Paired metamorphic belts in the Usagaran Orogen of Tanzania: evidence for one-sided oceanic subduction in the Palaeoproterozoic

• 2 Ga paired metamorphic belts reflect one-sided oceanic subduction to mantle depth. • Usagaran orogeny connected to a full-fledged Wilson cycle at ca. 2.15 to 2.0 Ga. • High-T-P eclogite shear zones formed during exhumation of subducted slab. • PT paths of lithotectonic units in Usagaran nappe stack are interrelated. • Age of paired belts coincides with incipient duality of metamorphic thermal regimes. Paired metamorphic belts (PMB) are the tectono-metamorphic expression of the one-sided subduction of oceanic lithosphere beneath continental margins and island arcs, which represents a key characteristic of terrestrial plate tectonics. PMB that are spatially and timely paired are mainly known from Phanerozoic orogenic belts, although the metamorphic rock record points to a duality of thermal regimes since the Palaeoproterozoic. Here we describe the tectono-metamorphic evolution of the Usagaran Orogen at the SE-margin of the Tanzania Craton, which includes PMB of Palaeoproterozoic age. The orogen’s lithological inventory reflects a Wilson cycle: (1) The passive margin sequence (Konse Group) was deposited during the Lomagundi carbon isotope excursion (2.22–2.06 Ga) and metamorphosed at 2.03 Ga during the Usagaran orogeny. (2) The precursors of most eclogites were ocean-floor basalts, but those of the Ky- and Opx-eclogites of boninite chemistry were related to subduction-initiation. Peak eclogite metamorphism at mantle depths (850–880 °C/18–19 kbar) was followed by transient granulite-facies conditions during exhumation (750–800 °C/6–10 kbar). High- T-P shear zones in Opx-eclogites and felsic mylonites formed at P < 15 > 10 kbar are related to exhumation. (3) High-P amphibolite-facies metamorphism of metasediments enveloping the eclogite belt (770 °C/13.2 kbar at 2.0 Ga) is attributed to an accretionary wedge setting. The concurrent evolutions and the preserved growth zonations in garnet of both units point to a shared short-lived metamorphism. (4) Low-P Grt-Crd-Sil-granulites, which overlie the accretionary wedge metasediments as a nappe, experienced a counter-clockwise P-T path at ca. 2.06 Ga (peak 820 °C/7.2 kbar) in an arc setting that predates the orogeny for 50–60 Myr. We attribute their protomylonitic texture to post-peak NW-thrusting towards the orogenic foreland during collision. Late-stage P-T paths of all units in the nappe stack (isobaric cooling at 20 km depth) are interrelated. Other Palaeoproterozoic PMB in orogens around the Congo Craton and in the Yangtze Craton, in addition to subduction-related eclogites in the Trans-North China Orogen, indicate that subduction of oceanic lithosphere and its partial exhumation after slab breakoff at mantle depths were already viable tectonic processes. Pre-Neoproterozoic eclogites representing exhumed oceanic crust are dominantly known from a short time interval (2.1–1.8 Ga), during which warm (12–14 °C/km) and cold (<11 °C/km) subduction zones existed, which both show the tectono-metamorphic characteristics akin to modern asymmetric one-sided oceanic subduction.