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Sapphirine‐bearing granulites and related high‐temperature metamorphic rocks from the Higo metamorphic terrane, west‐central Kyushu, Japan
Author(s) -
OSANAI Y.,
HAMAMOTO T.,
MAISHIMA O.,
KAGAMI H.
Publication year - 1998
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1998.00064.x
Subject(s) - granulite , geology , metamorphic rock , geochemistry , metamorphism , gneiss , metamorphic facies , cordierite , petrology , migmatite , facies , geomorphology , biochemistry , chemistry , structural basin , catalysis
The Higo metamorphic unit in west‐central Kyushu island, southwest Japan is an imbricated crustal section in which a sequence of units with increasing metamorphic grade from high (northern part) to low (southern part) structural levels is exposed. The basal part of the metamorphic sequence representing an original depth of 23–24 km consists mainly of garnet–cordierite–biotite gneiss, garnet–orthopyroxene gneiss, orthopyroxene‐bearing amphibolite and orthopyroxene‐bearing S‐type tonalite. These metamorphic rocks underwent high amphibolite‐facies up to granulite facies metamorphism with peak P – T conditions of 720 MPa, 870 °C. In addition sapphirine‐bearing granulites and related high‐temperature metamorphic rocks also occur as tectonic blocks in a metamorphosed peridotite intrusion. The sapphirine‐bearing granulites and their related high‐temperature metamorphic rocks can be subdivided into five types of mineral assemblages reflecting their bulk chemical compositions as follows: (1) sapphirine–corundum–spinel–cordierite (2) corundum–spinel–cordierite (3) garnet–corundum–spinel–cordierite (4) garnet–spinel–gedrite–corundum, and (5) orthopyroxene–spinel–gedrite. These metamorphic rocks are characterized by unusually high Al 2 O 3 and low SiO 2 contents, which could represent a restitic nature remaining after partial melting of pelitic granulite under the ultra high‐temperature contact metamorphism at the peak metamorphic event of the Higo metamorphic unit. The metamorphic conditions are estimated to be about 800 MPa and above 950 °C which took place at about 250 Ma as a result of the thermal effect of the regional gabbroic rock intrusions.