Holistic microstructural techniques reveal synchronous and alternating andalusite and staurolite growth during three tectonic events resulted from shifting partitioning of growth vs deformation

In this sample, increasing non-coaxiality as the deformation intensified resulted in the same asymmetry for each of the three events and thus an overall spiral-like shape. Differing stages in the development of these bulk-shortening geometries preserved in adjacent or touching phases negate any role for porphyroblast rotation during ductile deformation. Andalusite and staurolite grew without any inter-reaction in locations where they lie in contact. This multiply repeated growth behaviour initiated within zones of spatially partitioned crenulation deformation of pre-existing foliations. This varied slightly in timing and in the subsequent development of successive surrounding foliations from location to location over distances around 5 mm. Growth ceased adjacent to any zone where metamorphic differentiation associated with the development of new foliations initiated. This also varied in timing locally as a result of migrating and/or shifting partitioning of the deformation at similar to smaller scales. A pseudosection for the bulk composition of this sample shows that andalusite and staurolite grow simultaneously only over a very tightly constrained range of P-T conditions. Yet these occurred for at least three tectonic events, two of which produced strong schistosities. This required bulk coaxial orogen-scale deformation with slight crustal thickening during two periods of bulk horizontal shortening balanced by gravitational collapse in the intervening event.