New analytical and numerical geospeedometers tested on garnet pyroxenites from Bragança 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 tested to evaluate cooling rate on upper mantle-derived pyroxenites from the Braganca massif (NE Portugal). These estimations, coupled with standard geobarometric 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 uncertainties in geospeedometry (whether numerical or analytical): precision of temperature estimates and input diffusion data. None of the available geospeedometrical tools, when used independently, is able to provide definitive results with less than a factor 10 in precision. Cross-calibration is, thus, required in order to provide reliable cooling rate estimates. We obtain for pyroxenites from the Braganca massif, a minimum cooling rate from 1000 to 750 °C of the order of 250 °C/Ma for a depth interval of 10–15 km.