The Mecubúri and Alto Benfica Groups, NE Mozambique: Aids to unravelling ca. 1 and 0.5 Ga events in the East African Orogen

Abstract

We present new field and geochronological data for two newly identified sequences of Neoproterozoic metasedimentary rocks, the Mecubúri and Alto Benfica Groups, interfolded with the Nampula Complex, NE Mozambique. Both groups comprise clastic sequences of meta-psammites and meta-conglomerates, which were strongly deformed, metamorphosed to sillimanite grade and locally migmatised. The relatively unsheared contacts with the surrounding Mesoproterozoic gneisses suggest that both groups are largely autochthonous. The depositional environment of the sequences is interpreted as a proximal, continental-fluvial environment in small depositional, possibly inter-montane, fault-controlled basins. Evidence of post-Mesoproterozoic exposure to the surface is provided by secondary nodular sillimanite growth in the Mecubúri Group and the underlying high-grade basement, pointing to weathering and/or epigenetic hydrothermal alteration to clays prior to burial, deformation and metamorphism. U–Pb analyses of detrital zircons constrain maximum depositional ages of 530 ± 18 and 938 ± 32 Ma for the Mecubúri and Alto Benfica Groups, respectively. The two groups have different provenances as shown by their detrital zircon age-frequency distributions. The Mecubúri Group was derived from the adjacent eastern Nampula Complex basement (ca. 1100 Ma), with minor early and mid-Neoproterozoic (950–700 Ma) and abundant late Neoproterozoic components (650–550 Ma). The latter were probably derived from Neoproterozoic terranes north of the Lurio belt, such as the Cabo Delgado Nappe Complex (CDNC). Hf isotope data indicate a juvenile origin to the Nampula Complex basement (ɛHf ca. +4 to 8) which was then re-worked as the source for the late Neoproterozoic melts (ɛHf ca. 0 to +5). Juvenile magmatic input also continued in the late Neoproterozoic (ɛHf ca. +11). In addition to the dominant ca. 1100 Ma local source rocks, the Alto Benfica Group, in the western Nampula Complex, records input from older Mesoproterozoic (1200–1400 Ma), Palaeoproterozoic (1600–2000 Ma) and rare Archaean sources, with scant evidence of Neoproterozoic provenance. These sources themselves reflect re-working of older pre-existing crustal components with crustal separation ages up to 3000 Ma. Zircon rims to detrital grains and metamorphic monazite from both groups date high-grade metamorphism at 500 ± 10 Ma, within uncertainty of the age for late to post-tectonic granitoids of the Nampula Complex and the Lurio belt. This suggests an intimate genetic relationship between the metamorphism and migmatisation of the two metasedimentary sequences and granite emplacement at that time. The detrital zircon data from the Alto Benfica Group suggest that the Nampula Complex was juxtaposed with the adjacent terranes of northern Mozambique and SE Africa, which were uplifted, exposed to sub-aerial weathering and locally lateritised at the time of deposition. The terrane juxtaposition event can be equated with the main, NW–SE-directed collision phase of the East African Orogeny at ca. 550 Ma. The resulting mountain belt was rapidly eroding and providing detritus for the two clastic sequences at ca. 530 Ma, probably during the early phases of extensional orogenic collapse. Later in the extensional phase, local SW–NE oriented compression deformed the Mecubúri and Alto Benfica Groups, along with the basement, into tight upright folds, with associated axial planar foliation. Sillimanite-grade metamorphism and migmatisation, associated with the emplacement of late-tectonic granites took place at ca. 500 Ma. It follows that none of the regional fabrics and large-scale structures in the Nampula Complex are Mesoproterozoic in age; the present distribution of lithological units and structures are almost entirely the result of the East African collisional orogeny at ca. 550 Ma and its subsequent extension and collapse.