The basaltic volcanism of Pire Mahuida Complex, extraandean Patagonia,

SALANI F M,; REMESAL M B,; PARICA C A,; CERREDO M E

Göttingen,

Otro; 21 LATIN AMERICAN COLLOQUIUM; 2009

LAK

The Pire Mahuida Complex (PMC), located in the extraandean region of the southern part of Argentina, is a bimodal volcanic field developed during Miocene times, likely related to an extensional regime. The PMC is made up mainly of acidic pyroclastic and lava facies, rhyolite/dacite (< trachyte) flows and lava domes. Subordinated in volume, basaltic flows overlie this extensive sequence of acidic rocks. The basic facies displays a wide span of compositional, morphological and structural features which allow distinguishing different groups. The effusion of basaltic rocks represents the final phase of the eruptive history of the PMC. The basalts can be grouped in two main effusive stages. The first one corresponds to lava flows (~20 Ma, Ardolino et al. 2002) of scarce thickness (3-8m). They constitute a main NW-SE plain in the central part of the Sierra de Pire Mahuida, with some basaltic lava tongues flowing long distances northwards. Columnar jointing, colonnade and tabular structures and onion-like parting are common. Rocks are porphyritic with plagioclase phenocrysts. Groundmass consists mainly of clinopyroxene (aegirine-augite and titanoaugite), plagioclase, olivine, apatite and ilmenite. They show intersertal textures with glassy patches and rare phlogopitic mica. Xenolithic plagioclase with disequilibrium rims, clinopyroxene of xenolithic origin with exsolution of opaque minerals, orthopyroxene with clinopyroxene and opaque reaction rims were recognized. Some of basalts contain quartz crystals with glass rims and augite crowns. The younger stage (17 Ma, Salani et al. 1994) of the basaltic facies is represented by the product of minor centers. At Cerro Bayo, a small volcano composed by pyroclastic and lava material, shows a volcanic sequence related to a strombolian eruption. There, brecciated agglomerate with olivine crystals, basaltic lithic and glassy matrix is followed by a basaltic flow with ultramafic inclusions (spinel lherzolithe). The lava flow is a fine grained nephelinite, with minor amounts of olivine (Fo= 85) and titanoaugite phenocrysts; the roundmass is mainly composed of olivine, clinopyroxene microlites, opaque minerals and interstitial nepheline. Vents widely distributed within the volcanic field also originated nefelinite flows that reach the outhernmost part of the PMC. The Cerro Degollado and Negro basaltic plugs are included in this later effusive stage.