Geochemical characteristics and tectonic setting of metamorphosed rocks in the Tugela terrane, Natal belt, South Africa

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The isotopically juvenile Tugela terrane, part of the Grenvillian Natal belt, southeastern Africa consists of a heterogeneous assemblage of regionally metamorphosed (upper amphibolite facies) mafic and felsic rocks. The Tugela terrane is divisible on the basis of lithology and structural style, from east to west, into four major thrust sheets referred to as the Nkomo, Madidima, Mandleni and Tugela sheets, respectively. The Tugela sheet includes the Kotongweni tonalite, a 15km long body of coarsely crystalline garnetiferous hornblende meta-tonalite. Although strongly lineated and foliated, the intrusive contact of the tonalite with its amphibolite wall rocks remains recognizable, as are wallrock xenoliths within the intrusion. Geochemically, the tonalite is extremely depleted with very low abundance of Ta, Nb, rare earth elements (REE) and LIL elements, features considered diagnostic of arc-related igneous rocks. The lithological and geochemical characteristics of the tonalite and its amphibolite wallrocks suggest their formation in an intra-oceanic arc setting comparable to the present day Izu-Bonin arc. In contrast, amphibolites of the underlying Mandleni sheet are characterized by relatively high abundance of Nb, Ti, LIL and HFS elements more similar to the present day oceanic island basalt. The lithologically and geochemically distinct thrust sheets that comprise the oceanic Tugela terrane were each derived from distinct tectonic settings. The Kotongweni tonalite and its wall rocks, preserved in the Tugela sheet, provide a record of middle Proterozoic magmatic arc development prior to the obduction of the Natal belt onto the Kaapvaal craton at about 1.1 Ga.
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Geochemical characteristics and tectonic setting of metamorphosed rocks in the Tugela terrane, Natal belt, South Africa

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