Late Mesoproterozoic metamorphism in Tasmania: rifting or Rodinia-forming collision?

The collision of Australia-Antarctica with western Laurentia in the late Mesoproterozoic or early Neoproterozoic is hypothesised marks the final assembly of the supercontinent Rodinia. Evidence for metamorphism or deformation along the eastern margin of Australia-Antarctica and the western margin of Laurentia that might be attributed to this Rodinia-forming collision is equivocal. One of the few places in eastern Australia where Late Mesoproterozioc metamorphism is recognised is King Island- a small island located approximately 100 km northwest of Tasmania. Based on its record of Late Mesoproterozoic metamorphism, some workers interpret King Island to be a fragment of the collisional suture between Australia-Antarctica and Laurentia within Rodinia. However, the tectonic setting of Late Mesoproterozoic metamorphism on King Island has not been studied in detail and the interpretation that it reflects collisional processes has not been demonstrated.

This study presents results from recent field-based mapping on King Island and a reassessment of the pressure-temperature-time history of the Late Mesoproterozoic metamorphic event recorded here. Mesoproterozoic strata exposed on the west coast of King Island comprise sandy turbidites of the Surprise Bay Formation. The Surprise Bay Formation is intruded by a series of northeast trending dolerite dykes with MORB and within-plate basalt geochemical affinities. Probable mafic volcaniclastic rocks are locally interlayered with turbdites in the Surprised Bay Formation and are compositionally similar to the dolerite dykes. Both the Surprise Bay Formation and dolerite dykes record deformation assigned to D1, which produced a subhorizontal fabric and tight recumbent folds that were subsequently rotated into an upright position during Neoproterozoic—Paleozoic deformation. Metamorphism was pre- to syn-kinematic with respect to D1 with the peak garnet- and andalusite-bearing assemblages equilibrating in the low-pressure amphibolite facies at ca. 1290 Ma.

Integrating field and microstructural observations suggests that sedimentation, mafic magmatism, deformation, and low-pressure medium-temperature metamorphism on King Island may have been broadly contemporaneous. We suggest that Late Mesoproterozoic metamorphism on King Island occurred in a mature continental rift setting, where mantle upwelling and voluminous mafic magmatism drive low pressure medium—high-temperature metamorphism in thinned continental crust. Late Mesoproterozoic tectonism on King Island may therefore to be unrelated to orogenesis associated with the assembly of Rodinia. Instead, ca. 1290 Ma rift-related metamorphism on King Island overlaps with the final stages of the breakup of the supercontinent Nuna, which is also recorded by widespread basin formation throughout mainland Tasmania at this time.