New analytical and numerical geospeedometers tested on garnet pyroxenites from Braganca Nappe Complex (NE Portugal)

Two new geospeedometric approaches, an analytical one [Phys. Earth Planet. Inter. 110 (1999) 95] and a numerical one [Am. J. Sci. 29 (1998) 30], are t ested to evaluate cooling rate on upper mantle-derived pyroxenites from the Braganca massif (NE Portugal). These estimations, coupled with standard ge obarometric techniques, allow theoretically the determination of exhumation rates responsible for incorporation of these rocks within the lower crust. In the present paper, we discuss the most significant sources of uncertain ties in geospeedometry (whether numerical or analytical): precision of temp erature estimates and input diffusion data. None of the available geospeedo metrical tools, when used independently, is able to provide definitive resu lts with less than a factor 10 in precision, Cross-calibration is, thus, re quired in order to provide reliable cooling rate estimates. We obtain for p yroxenites from the Braganca massif, a minimum cooling rate from 1000 to 75 0 degreesC of the order of 250 degreesC/Ma for a depth interval of 10-15 km . (C) 2001 Elsevier Science B.V. All rights reserved.