Processes controlling the shrinkage of porphyroclasts in gabbroic shear zones

Abstract

This paper presents microstructural and geochemical data on amphibole and clinopyroxene porphyroclasts collected in amphibolite-facies ultramylonites of the Ivrea–Verbano zone, Italy. The well-rounded porphyroclasts show a concentration of defects, including (100) twins, stacking faults, free dislocations, subgrains and microcracks within their rims that indicate stress concentrations. Localised deformation within the margins of porphyroclasts increases reaction kinetics and supports amphibole/clinopyroxene replacement reactions and heterogeneous plagioclase nucleation. In addition to microcracking and/or subgrain rotation these metamorphic reactions lead to a marginal grain-size reduction and the shrinkage of porphyroclasts. In order to correlate the defect concentration at the rim of porphyroclasts with stresses, a finite-element inclusion–matrix model is used. It depends on the degree of bonding between inclusion and matrix whether strong stress concentrations within the rim of the inclusion occur. For well-coupled two-phase systems, shear stresses within the rim of the inclusion can reach a magnitude many times the magnitude of the matrix stresses. Marginal stress concentrations do not evolve if inclusion and matrix are decoupled. This may indicate that mechanically decoupled porphyroclasts can survive to very large strains.