Origin of Late Paleogene to Neogene basalts and associated coeval felsic volcanic rocks in Southwest Hokkaido, northern NE Japan arc: Constraints from Sr and Nd isotopes and major- and trace-element chemistry

摘要

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 (34-30 Ma), Early Miocene (25-17 Ma), Middle Miocene (16-12 Ma), Late Miocene (10-5 Ma), Pliocene (4 Ma) and Quaternary (2 Ma), thus spanning the pre-Japan Sea opening to post-opening stages.The majority of basaltic rocks after about 16 Ma 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 westernvolcanic 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 (34 Ma) 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 (25-17 Ma), 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 (16-12 Ma), 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.