Somún Curá Magmatic Province (Argentina). Lava-pyroclastic sequence southern of El Cain village: Age and composition.

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

Hamburg

Congreso; 25th Latin American Colloquium of Geosciences; 2019

Universität Hamburg

The volcanic sequence of the Somún Curá Magmatic Province (SCMP) occupies large areas in the south of Río Negro Province and north of Chubut Province, Argentina. SCMP encompass a great variety of basaltic volcanic associations in the north of extrandean Patagonia.The western region of the SCMP is characterized by the stratigraphic complexity including a large time range, a recurrent magmatism and several petrogenetic lineages. In the north of the Barril Niyeu Volcanic Complex (BNVC) (Fig. 1 a; b), one of the most important bimodal associations of the PMSC, and to the south of El Cain village, a sequence of lava and pyroclastic rocks outcrops (Fig. 1 b). These exposures record two basaltic events separated by a time gap, currently evaluated only based on stratigraphic relationships. Further estimations are provided by newly acquired radiometric dating, which are the focus of this contribution.The oldest basaltic event is represented by a fine grained rock of intersertal texture, with plagioclase in a fluidal arrangement, olivine, clinopyroxene and opaque minerals, and significant zeolitization. A whole rock K/Ar age of basalt flow of this unit yielded 53 ± 2 Ma. Resting over this basalt there are fall deposits and pyroclastic flows with discontinuous intercalations of pyro-epiclastic deposits assigned to the Sarmiento Group (Oligocene). This sequence is >70 m thick and constitutes prominent cliff covered by a new episode of basaltic lavas that forms a plateau which frames the southern sector of the El Cain depression. The flows, originated in an apparatus located 13km to the west, are light gray basalts with vesicular roofs and average thickness of 5 m. Rocks are porphyric traquibasalts of fluidal texture, with plagioclase, titaniferous augite, iddingsitized olivine and interstitial alkali feldspar. This rock, yielded a whole rock K/Ar age of 18.7 ± 0.4 Ma, that represents the youngest age of the BNVC effusions.Therefore, the radiometric dating constrains the BNVC activity between the Eocene (Ypresian) and the Miocene (Burdigalian). Although these ages correspond to a very restricted area of the PMSC, the correlation with regional events allows to extrapolate this time range to a wide area of the western sector of this Province. The Palaeogene volcanism is represented by several necks of the Cerro Cortado Fm., El Buitre Fm. and equivalents, which have been correlated with the basalt outcrops of Basalto Pilquiniyeu in the west of the PMSC. The oldest basalt in the studied profile correlates with this magmatism that extends between the Paleocene and the Eocene with a climax in the Early Eocene. In addition, the most modern events correspond to episodes linked to large bimodal complexes such as Barril Niyeu, Apas, Talagapa and Pire Mahuida (Fig. 1 a).