A new staurolite locality in the Hidaka metamorphic belt (HMB) is reported. Staurolite occurs as inclusions in garnet and plagioclase in alternating garnet-cordierite and garnet-cordierite-gedrite gneisses in the Shizunai river area, central part of the belt.
     Two types of staurolite-breakdown reactions, Staurolite+Chlorite+Quartz=Garnet+H₂O and Staurolite+Gedrite (Na-, Al-poor)+Plagioclase (Ca-poor)=Gedrite (Na-, Al-rich)+Plagioclase (Ca-rich)+Quartz+H₂O, are recognized. Subsequently, Gedrite (Na-rich)+Plagioclase (Ca-rich)+Quartz=Gedrite (Na-poor)+Plagioclase (Ca-poor) occurred.
     Estimated H content of the staurolite is low (2.2-3.2), suggesting low pressure type. Those from the both Menashunbetsu river and Hidakahorobetsu river areas, locating in the south of the Shizunai river area, show similar values.
     Fe-Mg partitioning coefficients between included staurolite and host garnet are plotted on and around the line of K_D =1. Despite of large uncertainty (±60-70°C) of the Koch-Müller (1997)'s staurolite-garnet geothermometry, the application to the pair from the Shizunai river area shows a consistent result with other methods. The minimum temperature estimate for staurolite-garnet pairs is almost constant through the whole HMB. The closure temperature of the Fe-Mg exchange reaction between staurolite and garnet in the HMB is ca. 700°C.
     The similarities of H contents of staurolite and Fe/Mg distribution and re-equilibration processes of staurolite-garnet pairs may indicate that at least high-grade part of the central area of HMB has been suffered similar thermal histories at similar depths.

抄録全体を表示

Rishiri volcano is an isolated polygenetic volcano that lies at a distance of about 200 km from Quaternary volcanic fields in Hokkaido. The volcanic activity has started abruptly since <0.2 Ma after∼9 My hiatus of volcanism and might have ended since several thousand years ago. The volcanic activity can be divided into three, initial, climactic and final stages in terms of its eruption rate. The climactic stage can be also subdivided into three, C-1, C-2 and C-3 stages in the ascending order.
     The eruptive products span from basaltic to rhyolitic rocks (SiO₂=49.2-73.8%) which can be distinguished by phenocryst assemblages and whole-rock chemistry. Judging from incompatible trace elements, felsic rocks from each stage cannot be formed by fractional crystallization of the basaltic magma and may be produced by crustal melting. Phenocryst assemblage, Ti contents in hornblende, and geological thermometers indicate that magmatic temperature of the dacitic and rhyolitic rocks increases from ∼800°C in the initial stage to ∼1050°C in the C-2 stage, and then decreases to ∼700°C in the final stage. On the other hand, chemical compositions of olivine and spinel suggest that the basaltic magma has also changed its source mantle with the volcanic activity. It can be estimated that the source mantle for the basaltic magmas has changed from fertile peridotites in the initial stage to depleted ones in the climactic stage, and finally changed to fertile ones in the final stage.
     The variation in magmatic temperature of the felsic magmas indicates that the volcanic activity with higher eruption rate has produced magma with higher temperature. This suggests that the production of these magma can be explained by ascending and cooling of a mantle diapir which plays as a heat source to melt crustal materials. It can be concluded that the whole-life of Rishiri volcano may be explained by evolution of a single mantle diapir. Our petrological analysis could provide important constraints on modeling magma generation of a polygenetic volcano, because effect of preceding and/or adjacent volcanism is negligible in the case of Rishiri volcano.

抄録全体を表示

Petrology, geochemistry and Rb-Sr chronology of seven spinel lherzolite-harzburgite xenolith samples from Ichinomegata, northeast Japan and Kurose, southwest Japan were studied. Whole rocks and the constituent minerals were analyzed for Rb and Sr contents and ⁸⁷Sr/⁸⁶Sr ratios. The pure minerals were obtained by hand-picking and repetitive washing with hot acids (2M HCl, 5% HF) and distilled water.
     The peridotite xenoliths from Ichinomegata show wide variations in mineral and chemical compositions; clinopyroxene contents range from 4 to 13 volume%, Cr/(Cr+Al) ratios of spinel range from 0.19 to 0.41 and MgO contents of whole rock range from 38 to 46 weight%. The chondrite-normalized patterns of whole rock vary from LREE-depleted to flat or U-shaped. In spite of above petrographical and geochemical variations, ⁸⁷Sr/⁸⁶Sr ratios of the clinopyroxenes are very uniform (0.7033∼0.7034). Rb-Sr mineral isochron ages of 208.9±80.1 Ma, 233.8±8.4 Ma, 252.9±19.8 Ma and 310.8±23.3 Ma are given by olivine, orthopyroxene, clinopyroxene and whole rock from each of four peridotite xenoliths. Since the peridotites have the relatively narrow range of ages and wide range of equilibrium temperatures, it is believed that a large-scale isotopic equilibration has occurred at various depths in the upper mantle beneath Ichinomegata.
     The peridotite xenoliths from Kurose show narrow ranges in mineral and chemical compositions; clinopyroxene contents range from 7 to 9 vol.%, Cr/(Cr+Al) ratios of spinel range from 0.28 to 0.31 and MgO contents of whole rock range from 41 to 43 wt.%. On the other hand, ⁸⁷Sr/⁸⁶Sr ratios of the clinopyroxenes are very diverse (0.7034, 0.7039 and 0.7043), compared with those in Ichinomegata peridotites. The mineral isochrons based on the data of pyroxenes and whole rock also show very variable ages and initial ⁸⁷Sr/⁸⁶Sr ratios. The Rb-Sr mineral isochron ages of 129.9±4.8 Ma, 312.8±16.7 Ma and 487.2±9.1 Ma indicate large intervals between the equilibration events.

抄録全体を表示