Magmatism in southern Uruguay and the early rifting of the South Atlantic.
A suite of early Cretaceous vo1canic rocks (Puerto Gomez and Arequita Fms.) are
preserved within the N60° E trending Santa Lucia Basin, southern Uruguay which lies at
the southern margin of the Parana - Etendeka continental flood basalt province. New Ar-Ar ages of the basalts range from 134 to 130 Ma while ages from the rhyolites range
from 130 to 124 Ma. This magamtism was contemporaneous with the main flood basalt
event, although rhyolite activity continued after rifting
(127 - 126 Ma). The province
therefore contains unique information about melting conditions at the periphery of the
influence of the Tristan da Cunha plume.
The volcanic rocks of southern Uruguay are bimodal in silica, and the majority of basalts
of the Puerto G6mez Fm., herein termed the Treinte Y Tres magma type, have major-,
trace- element and initial isotope ratios similar to the low -Ti/Y Gramado -Tafelberg
magma types of the Parana- Etendeka. There are also a number of unique basalt samples
termed the Santa Lucfa magma type. which have low La/Nb, and are considered to have
been generated by mixing between lithosphere - and asthenosphere - derived melts.
These magmas represent the first sampling of true plume material in this CFB province.
The rhyolites of the Arequita Fm. are relatively evolved with variably sized euhedral to
anhedral quartz phenocrysts, and ignimbritic textures that are the first described from this
province. The rhyolites have lower magmatic temperatures (8S0 - 950°C) than those of
the Parana - Etendeka, and are divided into two geochemical series, the Lascano Series
and the Aigtia Series. The rhyolites of the Lascano and AigUa Series are not related to
the Puerto G6mez Fm. basalts, but rather they originated from separate sources in the
mid to lower crust, where melting was facilitated by mid-crustal level intrusions of
basaltic material as recognised from a large gravity anomaly. Melt production rates in Southern Uruguay were low (0.01km3 yr1) similar to the rates calculated
for the waning
stages of magmatism on the Serra Geral escarpment, southern Brazil. These rates are
consistent with the notion that by this time the principal melt production was located in
the newly forming ocean with Uruguay at the margins of the influence of the plume.