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

Detailed outcrop mapping combined with microstructural and U-Pb SHRIMP zirc on data indicate that emplacement of the Mabel Downs Tonalite spanned progr essive regional D-3 deformation in the Palaeoproterozoic Halls Creek Orogen of northern Australia, and that the duration of magmatism exceeded the cry stallisation time of the pluton had its entire volume been emplaced instant aneously (similar to 10(5) y). The pluton comprises several compositionally distinct phases, which show (i) a regional solid-state S-3a foliation-form ing 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 un deformed felsic veins. providing a younger age limit for D3 deformation. The protracted nature of deformation and magmatism during regional D-3 defo rmation is significant in the context of the evolution of Halls Creek Oroge n, which is characterised by a prolonged thermal event spanning three regio nal deformation events (D-2, D-3 and D-4) within a 30-40 million-year inter val. The accumulated finite strain is more probably the product of relative ly long-lived events (of the order of several millions of years) with low a verage crustal strain rates, rather than high crustal strain rates during s hort-lived deformation episodes (= 10(5) y). Thus the partitioning of strai n accumulation into discrete deformation events during the rapid developmen t of the Halls Creek Orogen was probably not as pronounced as in orogenic b elts characterised by higher accumulated strain or longer intervals between deformation events. (C) 2000 Elsevier Science Ltd. All rights reserved.