Magmatism in southern Uruguay and the early rifting of the South Atlantic

Kirstein, Linda A. (1997). Magmatism in southern Uruguay and the early rifting of the South Atlantic. PhD thesis The Open University.



A suite of early Cretaceous vo1canic rocks (Puerto GOmez and Arequita FIm.) 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 Paranl-Etendeka. There are also a number of unique basalt samples termed the Santa Lucia 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 anbedral quartz phenocrysts, and ignimbritic textures that are the first described from this province. The rhyolites have lower magmatic temperatures (850 - 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 Gomez Fm. basaks, but rather they originated from separate sources in the mid to lower crust. where melting was facilitated by mid-crustal level intrusions of basaJtic material as recognised from a large gravity anomaly. Melt production rates in southern Uruguay were low (0.01 km3 yr-1) similar to the rates ca1cuIated 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.

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