THE EVOLUTION OF HIGH-T-LOW-P GRANULITES IN THE NORTHERN MARGINAL ZONE SENSU-STRICTO, LIMPOPO BELT, ZIMBABWE - THE CASE FOR PETROGRAPHY

The Limpopo Belt of southern Africa is generally believed to represent the root of a late Archaean continental collision zone, and has been used to demonstrate the validity of the uniformitarian concept in tect onics Large scale tectonic models have been applied in spite of the fa ct that large portions of the belt yet await the most basic investigat ions. Here we report the first detailed field and petrographic study o f the northernmost part of the Limpopo Belt, the Northern Marginal Zon e sensu stricto (NMZ s.s) and conclude, on the basis of relative age r elation, nature of PT evolution and deformation, that none of the curr ent models can correctly explain the evolution of the study area. The evolution of NMZ s.s, is complex and includes four metamorphic stages, two major plutonic episodes and at least three deformation events. Th e oldest rocks, mafic granulites, record all four stages of metamorphi c mineral growth. The first two stages predate deposition of sediments and the intrusion of voluminous enderbite and charnockite between 2.7 2 and 2.62 Ga. The bulk of our observations relate to the late Archaea n (stage 3) granulite facies event. Abundant reaction textures are pre served in mafic granulite, metasediments, metamorphosed charnockite an d enderbite and allow us to qualitatively reconstruct the PT evolution . Prograde heating occurred in the sillimanite stability held. During peak temperature conditions vapour-absent melting is observed in most felsic lithologies,Temperatures between 800 and 850 degrees C at press ures as low as 0.4-0.5 GPa are indicated by various mineral assemblage s. The thermal peak was followed by an increase in pressure. Typical r eactions of anti-clockwise PT evolution, like breakdown of cordierite + spinel to sapphirine and orthopyroxene + plagioclase to garnet + qua rtz, are frequently found. Maximum pressure is constraint to ca. 0.85 GPa by the complete absence of garnet in mafic granulites. Initial coo ling was rapid, and is indicated by the back reaction of melt + orthop yroxene to biotite + quartz symplectites preserved in migmatites. This metamorphic event was accompanied by intrusion of porphyritic charnoc kite and granite, and by coeval compressional deformation. The observe d evolution, especially the combination of an anti-clockwise PT loop a nd compressional tectonics, requires a strong, transient heat-source a ffecting the base of the crust. Neither the thermal evolution nor the relative timing is correctly predicted by existing collision models. T he NMZ s.s. granulites were finally exhumed in a separate event along upper greenschist-facies thrusts, in response to a transpressive oroge ny affecting the units further south at 2.0 Ga. In spite of potential ambiguities inherent to a qualitative approach, our observations show that petrography and field work, if used in conjunction with dating of a few key age relations and structural interpretation, are a pre-requ isite to the erection of realistic tectonic models. The example of the NMZ s.s. may encourage geoscientists with limited access to analytica l facilities to reassess the geological evolution of terrains which la ck basic description.