Garnet—orthopyroxene thermobarometry of granulites from the north marginal zone of the Limpopo belt, Zimbabwe

The Limpopo mobile belt is an east-northeast-trending zone of high-grade Archaean rocks, 700 km in length and 250 km wide, situated between the Zimbabwean and Kaapvaal cratons of southern Africa. The origin of the Limpopo belt is currently the subject of much debate, although there is a growing consensus that the geology of this region is best explained in terms of some type of Archaean continental collision (Shackleton 1986). A three-fold subdivision is recognized within the mobile belt, with similar north and south marginal zones flanking a wider and geologically distinct central zone (Fig. 1, inset). The rocks of the north and south marginal zones are predominantly igneous in origin and comprise a diorite to granodiorite suite, with minor supra-crustal remnants, metamorphosed to granulite grade and with dominantly east-northeast-trending structures. Preliminary unpublished geochemistry shows that the igneous rocks are of calc-alkaline affinity. In contrast, the lithologies of the central zone are predominantly supracrustal and have mainly north-trending structures. McCourt & Verncombe (1987) have suggested that the central zone is an alloch-thonous thrust sheet bounded by shear zones and emplaced from the east. There is a marked contrast in both structural style and metamorphic grade between the rocks of the cratons and those of the mobile belt. This contrast is most obvious in southern Zimbabwe at the junction between the Zimbabwean craton and the north marginal zone (NMZ) of the mobile belt. Here there is a rapid change of metamorphic grade southwards from the greenschist-facies rocks of the granite—greenstone terrain, through

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