Ganne Jérôme, Gerbault Muriel, Block S. (2014). Thermo-mechanical modeling of lower crust exhumation-Constraints from the metamorphic record of the Palaeoproterozoic Eburnean orogeny, West African Craton. Precambrian Research, 243, p. 88-109. ISSN 0301-9268.
Titre du document
Thermo-mechanical modeling of lower crust exhumation-Constraints from the metamorphic record of the Palaeoproterozoic Eburnean orogeny, West African Craton
Precambrian Research, 2014,
243, p. 88-109 ISSN 0301-9268
The aim of this paper is to explore the link between the metamorphic record of a Palaeoproterozoic orogeny and its thermo-mechanical evolution, in order to discuss the burial and exhumation processes of moderate- to high-pressure volcano-sedimentary rocks (6-8 to >10 kbar) in the Birimian Province (2.2-2.0 Ga) of the West African Craton. Metamorphic data collected in Burkina Faso, southwest Ghana and eastern Senegal suggests a Palaeoproterozoic Birimian crust dominated by moderate apparent geothermal gradients of 20-30 degrees C/km (M2a), which fit a purely conductive thermal crustal regime. This produced greenschist- to amphibolite-facies metamorphic assemblages that developed during the Eburnean orogeny (similar to 2.1 Ga), associated with regional shortening and granitoid intrusions. The M2a gradient is superimposed on an early thermal regime (M1: <10-15 degrees C/km) that produced high-P greenschist- to blueschist-facies metamorphic assemblages. M1 most likely recorded an earlier thermal history in the volcano-sedimentary basins where the protolith of the metamorphic terrains formed. Whether M1 is related to distributed burial associated with sediment accumulation or early subduction-related tectonics remains an open question, not directly addressed here. Thermo-mechanical models were built based on the assumption of an initial crust made up of a succession of volcanic island arcs emplaced on top of a basement dominated by CaO-rich Tonalite-Trondhjemite-Granodiorite suites (TTGs), and tectonically paired with sedimentary basins. From the results of these thermo-mechanical models, we propose a mechanism of burial, heating and exhumation of meta-sediments through the Birimian crust controlled by simultaneous folding/shortening and gravitational instabilities in the juvenile crust. We postulated here the pre-existence of thick sedimentary basins (depth 15 km) in the Birimian orogenic crust to model the late-stage exhumation of meta-sediments, collected in the thermal aureole of CaO-poor granitoids, and which record elevated metamorphic pressures (P > 6-8 kbar). At around 25 km depth, TTGs reached appropriate PT conditions to start melting. The buoyant and low-viscosity partially molten material, comprising a fraction of CaO-poor melt and residual material, then ascended through the overlying sediments. It entrained upwards lower crustal material as well as the surrounding sediments. This scenario explains the important breaks in metamorphic conditions observed in the Birimian province between the thin slivers (<2 km thick) of high-pressure rocks (P > 8 kbar) preserved in the thermal aureoles of granites and the adjacent synforms of greenstones, which consist of low-grade metasediments (P < 6 kbar; T < 450 degrees C). This scenario is in agreement with a significant recycling of the TTGs in the genesis of CaO-poor granitoid melts. We discuss the implications of our results for the tectono-metamorphic and mechanical evolution of Palaeoproterozoic orogenies.
