RELICT SAPPHIRINE IN PYROPIC GARNET FROM THE EASTERN S∅R RONDANE MOUNTAINS, ANTARCTICA

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概要

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• Sapphirine-bearing biotitic schist forms a reaction zone between an ultramafic lens and its host biotite-hornblende gneiss at Balchenfjella, which is underlain by migmatitic complex metamorphosed under granulite-facies conditions in the Late Proterozoic and under amphibolite-facies conditions in the Early Paleozoic. This schist consists of porphyroblastic garnet, the core of which is compositionally homogeneous pyrope-almandine (X_<Mg>=Mg/(Fe+Mg)≅0.50) surrounded by an almandine rim (X_<Mg>≅0.27), biotite (X_<Mg>=0.70-0.73), plagioclase (An_<75-87>), spinel (X_<Mg>=0.26-0.36), corundum, rutile and zircon. In addition, individual garnet porphyroblasts enclose different mineral associations in the magnesian cores as follows : (A) sapphirine (X_<Mg>=0.75-0.85), commonly with kyanite, and, locally, with spinel (X_<Mg>=0.50-0.68), (B) kyanite only, and (C) gedrite (X_<Mg>=0.75-0.78)+quartz aggregates. The mineral associations sapphirine+kyanite±spinel and gedrite+quartz±kyanite are inferred to be relict assemblages of the prograde P-T path, and the magnesian garnet cores to have been homogenized during the granulite-facies event. Differences in the associations are presumed to have resulted from pre-metamorphic chemical heterogeneity in the precursor to the biotitic schist. These mineral associations suggest a clockwise prograde P-T trajectory entirely within the kyanite field; the assemblage sapphirine+kyanite might have been stable as T approached 700℃ constraining P to be at least 8 kbar. Further increase in temperature was accompanied by a small decrease in P to the 7-8 kbar (at T=760-800℃) estimated for peak conditions in the granulite-facies by M. ASAMI et al. (Proc. NIPR Symp. Antarct. Geosci., 6,57,1993). The relict sapphirine-kyanite assemblages in the S∅r Rondane Mountains are similar to those reported from the Luzow-Holm Bay complex, Antarctica, and the Highland Complex, Sri Lanka (e.g. Y. OGO et al. : Recent Progress in Antarctic Earth Science, ed. by Y. Yoshida et al. Tokyo, Terra Sci. Publ., 75,1992), suggesting similarities in the P-T evolution of the three areas.

• 国立極地研究所の論文

著者

• Grew Edward

  Department Of Geological Sciences University Of Maine

• Grew E.

  Department Of Geological Science University Of Maine

• Grew Edwqrd

  メーン大学

• Grew E.

  メーン大

• Grew Edward

  Department Of Geological Science University Of Maine

• ASAMI Masao

  Department of Geological Sciences, College of Liberal Arts, Okayama, University

• MAKIMOTO Hiroshi

  Geological Survey of Japan

• Asami M

  Department Of Earth Sciences Faculty Of Science Okayama University

• Asami Masao

  Department Of Geological Sciences College Of Liberal Arts Okayama University

• Asami Masao

  Department of Earth Sciences, Faculty of Science, Okayama University

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