GENERATION OF A-TYPE RHYOLITE FROM MELTING OF S-/I-TYPE SOURCES DURING HYBRIDIZATION PROCESSES WITH THOLEIITIC MAGMAS

The classical division of silicic igneous bodies into S-, I-, and A-type has as strong geotectonic regime aspect with A-type silicic magmas associated with bimodal suites of rifting and intraplate settings and S-/I-type silicic magmas with convergent plate margins corresponding mostly with calc-alkaline suites. Despite a general correlation with geotectonic regime, many silicic provinces dominated by one kind often include examples of the other type. A-type and ‘calc-alkaline’ rhyolites in nearly equal proportions occur in eastern Oregon where they are part of the Columbia River magmatic province. Rhyolites range from those with strong A-type affinities (high Fe, Fe-rich mafic silicates, high HFSE concentrations) to those with strong ‘calc-alkaline’ affinities (low Fe, hydrous and Mg richer mafic silicates, low HFSE).

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, f_O2 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.