Evidence for a Neoproterozoic ocean in south-central Africa from mid-oceanic-ridge–type geochemical signatures and pressure-temperature estimates of Zambian eclogites

Abstract

Precambrian eclogites, metagabbros, and gabbros occur in an ∼200-km-long by 40-km-wide zone in central Zambia. Pressure-temperature (P-T) estimates of kyanite-bearing eclogites (kyanite eclogites) throughout the zone give temperatures of 590–750 °C at minimum pressures of 20 kbar. Phengite-bearing eclogites equilibrated at 720–755 °C and 26–28 kbar and show evidence for a clockwise P-T path. These P-T conditions imply a low geothermal gradient of ∼8 °C/km and a subduction depth of ∼90 km. The eclogites, metagabbros, and gabbros show incompatible element patterns similar to those of recent mid-oceanic-ridge basalts, and thus are interpreted to represent former oceanic crust. The low geothermal gradient indicates a cold subducted oceanic lithosphere, implying long-lived, fast convergence and a relatively large (>1000 km) associated ocean basin. A Sm-Nd isochron defines an age of 595 ± 10 Ma for the eclogite facies metamorphism. These results imply that a Neoproterozoic suture zone exists between the Congo and Kalahari cratons. Suturing occurred during the same orogenic cycle that formed the Zambezi belt and is related to the assembly of Gondwana.