Apatite geochemistry of the Taihe layered intrusion, SW China: Implications for the magmatic differentiation and the origin of apatite-rich Fe-Ti oxide ores
Apatite geochemistry of the Taihe layered intrusion, SW China: Implications for the magmatic differentiation and the origin of apatite-rich Fe-Ti oxide ores
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DOI：
10.1016/j.oregeorev.2016.04.004
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发表时间：
2016-10
影响因子：
3.3
通讯作者：
Yuwei She;Xie‐Yan Song;Songyue Yu;Liemeng Chen;W. Zheng
中科院分区：
文献类型：
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作者：
Yuwei She;Xie‐Yan Song;Songyue Yu;Liemeng Chen;W. Zheng
太和岩体是位于峨眉山大火成岩省中部的层状岩体之一。岩体中部带的旋回单元由底部的磷灰石-磁铁矿单斜辉石岩和顶部的辉长岩组成。富含磷灰石的氧化物矿石含有6-12模态%的磷灰石和20-50模态%的Fe-Ti氧化物，这与富含磷灰石的岩石中缺乏Fe-Ti氧化物的同时代侵入体明显不同。太和中上带的磷灰石大部分为氟磷灰石，但有4个样品的磷灰石中Cl含量稍高，表明它们是从含水母岩浆中结晶出来的。与位于泰和岩体以南100 km的攀枝花岩体辉长岩中的磷灰石相比，泰和岩体中的磷灰石富Sr，贫HREE，贫LREE。计算得到的太和中上带与磷灰石处于平衡状态的岩浆在原始地幔标准化微量元素图解中也显示出弱的Sr负异常。这些特征表明，太和中上带的磷灰石结晶在单斜辉石之后，斜长石之前。太和中带和上带的磷灰石显示弱的负Eu异常，表明其氧逸度较高。氧化岩浆中铁、钛含量高，导致铁钛氧化物结晶。大量的铁钛氧化物和单斜辉石的结晶作用降低了岩浆中磷的溶解度，提高了岩浆中SiO2的含量，从而导致磷灰石的饱和。磷灰石与堆晶单斜辉石的Sr、V、稀土总量和Ce/Yb比值呈正相关，表明它们之间的成分近似平衡，说明它们来自同一铁玄武岩浆。在太和中带旋回单元底部，铁钛氧化物与磷灰石的早期结晶富集形成富磷灰石氧化物矿石。
The Taihe intrusion is one of the layered intrusions situated in the central zone of the Emeishan Large Igneous Province (ELIP), SW China. The cyclic units in the Middle Zone of the intrusion are composed of apatite-magnetite clinopyroxenite at the base and gabbro at the top. The apatite-rich oxide ores contain 6–12 modal% apatite and 20–50 modal% Fe-Ti oxides evidently distinguished from the coeval intrusions in which apatite-rich rocks are poor in Fe-Ti oxides. Most of apatites of the Taihe Middle and Upper Zones are fluorapatite, although four samples show slightly high Cl content in apatite suggesting that they crystallize from a hydrous parental magma. Compared to the apatite from the gabbro of the Panzhihua intrusion, situated 100 km to the south of the Taihe intrusion, the apatite of the Taihe rocks is richer in Sr and depleted in HREE relative to LREE. The calculated magma in equilibrium with apatite of the Taihe Middle and Upper Zones also shows weakly negative Sr anomalies in primitive mantle normalized trace element diagrams. These features indicate that the apatite of the Taihe Middle and Upper Zones crystallizes after clinopyroxene and before plagioclase. The apatite of the Taihe Middle and Upper Zones shows weakly negative Eu anomalies suggesting a high oxygen fugacity condition. The high iron and titanium contents in the oxidizing magma result in crystallization of Fe-Ti oxides. Crystallization of abundant Fe-Ti oxides and clinopyroxenes lowers the solubility of phosphorus and elevates SiO2concentration in the magma triggering the saturation of apatite. The positive correlations of Sr, V, total REE contents and Ce/Yb ratio in apatite with cumulus clinopyroxene demonstrate approximately compositional equilibrium between these phases suggesting they crystallized from the same ferrobasaltic magma. Early crystallization and accumulation of Fe-Ti oxide together with apatite produced the apatite-rich oxide ores at the base of the cyclic units of the Taihe Middle Zone.