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The neoproterozoic Brasiliano orogeny in northeast Brazil: 40Ar39Ar and petrostructural data from Ceará

1997; Elsevier BV; Volume: 81; Issue: 3-4 Linguagem: Inglês

10.1016/s0301-9268(96)00037-x

1872-7433

P. Monié, Renaud Caby, Michel Henri Arthaud,

Geological and Geophysical Studies Worldwide

The Borborema Province in NE Brazil comprises Paleoproterozoic basement and its Proterozoic cover that experienced a complex tectonometamorphic history during the Brasiliano/Pan-African orogeny. In Ceará, four major domains separated by continental-scale, steeply dipping dextral shear zones have been identified. 40Ar39Ar step-heating of mineral separates and laser-probe fusion of single grains have been used in conjunction 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 granulite 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°C/m.y. 40Ar39Ar laser probe muscovite dates from the Chaval augen gneiss point to a similar age for shearing responsible for the exhumation and fast cooling of the granulite belts. A muscovite-biotite pair from the Central Ceará 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 deelly buried rocks. To the south of the SPSZ, only moderate crustal thickening occurred within the various schist belts. A distinct cooling pattern is indicated for these belts and the western Patos shear zone, attesting to a different crustal development. 40Ar39Ar dates of amphibole, phlogopite, muscovite and biotite range from 550 to 522 Ma, recording a postmetamorphic cooling rate of 6–9°C/m.y. Slower cooling rates of 3–4°C/m.y. between amphibole and biotite closure ages in the range 540-500 Ma are reported for the eastern part of the Palos shear zone system. The diachronous cooling recorded along this shear zone is interpreted to reflect the variations in thickness of the high-temperature 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 cooling rate distribution at the scale of the Borborema Province that provides new constraints on the development of the Brasiliano orogeny between 580 and 500 Ma.