CONICET | Buscador de Institutos y Recursos Humanos

Petrography and geochemistry of basic lavas from the northwestern part of Meseta de Somuncurá, Río Negro province.In the northwestern part of Meseta de Somuncurá basic volcanic rocks crop out. According to their petrography and geochemical characteristics they can be discriminated in two groups: A) Basic lavas with orthopyroxene microphenocrysts developing a coronitic rim with the volcanic groundmass; also plagioclase and subordinate olivine microphenocrysts are present in an intersertal groundmass of plagioclase, titaniferous augite, olivine, opaque minerals and interstitial volcanic glass. According to TAS diagram they classify as basaltic andesites, while the proportions and relative abundances of trace elements (included the rare earth elements) indicate transitional characteristics between alkaline and subalkaline suites, with geochemical tendency between EMORB and OIB standards. Likewise, the multielement diagram normalized to EMORB display relative positive Ba and Sr anomalies. On the other hand, group B) Basic lavas with plagioclase and subordinated olivine microphenocrysts in an intergranular groundmass of plagioclase, titaniferous augite, olivine and opaque minerals. According to TAS diagram they classify as trachy-basalts and their relative abundances of trace elements, included the rare earth elements, indicate OIB geochemical signature, showing relative positive anomaly in Ba and negative in Zr, both in multielement diagram normalized to EMORB. Given the above characteristics and the previous studies in the Meseta de Somuncurá area, it is proposed that group A can be correlated with the Somuncurá Formation (plateau) and group B with the postplateau event and the units of Quiñelaf Superunit, younger than the Somuncurá Formation.

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