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

Abstract

New petrological and geochronological data are presented 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) relicts, 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 À1 ) apparent geothermal gradient. Migmatites record metamorphic conditions at the amphibolite-granulite facies transition. They reveal a clockwise pressure-temperature-time (P-T-t) path characterized 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 À1 apparent geotherm. A regional amphibolite facies metamorphic overprint is recorded by rocks that followed a clockwise P-T-t path, characterized by peak metamorphic conditions of 7.0-10.0 kbar at 550-680°C, which match a 20-25°C km À1 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 timing of anatexis and of the amphibolite facies metamorphic overprint is constrained by in-situ U-Pb dating of monazite crystallization at 2138 AE 7 and 2130 AE 7 Ma respectively. The new data set challenges the interpretation that metamorphic breaks in the West African Craton are due to diachronous Birimian 'basins' overlying a gneissic basement. It suggests that the lower crust was exhumed along reverse, normal and transcurrent shear zones and juxtaposed against shallow crustal slices during the Eburnean orogeny. The craton in NW Ghana is made of distinct fragments with contrasting tectono-metamorphic histories. The range of metamorphic conditions and the sharp lateral metamorphic gradients are inconsistent with 'hot orogeny' models proposed for many Precambrian provinces. These findings shed new light on the geodynamic setting of craton assembly and stabilization in the Paleoproterozoic. It is suggested that the metamorphic record of the West African Craton is characteristic of Paleoproterozoic plate tectonics and illustrates a transition between Archean and Phanerozoic orogens.

3-D numerical modelling of the influence of pre-existing faults and boundary conditions on the distribution of deformation: Example of North-Western Ghana

“…Although Block et al (2015) highlighted the significance of extension for exhumation in the Bole-Bulenga terrane; they also indicated that extension is certainly not the only process accounting for the exhumation of lower crust. In addition, Sakyi et al (2014) concluded that the oldest pulses of granitic emplacement in the Wa-Lawra belt are around 2212 ± 1 Ma.…”

“…According to Block et al (2015), the high grade Bole-Bulenga terrane in North-Western Ghana, is composed mostly of gneisses, metabasites and monzogranites and located between the craton scale N-S Jirapa and the NE-SW Bole-Nangodi shear zones (Fig. 1).…”

3D numerical modelling of the re-distribution of partially molten lower crust rocks in relay zones between faults during transtension: Implications for the Sefwi terrane, SW Ghana

“…The intruded granitoids in the BM domain locally yield ages between 2195 and 2172Ma (Hirdes et al, 1992;Baratoux et al, 2011;Sakyi et al, 2014). Several generations of main magmatic pulses took place later, ranging between 2153 and 2068 Ma (Hirdes et al, 1996;Egal et al, 2002;Gasquet et al, 2003;Baratoux et al, 2011;Block et al, 2015). 10…”

“…Craton-scale shear zones in NW Ghana (Block et al, 2015; and SW (McFarlane et al, 2016) commonly mark the boundaries between low-grade greenstone belts and high-grade metamorphic rocks of up to amphibolite-migmatite facies.…”

“…In addition, we only concerned here the D2 deformation stage occurred in the Sefwi terrane of SW Ghana. We installed the initial model according to the geological data estimated at the end of the D1 deformation stage (P, T and crustal thickness based on the metamorphic estimates performed by Block et al (2015) and McFarlane et al (2016)). In particular, the variation in the distribution of the Moho depth resulting from the D1 compression stage was not taken into 25 considerations for modelling.…”