Elsevier

Precambrian Research

Volume 265, August 2015, Pages 232-248
Precambrian Research

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

https://doi.org/10.1016/j.precamres.2015.02.005Get rights and content

Highlights

  • We report the first 40Ar/39Ar thermochronology from the Albany-Fraser Orogen.

  • Strikingly fast ca 20–30 °C/Ma cooling is driven by rapid syn-orogenic exhumation.

  • Structural and geochronological data suggest a transpressional exhumation mechanism.

  • In contrast, other Mesoproterozoic orogens experience slow, post-orogenic cooling.

  • Transpressional settings are underrepresented in the Mesoproterozoic cooling record.

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 40Ar/39Ar 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 339+17°C/Ma and 225+7°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.

Introduction

Exhumation processes can be difficult to ascertain, especially in ancient orogenic belts. Thermochronology can be used to determine the distribution of cooling ages and rates across an orogen, providing an important tool in resolving orogenic exhumation histories. Orogenic cooling rates may be highly variable, and when taken in isolation are not diagnostic of a particular exhumation mechanism (Ring et al., 1999). However, when integrated with other datasets such as structural and metamorphic histories, cooling rates may be correlated with tectonic setting and with exhumation mechanism. For example, in collisional settings, large hot orogens (LHO), which are characterised by a plateau in the hinterland, typically experience slow exhumation driven by orogenic collapse. In contrast, small cold orogens (SCO) are rheologically stronger and do not undergo orogenic collapse, but are exhumed more quickly by erosion (Jamieson and Beaumont, 2013).

A compounding factor in comparing cooling histories from different orogens is the empirical correlation between decreasing cooling rate and increasing orogenic age. Dunlap (2000) showed that cooling rates in ancient Proterozoic collisional orogens are slower than those in Phanerozoic orogens (Dunlap, 2000). Empirical estimates of cooling rates in Proterozoic orogens are between 0.5 and 5 °C/Ma, whereas those for Phanerozoic orogens range from 5 to 50 °C/Ma (Dunlap, 2000, Willigers et al., 2002) and up to 150–350 °C/Ma in young, tectonically active orogens (e.g. Arnaud et al., 1993, Zeck et al., 1992). Dunlap (2000) suggested that the difference in cooling rates reflects the decreasing preservation potential of the thermochronological record with age, as Proterozoic orogens are more vulnerable to isotopic resetting than Phanerozoic orogens. Additionally, the present-day surface exposures of many Proterozoic orogens consist of the deeply eroded orogenic cores, which were at deep crustal levels during orogeny and were not exhumed until well after the orogenic cycle was complete (Willigers et al., 2002). Consequently, the Proterozoic thermochronological record is therefore biased towards slow, post-orogenic cooling, rather than the faster, syn-orogenic processes recorded in the upper-crustal rocks typically exposed in Phanerozoic orogens (Willigers et al., 2002).

The cooling records of global Mesoproterozoic orogens vary in their degree of preservation, as thermal histories are vulnerable to overprinting by later heating events. For example, both the Natal Metamorphic Province of South Africa and the Eastern Ghats Belt of India were active in the Mesoproterozoic assembly of Rodinia, but yield few data about post-orogenic cooling due to overprinting by ca 500 Ma Pan-African tectonism (Jacobs et al., 1997, Mezger and Cosca, 1999). In contrast, records of cooling and exhumation from the Grenville Orogen of North America, its inferred counterpart in the South American Amazon Craton and the Sveconorwegian Orogen of Scandinavia have not been overprinted, and are comparatively well understood due to several thermochronological studies (e.g. Bingen et al., 1998, Bingen et al., 2008, Busch et al., 1997, Cosca et al., 1998, Page et al., 1996, Rivers, 2008, Tohver et al., 2004). Although less extensively studied, cooling histories have also been determined for the intracontinental Reynolds Range, Mt. Isa Province and Mount Woods Inlier in central and northern Australia (Forbes et al., 2012, McLaren et al., 1999, Spikings et al., 2002, Vry and Baker, 2006).

In this article, we introduce the Albany-Fraser Orogen of Western Australia (Fig. 1) as an example of a Mesoproterozoic orogen that preserves a strikingly fast cooling history, that appears to defy the trend of decreasing cooling rate with increasing orogenic age. Although the tectonic setting of the Albany-Fraser Orogen is not well understood, it is thought to record the Mesoproterozoic suturing of the Yilgarn Craton to the combined Mawson and Gawler cratons during the assembly of Rodinia (Clark et al., 2000). The orogen curves around the margin of the Yilgarn Craton, such that it strikes east-west in the western region, and strikes northeast-southwest in the northeastern region (Fig. 1). The direction of convergence during orogeny is interpreted to be northwest-southeast, and consequently the western part of the orogen experienced a significant component of transpressive deformation, whereas the eastern part of the orogen was deformed in a more directly compressive stress regime (Bodorkos and Clark, 2004b).

In this article, we report the first 40Ar/39Ar thermochronology from the western part of the Albany-Fraser Orogen. We use these results in combination with previously published data relating to the inferred bulk stress regime of collisional orogeny to constrain the post-peak metamorphic cooling and exhumation history of the orogen. The cooling of the western Albany-Fraser Orogen is shown to be much faster than that in other Mesoproterozoic orogens, and is interpreted to represent fast cooling in a transpressional setting.

Section snippets

Tectonic setting of the Albany-Fraser Orogen

The Albany-Fraser Orogen extends ca 1200 km along the southern and southeastern margins of the Yilgarn Craton of Western Australia (Fig. 1). The orogen consists of mostly Paleo- to Mesoproterozoic rocks formed on or close to the margin of the Yilgarn Craton, which were subsequently deformed to high metamorphic grades during the late Mesoproterozoic Albany-Fraser Orogeny (Kirkland et al., 2011a, Spaggiari et al., 2011). The tectonic setting of the Albany-Fraser Orogeny is not well constrained;

Sample collection

The primary aim during sample collection was to ensure representative lithologies and geographical spread across the different domains of the western Albany-Fraser Orogen. Nineteen samples were collected from a 360 km transect across the Northern Foreland, Nornalup and Biranup Zones (Fig. 1). From these samples, 22 individual crystals (16 biotite, 4 muscovite and 2 hornblende) were analysed using 40Ar/39Ar thermochronology. Sample lithologies are summarised in Table 1, and sample locations and

Nornalup Zone

One hornblende and five biotite grains from the Nornalup Zone produced robust 40Ar/39Ar cooling ages (Table 1). The hornblende from orthogneiss AF02-1 produced a flat step-heating spectrum with a cooling age of 1169 ± 7 Ma (Fig. 2). Four biotite grains (orthogneisses AF02-1, AF02-2 and AF03, and metagranite AF06) produced weighted plateau ages ranging from 1168 ± 5 Ma to 1144 ± 5 Ma, with no apparent geographical trend in cooling ages (Fig. 1, Fig. 2). Biotite from metagranite AF01 records a much

Cooling and exhumation of the western Albany-Fraser Orogen

Two hornblende, eight biotite and four muscovite grains yielded statistically robust age plateaus (>70% of 39Ar released); plateaus are generally flat and low in complexity (Fig. 2). One biotite from the Northern Foreland yielded a mini-plateau (50–70% of 39Ar released), and seven biotite samples from the Nornalup Zone yielded no age plateaus (Fig. 2). Results are summarised in Table 1, with all ages reported at the 2σ uncertainty level.

One biotite grain from the Nornalup Zone (AF01) yielded a

Conclusion

This article reports the first 40Ar/39Ar thermochronology from the western part of the Albany-Fraser Orogen of Western Australia. The Nornalup and Biranup Zones share a similar cooling history, with hornblende cooling ages at ca 1169 Ma, and biotite cooling ages clustered around ca 1159 Ma. These ages correspond to cooling rates of ca 339+17°C/Ma in the Nornalup Zone and ca 225+7°C/Ma in the Biranup Zone, for cooling between ca 585–365 °C. The Northern Foreland records muscovite cooling ages

Acknowledgements

We thank Dr. Dave Moecher for assistance with fieldwork, and Dr. Tony Kemp, whose review of an earlier draft greatly improved this article. Thoughtful reviews by Toby Rivers and an anonymous reviewer helped to focus the writing. We also gratefully acknowledge Celia Mayers and Adam Frew for their help in the Western Australian Argon Isotope Facility at Curtin University, and the staff at the UWA Centre for Microscopy and Microanalysis for their assistance. This research was sponsored by the

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