A rift-related origin for regional medium-pressure, high-temperature metamorphism

• Published in: Earth and planetary science letters 2015-07, Vol.421, p.75-88
• Main Authors: Tucker, Naomi M., Hand, Martin, Payne, Justin L.
• Format: Article
• Language: English
• Subjects: [formula omitted] pseudosection; Australia; Basins; Coarsening; deep rift basin; Earth; Harts Range; intraplate deformation; Marine environments; Metamorphic; Regional; regional high-temperature metamorphism; Sedimentation
• ISSN: 0012-821X; 1385-013X
• DOI: 10.1016/j.epsl.2015.04.003

•The lower Harts Range Group was buried to mid-lower crustal depths (∼30–35 km).•Burial occurred along a moderately-high apparent geotherm (∼840–980°C/GPa).•Metamorphism was facilitated by sediment loading and magmatism in a deep rift basin.•Regional high-grade terranes do not always reflect compressional thickening. Crustal-scale exhumation during the Alice Springs Orogeny (c. 450–320 Ma) in central Australia has exposed a region of medium-pressure, high-temperature metasedimentary and metabasic rocks that comprise the Harts Range Group (HRG). Similarities in the detrital zircon age spectra between the HRG and surrounding unmetamorphosed, intraplate late Neoproterozoic–Cambrian basin sequences suggest that the HRG is a highly metamorphosed equivalent of these successions. Calculated phase equilibria modelling and thermobarometry constrain peak metamorphic conditions to ∼880°C and 10.5 kbar, and ∼680°C and 5.5–8.0 kbar, in the structurally lowest and highest parts of the HRG, respectively. Metamorphic conditions also indicate that burial occurred along a near-linear moderately-high apparent thermal gradient, recorded by the prograde development of andalusite-bearing mineral assemblages at shallower structural levels. Prograde and peak metamorphism was associated with voluminous intrusive and extrusive mafic magmatism, the development of a coarse layer-parallel fabric and north-directed normal shear-sense kinematics. Collectively, these point to an extensional regime. Furthermore, burial and metamorphism at c. 480–460 Ma was concurrent with a shallow epicontinental marine environment and ongoing sedimentation in central Australia. Accordingly, the deep burial, metamorphism and deformation of the HRG to mid-lower crustal depths (∼20–35 km) must be justified in the context of the broader intraplate basin evolution at this time. It is difficult to reconcile medium-pressure, high-temperature metamorphism of the HRG with deep burial by tectonic overthickening which is commonly assumed to be the case. In contrast, metamorphism of the HRG seems more compatible with burial within a deep rift-style basin driven by high heat flow and mafic magmatism, suggesting that regional medium-pressure metamorphic terranes are not necessarily reflective of compressional thickening of the crust.