Structure and emplacement model for late-orogenic Paleoproterozoic granitoids : Tenkodogo-yamba elongate pluton (eastern Burkina Faso)

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Résumé

The Tenkodogo–Yamba (TY) elongate pluton is made of apparently isotropic biotite-bearing granites that form a continuous, 125 km long and NE trending, and 15–20 km wide, succession of granite bodies that intruded the so-called batholith of eastern Burkina Faso dominated by foliated granitoids and associated volcano-sedimentary belts. Geochemically, the granitoids of the batholith have well-defined TTG affinities that characterize the ‘‘gneiss-granitoids’’ of the Paleoproterozoic basement of West Africa. The biotite granites of the TY-elongate pluton, that display the so-called ‘‘basin’’ affinity, seem to be derived from the (partial) remelting of the batholith. The internal microstructures of the TY-elongate pluton are mostly purely magmatic, contrasting with the magmatic to high-temperature solid-state microstructures of the batholith. The systematic anisotropy of magnetic susceptibility study undertaken in the TY-elongate pluton reveals that the magmatic foliations cross-cut the foliations of the batholith and locally define concentric trajectories. The magmatic lineations have predominant steep plunges and well defined subareas ascribed to magma feeders which can be delineated. The overall NE- to NNW-trending trajectories of both foliations and lineations, independent of structures of the batholith, form contacts with it and form subdivisions into subplutons inside the alignment, clearly depict alignment-scale dextral sigmoids. The latter are interpreted as being formed during a dextral, NE-trending regional shearing parallel to the alignment, that occurred during emplacement of the biotite granites concerned. This study suggests that the 2.2 Ga TTGs, which form most of the Birimian terrains of this part of West Africa, were rapidly cooled and reached a brittle behaviour before being passively intruded, a few tens of million years later, by a new generation of granites, derived from partial remelting of the deep basement, during a regional-scale dextral wrench event. The present picture of the alignment is concluded to result from subsequent dissection into subareas along a set of late E-trending dextral faults.

Mots-clés

West Africa; Burkina Faso; Paleoproterozoic; Granites; Magnetic fabrics; Geochemistry