Petrogenesis of basalt-comendite and basalt-pantellerite suites,Terceira, Azores,and some implications for the origin of ocean-island rhyolites |
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Authors: | J E Mungall R F Martin |
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Institution: | 1. Earth and Planetary Sciences, McGill University, 3450 University Street, H3A 2A7, Montreal, QC, Canada
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Abstract: | Petrogenetic modeling of the Recent lava succession of Santa Barbara and Pico Alto volcanoes and associated basaltic lavas
indicates that there are two discrete lava series present, one erupted from the axial rift linking the two central volcanoes
and one associated with monogenetic cones scattered around the flanks of Santa Barbara. The felsic lavas of both volcanoes
are peralkaline and appear to be derived from associated basalts by fractional crystallization of an assemblage including
essential amphibole. Trace element abundances in the felsic lavas, particularly those of Sr and REE, cannot be reconciled
with an origin through partial melting of basaltic material at the base of the volcanic pile. The difference between the comenditic
and pantelleritic differentiation trends of Santa Barbara and Pico Alto is attributed primarily to FO2 control of the crystallizing assemblage, probably related to thermal dissociation of magmatic water in the Santa Barbara
magma chamber. This effect may be augmented by minor differences in parent basaltic compositions, the Pico Alto pantellerites
being derived from the rift basalts whereas the Santa Barbara comendites are derived from the off-rift basalts. A compositional
gap between 54 and 64% silica content in the lavas is not present if the suite is extended to include co-magmatic hypabyssal
xenoliths, leading to the inference that the gap in this and other bimodal suites results solely from a relative inability
of magma of intermediate composition to erupt. |
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