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首页> 外文期刊>Journal of Volcanology and Geothermal Research >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
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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

机译：地壳岩石的熔化可能是中新世到北海道东北部的第四纪流纹岩的起源：Sr和Nd同位素的约束以及主要和微量元素化学

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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 Ndl) 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.
机译：在日本北海道东北部发现的长英质火山岩（主要是流纹岩）和玄武岩，是中新世中期（14-9 Ma），中新世晚期（8-6 Ma）和上新世至第四纪（5-2 Ma）强烈的火山作用造成的。检查了流纹岩，以确定与玄武岩，高镁质安山岩（HMA），下地壳岩石和地幔橄榄岩的任何遗传关系。流纹岩的初始Sr和Nd同位素比（Sri和Ndl）与北海道北部的玄武质玄武岩和HMA重叠。先前发表的Sr和Nd同位素数据显示，北海道北部的大多数中新世至第四纪（14-0 Ma）玄武岩具有相对较窄的SrI-和NdI-范围（SrI 0.70299至0.70400，NdI 0.51281至0.51311），而没有SrI或NdI的时间变化。然而，与北海道北部的其他玄武岩相比，来自三个位置的玄武岩和HMA的SrI和NdI含量都没有减少。一些流纹岩（在此称为未消耗流纹岩）与北海道北部的一些玄武岩和HMA具有相似的SrI和NdI。与北海道北部其他玄武岩具有相似的SrI和NdI的流纹岩被称为贫化流纹岩。尽管流纹岩与近代玄武岩和HMA之间的SrI和NdI相似性可以通过分步结晶来解释，但此过程与玄武岩，HMA和流纹岩的REE化学以及分步结晶建模的结果不一致。但是，玄武岩和HMA岩浆的分步结晶以及某些沉积沉积岩的同化作用可能会产生一些流纹岩。根据REE化学和含闪石尖晶石锂沸石来源的部分熔融模型，后生化地幔橄榄岩的部分熔融程度很小，不可能解释流纹岩的成因。日高变质带的辉长岩可能是同位素贫化流纹岩的可能来源，因为流纹岩和辉长岩都具有相似的SrI和NdI。基于SrI和NdI的相似性，Hidaka变质带中的Ⅰ型to石和一些辉长岩可能是同位素贫化流纹岩的可能烃源岩。这些假说分别由橄榄石辉长岩和I型斜长石的部分熔融模型支持。从北海道东北部生产中新世后流纹岩的可能的构造-岩浆模型涉及中新世中期的软流圈上升，这与千岛后弧盆地和日本海后弧盆地的扩散有关。自中新世中期以来，整个北海道北部的上覆岩石圈变薄，北北海道北部的软流层衍生的玄武岩的生成具有低SrI和NdI的高值，但岩石圈衍生的玄武岩和HMA的生成较少（具有较高的SrI和较低的NdI）。自中新世爆发或引起地壳融化以来形成的玄武岩浆，导致流纹岩浆形成。

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