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This paper focuses the tectonic evolution of the southern Brasília belt, with emphasis on the Furnas segment, along the 21°S parallel. The uppermost structural unit (Passos Nappe -PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western São Francisco craton. An inverted metamorphic gradient ranging from greenschist to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain displays a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with undated siliciclastic low-grade metasedimentary rocks. The São Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambuí group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of São Francisco Craton, coeval with the exhumation of the External Domain thrust sheets. New K-Ar determinations on mineral separates are presented and interpreted among previous data. The SFC basement rocks display Paleo-to Mesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a "cold" São Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their K-Ar systems reset during the allochthony.Keywords: Brasilia Belt, K-Ar, Neoproterozoic, thrust tectonics, Brasiliano orogeny 1 -UERJ -Rio de Janeiro State University (cmval@uerj.br); 2-USP -São Paulo University; 3-UNESP -São Paulo State University; 4-CNPq researcher. INTRODUCTIONThe Brasília belt is an important tectonic element of the Tocantins orogenic Province, developed during the Neoproterozoic Gondwana assembly, through convergence between the Amazonian, São Francisco and a third cratonic block presently covered by the Paraná Basin (Brito Neves et al. 1999). The southern portion of Brasilia belt (Fig. 1) has its eastern tectonic elements consistently thrusted eastwards towards the border of São Francisco craton (SFC). Southwards, the Ribeira belt, displaying NE-SW structural fabric along the Atlantic coast and a relatively younger tectonic development, with metamorphic peaks dating around 580 and 530 Ma (Machado et al. 1996) truncates it.This article presents an overview of the tectonic organization of southern Brasilia belt, with emphasis on the Furnas segment, along the 21°S parallel. In that region, the Passos nappe displays low to highgrade Brasiliano metamorphism and was exhumed and thrusted at least 250 km onto a relatively cold foreland zone (eastern border of the São Francisco craton) as thin-skinned tectonic slices. K-Ar data help to constrain the timing and thermal conditions of allochthony.

-The South Orkney Islands are the exposed part of a continental fragment on the southern limb of the Scotia arc. The islands are to a large extent composed of metapelites and metagreywackes of probable Triassic sedimentary age. Deformation related to an accretionary wedge setting, with associated metamorphism from anchizone to the greenschist facies, are of Jurassic age (176-200 Ma). On Powell Island, in the centre of the archipelago, five phases of deformation are recognized. The first three, associated with the main metamorphism, are tentatively correlated with early Jurassic subduction along the Pacific margin of Gondwana. D 4 is a phase of middle to late Jurassic crustal extension associated with uplift. This extension phase may be related to opening of the Rocas Verdes basin in southern Chile, associated with the breakup of Gondwanaland. Upper Jurassic conglomerates cover the metamorphic rocks unconformably. D 5 is a phase of brittle extensional faulting probably associated with Cenozoic opening of the Powell basin west of the archipelago, and with development of the Scotia arc.

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A subduction complex composed of ocean floor material mixed with arc-derived metasediments crops out in the Elephant Island group and at Smith Island, South Shetland Islands, Antarctica, with metamorphic ages of 120-80 Ma and 58-47 Ma, respectively. Seven metamorphic zones (I-VII) mapped on Elephant Island delineate a gradual increase in metamorphic grade from the pumpellyite-actinolite facies, through the crossite-epidote blueschist facies, to the lower amphibolite facies. Geothermometry in garnetamphibole and garnet-biotite pairs yields temperatures of about 350°C in zone III to about 525°C in zone VII. Pressures were estimated on the basis of Si content in white mica, Al 2 O 3 content in alkali amphibole, NaM4/AlIV in sodic-calcic and calcic amphibole, AlVI/Si in calcic amphibole, and jadeite content in clinopyroxene. Mean values vary from about 6-7.5 kbar in zone II to about 5 kbar in zone VII. Results from the other islands of the Elephant Island group are comparable to those from the main island; Smith Island yielded slightly higher pressures, up to 8 kbar, with temperatures estimated between 300 and 350°C. Zoned minerals and other textural indications locally enable inference of P-T -t trajectories, all with a clockwise evolution. A reconstruction in space and time of these P-T -t paths allows an estimate of the thermal structure in the upper crust during the two ductile deformation phases (D 1 & D 2 ) that affected the area. This thermal structure is in good agreement with the one expected for a subduction zone. The arrival and collision of thickened oceanic crust may have caused the accretion and preservation of the subduction complex. In this model, D 1 represents the subduction movements expressed by the first vector of the clockwise P-T -t path, D 2 reflects the collision corresponding to the second vector with increasing temperature and decreasing pressure, and D 3 corresponds to isostatic uplift accompanied by erosion, under circumstances of decreasing temperature and pressure.

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