Volcanic rocks in the Narragansett basin, southeastern New England: Petrology and significance to early basin formation

Citation
A. Maria et Od. Hermes, Volcanic rocks in the Narragansett basin, southeastern New England: Petrology and significance to early basin formation, AM J SCI, 301(3), 2001, pp. 286-312
Citations number
100
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
AMERICAN JOURNAL OF SCIENCE
ISSN journal
0002-9599 → ACNP
Volume
301
Issue
3
Year of publication
2001
Pages
286 - 312
Database
ISI
SICI code
0002-9599(200103)301:3<286:VRITNB>2.0.ZU;2-G
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 py roclastic 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 repo rted for the rhyolite, based on U/Pb geochronology (Thompson and others, 19 99), The basalt flows are typically 1 to 2 m thick and marked by pillows, s ediment dikes, and magma/sediment commingling features. The two younger flo ws contain interganular to subophitic clinopyroxene and sparse phenocrysts of plagioclase and pseudomorphed olivine in a pilotaxitic groundmass. The t wo older flows are similar but contain no pyroxene, The rhyolite flows, fro m 3 to 20 m thick, are characterized by subhorizontal quartz seams that rep resent cooling cracks or zones of preferential vesiculation filled with lat e-stage quartz. Both rhyolite flows contain phenocrysts of anorthoclase in a granophyric, devitrified groundmass with relict perlitic features. Identi fication of pyroclastic deposits beneath one rhyolite flow indicates that e xtrusion was preceded by explosive activity. Major and trace elements indic ate 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 ea rly basin sediments suggests that the volcanics reflect a rifting event ass ociated with the formation of the Narragansett basin. Though it is now appa rent that the basin was active well before the Pennsylvanian, it is not cle ar whether the rift event was related to the extensional environment that p revailed through much of the Paleozoic, producing the alkalic plutonism pro minent in this part of the Avalon Zone. Similarities to Devonian-Carbonifer ous bimodal suites in the Maritimes basin of Canada suggest possibly analog ous origins.