Geological relationships of basalts, andesites and sand injectites at the base of the Parana volcanic province, Torres, Brazil

摘要

The Cretaceous interaction of the heated, giant Guarani aquifer with lavas, dikes and sills of the Parana volcanic province is here evaluated based on the description of the large exposures in Torres, southernmost Brazil. Chalcedony silicified sandstone dikes, sills and breccias containing volcanic clasts in a sandstone matrix are common in the Parana volcanic province. Sandstone layers mark either the contact between lava flows or the base of the vertically-jointed, massive core. In Torres, one basaltic andesite and one andesite lava flow of the low-Ti Gramado type, and basalt and basaltic andesite sills and dikes, are correlated over 10 km with rocks in the Graxaim quarry. Scintillometric emission rates vary systematically with contents of K, Th and U and are characteristic of each lava or intrusive rock. Major and trace elements are also characteristic of each lava. Three large outcrops of breccias display angular, amygdaloidal volcanic clasts immersed in a silicified sandstone matrix. No evidence was found of high temperature (1150 ℃) interaction of the lava with the loose, erg of the Botucatu Formation, such as fluidal volcanic clasts, quenched rims or thermal recrystal-lization of the sand; only a thin (5 mm) layer of thermally modified sand is present on top of a paleodune in Torres. In Torres, the interaction of andesite with wet sand occurred after degassing of the lava, because the breccia clasts are amygdaloidal. All evidence points to hydrothermal processes for the formation of the breccias, such as lozenge fractures filled with silicified sandstone, preserved detrital characteristics of the sandstone grains and cold (below 130 ℃) contact between the volcanic rock and the sand. Most significant is the timing of sand intrusion, because thin (1 mm) dikes of silicified sandstone cut fractures in the rock and the zeolite-filled amygdales. The sandstone dikes, sills and breccias are interpreted as a result of the ascent of over-pressured heated water and its vapor from the huge, underlying Guarani aquifer, carrying fluidized sand and continuously silicifying the intruded sand. The three breccias are sills intruded into cold lava flows. The description and correlation of the magmatic bodies and the field relationships of the silicified sandstones establish the stratigraphy of the Parana volcanic province in Torres and elucidate the processes responsible for the injection and effusion of sand in the Cretaceous.