的侵入物在较高的fo2条件下结晶。对矿层内和辉长岩寄主岩石中钛磁铁矿的溶出显微结构进行了详细的岩石学分析，也表明矿岩中fo2含量相对增加。攀枝花母岩浆模拟表明，干型（<0.5wt.%）起始成分钛磁铁矿结晶晚，湿型（<0.5wt.%）起始成分钛磁铁矿结晶早。% H2O)成分。这与斜长石结晶温度的明显变化相对应，结晶温度随母岩浆含水量的增加而降低。最初的斜长石成分从干燥岩浆的an55到含有3wt的岩浆的an73不等。% H2O。攀枝花岩体斜长石平均组成为An58，表明其初始岩浆H2O含量较低。铁钛氧化物矿岩结构清楚地表明钛磁铁矿在硅酸盐相（plag +cpx）后结晶，不平衡结构表明铁钛氧化物矿岩与封闭的硅酸盐不处于平衡状态。我们提出了一个开放体系中由不同H2O含量、钛磁铁矿晶体载荷和体积的岩浆多次补充形成氧化钛矿的模型。侵入的富含水和晶体的岩浆有效地热化学侵蚀形成下盘的先前形成的辉长岩堆积，并合并先前结晶的硅酸盐颗粒。由于水的高含量和较高的温度，这些颗粒被岩浆消耗，导致被消耗的硅酸盐原晶被包裹在氧化铁矿层中。
The ±260Ma Panzhihua mafic layered intrusion is one of a suite of intrusions related to the Emeishan Large Igneous Province (ELIP), SW China. The Panzhihua intrusion hosts a large (±60m thick) FeTi oxide ore body at the base of the intrusion. This study provides new constraints on the genesis of FeTi oxide ore layers at the Panzhihua intrusion using: geochemistry, petrography, and modeling of parent magma crystallisation with variation in fO2and H2O content. Whole-rock major element geochemical trends are controlled by the modal abundance of FeTi oxides (Ti-magnetite and ilmenite). The lower ±270m of the intrusion is clearly dominated by a Ti-magnetite accumulation trend whereas above this level the geochemical variation is consistent with a Ti-magnetite and ilmenite accumulation trend. This suggests that the lower ±270m of the intrusion crystallised at higher fO2conditions relative to that above ±270m. Detailed petrographic analysis of exsolution microtextures of the Ti-magnetite from within ore layers and gabbroic host rocks also indicates a relative increase in fO2within the ore rocks. Modeling of Panzhihua parent magma shows that Ti-magnetite crystallises late for dry (<0.5wt.%) starting compositions and early for wet (>1.5wt.% H2O) compositions. This corresponds to a distinct variation in plagioclase crystallisation temperature, which decreases with increasing H2O content of the parent magma. The initial plagioclase composition varies from An55for a dry magma up to An73for a magma containing 3wt.% H2O. The average plagioclase composition for the Panzhihua intrusion is An58, indicating an initial magma with low H2O content. Textures within the FeTi oxide ore rocks clearly indicate that Ti-magnetite crystallised after the silicate phases (plag.+cpx.) and disequilibrium textures indicate that the FeTi oxide ores were not in equilibrium with the enclosed silicates. We present a model for FeTi oxide ore formation in an open system by multiple replenishments of magma with variable H2O contents, Ti-magnetite crystal load and volume. Intruding H2O- and crystal-rich magmas effectively thermo-chemically erode previously formed gabbroic cumulates forming the footwall and incorporates previously crystallised silicate grains. These grains are consumed by the magma due to the high H2O content and higher temperature resulting in the association of consumed silicate primocrysts enclosed in FeTi oxide ore layers.