Geochronology and thermochronology of shear zones from the Strangeways and the Harts Range Metamorphic Complexes, central Australia

Shear zones play an active role in the exhumation and deformation of geological terranes and can serve as time markers in their geologic evolution. The Strangways Metamorphic Complex (SMC), which includes the Oonagalabi Tongue, forms part of the Arunta Region, central Australia, a large (200000 km 2) Proterozoic granulite terrane. Palaeozoic high-strain zones at amphibolite-facies grade reworked the granulites throughout the Arunta.; Through a combination of analytical techniques involving crystal chemistry (electron microprobe) and isotopes (40Ar/39Ar and Rb/Sr geochronology and oxygen isotopes) on shear-zone rocks from the SMC, this study aims to determine whether the final exhumation path of the central Arunta (Strangways Range) is significantly different from the eastern Arunta (Harts Range).; Many shear-zone rocks contain muscovite and biotite which are aligned parallel to the stretching lineation, suggesting that these minerals grew during movement along the shear zones. The majority of the samples were metamorphosed to upper-greenschist facies/lower-amphibolite facies, yielding estimated peak metamorphic conditions of 3 kbar and 400°C. Calculated Ar closure temperatures of muscovite were higher (∼400°C) than biotite (<335°C). 40Ar/39Ar and Rb/Sr muscovite have similar ages of ∼345 Ma, implying that muscovite grew at or below its Ar closure temperature (∼400°C). Rb/Sr biotite ages are slightly younger at ∼320 Ma, consistent with slow cooling, and are interpreted to represent cooling ages at ∼300°C. 40Ar/39Ar biotite ages are generally older than 40Ar/39Ar muscovite ages, likely due to excess argon, although the reason for preferential uptake of excess Ar in the biotite remains unknown.; From this work, the SMC experienced slow cooling, at ∼5.0°C/Ma, from peak metamorphism (600°C at 380 Ma) to 300°C at 322 Ma during the Alice Springs Orogeny (300-400 Ma). This is slower than the estimated cooling rate of ∼7.2°C/Ma for the eastern Harts Range, which is interpreted to have been more strongly perturbed thermally by the Larapinta event (440-480 Ma). An exhumation rate of ∼0.24 km/Ma from 380 Ma to 345 Ma was calculated for the SMC, representing ∼8.4 km of exhumation.