Stratigraphy and Geochemistry of Cerro Escorioso Volcanic Field, Miocene, Patagonia, Argentina.

SALANI, F.M.; LURO, SILVINA E.

Portland

Congreso; 2017 IAVCEI meeting; 2017

IAVCEI

The Cerro Escorioso Volcanic Field (CEVF) forms the basaltic magmatism, which covers an extension of 140 km2 and developed in the extraandean Patagonia of Argentina, southwestern part of the Somún Curá Plateau. This sequence corresponds to the postume stage of the Neogene extensional volcanism in the area. Lavas outpoured over volcanic Mesozoic units as Triassic Garamilla Formation and Jurassic Lonco Trapial Group, Cenozoic volcanics of Sarmiento Group and Miocene acidic lavas of Pire Mahuida Volcanic Complex. Analytical determinations (K-Ar 17 +1 Ma, Salani et al. 1994 and 14.3 + 0.6 Ma, Luro et al., 2015) pointed to a Miocene age. This basaltic field is the product of small centers, which correspond to very eroded or dismantled scoria cones, sometimes buried by lava flows, which make it difficult to study. However, seven vents have been identified, of which two have a good degree of preservation: the Cerro Escorioso and Cerro Negro (42º 14´ 24.31? S; 68º 48´41.96? W). The former stands out from the rest of the cones and it is one that gives the name of the field. Cerro Escorioso is the largest scoria cone, 34.5 m height, 764.5 m in diameter, with 13,10º slopes and show a well exposed sequence of decimeters to 1 m thick levels composed by lapilli and bombs accumulation.The levels exhibit a moderate to dense welding, and could originate clastogenic lavas. The effusive facies associated with the eruption centers are lava flows that extended far away from the vent some of which reach a length of 28 km. The volcano stratigraphic study allowed distinguishing three types of basalts that correspond to three identifiable lithochemical units. At the base of the sequence basaltic flows with olivine and clinopyroxene micro phenocrysts, a groundmass mainly composed by olivine, clinopyroxene and interstitial nepheline occur. These flows are followed by basaltic lavas with ortopyroxene nodules (with clinopyroxene and olivine coronae), showing intergranular groundmass mainly composed by clinopyroxene, olivine and plagioclase. Finally, the basaltic lava flows at the top of the sequence are characterized by a mineral assemblage mainly constituted of orthopyroxene crystals with clinopyroxene rims, plagioclase, opaque minerals and glass, defining intergranular and intersertal textures. The stratigraphic position of each basaltic type matches from base to top with subsaturated basalts, saturated alkaline basalts and subalkaline lavas. These contrasting petrographic and chemical features suggest different sources and/or complex process involved in the magmatic evolution.