Mechanisms of pluton emplacement and structural evolution of a 2.1 Ga juvenile continental crust: the Birimian of southwestern Niger

Abstract

The 2.1 Ga Birimian terranes of southwestern Niger consist of a granite-greenstone association. The structures of the greenstone belts result from the interference between pluton emplacement and regional transcurrent deformation (sinistral N-S strike-slip faults, NE-SW-trending schistosity). The regional cleavage trend is observed to change in the vicinity of elliptical plutons: cleavage trajectories bend around their contacts resulting in triple junctions at the pluton extremities, and narrow aureoles of intense penetrative schistosity and high-grade amphibolitefacies metamorphism are developed around them. Structural features of plutons suggest syntectonic emplacement and age data indicate that all plutons were emplaced between 2.16 and 2.10 Ga. The plutons are assembled in two distinct types of batholith elongated either parallel or perpendicular to the inferred regional shortening direction. A batholith parallel to the inferred regional shortening direction (Dolbel batholith) consists of km-sized subcircular alkali-granite plutons, except for the southernmost one which is of a larger size and exhibits an elliptical shape with a NW-SE short axis parallel to the shortening direction. Crosscutting relationships indicate a progressively northward emplacement trend for those plutons. Internal fabrics generally correspond to layering and magmatic flow fabrics, and, accessorily, to schistosity and shear zones which represent late stage plastic deformation restricted to the pluton margin. Batholiths which are perpendicular to the inferred regional shortening direction are composed of large elliptical plutons (500 to 5000 km 2 in area) consisting mainly of granodiorite, and covering 70% of the region. The plutons never show crosscutting relationships but are moulded against each other. Their internal fabric corresponds to a solid-state foliation. It either dips towards the centre of the pluton (funnel or teardrop-shaped intrusions), or is subhorizontal in the centre and vertical at the margin (dome-shaped intrusions). The foliation and the geometry of the largest pluton (Tera pluton) were acquired during pluton ascent and inflation, whereas the geometry of later shear zones indicates that deformation in the pluton was then “passively” acquired at the final emplacement level, due to regional deformation. The contrast in both the orientation and structural features of the batholiths reflects distinct emplacement mechanisms: the small alkali-granite plutons resulted from magma injection within NW-SE fractures parallel to the inferred regional shortening direction, whereas the granodiorite plutons are assumed to be emplaced as diapirs (forceful intrusions with long axes perpendicular to the shortening direction). These two intrusion mechanisms for plutons emplaced during the same deformation event and at the same crustal level, are considered to reflect a local change in the rheological behaviour of the crustal segment studied, in connection with the thermal effect induced by the large granodioritic plutons. This vertical tectonic regime appears to be typical of Palaeoproterozoic juvenile continental crust areas, and related to an interplay between emplacement of tremendous volumes of granodioritic magma and a regional transcurrent deformation.