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Phase relations and P–T conditions of the Ryoke pelitic and psammitic metamorphic rocks in the Ina district, Central Japan
Author(s) -
Tokada Tomokazu
Publication year - 1998
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1046/j.1440-1738.1998.00213.x
Subject(s) - geology , cordierite , biotite , plagioclase , andalusite , geochemistry , metamorphic rock , muscovite , isograd , sillimanite , feldspar , mineral , quartz , metamorphism , mineralogy , metamorphic facies , chemistry , geomorphology , paleontology , biochemistry , organic chemistry , facies , structural basin , catalysis
The Ina district of the Ryoke Belt is divided into two mineral zones, based on the mineral parageneses of the pelitic and psammitic rocks at the peak metamorphism. A biotite–muscovite zone (quartz + plagioclase + biotite + muscovite with or without K‐feldspar) constitutes the northwestern part, and a biotite–cordierite–K‐feldspar zone (quartz + plagioclase + biotite + cordierite + K‐feldspar) comprises the central to southern and eastern parts. The isograd reaction between two mineral zones is defined by a divariant reaction: Mg‐rich biotite + muscovite + quartz = Fe‐rich biotite + cordierite + K‐feldspar + H 2 O (1), which, in the K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (KFMASH) system, occurs at ∼ 590 °C at 0.2 GPa and 660 °C at 0.4 GPa. Fibrolite accompanied by andalusite porphyroblasts in aluminous pelitic rocks of the biotite–muscovite zone and the low‐grade part of the biotite–cordierite–K‐feldspar zone, suggests that sillimanite was the stable aluminosilicate at the peak metamorphic condition throughout the area. In the high‐grade part of the biotite–cordierite–K‐feldspar zone, fibrolite mostly occurs as inclusions in cordierite or in plagioclase. The phase relations and the compositional zoning of plagioclase in relation to fibrolite inclusions suggest that fibrolite was formed under relatively high‐pressure conditions, and that partial melting took place.