Carboniferous Extensional Volcanism

Abstract

Carboniferous volcanic records outcropping along the northwestern of Famatina Belt-southeastern Puna, NW Argentina, display continuous and well-exposed deposits of volcanic rocks, constituted by rhyolites and lesser volume of basalts and trachytes, related with volcaniclastic deposits. These rocks, assigned to the Lower Pennsylvanian (320 Ma U–Pb age), represent a bimodal volcanic succession whose geochemical and isotopic characteristics point to an extensional source regimen. Petrographic, structural, and textural characteristics of the sequence give indications of the components nature, emplacement, and environmental conditions. The association of porphyritic and vitrophyric lavic rhyolites with equivalent breccias is consistent with the eruption mechanism of central lava domes. The dominance of rhyolitic lithic clasts in the breccias suggests explosive eruptions. Basalts, as concordant lava flows, display autoclastic facies at their margins indicating locally high strain rates in an already more brittle lava body during continuous effusion. The association of rhyolites with their volcanoclastic breccias followed by minor basaltic and intermediate lavas suggests episodic eruption pulses triggering instabilities in the volcanic edifices and degradation of their flanks, with emission of lava and breccia flows and concomitant collapse of the central lava domes. The volcanic succession was locally affected by a later shear deformation episode.