Abstract

The inversion of the Neoproterozoic-Cambrian passive margin of East Gondwana occurred during the early Paleozoic Delamerian-Ross orogeny. We present 40Ar/39Ar and structural data from deformed and metamorphosed Neoproterozoic clastic rocks beneath the Tasmanian ophiolite and the footwall of a high-pressure metamorphic complex in northern Tasmania. These data reveal the timing of accretionary deformation and the initiation of backarc extension along the Australian margin of Gondwana. 40Ar/39Ar analyses of muscovite from lower greenschist facies fault slices bounding the Forth metamorphic complex give plateau ages of 521.4 ± 2.5 and 520.7 ± 1.6 Ma. These data suggest that deformation within an accretionary prism off the margin of Tasmania, and possibly ocean arc collision, were under way by ca. 521 Ma. Muscovite from upper amphibolite and upper greenschist facies rocks in five locations of the Forth metamorphic complex, including retrograde shear zones, give 40Ar/39Ar cooling ages that average 508.1 ± 2.6 Ma. Identical muscovite cooling ages from rocks originally at very different metamorphic P-T conditions suggest rapid cooling of the Forth complex at ca. 508 Ma, due to the juxtaposition of higher-grade against lower-grade rocks. Rapid cooling is also indicated by concordant 40Ar/39Ar ages of hornblende and muscovite in the high-grade core. Cooling was probably due to rapid exhumation along extensional shear zones within a regional extensional setting that also produced the Mount Read–Mount Stavely volcanic complexes (505–495 Ma) along with rift basins in Tasmania and southeast Australia. This continental rift magmatism and extension were caused by west-dipping subduction under the Australian margin of Gondwana after the collisional phase of the Delamerian-Ross orogeny. Rollback of subduction in the Australian sector of the margin between ca. 508 and 460 Ma produced a backarc basin >1000 km wide that became the basement for the Lachlan orogen turbidites. Similar amounts of subduction rollback seem not to have occurred in Antarctica at this time (unless the record is lost), suggesting significant along-strike differences in the early Paleozoic geodynamics of the Delamerian-Ross orogenic system.

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