Compositional variations in Post Plateau Basalts of Somun Cura, Patagonia

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

Kiel, Alemania

Otro; 20th Colloquim on Latein American Earth Sciences; 2007

The basaltic plateau of Somun Cura is interpreted as flood basalt on a wide extension at the Northern Argentine Patagonia, its development seems to be related to tectonic environment complex not fully explained. Since the former papers until today a good consensus was agreed about the existence of more than one episode in the formation of this magmatic sequence; 1) the basalt plateau with uniform composition and 2) the younger postplateau stage (Remesal et al. 2001, Salani et al. 2006). Into the post plateau stage it is possible to differentiate lithological variations. The most conspicuous difference let to separate the big trachyte-basalt complexes (represented at the ranges of Alta Sierra de Somun Cura, Apas, Chauchañe, etc.) from the monogenetic minor centres (Trayen Niyeu, Corona Chico, Dos Hermanos and others). The monogentic centres are characterized by a wide distribution in the plateau environment, for their exclusive basaltic composition and different type of eruption as a result of compositional variations (essentially because the content of volatile and silica). The studies carried out let to identify at least four types of basalts: a) strongly alkaline basalts, with nephelinites and noseanic basalts. b) alkaline to transitional basalts, porphyritic according to megacrystals of ortopyroxene, clinopyroxene, olivine and plagioclase in disequilibrium. c) olivine alkaline basalts and d) subalkaline basalts with crystals of orthopyroxene in reaction to clinopyroxene. Particularly the alkaline olivine basalts are the most common type associated to the biggest structures bimodal type complexes. The undersaturated basalts to the alkaline to transitional basalts are mostly related to small spatter cones and low volume lava flows. The subalkaline type are related to monogenetic vent with more complexity in the structure. A temporal superposition and coexistence of different basaltic effusions should be strongly possible for the postplateau episode also for the bimodal complexes and the monogenetic centres. At these times there no enough evidences to establish the final sequence. Furthermore, a temporal short rank for the basalt emissions is bounded with ages between 20 to 12 Ma in the Miocene. The petrographic evidences in the postplateau basalts, with spread of minerals in disequilibrium support the hypothesis of mixing processes between liquids of different origin. The geochemistry parameters sustain this idea and appoint the existence of different of sources at different depths and/or liquids with different degrees of melting, also supported because the variations of isotopic ratios for Sr and Pb. The isotopic ratios of Sr and Pb are also effective for characterization of each one of the four groups (strongly alkaline basalts, the alkaline to transitional basalts, the alkaline olivine basalts and the subalkaline basalts).