Precambrian coesite from northern Mali: first record and implications for plate tectonics in the trans-Saharan segment of the Pan-African belt

Abstract

Coesite occurs as inclusions in omphacite in an impure marble from the Late Proterozoic internal nappes of the frontal part of the Pan-African belt in northern Mali. Possible pseudomorphs after coesite are also found in garnets from the surrounding eclogitic metasediments. These eclogitic micaschists show slightly overprinted mineral assemblages including kyanite, pyrope-rich garnet (Py 32-40 , Alm 41-52 , Gro 15-18 ), jadeite-rich clinopyroxene (Jd 48-68 ), phengite (Si 3.35-3.40 ) and rutile. Provisional estimates for the conditions of this metamorphism (T about 700-750°C and P above 27 kbar) suggest that large portions of the paleo-west-African plate were subducted to depths exceeding 90 km. The flat and thin eclogitic unit, surrounded by retrogressed micaschists, rests on low-grade greenschist-facies phyllites. It is proposed that this coesite-bearing unit was exhumed from depths along low-angle migrating thrusts, in the form of a fast extrusive slice pushed up above a low-angle sole thrust