Rare earth element–SiO₂ systematics of island arc crustal amphibolite migmatites from the Asago body of the Yakuno Ophiolite, Japan: a field evaluation of some model predictions

Abstract

The two most commonly invoked processes for generating silicic magmas in intra-oceanic arc environments are extended fractional crystallization of hydrous island arc basalt magma or dehydration melting of lower crustal amphibolite. Brophy (Contrib Mineral Petrol 156:337–357, 2008) has proposed on theoretical grounds that, for liquids >~65 wt% SiO₂, dehydration melting should yield, among other features, a negative correlation between rare earth element (REE) abundances and increasing SiO₂, while fractional crystallization should yield a positive correlation. If correct, the REE–SiO₂ systematics of natural systems might be used to distinguish between the two processes. The Permian-age Asago body within the Yakuno Ophiolite, Japan, has amphibolite migmatites that contain felsic veins that are believed to have formed from dehydration melting, thus forming an appropriate location for field verification of the proposed REE–SiO₂ systematics for such a process. In addition to a negative correlation between liquid SiO₂ and REE abundance for liquids in excess of ~65 % SiO₂, another important model feature is that, at very high SiO₂ contents (75–76 %), all of the REE should have abundances less than that of the host rock. Assuming an initial source amphibolite that is slightly LREE-enriched relative to the host amphibolites, the observed REE abundances in the felsic veins fully support all theoretical predictions.