A continental back-arc setting for the Namaqua belt: Evidence from the Kakamas Domain

A study of the NW Kakamas Domain in South Africa/Namibia provides a new, unified lithostratigraphy and evolutionary history applicable to the whole Namaqua Sector. The Mesoproterozoic history ranges from similar to 1350 Ma to 960 Ma, but isotopic evidence suggests it was built upon pre-existing Paleoproterozoic continental crust that extended west from the Archaean Craton. In eastern Namaqualand, early rift-related magmatism and sedimentation at similar to 1350 Ma occurred in a confined ocean basin. Subsequent tectonic reversal and subduction at similar to 1290-1240 Ma led to establishment of the Areachap, Konkiep and Kaaien Domains. In the Kakamas Domain, widespread deposition of pelitic sediments occurred at similar to 1220 Ma (Narries Group). These contain detrital zircons derived from proximal crust with ages between similar to 2020 Ma and 1800 Ma (western Palaeoproterozoic domains) and 13501240 Ma (eastern early Namaqua domains), suggesting pre-sedimentation juxtaposition. The pelites underwent granulite grade metamorphism at similar to 1210 Ma (peak conditions: 4.5-6 kbar and 770850 degrees C), associated with voluminous, predominantly S-type granitoid orthogneisses between similar to 1210 Ma and 1190 Ma (Eendoorn and Ham River Suites) and low-angle ductile (D2) deformation which continued until similar to 1110 Ma, interspersed with periods of sedimentation. This enduring P-T regime is inconsistent with the expected crustal over-thickening associated with the generally-accepted collision-accretion Namaqualand model. Rather, we propose the Namaqua Sector is a 'hot orogen' developed in a wide continental back-arc with subduction west of the present-day outcrop. The observed high geotherm resulted from thinned back-arc lithosphere accompanied by an influx of mantle-derived melts. Ductile D2 deformation resulted from ''bottom-driven" tectonics and viscous drag within the crust by convective flow in the underlying asthenospheric mantle. This extended tectonothermal regime ceased at similar to 1110 Ma when SW-directed thrusting stacked the Namaqua Domains into their current positions, constrained in the Kakamas Domain by late- to post-tectonic I-type granitoids intruded between similar to 1125 Ma and 1100 Ma (Komsberg Suite). The thermal peak then shifted west to the Bushmanland and Aus Domains, where voluminous granites (1080-1025 Ma) were associated with high-T/low-P granulite facies thermal metamorphism and mega-scale open folding (D-3). Unroofing of the Namaqua Sector is marked by large-scale, NW-trending, sub-vertical transcurrent dextral shear zones and associated pegmatites and leucogranites at similar to 990 Ma. (c) 2022 China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).