THE NEOPROTEROZOIC BRASILIANO OROGENY IN NORTHEAST BRAZIL - AR-40 AR-39 AND PETROSTRUCTURAL DATA FROM CEARA/

The Borborema Province in NE Brazil comprises Paleoproterozoic basemen t and its Proterozoic cover that experienced a complex tectonometamorp hic history during the Brasiliano/Pan-African orogeny. In Cear6, four major domains separated by continental-scale, steeply dipping dextral shear zones have been identified. Ar-40/Ar-39 step-heating of mineral separates and laser-probe fusion of single grains have been used in co njunction with petro-structural observations to delineate the cooling/ tectonic history of these domains. To the north of the Senador-Pompeu shear zone (SPSZ), amphibole and biotite pairs from low-pressure granu lite belts exposed along the Granja shear zone and SPSZ record plateau ages in the range 575-560 Ma corresponding to cooling rates close to 20 degrees C/m.y.Ar-40/Ar-39 laser probe muscovite dates from the Chav al augen gneiss point to a similar age for shearing responsible for th e exhumation and fast cooling of the granulite belts. A muscovite-biot ite pair from the Central Ceara high-pressure nappes lying between the shear zones documents a lower cooling rate in the interval 534-525 Ma related to the decompression and uplift of previously deeply buried r ocks. To the south of the SPSZ, only moderate crustal thickening occur red within the various schist belts. A distinct cooling pattern is ind icated for these belts and the western Patos shear zone, attesting to a different crustal development. Ar-40/Ar-39 dates of amphibole, phlog opite, muscovite and biotite range from 550 to 522 Ma, recording a pos tmetamorphic cooling rate of 6-9 degrees C/m.y. Slower cooling rates o f 3-4 degrees C/m.y. between amphibole and biotite closure ages in the range 540-500 Ma are reported for the eastern part of the Patos shear zone system. The diachronous cooling recorded along this shear zone i s interpreted to reflect the variations in thickness of the high-tempe rature mylonitic belts: earlier, more rapid cooling occurred where the belts are narrow in the west; late, slower cooling occurred in wider zones to the east. These thermochronological data reveal an age and co oling rate distribution at the scale of the Borborema Province that pr ovides new constraints on the development of the Brasiliano orogeny be tween 580 and 500 Ma.