Paleozoic deformation and isotopic disturbance in the southeastern Arunta Block, central Australia

Abstract

Proterozoic metamorphic rocks of the Arunta Block, central Australia, were thrusted southward over the northern margin of the Neoproterozoic to mid-Paleozoic Amadeus Basin during the 400-300 Ma Alice Springs Orogeny. The extent and nature of mid-Paleozoic ductile deformation in the southeastern Arunta Block is examined in light of available UPb, 40Ar 39 Ar , KAr, RbSr and SmNd isotopic age data. Most of the Proterozoic amphibolite and granulite facies rocks north of the zone of basement-cover interaction were exhumed and cooled from above ∼ 500 to below ∼ 125°C as a result of mid-Paleozoic nappe formation. This suggests that deformation early in the cooling history occurred at amphibolite facies conditions and progressed to greenschist facies conditions later in the cooling history. In general, shear zone rocks yield the youngest isotopic ages, indicating that deformation-induced recrystallization, grain size reduction, or localized thermal perturbations are responsible for the relatively young ages. We conclude that shear zones of mid-Paleozoic age were active under greenschist and amphibolite facies conditions, though further work is needed to differentiate whether amphibolite facies shear zones are Paleozoic or Proterozoic. Some Proterozoic shear zones were uplifted passively in the mid-Paleozoic, resulting in final closure and mixed ages in key minerals like hornblende.