Journal of Petrology Volume 41 Number 9 Pages 1413-1438 2000
© Oxford University Press 2000
Early Cretaceous Basaltic and Rhyolitic Magmatism in Southern Uruguay Associated with the Opening of the South Atlantic
1DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES MK7 6AA, UK
2FACULTEIT DER AARDWETENSCHAPPEN, VRIJE UNIVERSITEIT, 1085 DE BOELELAAN, 1081 HV AMSTERDAM, THE NETHERLANDS
3DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, L, DK-1350, COPENHAGEN K, DENMARK
4DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH, 7700 SOUTH AFRICA
5DEPARTMENTO DE GEOFÍSICO, INSTITUTO ASTRONÔMICO E GEOFÍSICO, UNIVERSIDADE DE SÃO PAULO, RUA DO MATÃO 1226, CEP 05508-900, SÃO PAULO SP, BRAZIL
The Early Cretaceous volcanic rocks of southern Uruguay comprise mafic and felsic volcanics. The position of these outcrops at the southern edge of the Paraná–Etendeka continental flood basalt province provides an opportunity to investigate possible lateral variations in both mafic and more evolved rock types towards the margins of such an area of plume-related magmatism. The mafic lavas are divided into two compositionally distinct magma types. The more voluminous Treinte Y Trés magma type is similar to the low-Ti basalts of the Paraná flood basalt province. The Santa Lucía magma type is a distinct and rare basalt type with ocean-island basalt type asthenospheric affinities (high Nb/La, low 87Sr/86Sri). The felsic volcanics are divided into two series, the Lavalleja Series and the Aigüa Series. The Lavalleja Series are chemically and isotopically similar to the Paraná–Etendeka low-Ti rhyolites, and are considered to be related to the Treinte Y Trés lavas by extensive fractionation and crustal assimilation. The Aigüa Series have low 143Nd/144Ndi and low 87Sr/86Sri and unlike the rhyolites of the Paraná, are interpreted as melts of pre-existing mafic lower crust that subsequently underwent extreme fractionation. The differences observed in the felsic suites may be linked to differences in the volumes of the associated basalts and the amounts of extension.
KEY WORDS: South America; flood basalts; felsic volcanics; crustal melts; plume
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