Sm–Nd evidence for high‐grade Ordovician metamorphism in the Arunta Block, central Australia

Abstract

Sm–Nd ages from the Harts Range in the south‐eastern Arunta Inlier in central Australia indicate that regional metamorphism up to granulite facies occurred in the Early Ordovician (c. 475 Ma). This represents a radical departure from previous tectonic models for the region and identifies a previously unrecognized intraplate event in central Australia. Peak metamorphic assemblages (800 °C and 10.5 kbar) formed at around 476±14 Ma and underwent approximately 4 kbar of near‐isothermal decompression at 475±4 Ma. A coarse‐grained unfoliated garnet–clinopyroxene‐bearing marble inferred to have recrystallized late in the decompressional evolution, gives an age of 469±7 Ma. Two lines of evidence suggest the Early Ordovician tectonism occurred in an extensional setting. First, the timing of the high‐grade lower crustal deformation coincides with a period of marine sedimentation in the Amadeus and Georgina basins that was associated with a seaway that developed across central Australia. Second, isothermal decompression of lower crustal rocks was associated with the formation of a regional, sub‐horizontal mid‐crustal foliation. In the Entia Gneiss Complex, which forms the structurally lowest part of the Harts Range, upper‐amphibolite facies metamorphism (c. 700 °C, 8–9 kbar) occurred at 479±15 Ma. There is no evidence that P–T conditions in the Entia Gneiss Complex were as high as in the overlying units. This implies that the extensional system was reworked during a later compressional event. Sm–Nd data from the mid‐amphibolite facies (c. 650 °C and 6 kbar) detachment zone that separates the Irindina Supracrustal Assemblage and Entia Gneiss Complex give an age of 449±10 Ma. This age corresponds to the timing of a change in the pattern and style of sedimentation in the Amadeus and Georgina basins, and indicates that the change in basin dynamics was associated with mid‐crustal deformation. It also suggests that compressional deformation culminating in the Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny may have begun as early as c. 450 Ma. At present, the extent of Early Ordovician tectonism in central Australia is unknown. However, granulite facies metamorphism and associated intense deformation imply an event of regional extent. An implication of this work is that high‐grade lower crustal metamorphism and intense deformation occurred during the development of a broad, shallow, slowly subsiding intraplate basin.

Low thermal Peclet number intraplate orogeny in central Australia

“…unta Inlier show that maximum denudation associated with the ASO was V20 km [16], although throughout much of the orogen it was closer to 10 km. One consequence of this denudation was the exhumation of an orogenic core comprising older, higher-grade metamorphic rocks.…”

“…While the data summarised in Fig. 5 show a relatively continuous record of mineral isotope resetting through this interval, discrete pulses of isotopic resetting have been inferred from various parts of the orogen at V450^430 Ma, V390^380 Ma, V365^350 Ma and V330 Ma by various authors [6,14,16,17,18]. Thus, although there is clear evidence for episodic cooling within given subdomains, it is not yet clear whether the record is episodic at the scale of whole orogen.…”

“…5. The isotopic record of mineral closure in a diverse range of mineral systems spans an interval of V150 Myr from the late Ordovician [14,16] to the late Carboniferous [14]. While the data summarised in Fig.…”

“…See text for discussion. Primary thermochronologic sources include [6,14,16,17,18]. crust extending to a depth of 10 km that contributes 50 mW m 32 to the surface heat £ow [10], above a depleted lower crust.…”

Tectonic feedback, intraplate orogeny and the geochemical structure of the crust: a central Australian perspective

“…1). Shortening began possibly as early as 450 Ma (Shaw & Black 1991;Mawby et al 1999), but certainly by 400 Ma, and was marked by the inversion of a late Cambrian to mid-Ordovician marine basin and the deposition of clastic sediment (Shaw et al 1991a). Isotopic and sedimentological evidence indicates that deformation continued, at least intermittently, for at least 100 Ma, terminating in the early Permian (Shaw et al 1991a;Wells & Moss TECTONIC FEEDBACK 197 1983).…”