La Peligrosa Caldera: a key for the Jurassic volcanism in Southern Patagonia (47º 15´`S), Argentina.

PATRICIA SRUOGA

Christchurch, New Zealand

Simposio; Gondwana 11 Conference.; 2002

University of Canterbury

La Peligrosa Caldera is located at Sierra Colorada (47º 15´S) in the Chon-Aike Province (Kay et al., 1989), which constitutes a silicic LIP (large igneous province) with an estimated volume of 235.000 km3 (Pankhurst et al., 1998). Middle to Late Jurassic in age (188-153 Ma, Pankhurst et al., 2000) this long-lived and widespread volcanism took place in Patagonia along the western margin of Gondwanaland. Remarkably homogeneous in composition (Sruoga, 1989; Pankhurst et al., 1998) it is dominated by voluminous ignimbritic plateaux, with granites, lava domes, minor intermediate lavas and epiclastic tuffs. Exceptionally, collapse-calderas have been recognized. La Peligrosa Caldera (Sruoga, 1994) is presented as a key case to understand the eruptive mechanisms which prevailed during the ignimbritic flare-up in Patagonia in Jurassic times. At the northern flank of Sierra Colorada Tertiary Andean thrusting and intense glacial erosion have exposed the roots of the deeply dissected La Peligrosa Caldera. Two ignimbritic units, Lago Ghío and Sierra Colorada, may be distinguished. The Lago Ghío intracaldera ignimbrite, 400 meters thick, is a massive, crystal-rich and densely welded cooling unit. Interbedded collapse-breccias carry blocks up to 15 meters in size, of rhyolitic lavas, Paleozoic schists and lacustrine sediments. Rhyolitic lava domes and dyke swarms evidence post collapse volcanism, crudely defining the ring fault system. The overlying Sierra Colorada ignimbrite probably represents a caldera-related outflow facies of unknown location. A model of caldera complexes, including nested calderas of variable size, similar to the San Juan Volcanic field in USA, is proposed for the Chon Aike Province. This eruptive model is suitable to explain the recurrence of pyroclastic and lava units, the absence of well-preserved collapse-structures and the typical interfingering of different ignimbritic cooling units.