Crustal growth during island arc accretion and transcurrent deformation, Natal Metamorphic Province, South Africa: New isotopic constraints

Abstract

The Natal Metamorphic Province consists of, from north to south, the Tugela, Mzumbe, and Margate Terranes. These were accreted to the southeastern margin of the Kaapvaal Craton in the late Mesoproterozoic, and followed by intrusion of a large suite of A-type granitoid bodies. New U–Pb data from zircon, titanite, and monazite further constrains the temporal framework of these geological events. The Tugela and Mzumbe Terranes record protracted magmatism in an island arc complex from ~1200 Ma to 1160 Ma, followed by the accretion of these terranes onto the southern margin of the Kaapvaal Craton at ~1150 Ma. Arc magmatism in the Margate Terrane continued until ~1120 Ma and was followed by extension and bimodal volcanism immediately prior to accretion to the Kaapvaal/Mzumbe continental margin at ~1090 Ma. This accretion was accompanied by high-pressure and high-temperature metamorphism, juxtaposition of the Mzumbe and Margate Terranes along the Melville Thrust, and the formation of a number of syntectonic intrusive units derived from melting of the pre-existing arc crust. After accretion, extensional collapse is evidenced by the intrusion of mafic/ultramafic and alkaline intermediate magmatic suites at ~1085 Ma, resulting from mafic underplating and/or lower crustal delamination. Nd and Hf isotopic data imply the magmatic rocks of the Natal Metamorphic Province were derived from relatively juvenile continental crust, initially generated by island arc magmatism and subsequently reworked during the accretion event(s).The combined Kaapvaal-NMP region (the southern margin of the enlarged Kalahari Craton) then experienced extensive sinistral transcurrent deformation centred along a series of discrete steep shear zones that are found from the Kaapvaal cratonic margin to the southernmost portion of the Natal Metamorphic Province. This deformation is accompanied by low-pressure, (ultra)high-temperature metamorphism, isobaric cooling, and intrusion of the voluminous A-type Oribi Gorge Suite porphyritic granites and charnockites throughout the Mzumbe and Margate Terranes.