Petrology of middle Miocene to Quaternary primitive basalt and high magnesian andesite of north Hokkaido, Japan : source mantle characteristics and degrees of partial melting 北部北海道における中期中新世～第四紀の未分化玄武岩及び高マグネシア安山岩の岩石学的研究 : 起源マントルの特徴と部分溶融度

博士論文

Large amounts of geochemical data have been accumulated for middle Miocene to Quaternary volcanic rocks in North Hokkaido, which is situated between Kurile and Japan Sea back-arc basins. Tectonomagmatic models for the generation of these volcanic rocks include back-arc spreading related to the formation of the Kurile back-arc basin (Goto et al., 1995; Ikeda, 1998; Yamashita et al., 1999; Ikeda et al., 2000), increase in the dip-angle of the subducting Pacific Plate (Watanabe, 1995), and collision of the Eurasian, Okhotsk and Pacific plates (Okamura et al., 1995). These models have the common thread of asthenospheric upwelling. However, the characteristics of original mantle in detail are not clear. Recently, Shuto et al. (2004) demonstrated Sr and Nd isotope ratio of the mantle beneath North Hokkaido, but the other petrological features are unknown. Middle Miocene to Quaternary primitive basalts and high magnesian andesite (HMA) in North Hokkaido resulted from three periods of intense volcanism; early-stage (12-10 Ma), middle-stage (9-7 Ma) and late-stage (3-0 Ma). This study aims to present mineral and whole rock compositions of these primitive rocks, and to discuss their petrogenesis and the petrological character of the mantle source rocks. The following conclusions can be obtained from this study. (1) The assumed primary magma compositions, calculated using the olivine maximum fractionation model (Tamura et al., 2000), show that most of the early-stage primary magmas were generated at shallower depth in the mantle wedge (about 7 to 9 kb; 23 to 30 km depth), compared to the middle- and late-stage primary magmas (about 14 to 18 kb; 46 to 59 km depth). (2) Based on Fo-NiO relations for olivine phenocrysts and the Cr/(Cr+Al) of chromian spinel versus Fo content of olivine phenocrysts, early-stage primary basalt magmas were generated by higher degrees of partial melting of fertile mantle peridotite, whereas middle- and late-stage primary basalt magmas represent lower degrees of partial melting of similar mantle peridotite. On the other hand, the HMA magma may be derived from refractory mantle peridotite. (3) Primitive mantle normarized trace element patterns and chondrite normarized REE (rare Earth elements) patterns of North Hokkaido basalts showed similar characteristics, but those of HMA the other character such as a negative Eu anomaly. This difference is also observed on the isotopic characteristics by Shuto et al. (2004), and may possibly reflect the compositional gap of their original mantle. On the other hand, the trace element contents of early-stage primitive basalts are lower than those of middle- and late-stage primitive basalts. This indicates that the early-stage basalts would be generated by higher degrees of partial melting of mantle peridotite, compared to the middle- and late-stage basalts. (4) In the context of the early asthenospheric mantle upwelling model, the new tectonomagmatic model is proposed. Large-scale upwelling of asthenospheric mantle associated with the spreading of two back-arc basins (Japan Sea and Kurile basins) would replace the original asthenosphere and ascend into subcontinental lithospheric mantle beneath North Hokkaido. The asthenospheric mantle corresponds to the fertile peridotite that is parental to the North Hokkaido basalts, and the continental lithospheric mantle is equivalent to the refractory peridotite that generated the HMA. At this time, early-stage primary magmas would be generated by higher degrees of partial melting of shallower part of asthenospheric mantle. On the other hand, middle- and late-stage primary magmas would be produced by lower degrees of partial melting of deeper part of the mantle as a result of lowering of thermal gradient.

新大院博（理）甲第255号