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Apatite-hosted melt inclusions from the Panzhihua gabbroic-layered intrusion associated with a giant Fe–Ti oxide deposit in SW China: insights for magma unmixing within a crystal mush

Contributions to Mineralogy and Petrology volume 173, Article number: 59 (2018) Cite this article

Abstract

The ∼ 2-km-thick Panzhihua gabbroic-layered intrusion in SW China is unusual because it hosts a giant Fe–Ti oxide deposit in its lower zone. The deposit consists of laterally extensive net-textured and massive Fe–Ti oxide ore layers, the thickest of which is ∼ 60 m. To examine the magmatic processes that resulted in the Fe enrichment of parental high-Ti basaltic magma and the formation of thick, Fe–Ti oxide ore layers, we carried out a detailed study of melt inclusions in apatite from a ∼ 500-m-thick profile of apatite-bearing leucogabbro in the middle zone of the intrusion. The apatite-hosted melt inclusions are light to dark brown in color and appear as polygonal, rounded, oval and negative crystal shapes, which range from ∼ 5 to ∼ 50 µm in width and from ∼ 5 to ∼ 100 µm in length. They have highly variable compositions and show a large and continuous range of SiO2 and FeOt with contrasting end-members; one end-member being Fe-rich and Si-poor (40.2 wt% FeOt and 17.7 wt% SiO2) and the other being Si-rich and Fe-poor (74.0 wt% SiO2 and 1.20 wt% FeOt). This range in composition may be attributed to entrapment of the melt inclusions over a range of temperature and may reflect the presence of µm-scale and immiscible Fe-rich and Si-rich components in different proportions. Simulating results for the motion of Si-rich droplets within a crystal mush indicate that Si-rich droplets would be separated from Fe-rich melt and migrate upward due to density differences in the interstitial liquid when the magma unmixed. Migration of the Si-rich, immiscible liquid component from the interstitial liquid caused the remaining Fe-rich melt in the lower part to react with plagioclase primocrysts (An59–60), as evidenced by fine-grained lamellar intergrowth of An-rich plagioclase (An79–84) + clinopyroxene in the oxide gabbro of the lower zone. Therefore, magma unmixing within a crystal mush, combined with gravitationally driven loss of the Si-rich component, resulted in the formation of Fe-rich, melagabbro and major Fe–Ti oxide ores in the lower part and Si-rich, leucogabbro in the upper part of the intrusion.

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Acknowledgements

This study was financially supported by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB18000000), and NSFC Grants 41325006 and 41473037. Huan Dong and Chang-Ming Xing helped in the field trip and thin section observation. Lei Wu from University of Alberta is thanked for his kind help in the contour maps of Fig. 9. The manuscript is substantially improved from constructive comments from one anonymous reviewer and Jaayke Knipping, and the editor Chris Ballhaus.

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Authors and Affiliations

  1. Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China

    Kun Wang & Christina Yan Wang

  2. School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China

    Kun Wang

  3. Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou, 510640, China

    Christina Yan Wang

  4. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China

    Zhong-Yuan Ren

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  1. Kun Wang
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  2. Christina Yan Wang
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  3. Zhong-Yuan Ren
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Correspondence to Christina Yan Wang or Zhong-Yuan Ren.

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Communicated by Chris Ballhaus.

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Wang, K., Wang, C.Y. & Ren, ZY. Apatite-hosted melt inclusions from the Panzhihua gabbroic-layered intrusion associated with a giant Fe–Ti oxide deposit in SW China: insights for magma unmixing within a crystal mush. Contrib Mineral Petrol 173, 59 (2018). https://doi.org/10.1007/s00410-018-1484-z

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  • DOI: https://doi.org/10.1007/s00410-018-1484-z

Keywords

  • Magma unmixing
  • Crystal mush
  • Apatite-hosted melt inclusion
  • Fe enrichment
  • Layered intrusion