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Prograde P–T path of medium-pressure granulite facies calc-silicate rocks, Higo metamorphic terrane, central Kyushu, Japan |
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| Authors: | K MAKI Y FUKUNAGA T NISHIYAMA Y MORI |
| Institution: | Institute of Earth Sciences, Academia Sinica, 128 Academia Road Section 2, Nankang, Taipei 11529, Taiwan (); Department of Earth and Environment, School of Science, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan; Kitakyushu Museum of Natural History and Human History, 2-4-1, Higashida, Yahatahigashi-ku, Kitakyushu 805-0071, Japan |
| Abstract: | This paper reports an occurrence of medium-pressure granulite facies calc-silicate rocks intercalated with pelitic gneisses in the Higo metamorphic terrane, central Kyushu, Japan, which is classified as a low- P /high- T (andalusite-sillimanite type) metamorphic belt. Three equilibrium stages are recognized in the calc-silicate rock based on reaction textures: M1 stage characterized by an assemblage of porphyroblastic garnet + coarse-grained clinopyroxene + plagioclase included in the clinopyroxene; M2 stage by two kinds of breakdown products of garnet, one is plagioclase + coronitic clinopyroxene within garnet and the other is plagioclase + vermicular clinopyroxene surrounding garnet; and M3 stage by amphibole replacing clinopyroxene. The key assemblage in the calc-silicate rock common to M1 and M2 stages is Grt + Cpx + Pl ± Qtz, which constrains the pressure and temperature ( P – T ) conditions for these stages by Fe–Mg exchange reaction and the two univariant net-transfer reactions: 2Grs + Alm + 3Qtz = 3Hd + 3An or 2Grs + Prp + 3Qtz = 3Di + 3An. The P – T conditions for M1 and M2 stages were estimated to be about 8.4 ± 1.9 kbar and 680 ± 122 °C, and 6.7 ± 1.9 to 8.9 ± 2.2 kbar and 700 ± 130 to 820 ± 160 °C, respectively. Estimates are consistent with an isobaric heating P – T path. The high peak temperature conditions at normal crustal depths and the prograde isobaric heating path probably require heat advection due to melt migration during the high- T metamorphism. |
| Keywords: | an isobaric heating P–T path calc-silicate rock Higo metamorphic terrane low-pressure and high-temperature metamorphism pervasive melt migration |