Using P–T paths to interpret the tectonothermal setting of prograde metamorphism: An example from the northeastern Gawler Craton, South Australia

• Published in: Precambrian research 2011-02, Vol.185 (1-2), p.65-85
• Main Authors: Forbes, C.J., Giles, D., Hand, M., Betts, P.G., Suzuki, K., Chalmers, N., Dutch, R.
• Format: Article
• Language: English
• Subjects: Geothermal gradients; MnNCKFMASHT; Mount Woods Inlier; Prograde metamorphism; Proterozoic Australia
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2010.12.002

▶ Metamorphic study of pelites in northeastern Gawler Craton, South Australia. ▶ Prograde metamorphic path and peak P/T conditions resolved. ▶ Major high-T/low-P metamorphic event at ca. 1.61–1.59Ga. ▶ Contemporaneous with low-P thermal events in eastern Proterozoic Australia. ▶ Temperatures attained by intrusion of voluminous magmas and burial of hot rocks. We present metamorphic analysis from a porphyroblastic garnet–cordierite–spinel pelite from the Palaeo-Mesoproterozoic Mount Woods Inlier in South Australia. Petrographic analysis combined with a series of P–T pseudosections representative of a progressively changing effective bulk composition due to early porphyroblast growth and creation of an Al-rich, Si-poor chemical environment in the porphyroblast cores provides evidence of a prograde path that reached granulite facies conditions. Andalusite porphyroblasts are interpreted to have grown during the early phases of prograde metamorphism at conditions of ∼2–3kbar and ∼550–610°C. Geothermal gradients associated with early stages of prograde metamorphism are estimated to be ∼50–90°Ckm−1. The aluminosilicate porphyroblasts subsequently reacted to form cordierite–spinel symplectites that are chemically restricted from the remainder of the bulk rock by earlier formed cordierite moats. Peak metamorphism is petrographically represented by a phase of garnet growth, and attained conditions of ∼4.7kbar and 750°C, equivalent to a geothermal gradient of 45–50°Ckm−1. Monazite CHIME analysis has yielded an age of ∼1615Ma for the Moonlight Hills samples, and is suggested to represent monazite growth during prograde metamorphism through temperatures of ∼500–600°C. Peak metamorphism was contemporaneous with high-temperature/low-pressure metamorphism elsewhere in the Gawler Craton (Fleurieu Peninsula) and eastern Proterozoic Australia (e.g. Broken Hill Block, Mount Isa Inlier) at ca. 1.61–1.59Ga. The Mount Woods Inlier was heated at low pressure conditions due to intrusion of voluminous magmas to mid- to upper-crustal levels, some of which have been removed during exhumation and erosion. The hot rocks were then buried, at which point peak metamorphic conditions were attained.