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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| Institution: | 1. Laboratory of Isotope Geology, Ministry of Land and Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China;2. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, PR China;1. Universität Tübingen, FB Geowissenschaften, Wilhelmstrasse 56, 72074 Tübingen, Germany;2. Universität Bonn, Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Poppelsdorfer Schloss, 53115 Bonn, Germany;3. Universität Mainz, Institut für Geowissenschaften, Petrologie, J.-J.-Becher-Weg 21, 55099 Mainz, Germany;4. Macquarie University, Department of Earth and Planetary Sciences, North Ryde, NSW 2109, Australia;1. School of Earth Sciences and Resources, China University of Geosciences (Beijing), Beijing 100083, China;2. Department of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024-5192, USA;3. Henan Institute of Geological Survey, Henan Bureau of Geo-Exploration and Mineral Development, Zhengzhou 450001, China;1. School of Physical Sciences, University of Adelaide, Adelaide, SA 5000, Australia;2. School of Chemical Engineering, University of Adelaide, Adelaide, SA 5000, Australia;3. BHP Billiton, Olympic Dam, Adelaide, SA 5000, Australia;1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China;2. University of Chinese Academy of Sciences, Beijing 100049, China;3. Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA;1. Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China;2. State Key Laboratory for Mineral Deposits Research, Nanjing University, Nanjing 210093, China;3. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China |
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| Abstract: | 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 SiO2 concentration 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. |
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