New geochronological and geochemical constrains for extra back-arc volcanism in Somuncura region, Northern Patagonia

ORIHASHI Y; MOTOKI A; HALLER, MJ; SUMINO H; IWAMORI H; CARIO FD; HIRATA G; ANMA R; NAGAO K

Pucón

Congreso; International Association of Volcanology and Chemistry of the Earth's Interior, General Assembly; 2004

IAVCEI

Oligocene to Mid-Miocene Somuncura basaltic plateau widely covers an area of roughly 20,000 km2 in extra-back arc province, northern Patagonia. Previous studies suggested two contrasting models for the magmatism in extra-back arc region; 1) upwelling of either small-scale “hot spot” (Kay et al., 1993) or “wet” plume (Iwamori, 1992), and 2) asthenospheric upwelling resulted by the slab roll-back of the Farallon plate (de Ignacio et al., 2001) or by opening of slab window beneath the region, that followed ridge subduction (Gorring et al., 1997). However, further discussion supported by concrete evidence is required to specify magmatism of the Somuncura basalt. In this study, we determined both major & trace element compositions and K-Ar ages of 47 basaltic samples collected from northern Somuncura plateau and surrounding area (Cerro Colorado, Quetlequile, Las Bayas, etc.) to understand spatiotemporal magmatic evolution of the region. Our K-Ar ages indicate that the activity of Somuncura basalts started in Oligocene (34 Ma), were most active at 22-23 Ma, and attenuated toward Mid-Miocene (18-11 Ma) in the Somuncura region, but is traceable in surrounding area down to 5.6-0.34 Ma. As for whole rock chemistry, variation in major elements is likely to result from different degree of partial melting of hydrous peridotite. The alkalinity increased and concentration patterns of “fluid-favor elements” (e.g., K, Rb, Ba and Pb) seemed to change with time, which is here attributed to either multiple upwelling of fluid-rich asthenospheric mantles or to degree of contamination of different lithospheric mantles in each volcanic event.