Geochemical evidence for “cryptic amphibole fractionation” and lower crust melting for the generation of island arc tholeiitic rocks from northern Fossa Magna, central Japan

Abstract

Amphibole plays an important role in arc-related magma differentiation. This relates to the fact that it contains a variety of elements and volatiles, and because it is one of the main minerals comprising the lower crust. Pliocene-Pleistocene Yoneyama volcanic rocks (northern Fossa Magna, central Japan) show a high-K sub-alkalic composition, and pargasite bearing ultramafic–mafic rocks and megacrysts are frequently contained within them. Volcanic activities are divided into five stages. Based on petrography and petrochemistry, “cryptic amphibole fractionation” was found to contribute to the magma differentiation process for the 1st stage rocks even though amphibole is rarely included as a phenocryst. Partial melting of amphibole bearing lower crust, on the other hand, was controlled the generation of the 3rd stage basaltic magma, with the lower crust being one of the important sources supplying H2O to basaltic magma. On the basis of mineral chemistry and isotopes, amphibole bearing ultramafic–mafic cumulates and megacrysts are considered to have crystallized from these basaltic magmas at a lower crustal depth. We conclude that these cumulates are the direct evidence for “cryptic amphibole fractionation” of Yoneyama volcanic rocks.