Ar-40/Ar-39 thermochronology of the Ethiopian and Yemeni basements: reheating related to the Afar plume?

In order to constrain the timing of mantle plume-related basement uplift an d to discuss the active or passive mechanisms of the continental rifting in the Afar triple junction area, a Ar-40/Ar-39 thermochronological study has been carried out on the Panafrican basement in Ethiopia and Yemen. Muscovi tes, biotites and K-feldspars from granitoids and high-grade metamorphic sa mples have been analysed using the Furnace step heating procedure. R-feldsp ar thermal histories have been numerically modelled, according to the multi -domain Ar diffusion theory. The cooling ages range from 462 Ma to 679 Ma f or granitoids and from 495 Ma to 796 Ma for high-grade metamorphic rocks, w hich suggests that magmatic and metamorphic episodes belong to the same the rmotectonic event. A gradual younging of cooling ages from north to south i s consistent with the assumption that Panafrican continental collision bega n in Southern Ethiopia. Age gradients recorded by K-feldspars reflect Ar loss and are most likely r elated to the Cenozoic reheating of the basement. The estimated mean temper ature over the last 50 Ma is between 138 degrees C and 177 degrees C at 2 k m below the surface, which suggests a minimum mean thermal gradient of 69-8 8 degrees C/km. The thermal anomaly is expected to reflect heat conduction from the mantle plume, magma underplating and melting of the lower crust, a nd heat advection related to the magma transfer through the crust. However, due to limited exhumation, it was not possible to constrain the timing of any mantle plume-related basement uplift. This study suggests that the Ar-4 0/Ar-39 thermochronology is generally unsuited to monitor denudation of a c ontinental crust in an extensional setting. Furthermore, it highlights some of the thermal effects that continental flood basalts may have on the crus t, which can lead to inconsistencies between K-feldspar and mica age data. (C) 1999 Elsevier Science B.V. All rights reserved.