Abstract
Greenstone belts in the West African Craton (WAC) are separated by several generations of granitoids intruded at ca. 2.18–1.98 Ga. Simultaneous folding and exhumation play an important role in the formation of greenstone-granitoid belts. However, the overall tectonic regime and origin of granitoids remain controversial. In this study, we present the estimates of the mantle potential temperature (Tp) for the WAC, which yields values of about 1500–1600 °C, pressure estimates of initial and final melting yield values of about 3.7–5.2 GPa and 1–1.3 GPa, respectively. Subsequently, 2D thermo-mechanical models have been constructed to explore the width of volcano-sedimentary basin on spatial-temporal evolution of diapirs that emplaced in the lower-middle crust during compression. The models show that the width of the volcano-sediment layer plays an important role in the formation mechanisms of greenstone-granitoid belts. The lower crust beneath sedimentary sequences is deformed into a buckle fold during the first compressional stage, through which relief uplifts slowly. Subsequently, the buckle fold is further deformed into several individual folds. Diapirs made of lower crust rocks ascend and emplace in the middle-upper crust resulting from instability. Benefitting from the mantle temperature, the pressure estimates and the numerical modelling results, a new geodynamic model was constructed. This model indicates that a series of sheet-like granitoids possibly derived from either subducted mélanges, lower crust and/or mantle melting that are accumulated at depths of the subcontinental mantle would channel along diapirs before feeding the upper crust. When the granitoids arrive at the solidified lids of the diapirs, they would favour migrating horizontally and intrude into the upper crust through weakening zones between the diapirs. Our geodynamic model also suggests an asymmetry of structures between the upper and middle-lower crust, with the dome-like granitoids overlying high-grade sedimentary synforms and high-grade diapirs underlying low-grade greenstone belts.
Highlights
In the West African Craton (WAC), Precambrian terranes show widespread greenstone belts separated by several generations of granitoids intruded at around 2.18–1.98 Ga [1]
The apparent lack of High-Pressure rocks before the Neoproterozoic time (1000–540 Ma) is still a key debate challenging the existence of modern subduction zones in the Archaean and Paleoproterozoic times [58]
We suggest a series of sheet-like granitoids possibly derived subducted lower crust and/or melting mantle accumulated at depths of the subcontinental from eithermélanges, subducted mélanges, lowermantle crust and/or melting accumulated at depths of the mantle would canalize along diapirs upper crust.the granitoids at the subcontinental mantle would canalizebefore alongfeeding diapirs the before feeding upper
Summary
In the WAC, Precambrian terranes show widespread greenstone belts separated by several generations of granitoids intruded at around 2.18–1.98 Ga [1]. Ganne et al [4] explored the exhumation of lower crust under a convergent setting by employing a volcano-sedimentary basin of about 280–290 km in width (evidence from the Siguiri basin which straddles upper Guinea-Southwest Mali). Their model produced two synchronous diapirs made of partially molten lower crustal rocks, 70 km apart.
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