Warm continental subduction initiated by back-arc collapse: Evidence from remote south-west Tasmania

Abstract

The Tasmanian microcontinent, situated along the East Gondwana accretionary margin during the late Neoproterozoic and early Palaeozoic, contains an unequivocal high-pressure metamorphic record comprising key information pertaining to the geodynamics of subduction along the margin. Subduction of the Tasmanian microcontinent is interpreted by some as a response to back-arc basin inversion prior to ophiolite obduction and high-pressure metamorphism during the Cambrian Tyennan Orogeny. However, thermobarometric evidence in support of such a model from rocks once positioned on the subducting continental margin is lacking. Despite occurrences of eclogite-facies mineral assemblages in the strongly deformed Tyennan Region of western Tasmania, garnet-bearing quartzofeldspathic assemblages documented in metasedimentary lithologies from the remote south-west coast of Tasmania have been interpreted as an expression of low- to moderate-pressure metamorphism. We report a strongly overprinted chlorite-quartz-garnet-bearing assemblage from the southern Tyennan Region (Nye Bay) which shows evidence for high-pressure metamorphism. Coarse-grained garnet porphyroblasts contain inclusions of kyanite, muscovite, and rutile, and yield in-situ Lu–Hf dates of c. 520 Ma. The cm-scale garnet porphyroblasts are zoned in the major and trace elements, preserving core-rim compositional gradients reflecting garnet growth up-pressure. Aided by mineral equilibria forward modelling, the garnet rim compositions and the Zr content of Cambrian rutile constrain peak metamorphic conditions of ∼ 17.5–19 kbar and ∼ 780–820 °C, equivalent to warm subduction thermal gradients between 410–470 °C/GPa. Garnet core compositions and the Ti content of quartz inclusions in the garnet cores constrain the pressures and temperatures for garnet nucleation to ∼ 6–7 kbar and ∼ 560–580 °C, corresponding to relatively high prograde thermal gradients between 800–965 °C/GPa. The thermal gradients determined from the south-west Tasmanian metamorphic record provide a direct window into the progressive evolution of the thermal state of the Cambrian subduction system, with the physical conditions of garnet nucleation potentially reflecting those of subduction initiation. The corresponding warm thermal gradients provide evidence for subduction initiation driven by the collapse of a pre-orogenic back-arc. This interpretation is consistent with an existing tectonic model for the Tyennan Orogeny which proposes a back-arc basin origin for the protoliths to the western Tasmanian sub-ophiolitic metamorphic sole.