Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African craton

S Block (1) , J Ganne (1) , L Baratoux (2, 1) , Armin Zeh (3) , Luis A Parra-Avila (4) , M Jessell (4) , L Ailleres (5) , L Siebenaller (1)

1 GET - Géosciences Environnement Toulouse
2 IFAN - Institut Fondamental d'Afrique Noire
3 Institut für Geowissenschaften
4 Center for Exploration Targeting
5 Monash University [Clayton]

We present new petrological and geochronological data on high-grade ortho-and paragneisses from northwestern Ghana, forming part of the Paleoproterozoic (2.25-2.00 Ga) West African Craton. The study area is located in the interference zone between N-S and NE-SW trending craton-scale shear zones, formed during the Eburnean orogeny (2.15-2.00 Ga). High-grade metamorphic domains are separated from low-grade greenstone belts by high-strain zones, including early thrusts, extensional detachments and late-stage strike-slip shear zones. Paragneisses sporadically preserve high-pressure, low-temperature (HP-LT) relics, formed at the transition between the blueschist facies and the epidote-amphibolite sub-facies (10.0-14.0 kbar, 520-600°C), and represent a low (~15°C km-1) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite-granulite facies transition. They reveal a clockwise pressure-temperature-time (P–T–t) path characterised by melting at pressures over 10.0 kbar, followed by decompression and heating to peak temperatures of 750°C at 5.0-8.0 kbar, which fit a 30°C km-1 apparent geotherm. A regional amphibolite-facies metamorphic overprint is recorded by rocks that followed a clockwise P–T–t path, characterised by peak metamorphic conditions of 7.0-10.0 kbar at 550-680°C, which match a 20-25°C km-1 apparent geotherm. These P–T conditions were reached after prograde burial and heating for some rock units, and after decompression and heating for others. The

Mots clés: Paleoproterozoic; West African Craton; Exhumation constraints; Anatectic dome; Collision

Domaines: Tectonique; Pétrographie

S Block, J Ganne, L Baratoux, Armin Zeh, Luis A Parra-Avila, et al.. Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African craton. Journal of Metamorphic Geology, 2015, 33 (5), pp.463-494. ⟨10.1111/jmg.12129)⟩. ⟨hal-01221349⟩