Prograde P–T path of medium‐pressure granulite facies calc‐silicate rocks, Higo metamorphic terrane, central Kyushu, Japan

Abstract

AbstractThis 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.