No abstract

No abstract

The Hidaka metamorphic belt consists of an island-arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, Dors, D,, D,rs, and D,, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (Do) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D, is the stage of prograde metamorphism with increasing temperatures a t a constant pressure during an early phase, and with a slight decrease of pressure a t the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite-granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S-type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D, stage, mafic and intermediate magmas supplied and transported heat to form the arc-type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D, stage is characterized by subhorizontal top-to-the-south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (dkollement) and duplex structures associated with intrusions of S-type tonalite. Deformation structures and textures of high-temperature mylonites formed along the dkcollement, as well as the duplex structures, show that the D, stage movement occurred under a N-S trending compressional tectonic regime. The depth of intra-crustal dkcollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal dhcollement, however, might have extended to the crust-mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral-reverse-slip (dextral transpression) movement accompanied by low-temperature mylonitization with retrograde metamorphism, the stage defined as D,. The crustal rocks from the basal dkcollement to the upper were tilted eastward on the N-S axis and exhumed during the D, stage. During D, and D, stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.

No abstract

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 0, 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 0 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,0 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.