Fractional crystallization of the basaltic suite of Usu volcano, southwest Hokkaido, Japan, and its relationships with the associated felsic suite

Abstract

REE concentrations were determined for four basalts, two basaltic andesites, three dacites and one rhyolite from Usu volcano, SW Hokkaido, Japan, and are reported with their mineralogical data. The basic rocks are tholeiitic, whereas the felsic rocks are calc-alkaline. No intermediate andesite has been erupted. The REE abundances of the basic rocks increase with increasing FeO ∗ MgO ratios and the mineral compositions are consistent with low-pressure fractionation. The variation of the REE concentrations can also be explained by low-pressure fractionation. In contrast, the chemical features of the felsic rocks can be hardly accounted for by low- or high-pressure fractionation; the felsic rocks are more depleted in HREE than the most differentiated basaltic andesite. A model calculation is suggestive of their origin: the felsic rocks can be produced by remelting of an intrusive body of tholeiitic basic magma.