Tectonic implications from the geochemistry of Mfongosi Group metasediments, Natal Metamorphic Province, South Africa

Abstract

The formation of the Mesoproterozoic Natal Metamorphic Province occurred during the closing stages of the assembly of the supercontinent Rodinia (~1150 to ~950 Ma), forming part of the ~1190 to ~950 Ma Grenvillian orogeny. This was preceded by the accretion, at ~1210 Ma, of Mesoproterozoic island arcs along the southern margin of the Archaean Kaapvaal Craton. The Mfongosi Group represents an enigmatic sequence of rocks found between the Kaapvaal Craton and the partially ophiolitic Natal Nappe Zone to the north of these accreted island arcs. Mfongosi metagreywackes adjacent to the Kaapvaal Craton have ocean island arc major element geochemical signatures. These contrast with metagreywackes approximately two kilometres further south, near the contact zone between the Natal Thrust Front and Natal Nappe Zone, which have active continental margin major element geochemical signatures. A third type of metasediment is represented in both areas by geochemically distinct low-Ca+Na, high-K meta-arkoses to meta-lithic arkoses, which were formed by relatively minor sedimentation from a passive continental margin. The Mfongosi Group is a fore-arc complex, incorporating elements of a fore-arc basin and an accretionary prism deposited in a trench, which filled during final oblique collision between the Kaapvaal Craton and an oceanic island arc to the south. The inclusion of an active continental margin signature in the metasediments is enigmatic, although the geochemistry of the metagreywackes suggests that the margin of the Grunehogna Province is a probable source. Subsequent closure of this basin resulted in inversion and accretion of the metasedimentary sequence onto the southern margin of the Kaapvaal Craton.