Melting of crustal rocks as a possible origin for Middle Miocene to Quaternary rhyolites of northeast Hokkaido, Japan: Constraints from Sr and Nd isotopes and major- and trace-element chemistry
التفاصيل البيبلوغرافية
العنوان:
Melting of crustal rocks as a possible origin for Middle Miocene to Quaternary rhyolites of northeast Hokkaido, Japan: Constraints from Sr and Nd isotopes and major- and trace-element chemistry
Felsic volcanic rocks (mainly rhyolites) and basalts found in northeast Hokkido, Japan, result from intense volcanism during the Middle Miocene (14–9 Ma), Late Miocene (8–6 Ma) and Pliocene to Quaternary (5–2 Ma). 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 (14–0 Ma) 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.
