Coeval volcanism due to interaction of back-arc basin basalt(BABB) magma with the island-arc crust in the late Miocene Engaru volcanic field, northeastern Hokkaido, Japan: The evidence of Sr and Nd isotopic ratios combined with major- and trace-element compositions.

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 sobordinate amounts of andesite, Sakaeno andesite (SK), which 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 icclandite-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 similarity in SrI of the rhyolite to S-type granitoids and pelitic-psammitic rocks of the Hidaka belt suggests the crustal origin of the rhyolite. The combined major- and trace-element and Sr- and Nd- isotopic data indicate that the main generation process of the SK type I andesite was the mixing of basaltic magma (the CK type I basalt) with some felsic magmas. The felsic magmas are the WK type I rhyolite magma and rhyolitic magmas having higher Sr and Nd contents, and higher SrI and lower NdI values than the WK type I rhyolite. On the other hand, the SK type II andesite and WK type II rhyolite may have been derived 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 are during the basin spreading, could have heated the crust to generate calc-alkaline rhyolitic magma. Andesitic rocks were derived both by mixing of BABB magma with the crust-derived rhyolitic magma and by fractional ctystallization of BABB magma.