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

Abstract

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. © 2010 Elsevier B.V.