Spinel inclusions in olivine and plagioclase crystals in a layered gabbro: a marker and a tracer for primary phenocrysts in a differentiating magma reservoir (original) (raw)

Abstract

The problem of whether cumulate rocks were formed by crystal settling or by in situ crystallization after magma emplacement is an important issue concerning the mechanisms of magmatic differentiation. However, it is hard to distinguish these two processes for plutonic rocks because the primary texture and chemical composition have generally been modified by postcumulus processes. To contribute this problem, we studied the distribution and compositions of Cr-spinel inclusions hosted in olivine and plagioclase in the Murotomisaki Gabbroic Intrusion (MGI), SW Japan. It is shown that the olivine-hosted inclusions are restricted to specific horizons where accumulation of olivine phenocrysts is thought to have occurred and that the compositional variations of the Cr-spinel are explained by a secondary compositional modification that probably took place after the magma emplacement. It is also shown that the Cr-spinel inclusions in a chilled margin have suffered the least compositional modification and nearly retains the primary composition. Those in the interior of the intrusion, on the contrary, have been significantly modified by re-equilibration with residual melt driven by cation diffusions through the host phases. Those in plagioclase have been less modified. It is shown that all the spinel inclusions had primarily the same and common composition at the time of magma emplacement. This implies that all the inclusion-bearing crystals, olivine and plagioclase, represent primary phenocrysts that had already existed in the emplaced magma. In this way, spinel inclusion in the MGI may be regarded to be a useful petrographic “marker” for identifying intratelluric phenocrysts and also as a “tracer” to trace the motion of the primary phenocrysts after the magma emplacement.

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Acknowledgments

This study is a part of the doctoral thesis of T. H. at Kyoto University being financially supported by the Grant-in-Aid for JSPS Fellows (No.19.4363) to T. H. He is grateful to the Department of Geology and Mineralogy, Kyoto University. The manuscript was mostly written during his postdoctoral periods at Kyushu University and at Tohoku University. T. H. thanks A. Toramaru (Kyushu University) and M. Nakamura (Tohoku University) for their generous supports during these periods. We also thank D. Lenaz and two anonymous reviewers for their constructive criticism and suggestions for improving the manuscript and to M. Schmidt for his editorial handling.

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Authors and Affiliations

  1. Department of Earth Resource Science, Faculty of International Resource Sciences, Akita University, Akita, 010-8502, Japan
    Takashi Hoshide
  2. Graduate School of Science, Kyoto University, Kyoto, 606-8502, Japan
    Masaaki Obata

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  1. Takashi Hoshide
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  2. Masaaki Obata
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Correspondence toTakashi Hoshide.

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Communicated by M. W. Schmidt.

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Hoshide, T., Obata, M. Spinel inclusions in olivine and plagioclase crystals in a layered gabbro: a marker and a tracer for primary phenocrysts in a differentiating magma reservoir.Contrib Mineral Petrol 168, 1049 (2014). https://doi.org/10.1007/s00410-014-1049-8

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