Long-lived orogenic construction along the paleo-Pacific margin of Gondwana (Deep Freeze Range, north Victoria Land Antarctica).

he paleo-Pacific margin of Gondwana records a prolonged history of convergence during the Cambrian-Early Ordovician Ross-Delamerian and the Middle Ordovician-Early Silurian Lachlan orogenies. This study describes structure, petrology, and geochronology of a set of NW-SE striking ductile shear zones that crosscut the Ross age, Early Cambrian granitoid rocks of the Wilson Terrane in the Deep Freeze Range (North Victoria Land, Antarctica). The shear zones developed under amphibolite facies metamorphic conditions (650-700°C and 0.5-0.7 GPa) and show a systematic top-to-the-NE sense of shear. The shear zone activity interferes with emplacement of late, subhorizontal leucocratic dikes and combined U-(Th)-Pb (zircon and monazite) and 40Ar-39Ar (biotite and phengite) geochronology constrains the shearing event at ~470 Ma, with the sheared granite yielding U-Pb zircon crystallization ages of ~508 Ma. The reconstructed P-T path followed by the granite protoliths indicates an anticlockwise trajectory, suggesting the synshearing amphibolite metamorphism was associated with the burial of an early formed, Ross continental crustal section. These new findings are interpreted as evidence of a renewed, Ordovician episode of orogenic construction at the paleo-Pacific margin of Gondwana that predated the onset of the Lachlan orogeny in the region. A polycyclic reactivation of the Ross age Wilson Terrane of North Victoria Land is suggested, which is used to propose a unitary framework for the space-time transition from the Ross-Delamerian to the Lachlan orogeny along the proto-Pacific active margin of Gondwana.