Structure and emplacement model for late-orogenic Paleoproterozoic granitoids: the Tenkodogo–Yamba elongate pluton (Eastern Burkina Faso)

Abstract

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.