Insights into Lower Crustal Processes in the Granulate Belts of Tanzania

The granulite belts of Tanzania expose the lower crustal units of the suture zone between east and west Gondwana that formed during the Pan-African orogeny. This 600 Ma old collision zone is referred to as "Mozambique Belt" in East Africa. Since 2002 a group of Earth Scientists from the University of Graz has chosen the Mozambique Belt in Tanzania as a study area in order to reconstruct the Pre-Pan-African versus the Pan-African evolution, with a combined approach of structural, geochemical, petrological and geochronological investigations. The outcome of this study has opened a range of questions including important regional and conceptional ones on the granulite formation that build the base for this application and are aimed to get closer insights into lower crustal processes. The first aim is the reconstruction of detailed quantitative P-T-D-t paths as previous studies have shown that two different granulite provinces of the Mozambique Belt, namely the Eastern Granulites and the Western Granulites show different retrograde P-T evolution after the metamorphic peak. This result has been gained by careful petrography and conventional thermobarometry for selected samples and was combined with information from microtectonical studies on intracrystalline deformation processes that could be related to metamorphic textures. The Western Granulites are characterized by a clockwise retrograde P-T path with near isothermal decompression (ITD) and the Eastern Granulites are characterized by a segment of near isobaric cooling (IBC) after the metamorphic peak conditions. Both different granulite provinces however expose a decompression path during final exhumation and show a general clockwise shape of the P-T path. This new result is in contrast to first conventional petrological examinations on the P-T evolution that has been done in the nineties and suggested an anticlockwise path for the Eastern Granulites and requires therefore a different tectonic interpretation of their formation. The accuracy of individual P-T segments of a metamorphic path in granulite facies rocks is restricted by conceptual problems of conventional thermobarometry by retrograde re-equilibration of mineral assemblages. It is striking that the P-T conditions in central Tanzania are noticeably uniform at 800C and 10 kbar. It seems worthwhile, therefore as a major aim for this study, to test the P-T conditions with alternative means such as thermodynamic modelling of pseudosections for a large range of samples and combine these with microtectonic information to infer the tectonic history of the Eastern and Western Granulites. The second major question that has arisen from previous works is the significance of early Pan-African formation ages around 800-900 Ma. These magmatic ages have been largely unknown from Tanzania until now, however recent studies have found 800-900 Ma formation ages from U/Pb studies on zircons in anorthosites and enderbites of the Eastern Granulites. The regional extent and the tectonic significance of these ages have to be studied as the second aim for this application. Both questions are of major importance for the reconstruction of the formation of Gondwana. In particular this study will increase insights of lower crustal processes by a discussion on conceptual questions that are related to the thermodynamics of granulite facies metamorphism such as the equilibration problem, unknown fluid composition and cooling mechanisms of granulites.