Contribution to the geochronology of the Triassic magmatism in the Western Sierras Pampeanas, province of San Juan, Argentina.

LOPEZ, MARIA GIMENA; CASTRO DE MACHUCA, BRÍGIDA; GÓMEZ, RICARDO; PALACIO BALDERRAMO, GLADIS

São Pablo, Brasil

Simposio; VI Simposio de Vulcanismo e Ambientes Associados.; 2015

Universidad de São Pablo

Alkaline volcanic rocks in the Sierra de Valle Fértil, Western Sierras Pampeanas, have been little studied. According to Mirré (1976), the alkaline rocks ranging from olivine basalts to rhyolites intruded or covered the igneous-metamorphic basement of the Valle Fértil Complex (Proterozoic - Lower Paleozoic), as part of a non-orogenic continental association of alleged Triassic age. This volcanism has been linked to extension of the continental lithosphere during the rifting process that led to the development of extensional NNW-striking fault systems and continental rift basins. The aim of this work is to release the first radiometric age obtained for one of these basalts, the Usno Basalt, not previously described in the geological literature. The Usno Basalt (30°33´54.28"S - 67°32´29.08" W), is the most northerly manifestation of this magmatic event in the Sierra de Valle Fértil. The village of Usno is emplaced on the basalt and the few outcrops which can be accessed cover an area of approximately 25 m2. The basalt flow overlies the crystalline basement reaching about 5 m in thickness. Throughout the basalt flow three levels can be distinguished: a violet grey and strongly vesicular lower level; an intermediate reddish one due to intense limonite alteration interpreted as a paleosoil including fragments of the lower level, and a higher massive to poorly vesicular dark gray level with aphyric texture, marked jointing, and incipient columnar disjunction. The Usno Basalt is holocrystalline and inequigranular. It consists of scarce (5 - 8 percent volume) phenocrysts of subhedral plagioclase (3.5 mm maximum length), and microphenocrysts (0.3 - 0.7 mm) of plagioclase, alkali feldspar and olivine totally replaced by iddingsite. The groundmass (0.1 mm average) is composed of oriented feldspar laths, olivine altered to iddingsite, abundant opaques and interstitial alkali feldspar.Geochemistry of the Usno Basalt indicates an alkaline character with average Na2O + K2O = 7.5 percent, and suggests a genesis associated to intraplate magmatism (continental rifting). Radiometric dating using the 40Ar/39Ar method on whole rock was carried out on a sample from the upper level at the laboratory of the Servicio Nacional de Geología y Minería (Chile), giving a combined age of 238.1 ± 0.4 Ma that corresponds to the upper part of the Middle Triassic (Ladinian). The geochemical characteristics and obtained age, clearly relate the Usno Basalt to the magmatism associated with an intra-continental extensional setting in the Western Sierras Pampeanas region during the Triassic.