Search

The Ryozen Formation, which crops out on the trench side of the NE Japan arc, contains middle Miocene rhyodacite with adakite-like trace element geochemical characteristics (Ryozen adakitic rhyodacite) and spatially and temporally related basalt (Ryozen basalt) and andesite (Ryozen andesite). K–Ar age data for the basalt and a zircon U–Pb age for the adakitic rhyodacite, combined with the stratigraphy, suggest that all of these volcanic rocks were erupted at about 16–14 Ma. The primitive nature of the Ryozen basalt is shown by its high MgO (maximum 14·1 wt %), Ni (392 ppm) and Cr (1193 ppm) contents. Using the olivine maximum fractionation model, the segregation depth of the parental primary magma to this basalt is estimated at c. 50 km (about 1·5 GPa). The Ryozen andesite has slightly higher 87Sr/86Srinitial (SrI) and lower 143Nd/144Nd initial (NdI) ratios than the Ryozen basalt. This, and the characteristics of the variation trends defined by basalt and andesite samples in SiO2 versus major and trace element variation diagrams, suggests that the andesite may have resulted from fractional crystallization of basaltic magma with minor assimilation of pre-Cretaceous sedimentary rocks (i.e. an AFC process). The Ryozen rhyodacite has phenocrysts of plagioclase, amphibole, garnet and titanomagnetite, and is characterized by low Sr/Y ratios (∼30), low Y concentrations (<10 ppm), high chondrite-normalized La/Yb [(La/Yb)cn] (>25) and low chondrite-normalized Yb [(Yb)cn] values (<5 ppm). These geochemical characteristics are similar to those of a new adakite subgroup (rhyodacite lavas in eastern Jamaica; Jamaican-type adakite). Thus, we define the Ryozen rhyodacite as the Ryozen low Sr/Y adakitic rhyodacite. A potential mechanism for the generation of this rhyodacite is crystal fractionation of plagioclase, orthopyroxene, clinopyroxene, amphibole, garnet, titanomagnetite and minor apatite from an andesitic parent magma. This mechanism is consistent with mass-balance modeling, which matches the observed major and trace element chemistry, as well as SrI and NdI, for the Ryozen andesite and low Sr/Y adakitic rhyodacite. The most likely tectono-magmatic model for the production of the volcanic rocks of the Ryozen Formation involves the upwelling of depleted hot asthenosphere, which modified the thermal structure of the mantle wedge beneath the trench side of the arc during the middle Miocene. This resulted in partial melting of both mantle wedge peridotite and the relatively cool subducting Pacific plate, leading to the simultaneous production of primitive basalt, normal andesite, high-magnesium andesite and low and high Sr/Y adakitic rhyodacites. [ABSTRACT FROM PUBLISHER]

Late Oligocene to Middle Miocene adakitic andesites are found in the southern part of Okushiri Island, the northern Noto Peninsula and in the Toyama region in the present-day back-arc margin of the SW and NE Japan arcs. On Okushiri Island, adakitic andesite is accompanied by moderately alkaline basalt, whereas on the Noto Peninsula, adakitic andesite has been erupted along with high magnesian andesite (HMA), bronzite andesite and tholeiitic basalt. Adakitic andesites from all three locations are characterized by high Sr/Y and low Y, and have higher MgO contents than adakitic melts generated by experimental melting of metabasalt and amphibolite. They also have higher Ni and Cr contents than either Archaean tonalite–trondhjemite–granodiorite (TTG) suites or Early Cretaceous adakitic granites, which have been attributed to partial melting of subducted oceanic crust. The Noto Peninsula adakitic andesite has Sr and Nd isotopic compositions identical to normal mid-ocean ridge basalt (N-MORB), whereas the Okushiri Island and Toyama adakitic andesites are more isotopically primitive than N-MORB. The Noto Peninsula primary adakitic melt was derived from subducted oceanic N-MORB crust, whereas the Okushiri Island and Toyama primary adakites are interpreted as melts of subducted N-MORB and sediment that have subsequently interacted with the overlying mantle wedge peridotite. To explain the comagmatism of adakite, HMA and basalt, the following model is proposed. A hydrated adakitic diapir ascends from the subducting slab and is heated because it enters the overlying hot mantle wedge. The subsequent establishment of thermal and H2O gradients in the adakitic diapir and surrounding mantle wedge peridotite results in concurrent generation of adakitic andesite magma in the inner adakitic diapir region (low temperature and high H2O content), HMA and bronzite andesite magmas in the intermediate peridotite region (intermediate temperature and H2O content), and tholeiitic basalt magma in the outer peridotite region (high temperature and lower H2O content). Comagmatic adakite and mildly alkaline basalt are found in cooler and wetter adakitic diapirs and hotter and drier peridotite regions respectively. The most likely tectono-magmatic situation for the genesis of adakitic magmas in this example of a cool subduction zone involves upwelling of hot asthenosphere into the subcontinental lithosphere beneath the back-arc side of the NE Japan arc and northern end of the SW Japan arc, during the period spanning the pre-Japan Sea opening to syn-opening stages. The unusually high temperature conditions established in the mantle wedge owing to upwelling of hot asthenosphere caused partial melting of the relatively cool subducting Pacific plate, resulting in the generation of adakitic magmas. [ABSTRACT FROM AUTHOR]

Abstract: Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9Ma), Late Miocene (8–6Ma) and Pliocene to Quaternary (5–2Ma). Rhyolites were examined to determine any genetic relationship to coeval basalts, high magnesian andesite (HMA), lower crustal rocks and mantle peridotite. Rhyolites have initial Sr and Nd isotopic ratios (SrI and NdI) which overlap with coeval north Hokkaido basaltic rocks and HMA. Previously published Sr and Nd isotopic data show that most Middle Miocene to Quaternary (14–0Ma) basaltic rocks from north Hokkaido have a relatively narrow SrI- and NdI- range (SrI 0.70299 to 0.70400, and NdI 0.51281 to 0.51311), with no temporal variation in either SrI or NdI. Basalt and HMA from three locations are, however, more undepleted in terms of both SrI and NdI than other north Hokkaido basaltic rocks. Some of rhyolites (termed undepleted rhyolites here) have similar SrI and NdI to some north Hokkaido basalts and HMA. Rhyolites with similar SrI and NdI to other north Hokkaido basaltic rocks are termed depleted rhyolites. Although the similarity of SrI and NdI between rhyolites and coeval basalts and HMA can be accounted for by fractional crystallization, this process is inconsistent with the REE chemistry of basalts, HMA and rhyolites, and with the results of fractional crystallization modeling. However, a few rhyolites may result from the fractional crystallization of basaltic and HMA magmas with assimilation of some metasedimentary rocks. Small degrees of partial melting of a metazomatized mantle peridotite is an unlikely mechanism to explain the genesis of rhyolites according to REE chemistry and partial melt modeling of an amphibole bearing spinel lherzolite source. Gabbros of the Hidaka metamorphic belt are a possible source for isotopically depleted rhyolites, as both the rhyolites and gabbros have similar SrI and NdI. I-type tonalite and some gabbros in the Hidaka metamorphic belt are possible source rocks for isotopically undepleted rhyolites based on similarity in SrI and NdI. These hypotheses are supported by partial melt modeling of olivine gabbro and I-type tonalite respectively. A possible tectono-magmatic model for the production of post-Middle Miocene rhyolites from NE Hokkaido involves upwelling of the asthenosphere during the Middle Miocene, associated with the spreading of the Kurile back-arc basin and Japan Sea back-arc basin. This would have resulted in thinning of the overlying lithosphere beneath north Hokkaido, and the production of asthenosphere-derived basaltic rocks with low SrI and high NdI throughout north Hokkaido since the Middle Miocene, but less common production of lithosphere-derived basaltic rocks and HMA (with high SrI and low NdI). Basaltic magmas formed since the Middle Miocene either erupted, or caused melting of the crust, resulting in the generation of rhyolitic magma. [Copyright &y& Elsevier]

Abstract: Basalts and felsic volcanic rocks (mainly dacite and rhyolite) found in southwest Hokkaido, northern part of the NE Japan arc, result from protracted volcanism during the Oligocene (34–30Ma), Early Miocene (25–17Ma), Middle Miocene (16–12Ma), Late Miocene (10–5Ma), Pliocene (4Ma) and Quaternary (2Ma), thus spanning the pre-Japan Sea opening to post-opening stages. The majority of basaltic rocks after about 16Ma show depleted Sr (SrI) and Nd (NdI) isotopic signatures compared with some Middle to Early Miocene basalts, which strongly resemble, in terms of both timing and extent, the change in SrI and NdI values for back-arc basaltic rocks of the central NE Japan arc. However, significant differences exist for younger basaltic rocks, in that basaltic rocks with depleted SrI and NdI signatures are found from the Middle Miocene onwards throughout the eastern-, transitional- and western-volcanic zones in SW Hokkaido, whereas in the central NE Japan arc, basaltic rocks with similar isotopic signatures are confined to the back-arc side. Felsic volcanic rocks in southwest Hokkaido have SrI and NdI values, which overlap with coeval southwest Hokkaido basaltic rocks. Although the relationship between mafic and felsic rocks could be attributed to fractional crystallization, this process is inconsistent with REE chemistry, as total REE do not increase systematically from basaltic rocks to felsic volcanic rocks. Alternatively, lower crustal mafic rocks, represented by gabbroic and amphibolitic xenoliths found in basaltic rocks at Itinome-gata (Oga Peninsula), are a possible source for Late Paleogene to Quaternary felsic magmas, as both felsic volcanic rocks and xenoliths have similar SrI and NdI. A possible tectono-magmatic model for the production of post-Late Paleogene volcanic rocks from SW Hokkaido commences in the Oligocene (34Ma) with asthenospheric mantle upwelling followed by partial melting to generate basalt magma (Matsue basalt) with depleted SrI and NdI, followed by interaction of asthenosphere-derived basalt magmas with overlying subcontinental lithosphere. In the Early Miocene (25–17Ma), asthenospheric upwelling triggered partial melting of the overlying lithospheric mantle from which most basalts with undepleted SrI and NdI values were derived. During the Middle Miocene (16–12Ma), thinning of the overlying lithosphere due to the opening of the Japan Sea resulted in asthenospheric upwelling which reached the region beneath the present NE Japan arc volcanic front. Partial melting of the asthenosphere led to generation of voluminous basalt magma with depleted SrI and NdI values throughout southwest Hokkaido. Most basaltic rocks that erupted since the Late Miocene are considered to have also formed from asthenospheric mantle. Basaltic magmas formed since the Oligocene have either been erupted, or fluxed and heated the lower crust from which coeval felsic magmas were generated. [Copyright &y& Elsevier]

Most middle Miocene to Quaternary basaltic rocks younger than about 12 Ma from north Hokkaido show a narrow range in initial 87Sr/86Sr (SrI) and initial 143Nd/144Nd (NdI) isotope ratios, which are slightly undepleted, compared to MORB. SrI ranges from 0.70296 to 0.70393 and NdI from 0.51288 to 0.51307, but there is no temporal variation in either SrI or NdI. An exceptional basalt from Ohmu area, which is more undepleted in terms of both SrI and NdI than other north Hokkaido (NH) basaltic rocks. Most of the NH basaltic rocks are isotopically similar to basaltic rocks found on the back-arc side of the NE Japan arc (BA basaltic rocks) younger than about 15 Ma, whereas basaltic rocks from the Ohmu area are isotopically similar to BA basaltic rocks older than 15 Ma. The generation of both NH and BA basaltic rocks appears to be closely related to mantle evolution. This involves upwelling of asthenosphere into the continental lithosphere beneath north Hokkaido and the back-arc side of the NE Japan arc, at the same time as spreading of the Kurile back-arc basin and Japan Sea back-arc basin. The magma source for the Ohmu area basalts and the BA basaltic rocks prior to 15 Ma may correspond to the subcontinental lithospheric mantle thinned by the upwelling of asthenospheric mantle material from which most of the NH basaltic magmas and the BA basaltic magmas younger than 15 Ma were derived. This asthenospheric mantle is slightly different from MORB source in terms of major- and trace-element characteristics such as TiO2/K2O and Zr/Y as well as Sr and Nd isotopes, which may be due to involvement of EM II-like component. [Copyright &y& Elsevier]