Ascent and emplacement of pegmatitic melts in a major reverse shear zone (Sierras de Cَrdoba, Argentina)

Ordovician to Devonian aged crustal-scale Guacha Corral shear zone (GCSZ), central Argentina, hosts rare element pegmatites of the Comechingones pegmatitic field (CPF). In the CPF an eastwards decreasing strain gradient related to the GCSZ deformation is defined, with a high strain domain (HSD) to the west and a low strain domain (LSD) to the east. Pegmatites of the CPF were emplaced in both HSD and LSD synkinematically during ductile GCSZ deformation. Two main mechanisms for pegmatitic melt ascent and emplacement are recognized: “fracture-controlled” and “magma pumping” mechanisms. The former implies fracturing generated due to simple shear deformation not related to any previous heterogeneity. With further deformation, pegmatites were emplaced in low-dip surfaces of anisotropy (Cʹ-planes or T-fractures), that might behave as releasing bends connecting adjacent high-dip conduits or shear zones. Displacements along staggered shear zones with releasing bends induce the local development of domains with negative pressure gradients, where open spaces could form transiently attracting for the collection of buoyant melts, a mechanism similar to “magma pumping”. With ongoing deformation pegmatites were progressively rotated, sheared and transposed to the mylonitic foliation. Late pegmatites emplaced by either of the two mechanisms in the HSD and the TZ have retained their original orientations.