Rapid cooling and exhumation in the western part of the Mesoproterozoic Albany-Fraser Orogen, Western Australia

Abstract

The Albany-Fraser Orogen of southwestern Australia is an understudied orogenic belt, which is interpreted to record the Mesoproterozoic suturing of the Yilgarn Craton of Western Australia to the Mawson Craton of East Antarctica during Rodinia assembly. Previous U-Pb geochronology has dated peak amphibolite to granulite-facies metamorphism in the orogen at ca 1180 Ma. Here, we report the first 40 Ar/ 39 Ar thermochronology of hornblende, biotite and muscovite grains from a 360 km transect across the western Albany-Fraser Orogen, and uncover a record of strikingly fast syn-orogenic cooling and exhumation. To the north, muscovites from the Northern Foreland record cooling at ca 1159 Ma. In the central and southern domains of the orogen, the Biranup and Nornalup Zones, hornblende yields ca 1169 Ma cooling ages, and biotite yields ca 1172-1144 Ma cooling ages. The new cooling ages imply that the three domains were exhumed rapidly following peak metamorphism at ca 1180 Ma, attained a similar structural level by ca 1159 Ma, and have experienced a uniform exhumation history since that time. To constrain mineral closure temperatures and post-peak metamorphic cooling rates, we conducted a Monte Carlo simulation, which fully propagates uncertainty and minimises error correlations. Modelling of cooling from hornblende to biotite closure temperatures (ca 585 • C and 365 • C respectively) in the Nornalup and Biranup Zones yields fast cooling rates of 33 +17 −9 • C/Ma and 22 +7 −5 • C/Ma respectively. These fast cooling rates imply rapid exhumation in an active tectonic setting undergoing peak metamorphism. Although the structural evolution of the Albany-Fraser Orogen remains poorly constrained, the transpressional tectonic activity associated with deformation in the western part of the Albany-Fraser Orogen may have been an active driver of this fast exhumation. This is distinctly different from exhumation models for granulite-facies domains in other Mesoproterozoic orogens, which typically experience post-orogenic, slow 1-5 • C/Ma cooling, driven by mechanisms such as orogenic collapse and erosion. We consider that the observed differences reflect the interpreted syn-tectonic transpressional exhumation history of the Albany-Fraser Orogen, which is an underrepresented tectonic regime in the Mesoproterozoic cooling record.

Closed system behaviour of argon in osumilite records protracted high‐Tmetamorphism within the Rogaland–Vest Agder Sector, Norway

“…An Ar/Ar closure temperature of ~620°C (for a 175 µm radius grain and a cooling rate of 10°C/Ma) indicate that osumilite is one of the K-bearing minerals most retentive to argon. The modelled closure temperature is lower than clinopyroxene (~750°C; Cassata et al, 2011), but similar to orthopyroxene (~620°C; Cassata et al, 2011) and hornblende (585 ± 50, Scibiorski et al, 2015). The properties of osumilite, including its potentially large grain size, make it capable of recording P-T-t information during extreme crustal conditions and prolonged orogenic processes.…”

Zircons from the Acraman impact melt rock (South Australia): Shock metamorphism, U–Pb and 40 Ar/ 39 Ar systematics, and implications for the isotopic dating of impact events

“…The relatively low level of shock observed in the shatter-coned Yardea Dacite of the central Acraman impact structure (Williams, 1994) suggests that the thermal pulse of the impact did not reset the white feldspar population, despite the generally low closure temperatures for the K/Ar system in feldspars (e.g., McDougall and Harrison, 1999). Multiple isotopic systems in the feldspars and the granular zircons from the Acraman impactites seem to record a thermal event in the Late Mesoproterozoic, roughly contemporaneous with the $1.33-1.10 Ga Albany-Fraser orogeny in southwestern Australia (Clark et al, 2000;Bodorkos and Clark, 2004;Scibiorski et al, 2015) and the coeval Musgrave orogeny in central Australia (White et al, 1999;Betts and Giles, 2006).…”

Fast cooling following a Late Triassic metamorphic and magmatic pulse: implications for the tectonic evolution of the Korean collision belt

“…Rapidly evolving terranes and tectonic systems often yield a small range of ages for mineral geochronometers that have very different closure temperatures, including polygenetic Th-and U-bearing accessory minerals formed at or close to peak metamorphic conditions (e.g., Dallmeyer et al, 1986;Dokka et al, 1986;Goodwin and Renne, 1991;Baldwin et al, 1993Baldwin et al, , 2004Brown and Dallmeyer, 1996;Platt et al, 1998;Treloar et al, 2000;Di Vincenzo and Palmeri, 2001;Zeitler et al, 2001;de Jong, 2003;Štípská et al, 2004;Çelik et al, 2006;Schulmann et al, 2008;Pitra et al, 2010;Wilke et al, 2010;Charles et al, 2012;Cubley et al, 2013a,b;Daoudene et al, 2013). Tectonically exhumed terranes generally experienced rapid cooling, whereas exhumation by erosion record slow cooling (e.g., Dallmeyer et al, 1986;Dokka et al, 1986;Baldwin et al, 1993Baldwin et al, , 2004Brown and Dallmeyer, 1996;Platt et al, 1998;Charles et al, 2012;Cubley et al, 2013a,b;Daoudene et al, 2013;Scibiorski et al, 2015), unless erosion was forced by extreme exhumation and became the driving force, as exemplified by the eastern and western Himalayan syntaxes (e.g., Treloar et al, 2000;Zeitler et al, 2001).…”