Tectono-metamorphic evolution of the internal zone of the Pan-African Lufilian orogenic belt (Zambia): Implications for crustal reworking and syn-orogenic uranium mineralizations

Abstract

Listen

Translate article

The internal zone of the Pan-African Lufilian orogenic belt (Zambia) hosts a dozen uranium occurrences mostly located within kyanite micaschists in a shear zone marking the contact between metasedimentary rocks attributed to the Katanga Neoproterozoic sedimentary sequence and migmatites coring domes developed dominantly at the expense of the pre-Neoproterozoic basement. The P–T–t–d paths reconstructed for these rocks combining field observations, microstructural analysis, metamorphic petrology and thermobarometry and geochronology indicate that they have recorded burial and exhumation during the Pan-African orogeny. Both units of the Katanga metasedimentary sequence and pre-Katanga migmatitic basement have underwent minimum peak P–T conditions of ~9–11kbar and ~640–660°C, dated at ca. 530Ma by garnet-whole rock Lu–Hf isochrons. This suggests that this entire continental segment has been buried up to a depth of 40–50km with geothermal gradients of 15–20°C.km−1 during the Pan-African orogeny and the formation of the West Gondwana supercontinent. Syn-orogenic exhumation of the partially molten root of the Lufilian belt is attested by isothermal decompression under P–T conditions of ~6–8kbar at ca. 530–500Ma, witnessing an increase of the geothermal gradients to 25–30°C·km−1. Uranium mineralizations that consist of uraninite and brannerite took place at temperatures ranging from ~600 to 700°C, and have been dated at ca. 540–530Ma by U–Pb ages on uraninite. The main uranium deposition thus occurred at the transition from the syn-orogenic burial to the syn-orogenic exhumation stages and has been then partially transposed and locally remobilized during the post-orogenic exhumation accommodated by activation of low-angle extensional detachment.