Significance of Late Paleozoic Fission-track Ages in Volcanic Rocks from the Lavras Do Sul Region, Southernmost Brazil

Abstract

This paper describes the application of apatite fission-track thermochronology to four outcrop samples of the basaltic to andesitic volcanic rocks of the Hilário Formation, located in the Precambrian Sul-rio-grandense Shield, in southernmost Brazil. The investigated unit was generated during the late stages of the Neoproterozoic Brasiliano/Pan-African Cycle (900-500 Ma), in a context of strike-slip motion along NE-trending structures and consequent basin inception. The apparent fission-track (FT) ages fall in the range 270.1 to 298.2 Ma, late Carboniferous to early Permian. The mean confined fission-track lengths range between 10.76 and 12.20 mm and the estimate for initial fission-track recording (below 110°C) is positioned around 359 and 344 Ma. Considering this, the Neoproterozoic volcanic rocks of the Hilário Formation were buried under at least 4 km of volcanic and sedimentary rocks, resulting in thermal fading of all the original fission-tracks, and remained so until the late Paleozoic. Then, the investigated rocks and the whole region acted as the southern margin and source area for the Paleozoic/Mesozoic intraplate Paraná Basin. A long-lived cooling trend, from the early Carboniferous (359-344 Ma) to the Permian/Triassic boundary (ca. 250 Ma) can be inferred from the analysis of the thermal history curves. This tendency can be associated to the conjunction of uplift and denudation of this region. During the Carboniferous, it probably reflects a response to the compressive events in the southwestern Gondwanan margin, in the context of the Eo-Hercynian (Chañic) Orogeny. This period, in the studied region, was also characterized by extensive ice caps related to the Gondwanan glaciation and by lack of sedimentation. With the collapse of the glaciers, alluvial fan and glaciogenic sedimentation took place during the late Carboniferous and early Permian. This cooling phase is probably recorded by the apparent FT ages. A subsequent uplifting event is revealed by the analysis of the thermal history curves, which show a strong inflection between 90 and 70 Ma. These data are here tentatively correlated with significant re-arrangements of the Brazilian southeastern coast, such as the Serra do Mar and Serra da Mantiqueira uplifts.