Timing and duration of syn-magmatic deformation in the Mabel Downs Tonalite, northern Australia

Abstract

Detailed outcrop mapping combined with microstructural and U–Pb SHRIMP zircon data indicate that emplacement of the Mabel Downs Tonalite spanned progressive regional D 3 deformation in the Palaeoproterozoic Halls Creek Orogen of northern Australia, and that the duration of magmatism exceeded the crystallisation time of the pluton had its entire volume been emplaced instantaneously (∼10 5 y). The pluton comprises several compositionally distinct phases, which show (i) a regional solid-state S 3a foliation-forming event, predated by a strongly deformed porphyritic monzogranite with a U–Pb SHRIMP zircon age of 1837.3±6.0 Ma (95% confidence level); and (ii) overprinting by the localised S 3b Ord River Shear Zone, which crosscuts a 1831.9±3.3 Ma foliated granodiorite and contains 1826.6±7.3 Ma undeformed felsic veins, providing a younger age limit for D 3 deformation. The protracted nature of deformation and magmatism during regional D 3 deformation is significant in the context of the evolution of Halls Creek Orogen, which is characterised by a prolonged thermal event spanning three regional deformation events ( D 2, D 3 and D 4) within a 30–40 million-year interval. The accumulated finite strain is more probably the product of relatively long-lived events (of the order of several millions of years) with low average crustal strain rates, rather than high crustal strain rates during short-lived deformation episodes (=10 5 y). Thus the partitioning of strain accumulation into discrete deformation events during the rapid development of the Halls Creek Orogen was probably not as pronounced as in orogenic belts characterised by higher accumulated strain or longer intervals between deformation events.