Document Type

Article

Date of Original Version

2001

Abstract

A suite of volcanic rocks in the northwest corner of the Narragansett basin includes at least four basalt flows, two rhyolite flows, and associated pyroclastic rocks. The volcanics are interbedded with non-marine, sedimentary rocks within the lower part of the Wamsutta Formation and, until recently, were considered Pennsylvanian in age. An age of about 373 Ma has been reported for the rhyolite, based on U/Pb geochronology (Thompson and others, 1999). The basalt flows are typically 1 to 2 m thick and marked by pillows, sediment dikes, and magma/sediment commingling features. The two younger flows contain intergranular to subophitic clinopyroxene and sparse phenocrysts of plagioclase and pseudomorphed olivine in a pilotaxitic ground- mass. The two older flows are similar but contain no pyroxene. The rhyolite flows, from 3 to 20 m thick, are characterized by subhorizontal quartz seams that represent cooling cracks or zones of preferential vesiculation filled with late-stage quartz. Both rhyolite flows contain phenocrysts of anorthoclase in a granophyric, devitrified ground- mass with relict perlitic features. Identification of pyroclastic deposits beneath one rhyolite flow indicates that extrusion was preceded by explosive activity. Major and trace elements indicate that the volcanics are mildly alkaline, and geochemical trends suggest the basalt and rhyolite originated by partial melting of different sources, followed by limited fractional crystallization. Apparent restriction to early basin sediments suggests that the volcanics reflect a rifting event associated with the formation of the Narragansett basin. Though it is now apparent that the basin was active well before the Pennsylvanian, it is not clear whether the rift event was related to the extensional environment that prevailed through much of the Paleozoic, producing the alkalic plutonism prominent in this part of the Avalon Zone. Similarities to Devonian-Carboniferous bimodal suites in the Maritimes basin of Canada suggest possibly analogous origins.

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