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 w hich the magma has been crystallised is shown to be relevant to the develop ment of excess pore pressure at continued shearing. The reigning pressure r egime is compared to the stresses required for fracturing of the skeletal e lements. At rates of loading in excess of average tectonic rates (greater t han or equal to 10(-14) s(-1)), shear-induced dilation in granitic magmas w ith high solidosities (crystal contents >50%), can lead to fracture. The av ailable excess skeletal pressure at a given strain rate is a function of tw o coupled parameters, grain size and tortuosity, with higher skeletal press ures favoured by smaller mean particle size. Our analysis suggests that the common occurence of brittle-like features thought to have formed in the ma gmatic state during pluton crystallisation can only be achieved where strai n rates (emplacement loading) are at least of the order 10(-13) s(-1) or gr eater, consistent with similar estimates of strain rates during pluton empl acement based on field studies. (C) 2001 Elsevier Science Ltd. All rights r eserved.