Electron microprobe U-Th-Pb monazite dating of the Transamazonian metamorphic overprint on Archean rocks from the Amapa Block, southeastern Guiana Shield, Northern Brazil

The Amapa Block, southeastern Guiana Shield, represents an Archean block involved in a large Paleoproterozoic belt, with evolution related to the Transamazonian orogenic cycle (2.26 to 1.95 Ga). High spatial resolution dating using an electron-probe microanalyzer (EPMA) was employed to obtain U-Th-Pb chemical ages in monazite of seven rock samples of the Archean basement from that tectonic block, which underwent granulite- and amphibolite-facies metamorphism. Pb-Pb zircon dating was also performed on one sample. Monazite and zircon ages demonstrate that the metamorphic overprinting of the Archean basement occurred during the Transamazonian orogenesis, and two main tectono-thermal events were recorded. The first one is revealed by monazite ages of 2096 +/- 6, 2093 +/- 8, 2088 +/- 8, 2087 +/- 3 and 2086 +/- 8 Ma, and by the zircon age of 2091 +/- 5 Ma, obtained in granulitic rocks. These concordant ages provided a reliable estimate of the time of the granulite-facies metamorphism in the southwest of the Amapa Block and, coupled with petro-structural data, suggest that it was contemporaneous to the development of a thrusting system associated to the collisional stage of the Transamazonian orogenesis, at about 2.10-2.08 Ga. The later event, under amphibolite-facies conditions, is recorded by monazite ages of 2056 +/- 7 and 2038 +/- 6 Ma, and is consistent with a post-collisional stage, marked by granite emplacement and coeval migmatization of the Archean basement along strike-slip shear zones. (C) 2008 Elsevier Ltd. All rights reserved.