Sequential porphyroblast growth during progressive deformation and low-P high-T (LPHT) metamorphism, Cooma Complex, Australia: the use of microstructural analysis to better understand deformation and metamorphic histories

Abstract Detailed microstructural analysis has been conducted in the Cooma Complex, which is one of the most thoroughly studied areas, structurally and metamorphically, in Australia. This was done to demonstrate the usefulness of microstructural analysis for better understanding deformation and metamorphic histories, and the results add significant new information about deformation and metamorphism of this area. Microstructural relationships in the schists and gneisses indicate that four coaxially superposed foliations formed with alternating subhorizontal (S 1 and S 3 ) and subvertical (S 2 and S 4 ) orientations, and the meso- and macrostructural relationships are easily understood within this deformational framework. Three stages of sequential porphyroblast growth involve cordierite and andalusite in the schists, and cordierite, andalusite and K-feldspar in the gneisses. The porphyroblasts overgrew and preserved progressive stages in the structural development of these rocks, with each period of growth occurring during a different deformation event (D 2 , D 3 and D 4 ). The sequential timing of these porphyroblasts is consistent with kfmash reaction-curve topology and indicates a prograde P - T - t path convex to the temperature axis. Combining this with isobaric cooling (inferred from previous studies) gives an anti-clockwise P - T - t path. Subvertical and subhorizontal foliations in porphyroblasts contrast with the variable orientations of the same foliations measured in the field. However, careful field work and microstructural analysis demonstrate that matrix-foliation rotation, rather than porphyroblast rotation, relative to geographic coordinates, is largely responsible for the contrasting observations, even where bedding has been overturned after porphyroblast growth.