PALEOPROTEROZOIC ECLOGITES AND GARNET PYROXENITES OF THE UBENDE BELT (TANZANIA)

Petrographical and mineral chemical data are given for eclogites and g arnet pyroxenites from the Paleoproterozoic Ubende belt which bounds t he western edge of the Tanzania craton. Both types of rocks are consti tuents - with meta-peridotites, mafic granulites, amphibolites and qua rtzites - of a mafic-ultramafic sequence known as Ubende and Ikulu ser ies and interpreted as an ophiolite suite. Eclogites also occur as sma ll lenses in garnet-kyanite gneiss. Most of the eclogites underwent st rong retrogression resulting in opx-cpx coronas around garnet and in b reakdown of omphacite. Pyroxene is omphacite close to Jd(50)Ac(15)Di(3 5) in kyanite eclogite, low-Jd omphacite - Na-augite in retrograded ec logite and Na-augite to augite in garnet pyroxenites. The garnets in b oth eclogites and garnet pyroxenites belong to grossular-rich Prp-Alm series (Prp(16-46) Grs(21-31)) with < 5 mol% spessartine. Amphibole is taramite in kyanite eclogite, edenite-pargasite in retrograded eclogi tes and garnet pyroxenites and pargasite in late retrogression assembl ages. The mafic-ultramafic sequence has experienced a complex metamorp hic history recorded in mineral assemblages of kyanite eclogite, retro graded eclogites, mafic granulites, amphibolites, and blastomylonites. The tectonometamorphic evolution is interpreted in terms of Paleoprot erozoic plate tectonics, involving a subduction of oceanic plate under the Tanzania craton and several stages of subsequent exhumation, char acterized by different P-T paths.