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

Abstract

Low-pressure/high-temperature (low-Plhigh-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-Plhigh-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.

Metasomatic albitites and related biotite‐rich schists from a low‐pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north‐eastern Australia: microstructures and P–T–d paths

“…The P–T calibrations of this reaction are also a problem (e.g. Johnson & Vernon, 1995), and those we have tried (e.g. Pattison & Tracey, 1991; Powell & Holland, 1990) provide results that are inconsistent with the widespread occurrence of high Mg/Mg + Fe cordierite and andalusite at Mount Isa and Mary Kathleen (Rubenach, 1993; Reinhardt, 1992).…”

Nitrogen isotope fractionations during progressive metamorphism: A case study from the Paleozoic Cooma metasedimentary complex, southeastern Australia

“…1; Johnson and Vernon, 1995;Johnson, 1999;Williams, 2001;Collins, 1992, 2002). It lies in the southeastern Paleozoic Lachlan Fold Belt, part of a major orogenic belt that extended for some 20,000 km through Australia, Antartica, and South America before breakup of the Gondwana supercontinent (Coney et al, 1990;Coney, 1992;Fergusson and Coney, 1992).…”

“…The Lachlan Fold Belt is a low-pressure metamorphic belt dominated by regional greenschist-facies metamorphism. Exposure of high-grade metamorphic rocks is limited to two north-trending belts, known as the Wagga-Omeo and Cooma metamorphic complexes respectively (Johnson and Vernon, 1995;Collins, 1992, 2002). These complexes are characterized by large domains, about 10 km wide, of low-pressure metamorphism as indicated by the presence of andalusite (300-500 MPa; Ellis and Obata, 1992;Williams, 2001).…”

The low-pressure partial-melting behaviour of natural boron-bearing metapelites from the Mt. Stafford area, central Australia

“…Several of these studies of metapelitic andalusite-sillimanite phase equilibria in low-pressure settings (relavant to andalusite + melt stability) rejected the Holdaway (1971) curve because it was impossible to reconcile the andalusite stability field with other phase equilibrium constraints (Vernon, 1982;Vernon et al, 1990;Pattison & Tracy, 1991;Pattison, 1992;Johnson & Vernon, 1995), but Pattison (1992) provided a calculated position midway between the Holdaway (1971) and Richardson et al (1969) positions. This allows for an andalusite + haplogranitic melt stability field below 3 kbar, without the need of F, B, Li or excess Al components in the melt (Clarke et al, 2005).…”