Age of pegmatites from eastern Brazil and implications of mica intergrowths on cooling rates and age calculations R.R. Viana a , I. Ma ¨ntta ¨ri b , Henjes Kunst c , H. Jordt-Evangelista a, * a Departamento de Recursos Minerais, Universidade Federal de Mato Grosso, Cuiaba ´, MT, Brazil b Geological Survey of Finland, Espoo, Finland c Bundesanstalt fu ¨r Geowissenschaften und Rohstoffe, Hannover, Germany Received 1 August 2001; accepted 1 September 2002 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 207 Pb/ 206 Pb 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 8C (reported closure temperatures for K – Ar isotope systems of coarse-grained muscovite and biotite, respectively), which suggests a mean cooling rate of 3.3 8C/Ma. As such, it took 60 Ma for the pegmatite and its country rocks to cool from 600 8C (approximate crystallization temperature of pegmatite) to the closure temperature of 400 8C 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. q 2003 Elsevier Ltd. All rights reserved. Keywords: Brazil; Cooling rate; Eastern brazilian pegmatite province; Geochronology; K–Ar age; Mica; Monazite; Pegmatite; U–Pb age; Zircon 1. Introduction Brazil is one of the largest producers of colored gemstones such as aquamarine, emerald, kunzite, alexan- drite, tourmaline, and topaz (Morteani et al., 2000; Ce ´sar-Mendes et al., 2001; Pinto and Pedrosa-Soares, 2001). Large quantities and varieties of gemstones are produced in the northeastern and eastern pegmatite provinces. Of these, the eastern Brazilian pegmatite province (EBPP), sometimes also called the Oriental pegmatite province, is the largest in area (approximately 800 km long and 150 km wide) and of the greatest importance. The pegmatites are spread over eastern Minas Gerais, western Espı ´rito Santo, northern Rio de Janeiro, and southern Bahia (Fig. 1). The EBPP is characterized not only by its geographic location, but also its particular geotectonic setting in a Neoproterozoic-Cambrian orogenic belt gener- ated during the Brasiliano-Pan-African cycle, which con- sisted of a set of orogenies that lasted from 850 to550 Ma (Pinto and Pedrosa-Soares, 2001). The majority of the pegmatites of the EBPP are related to granite intrusions into the Brasiliano mobile belts generated during the consolida- tion of the Gondwana supercontinent. They are considered residual melts derived from S-type (product of the total or partial melting of a sedimentary source) and I-type (derived from igneous sources) granites (Lobato and Pedrosa-Soares, 1993; Pedrosa-Soares et al., 1999; Pinto and Pedrosa-Soares, 2001). Pegmatite melts derived directly 0895-9811/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0895-9811(03)00105-6 Journal of South American Earth Sciences 16 (2003) 493–501 www.elsevier.com/locate/jsames * Corresponding author. E-mail addresses: hanna@degeo.ufop.br (H. Jordt-Evangelista), Rubia@cpd.ufmt.br (R.R. Viana), irmeli.manttari@gsf.fi (I. Ma ¨ntta ¨ri), henjes-kunst@bgr.de (H. Kunst).