The Hidaka Metamorphic Belt (HMB) in Hokkaido, northern Japan, consists of tilted metamorphic layers of an island-arc type crust from lower (granulite facies) to upper (very low-grade metasedimentary) horizons. Abundant granitic rocks, mainly S-type tonalites of crustal origin, intrude various metamorphic layers and are classified into four depth types, namely upper, middle, lower and basal. The basal orthopyroxene-garnet (S-type) tonalities were intruded into granulite facies country rocks. Textural and compositional evidence from minerals in the basal tonalite indicates that the crystallisation sequence is Grt–Pl–Opx–Bt–Qtz–Crd–Kfs, and that crystallisation took place at about 600 MPa and 900°C–700°C. Some crystallisation experiments were carried out in an internally heated pressure vessel, using the basal tonalite, under the conditions of 300 and 600 MPa, 700–900°C, and with 0–20 wt% H 2 O, respectively. The results show that the primary S-type tonalite magma was at a temperature above 900°C and contained 3–4 wt% H 2 O at the beginning of crystallisation. In order to study the influence of normative orthoclase content on orthopyroxene crystallisation, some starting materials also included 15, 20 and 25% normative orthoclase, by adding KAlSi 3 O 8 gel to the rock powder. Normative orthoclase content has an influence on the subliquidus crystallisation limit of orthopyroxene. The changes in P–T conditions and chemical composition of the magma during ascent would generate the sequence from the basal to upper S-type granite. Opx-free S-type granitic magma can be generated from lower crustal Grt–Opx S-type granitic magma, by differentiation with falling magmatic temperature.

Abstract Xenocrystic garnet and kyanite, in addition to clinopyroxene and rare orthopyroxene, are newly found to occur in middle Proterozoic slightly metamorphosed adakitic trondhjemites and tonalites (meta-tonalites) at Cape Hinode on the eastern Prince Olav Coast in the latest Proterozoic–Early Palaeozoic Lützow-Holm Complex, East Antarctica. Textural and compositional features of garnet and kyanite suggest that these minerals formed most probably as restite phases of partial melting of mid-ocean ridge basalt (MORB) between 15 and 20 kbar pressure, and were entrained by the tonalitic magmas, which underwent fractional crystallization upon ascent to form cumulates that were also entrained and metamorphosed to basic–intermediate granulite blocks. Available geochronological data for the meta-tonalites indicate that all these events including MORB formation took place in the middle Proterozoic. The meta-tonalites and associated basic, calc-silicate, and pelitic rocks were emplaced as an allochthonous block in the Lützow-Holm Complex at the waning stage of its main regional metamorphism, most probably as a part of the final amalgamation of East and West Gondwana into the Gondwana supercontinent.

The high dT/dP -type Hidaka Metamorphic Belt in Hokkaido, northern Japan, represents a tilted crustal section of a magmatic arc of Tertiary age. The highest metamorphic grades reached are granulite facies, and the syn-metamorphic granitic rocks are widely distributed in this metamorphic terrane. The granitic rocks are mainly tonalitic and granodioritic in composition, and are classified into peraluminous (S-type) and metaluminous (I-type) granitoids. A large amount of pyroxene-bearing S-type tonalites (garnet-orthopyroxene tonalite) is distributed in the Niikappu river region in the northern part of the Hidaka Metamorphic Belt. Pyroxene-bearing I-type tonalite (two-pyroxene hornblende tonalite) bodies are also distributed in this area. The pyroxene-bearing tonalites are classified into several sub-types on the basis of their field occurrence, texture, mineral assemblage and geochemical features. Homogeneous I H - and S H -type tonalite are thought to represent original magmas, i.e. those which have been generated by partial melting of mafic metamorphic rocks and pelitic-psammitic metamorphic rocks, respectively. Model calculations assuming batch partial melting indicate that possible restites are garnet-two-pyroxene mafic granulite for I H -type and garnet-orthopyroxene aluminous granulite for S H -type. The unexposed lowermost crust of the ‘Hidaka crust’ is thought to be composed of garnet-two-pyroxene mafic granulite, garnet-orthopyroxene aluminous granulite and metagabbros.