GENERATION OF A-TYPE RHYOLITE FROM MELTING OF S-/I-TYPE SOURCES DURING HYBRIDIZATION PROCESSES WITH THOLEIITIC MAGMAS
We performed melting experiments on natural rocks collected near rhyolites to investigate the influence of voluminous tholeiitic magmas on partial melting products generated from meta-sedimentary and igneous protoliths, as could be the case in eastern Oregon during Columbia River Basalt volcanism. Experiments were performed at fluid absent and present conditions, 0.5 MPa, fO2 at QFM, and various temperatures. To approximate material influence of mafic magmas on crustal protoliths during partial melting, starting material of some experiments consisted of mixtures of crustal rocks with a tholeiitic glass (with 50, 56, and 59 wt.% SiO2). Experiments with intermediate to silicic protoliths generated rhyolite melts largely consistent with observed calc-alkaline rhyolites. Experiments with tholeiitic glass produced Fe-richer melts and those with 56% and 59% glass, a rhyolite melt with 5 wt.% FeO*. Calculating trace elements composition of melts using composition of protoliths and glasses combined with residual mineralogy indicate that Fe-rich rhyolite melts are also enriched in HFSE elements compared to more ‘calc-alkaline’ melts. This suggests that involvement of evolved and trace-element enriched intermediate tholeiitic magmas like those observed in Grande Ronde Basalt are likely to have had a significant influence on generating A-type rhyolites while 'calc-alkaline' rhyolites appear to be a product of partial melting without strong material influence from tholeiitic magmas.