Mantle plume injection during the spreading of the back-arc basin-Evidences from the volcanic fields in north Hokkaido, Japan-

点击翻译按钮获取中文摘要

The results obtained from this research project are summarized as follows.1. The late Miocene Engaru volcanic field in northeastern Hokkaido contains basalts (Tomeoka basalt (TM) and Chiyoda-Kakurezawa basalt (CK)) and rhyolite (Wakamatsu rhyolite (WK)) with subordinate amounts of andesite (Sakaeno andesite (SK)) that erupted during 7-9 Ma. Both the TM and CK basalts have geochemical characteristics similar to those of back-arc basin basalt (BABB). Based on differences in major and trace element abundances and the initial values of Sr and Nd isotopic ratios (SrI and NdI), the CK basalt, SK andesite and WK rhyolite can be divided into two types (I and II), respectively. The SK type I andesite and WK type I rhyolite are of calc-alkaline series, whereas the SK type II andesite (including icelandite-like andesite) and WK type II rhyolite are of tholeiitic series. The WK type I rhyolite has remarkably high SrI and low NdI values compared with other volcanic rocks from the study area. The si … More milarity in SrI and NdI of the rhyolite to S-type granitoids and pelitic-psammitic rocks of the Hidaka belt suggests the lower crustal origin of the rhyolite. The SK type I andesite is plotted on or near mixing lines between the CK type I basalt and WK type I rhyolite in major and trace element variation diagrams and SrI vs. NdI diagram, implying its mixing origin. On the other hand, major- and trace- element and Sr- and Nd- isotopic characteristics indicate that the SK type II andesite and WK type II rhyolite were generated from the CK type II basaltic magma by fractional crystallization accompanied by a minor degree of assimilation of crustal materials. The genetic relationship of these coeval basalts, andesite and rhyolite can be attributed to spreading of the Kurile basin. BABB magma which is a partial melt in a hot asthenosphere uprising below the island arc during the basin spreading, could have heated the lower crust to generate calc-alkaline rhyolitic magma. Andesitic rocks were derived both by mixing of BABB magma with the lower crust-derived rhyolitic magma and by fractional ctystallization of BABB magma.2. Basalts and andesites from the Iwao Formation, occurring in the Hamamasu area along the Japan Sea coast in northwest Hokkaido, were derived from a volcano which has been erupted during a relatively short period of 3.7 to 3.8 Ma. Similar disequilibrium phenocryst assemblages (Mg-rich olivine + Mg-rich clinopyroxene + Mg-poor orthopyroxene + hornblende + An-rich plagioclase + An-poor plagioclase + quartz + Fe-Ti oxide) are commonly found in the basalts and andesites. Detailed microprobe analyses as well as petrographic observations of coexisting phenocrysts reveal that the disequilibrium phenocryst assemblages in these volcanic rocks may be attributable to mixing of a basaltic magma in which olivine, clinopyroxene and An-rich plagioclase were crystallizing, and a dacitic magma in which orthopyroxene, hornblende, An-poor plagioclase, quartz and Fe-Ti oxide were crystallizing. Not only chemical variation but also petographic variation from basalts to andesites of the Iwao Formation are accounted for by the difference in mixing ratios of basaltic and dacitic magmas. Less