Consolidation phenomena in sheared granitic magma: effects of grain size and tortuosity

Granitic (and other) magmas with crystal contents between 50 and ca.70% are expected to show dilatant behavior during deformation. The grain size at which the magma has been crystallised is shown to be relevant to the development of excess pore pressure at continued shearing. The reigning pressure regime is compared to the stresses required for fracturing of the skeletal elements. At rates of loading in excess of average tectonic rates (≥ 10⁻¹⁴ s⁻¹), shear-induced dilation in granitic magmas with high solidosities (crystal contents >50%), can lead to fracture. The available excess skeletal pressure at a given strain rate is a function of two coupled parameters, grain size and tortuosity, with higher skeletal pressures favoured by smaller mean particle size. Our analysis suggests that the common occurrence of brittle-like features thought to have formed in the magmatic state during pluton crystallisation can only be achieved where strain rates (emplacement loading) are at least of the order 10⁻¹³ s⁻¹ or greater, consistent with similar estimates of strain rates during pluton emplacement based on field studies.