Age of pegmatites from eastern Brazil and implications of mica intergrowths on cooling rates and age calculations

Abstract

U–Pb and K–Ar dating of selected minerals from different types of pegmatites in the northern region of the eastern Brazilian pegmatite province (EBPP) are reported. A concordant U–Pb age of 498±3 Ma for monazite from a simple, quartz-feldspar pegmatite without gem minerals corresponds to the crystallization age related to the Brasiliano-Pan-African posttectonic magmatic stage. This correlation is substantiated by a discordant 207Pb/ 206Pb age of 498±11 Ma for a zircon fraction that comprises large, prismatic crystals of pegmatitic origin with recent lead loss. The U–Pb isotope systematics of another zircon fraction composed of fine-grained, transparent grains indicates inheritance from older basement rocks. K–Ar age determinations for the core and rims of very large crystals of muscovite from more evolved, beryl-bearing pegmatites yield a mean age of 498±4 Ma. However, K–Ar dating of biotite enclosed in muscovite crystals results in a younger age of 485±4 Ma. This difference in age of ca. 13 Ma is interpreted to correspond to the time span for cooling from 400 to 350 °C (reported closure temperatures for K–Ar isotope systems of coarse-grained muscovite and biotite, respectively), which suggests a mean cooling rate of 3.3 °C/Ma. As such, it took 60 Ma for the pegmatite and its country rocks to cool from 600 °C (approximate crystallization temperature of pegmatite) to the closure temperature of 400 °C of muscovite, thus leading to an emplacement age of 560 Ma for the fertile pegmatite. This date is within the range of ages obtained for nearby fertile granites. The beryl-bearing pegmatites may be late tectonic and related to the main stage of granitogenesis of the Brasiliano orogeny, not posttectonic as determined for the northern, unfertile pegmatite.