MECHANISMS OF PLUTON EMPLACEMENT AND STRUCTURAL EVOLUTION OF A 2.1-GAJUVENILE CONTINENTAL-CRUST - THE BIRIMIAN OF SOUTHWESTERN NIGER

The 2.1 Ga Birimian terranes of southwestern Niger consist of a granit e-greenstone association. The structures of the greenstone belts resul t from the interference between pluton emplacement and regional transc urrent deformation (sinistral N-S strike-slip faults, NE-SW-trending s chistosity). The regional cleavage trend is observed to change in the vicinity of elliptical plutons: cleavage trajectories bend around thei r contacts resulting in triple junctions at the pluton extremities, an d narrow aureoles of intense penetrative schistosity and high-grade am phibolite-facies metamorphism are developed around them. Structural fe atures of plutons suggest syntectonic emplacement and age data indicat e that all plutons were emplaced between 2.16 and 2.10 Ga. The plutons are assembled in two distinct types of batholith elongated either par allel or perpendicular to the inferred regional shortening direction. A batholith parallel to the inferred regional shortening direction (Do lbel batholith) consists of km-sized subcircular alkali-granite pluton s, except for the southernmost one which is of a larger size and exhib its an elliptical shape with a NW-SE short axis parallel to the shorte ning direction. Crosscutting relationships indicate a progressively no rthward emplacement trend for those plutons. Internal fabrics generall y correspond to layering and magmatic flow fabrics, and, accessorily, to schistosity and shear zones which represent late stage plastic defo rmation restricted to the pluton margin, Batholiths which are perpendi cular to the inferred regional shortening direction are composed of la rge elliptical plutons (500 to 5000 km(2) in area) consisting mainly o f 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 intrusion s), or is subhorizontal in the centre and vertical at the margin (dome -shaped intrusions). The foliation and the geometry of the largest plu ton (Tera pluton) were acquired during pluton ascent and inflation, wh ereas the geometry of later shear zones indicates that deformation in the pluton was then ''passively'' acquired at the final emplacement le vel, due to regional deformation. The contrast in both the orientation and structural features of the batholiths reflects distinct emplaceme nt mechanisms: the small alkali-granite plutons resulted from magma in jection within NW-SE fractures parallel to the inferred regional short ening direction, whereas the granodiorite plutons are assumed to be em placed 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 le vel, are considered to reflect a local change in the theological behav iour of the crustal segment studied, in connection with the thermal ef fect induced by the large granodioritic plutons. This vertical tectoni c regime appears to be typical of Palaeoproterozoic juvenile continent al crust areas, and related to an interplay between emplacement of tre mendous volumes of granodioritic magma and a regional transcurrent def ormation.