La Peligrosa Caldera: a piecemeal transtensional structure in the Jurassic Chon Aike silicic LIP of Southern Patagonia (47º 15´S), Argentina

SRUOGA, P.; JAPAS, S.; SALANI, F. M.; KLEIMAN, L.

Reykjavik. Islandia

Simposio; IAVCEI 2008. General Assembly; 2008

IAVCEI

La Peligrosa Caldera is located at Sierra Colorada (47º 15´S, 69° 20´W) in the Chon-Aike silicic LIP which has an estimated volume of 235.000 km3 Middle to Late Jurassic in age (188-153 Ma) this long-lived and widespread volcanism took place in Patagonia along the western margin of Gondwanaland. Remarkably homogeneous in composition, it is dominated by voluminous ignimbritic plateaux, with granites, lava domes, minor intermediate lavas and epiclastic tuffs. However, collapse?calderas have been exceptionally recognized. La Peligrosa Caldera represents an unique window to understand the eruptive mechanisms which prevailed during the ignimbritic flare-up in Southern Patagonia during middle to late Jurassic times. At the northern flank of Sierra Colorada, Tertiary Andean thrusting and intense glacial erosion have helped to unravel the roots of the deeply dissected La Peligrosa Caldera. Key pieces of evidence are useful to reconstruct the volcanic structure. The Lago Ghío intracaldera ignimbrite, 400 meters thick, is a massive, crystal-rich and densely welded cooling unit. It shows an order of magnitude decrease in thickness at the suspected caldera margins. At the base, an interbedded collapse-breccia deposit carries blocks up to 300 meters in size of rhyolitic lavas, paleozoic schists and lacustrine sediments. Rhyolitic lava domes and flows represent the post collapse volcanism. A prevailing transtensional regime controlled the outpouring of the silicic large-volume ignimbrites and lava flows. The collapse of La Peligrosa Caldera occurred following the formation of a dilatational zone related to a releasing step-over associated with N-S to NNW regional faults. A close relationship between volcanic facies and main structural trends is recognized and allow to propose a piecemeal type caldera. This eruptive model may be applied to the rest of the Chon Aike silicic LIP because it explains the recurrence of pyroclastic and lava units, the absence of wellpreserved collapse-structures and the typical interfingering among different ignimbritic cooling units.