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

Abstract

Excellent inclusion trails in a sample containing both staurolite and andalusite porphyroblasts are used to demonstrate techniques that allow the intimate relationships between deformation and porphyroblast growth to be recognized, described in detail and understood. This approach reveals three main phases of growth of both mineral phases, some of which was demonstrably synchronous, during three tectonic events. Each main period of growth occurred during the early stages of three deformations that were successively near orthogonal. However, extra periods are distinguishable in andalusite in some of these events because this phase occurs as clusters of large crystals that vary in orientation by 2° to >10°. All foliations defined by all inclusion trails within every porphyroblast inflect/intersect about an axis trending at 025° (called a FIA). This indicates that the direction of the horizontal component of bulk shortening was identical for the first and third of the three deformations recorded by porphyroblast growth. Portions of sigmoidal to slightly spiral-shaped inclusion trails in most porphyroblast clusters locally diverge in opposite directions due to overprinting orthogonal bulk shortening typical of that which forms millipede geometries. These microstructures confirm the role of coaxial bulk shortening in initiating porphyroblast growth in an environment that locally becomes strongly non-coaxial as the deformation intensifies in the same event.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 5mm. 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.