The magma evolution process of the Taihe Layered Intrusion in the Emeishan area,SW China:Evidence from geochemical characteristics of amphiboles
The Taihe mafic-ultramafic layered intrusion is located in the internal zone of the Emeishan Large Igneous Province.It hosts super-large sized V-Ti magnetite ore deposits.The lower,middle and upper lithofacies zones have been identified from the bottom to the top of the Taihe intrusion based on their mineral assemblages,mineral contents and petrographic textures of various rocks in the intrusion.Especially,the Ⅰ-Ⅳ cycles of intrusive rocks have been further indentified in the middle lithofacies zone.Amphibole is widely distributed in different layered rocks of the Tai intrusion.Most rock samples have amphibole volumetric fractions ranging from 1％to 3％.Minor rock samples have those of 5％-10％.Petrographic studies show that amphiboles have poikilite structure and uniform optical characteristics in various rocks.Combined with their geochemical compositions,these amphiboles are related to the mantle-derived magma and are crystallization products of the evolved magma in the late stage.The amphiboles in the lower lithofacies zone have relatively high contents of Cr(3-112 ppm)and Ni(114-163 ppm),whereas those in the middle and upper lithofacies zones have relatively low contents of Cr(3.81-28.1 ppm)and Ni(0.42-20.9 ppm),implying that the parent magma of rocks in the lower lithofacies zone is relatively close to the primitive magma and is an Fe-Ti-rich magma.Furthermore,TiO2 concentrations of amphiboles in the middle lithofacies zone are rhythmic cyclically changed.Especially,Nb contents are clearly high in amphiboles in the Ⅵ cycle of the middle lithofacies zone,but are rapidly decreased in amphiboles of the upper lithofacies zone.Meanwhile,the parent magmas of rocks in the middle lithofacies zone are highly enriched in Y,REE,Sr and other incompatible elements based on trace element spidergrams of amphiboles.These features indicate that parent magma of rocks in the middle lithofacies zone was an evolving magma.The oxygen fugicity of the magma estimated by the composition of hornblende shows a cyclic variation,decreasing gradually from the bottom of the cycle to the top of the cycle,which is similar to the variation of Fe3+/(Fe3++Fe2+)ratio of hornblendeThe oxygen fugacities of the magma estimated by using amphibole compositions vary cyclically,with a gradually decreased tendency from the bottom to the top of the cycle.This feature is similar to the variation of Fe3+/(Fe3++Fe2+)ratios of amphiboles.The high oxygen fugacity and high water concentration of the magma are key factors resulting in the crystallization of magnetite in the early magmatic stage for the Taihe intrusion.
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