Lower crustal part of the Hidaka Arc (Hidaka metamorphic belt) is composed of the granulite-facies unit (zone IV) and the main anatexis zone. The former can be observed on the surface, while the latter is considered to be existed beneath the Hidaka Main Thrust by the evidence of restitic Gr-Opx granulites as xenoliths included in intrusive anatectic tonalities. In pelitic granulites of zone IV, Bt-dehydration melting took place with forming leucosomes including euhedral Opx, An-rich Pl and Crd. Intercalated mafic granulites in zone IV also show the Hbl-dehydration melting and Opx-bearing leucosome formation. A small scale partial melting in zone IV is identified, where the produced melt would be effected to make plastic condition of lower crustal rocks and to form duplex structure in the lower crust of the Hidaka Arc. Partial molten melts derived from the main anatexis zone moved and assembled to form tonalitic magmas with peraluminous S-type and metaluminous I-type signatures, which intruded into the shallower crustal level along the floor-thrust and roof-thrust of duplex structure.
The Hidaka belt is situated in the central part of Hokkaido island, northern Japan. The southern part of the belt is composed mainly of Tertiary metamorphic and plutonic rocks defined as the Main Zone of the Hidaka metamorphic belt. Various types of amphibolites, which were metamorphosed under amphibolite- to granulite-facies conditions, are distributed in the Main Zone. The northern part of the belt consists mainly of Cretaceous to Palaeogene sedimentary rocks including non- or weekly- to highly-metamorphosed mafic rocks (referred to as Northern Hidaka greenstones in present study). In this study, we discuss whole rock chemistry including isotopic compositions of the Southern Hidaka amphibolites to realize their protoliths, and we also compare the results with those of the Northern Hidaka greenstones. The various discrimination diagrams using major and trace element compositions suggest N-MORB magmatic characteristics for the Southern Hidaka amphibolites. REEs and NdI values also show this feature. Major and trace element compositions of the Northern Hidaka greenstones show similar characteristics with the Southern Hidaka amphibolites (N-MORB). SrI values of the amphibolites and the greenstones are slightly higher than those of the representative N-MORB composition, possibly due to the isotope interaction between original magma and seawater or sedimentary rock (or sediment). It is noted that the SrI values of both the amphibolite and the greenstones are similar to each other. Based on the N-MORB-like composition of major, trace elements, REEs and NdI values for the amphibolites and major and trace elements for the greenstones as well as their similar SrI values, it is a possible to consider that the protoliths of the Southern Hidaka amphibolites and the Northern Hidaka greenstones were derived from a common N-MORB magmatism.
The low-pressure / high temperature type Hidaka Metamorphic Belt (HMB) in Hokkaido, northern Japan, represents a tilted crustal section of a magmatic arc of Tertiary age. Exposed crustal section forms duplex structure which was formed by the uplift tectonic process. Syn-tectonic tonalite magma intrudes along the floor thrust, ramp, and roof thrust of the crustal-scale duplex. The tonalite magma was generated by anatexis of the unexposed lowermost crust. Pyroxene-bearing tonalites (basal tonalite body) is distributed in the Niikappu river region, northern part of the Hidaka Metamorphic Belt. The various evidences that show a cooling process can be observed within the tonalite body, such as the orthopyroxene pseudomorph and the aplite veins. Cooling process of the tonalite body has been revealed from these textures. The P-T-t paths of the syn-tectonic tonalite suite and the metamorphic layer show the uplift tectonics of the crust. A P-T-t path of the delaminated lowermost crust also can be presumed.
The Hidaka metamorphic belt is considered to be a tilted island-arc crustal section, as resulted by the arc-arc collision. In the lower sequence of the belt, granulite-facies metamorphic rocks and S- and I-type tonalites underwent mylonitic deformation. Hornblede gabbro-diorite complex (Niobetsu complex) and biotite granite (Nozuka-dake granite) were emplaced after mylonitc deformation. The magma activity of the S- and I-type tonalites and the Niobetsu complex occurred at ca. 55 to 50 Ma and 18 Ma, respectively. The Niobetsu complex includes a large amount of xenoliths derived from the host S-type tonalite. These xenoliths show recrystallized-granoblastic texture with locally remaining mylonitic foliation, and consist of plagioclase-orthopyroxene-cordierite-quartz-biotite-K-feldspar-spinel ±garnet-apatite-opaque minerals. The metamorphic P-T conditions of the xenoliths are up to 900ºC and 500 MPa, which are fairly lower pressure conditions compared with the highest-grade metamorphic rocks of the Hidaka metamorphic belt. The mineral assemblages and textures of the tonalite xenoliths are produced through reactions such as biotite+quartz=orthopyroxene+melt, and/or biotite+garnet+quartz=orthopyroxene+cordierite (or+spinel) +melt. Whole-rock geochemical studies combined with metamorphic analyses indicate that the tonalite xenoliths are regarded as restites after releasing granitic melt from the S-type tonalite. Geochemical analyses including Sr and Nd isotope compositions reveal that the Nozuka-dake granite are derived through the process of mixture with the Niobetsu complex and the granitic melt released from the tonalite xenoliths. This magma process occurred probably at the syn-collisional events in the Hidaka Collision Zone.
Compilation of published and newly obtained thermochronologic data for the granitoid plutons in the Hidaka belt of Hokkaido and granitoid clasts in the Middle Miocene foreland basin-fill provides new insights on the uplift and exhumation processes of the Hidaka Mountains. The granitoid plutons in the Hidaka belt and low-grade metamorphic rocks in the Hidaka metamorphic belt yield biotite K-Ar ages of 40-46 Ma. They constitute shallow crustal sequence and record the Middle Eocene regional cooling that postdates the Paleocene metamorphic peak of the Hidaka metamorphics. Meanwhile, many age determinations yielding Miocene cooling ages (~20 Ma) of the high-grade Hidaka metamorphic rocks indicate that the deep-seated rocks were exhumed during the Miocene. Simultaneous deposition of thick and coarse-grained foreland basin-fill suggests a vigorous erosional denudation. The clasts of monzogranite - granodiorite rocks yielding Eocene cooling ages are dominant in the Middle Miocene turbidites. In contrast, the tonalitic and metamorphic rock clasts yielding Miocene cooling ages occur within the Late Miocene fan delta deposits. The temporal change of the detritus show sequential unroofing of the ancestral Hidaka Mountains through Miocene time. Besides the above two clusters of the cooling ages, another cluster of Early Oligocene ages (29~36 Ma) is known for the mid-crustal rocks in the southeastern part of the Hidaka Mountains. The cluster indicates local exhumation of the mid-crustal rocks during the Oligocene most probably caused by sub-horizontal southward crustal stacking. Zircon and apatite fission-track ages of some granitoid clasts and their depositional ages suggest that cooling rates of initially eroded shallow crustal rocks are up to 100ºC/Myr in the early Middle Miocene. The rates probably reflect thrusting under the condition of increasing thermal gradient due to synchronous magmatic activities. They are much greater than those of 20~30ºC/Myr estimated from thermochronology on the Hidaka metamorphics.
Latest Jurassic to Cretaceous forearc and accretionary complexes are the major constituents of the frontal fold-and-thrust belt in the Hidaka collision zone, which formed between the Northeast Japan and the Kurile arcs. Their complicated geologic structure is here modeled through reconstruction of the original (pre-collision) tectonostratigraphic relations between many geologic units. Constituent lithologic assemblages, fossil and radiometric ages, and geochemical discrimination of metabasites suggest that the latest Jurassic -Cretaceous systems originally had regional, near-horizontal pile-nappe structure. It comprises the Lower Sorachi ophiolite nappe with the Nitarachi-Yezo forearc basin cover sequence, the Naizawa-Iwashimizu composite nappe (Early Cretaceous accretionary complex), the Horobetsugawa-Pankehoronai nappe (Late Cretaceous accretionary complex), and the basal Poroshiri Ophiolite, structurally downward in ascending order. The Kurile Arc collision rearranged the pile-nappe structure mainly by folding, partly with strike-slip duplexes, rather than imbricate thrusting. Asymmetric unit distribution across the two major antiforms suggests the existence of the corresponding ramps of the main detachment thrust beneath them. Approximately 10 km high ramp may have lifted and tilted up the almost entire section of the immature forearc crust consisting of the nappe units around the Idonnappu Zone. The Hidaka Main Thrust, the major collision boundary, probably branched off this big ramp to the surface. The other 4-5 km high ramp may have formed an antiform consisting of the Naizawa-Iwashimizu composite nappe and sedimentary covers around the Kamuikotan Zone.