THE COMPETITION BETWEEN RATES OF DEFORMATION AND SOLIDIFICATION IN SYN-KINEMATIC GRANITIC INTRUSIONS: RESOLVING THE PEGMATITE PARADOX

Granitic pegmatites found within shear zones are used to infer that their precursor melts serve to localise deformation, weakening continental crust. But many syn-tectonic granitic pegmatite bodies within shear zones reveal little internal deformation, preserving magmatic primary textures. This implies the bodies were partially molten during tectonic shearing and should therefore should have been weaker than the host shear zone. However, the shapes of pegmatites indicate that they deformed with a greater competence (apparent viscosity) than their surroundings. This co-located pair of material behaviours is the pegmatite paradox. These relationships are shown by our case study, from a syn-tectonic granite pegmatite suite from the Caledonides of NW Scotland (Torrisdale, Sutherland). The pegmatites outcrop as aligned a­­­rrays of pods and contorted sheets, with interfacial buckles, while the foliations in the host shear zone pick out flanking folds. Collectively these structures indicate that the pegmatites are syn-kinematic and deformed with competence greater than the host shear zone. None of the bodies show “weak inclusion” behaviour as would be expected if the precursor melts were present during the accumulation of significant strain increments. Yet internally, pegmatites display coarse igneous textures, with only local, chiefly fracture-dominated, deformation. Many bodies show primary layering with cm-feldspar crystals nucleated on intrusion walls. We infer that preferential crystallization on intrusion margins served to encapsulate residual melt inside stiff rinds. These rinds must have formed quickly, greatly outpacing imposed strain rates – a deduction consistent with modern estimates for the rates of feldspar crystallization (cms.a^-1) from undercooled hydrous silicic melts. Fully liquid granitic melts of these types may only be present fleetingly so that their presence becomes irrelevant to the accumulation of tectonic strains. Further crystallization can be pulsed as the concentrations of fluxes increase in residual fluids. Rupture of partly crystallized, rind-enclosed pegmatite bodies during deformation may serve to pump residual melt to other sites - structurally-controlled fractionation.