Stepping stones and pitfalls in the determination of an anticlockwise P‐T‐t‐deformation path: the low‐P, high‐T Cooma Complex, Australia

Abstract

Abstract Low‐pressure/high‐temperature (low‐P/high‐T) metamorphic rocks of the Cooma Complex, southeastern Australia, show evidence of an anticlockwise pressure‐temperature‐time‐deformation (P‐T‐t‐D) path, similar to those of some other low‐P/high‐T metamorphic areas of Australia. Prograde paths are reasonably well constrained in cordierite‐andalusite schists, cordierite‐K‐feldspar gneisses and andalusite‐K‐feldspar gneisses. These paths are inferred to be convex to the temperature axis, involving increase in pressure with increase in temperature. Evidence of the retrograde path is inconclusive, but is consistent with approximately isobaric cooling, as are available isotopic data on the Cooma Granodiorite, which indicate initially rapid cooling following attainment of peak temperatures. The retrograde path is inconsistent with either a clockwise P‐T‐t‐D path involving rapid or even moderate decompression immediately post‐dating the peak of metamorphism, or a path in which the retrograde component simply reverses the prograde component, because both these paths should cross reactions forming cordierite from aluminosilicate, for which no evidence has been observed.Determination of the deformational‐metamorphic history of the complex is not straightfoward and depends on careful examination of critical samples. Evidence necessary for successful elucidation of the prograde, and part of the retrograde, deformational‐metamorphic history in the Cooma Complex includes: (1) sequentially grown porphyroblasts that can be timed relative to surrounding foliations; (2) partial replacement microstructures providing relative timing of metamorphic reactions that cannot be timed relative to foliation development; (3) a tectonic marker foliation (S4 at Cooma) that allows correlation of foliations from one location to another; and (4) single samples containing all of the foliations and all generations of porphyroblast growth within a single metamorphic zone. The latest two or three foliations involve low strain accumulation, allowing relative timing relationships between foliations and porphyroblasts to be more clearly determined.Sequential porphyroblast growth and foliation development in the cordierite‐andalusite schists is examined for situations involving rotation and non‐rotation of porphyroblasts relative to geographically fixed coordinates. Although the number of foliations developed varies in the rotational situation, depending on the deformation history proposed, the sequential order of porphyroblast growths does not differ from the non‐rotational situation. Thus, whether or not porphyroblasts rotated in the Cooma rocks, the sequence of reactions, and therefore P‐T‐t paths inferred from the relative timing of porphyroblast growths, remain the same, for the deformational histories evaluated.