Diatexite and metatexite from the Higo metamorphic rocks, west-central Kyushu, Japan

Abstract

A variety of anatectic migmatites occur in high grade zones of a Mesozoic high T/P typemetamorphic belt the Higo metamorphic belt, Kyushu, Japan. This paper deals with two major lithotypes of the migmatites metatexite and diatexite and attempts to characterize them in terms of field occurrence, texture, mineralogy and whole rock chemistry. The metatexite is a layered migmatite that is composed of leucosome and melanosome with metamorphic fabrics, while diatexite is a massive and more homogeneous type and has plutonic igneous textures. Both types of migmatite occur in a high grade garnet cordierite zone (D zone) and an even higher grade garnet hypersthene zone (E zone) of the Higo metamorphic belt, and they are both considered to represent the products of partial melting of pelitic and psammitic gneisses. The diatexite occurs in masses, sporadically, in the matrices of metatexite both in the D zone and E zone, and it does not appear to increase in abundance with the increase of metamorphic grade. Another important finding is that no systematic compositional difference has been detected between the metatexite and the diatexite despite their textural and structural differences, if compared within the same D zone. Treating the metatexite and diatexite together, it was shown that the E zone migmatites are chemically and mineralogically more depleted than the D zone migmatites, which may imply that the former represents the refractory residues of partial melting. Some leucogranites, which form veins, dikes and pods in metamorphic rocks, have compositions of anatectic melts and are considered to be compositionally complementary to the residual E zone migmatites. Although the D zone rocks must also have been partially molten at the peak of metamorphism, melt segregation probably did not take place effectively there, and, as a whole, they largely retain their original metasedimentary composition. The factor controlling the structural types of the migmatite is also discussed.

Formation of a high‐temperature metamorphic complex due to pervasive melt migration in the hot crust

“…The volume fractions of muscovite in the pelitic source rocks were calculated from the SiO 2 ‐Al 2 O 3 ‐FeO‐MgO‐CaO‐Na 2 O‐K 2 O chemical compositions described by Kobayashi et al . (2005) and Yamamoto (1962) for the HMC, and by Kawakami and Ikeda (2003) and Higashimoto et al .…”

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

“…Anatectic migmatites pervasively occur in the higher grade part of the Higo metamorphic rocks, whereas leucogranites are the most abundant, not in the highest grade zone, but in the intermediate grade zone, in which migmatites do not develop (Obata et al. , 1994; Miyazaki, 2004; Kobayashi et al. , 2005).…”

“…, 2005). The migmatites in the highest grade zone represent refractory residues of partial melting, and some leucogranites may be compositionally complementary to the residual migmatites (Kobayashi et al. , 2005).…”

“…On the other hand, there are pervasive occurrences of migmatites in the higher grade part of the Higo metamorphic terrane (Obata et al. , 1994; Miyazaki, 2004, 2007;Kobayashi et al. , 2005).…”

“…Although the Higo plutonic rocks intruded into the Higo metamorphic terrane, they are not likely to be a heat source of the low-P ⁄ high-T metamorphism because the intrusion of the Higo plutonic rocks occurred after the formation of foliation during the peak metamorphism (Miyazaki, 2004). On the other hand, there are pervasive occurrences of migmatites in the higher grade part of the Higo metamorphic terrane (Obata et al, 1994;Miyazaki, 2004Miyazaki, , 2007Kobayashi et al, 2005). Miyazaki (2004Miyazaki ( , 2007 argued that melt migration was a principal cause for the low-P ⁄ high-T metamorphism, based on thermal modelling with mesoscale pervasive flow of melt and solidification (pervasive melt migration model).…”