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

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

Hamburgo

Congreso; 25Latin-American Colloquium og Geosciences; 2019

Universitát Hamburg

The volcanic sequence of the Somún Curá MagmaticProvince (SCMP) occupies large areas in the south of RíoNegro Province and north of Chubut Province, Argentina.SCMP encompass a great variety of basaltic volcanic associationsin the north of extrandean Patagonia.The western region of the SCM P is characterized by thestratigraphic complexity including a large time range, arecurrent 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 associationsof 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 separatedby a time gap, currently evaluated only based onstratigraphic relationships. Further estimations are providedby newly acquired radiometric dating, which arethe focus of this contribution.The oldest basa ltic event is represented by a finegrained rock of intersertal texture, with plagioclase in afluidal arrangement, olivine, clinopyroxene and opaqueminerals, and significant zeolitization. A whole rock K/Arage of basalt flow of this unit yielded 53 ± 2 Ma. Restngover this basalt there are fall deposits and pyroclasticflows with discontinuous intercalations of pyro-epiclasticdeposits assigned to the Sarmiento Group (Oligocene).This sequence is > 70 m thick and constitutes prominentcliff covered by a new episode of basaltic lavas that formsa plateau which trames the southern sector of the ElCain depression. The flows, originated in an apparatuslocated 13km to the west, are light gray basalts with vesicularroofs and average thickness of 5 m. Rocks are porphyrictraquibasalts of fluidal texture, with plagioclase,titaniferous augite, iddingsitized olivine and interstitialalkali feldspar. This rock, yielded a whole rock K/Ar ageof 18. 7 ± 0.4 Ma, that represents the youngest age of theBNVC effusions.Therefore, the radiometric dating constrainsthe BNVC activity between the Eocene (Ypresian)and the Miocene (Burdigalian). Although these ages correspondto a very restricted area of the PMSC, the correlationwith regional events allows to extrapolate thistime range to a wide area of the western sector of thisProvince. The Palaeogene volcanism is represented bysevera! necks of the Cerro Cortado Fm., El Buitre Fm. andequivalents, which have been correlated with the basaltoutcrops of Basalto Pilquiniyeu in the west of the PMSC.The oldest basalt in the studied profile correlates withthis magmatism that extends between the Paleoceneand the Eocene with a clímax in the Early Eocene. In addition,the most modern events correspond to episodeslinked to large bimodal complexes such as Barril Niyeu,Apas, Talagapa and Pire Mahuida (Fig. 1 a).