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A. Ardolino A. Busteros R. Cucchi M. Franchi H. Lema M. Remesal

Resumen

Eocene alkaline-basic intrusives crop out at the southem border of Somún Cura Plateau, near Bajada del Diablo and Gan Gan. These intrusive bodies are included in the El Buitre Formation. It shows domic or tabular morphologies with diameters ranging from 1 to 3 km. El Buitre Formation shows intrusion relation with the Senonian sediments of La Colonia Formation, but the reIations with other formations, such as Sarmiento tuff or Somún Cura basalts are unknown. The intrusives from Bajada del Diablo and Gan Gan show lithologic and morphologic similitudes with Eocene intrusives of other localities such as El Sombrero, Colonia Sarmiento and Ingeniero Jacobacci. Therefore, the intrusives could constitute a 350 km length discontinuous belt in Chubut Province, Argentina. The petrographic characteristics of these intrusive bodies show that they form a gabbro - foid gabbro - foid monzogabbro serie with rare syenitic compositions. Its grain size varies from very coarse to fine; granular, porphyric and subophitic textures are prevailing. Typical mineralogic association is plagioclase (An 48-56%) - clinopyroxene - olivine - opaque minerals and analcime. Minor components are alkali feldspar, feldspathoids, biotite and apatite. El Buitre Formation can be considered as a subvolcanic event of a subsaturated sodic alkaline magmatism that characterizes continental intraplate regions.KEY WORDS. Patagonia. Paleogene. Volcanism. lntraplate. 

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