Rare Earth Element (REE) - Silicon DIoxide systematics of island arc lower crust amphibolite migmatite from Yakuno Ophiolite, SW Japan: A field evaluation of some model predictions

The generation of silicic magma has been a contentious issue for decades. Among the various models that have been proposed, the two major end member processes are fractional crystallization of basaltic magma and partial melting of lower crustal rocks (amphibolites). Brophy (2008) proposed a model to distinguish between these two processes using rare earth element (REE) - SiO2 systematics. While REE is incompatible in most igneous minerals regardless of magma composition, it is compatible in hornblende when the magma is silicic. The active participation of hornblende in the early stage of partial melting, in contrast to the absence of hornblende all through fractional crystallization, makes the REE-SiO2 systematics differ dramatically in these two processes. Specifically, the trends are positive for fractional crystallization and negative for partial melting, when SiO 2 content is over 60%. If the model is correct, then it provides a simple test for identifying which process is responsible for a given natural silicic magma.;To evaluate the model, natural samples of lower crustal amphibolite migmatite were collected from the Yakuno Ophiolite, SW Japan. According to the regional geology (Suda, 2004), these rocks are considered to represent the partial melting of island arc lower crust. To determine whether the silicic portion of the migmatites actually represent partial melts, a series of high pressure (5Kb) melting experiments were conducted on the freshest natural amphibolite sample. The major element compositions of the experimental melts agree well with those of the silicic portion of the natural migmatite samples, supporting the latter one to be of a partial melting origin. The experimental data were then used to construct an amphibolite melting history which permitted the quantitative modeling of both REE and SiO2 abundances in the evolving melt. The REE abundances modeled with experimental results and observed in the silicic portion of the migmatite shows very good agreement. Thus, Brophy's (2008) general model for REE-SiO2 systematics during the partial melting of lower crustal amphibolite was confirmed.