Tectonic implications of RbSr biotite ages for the Hillgrove Plutonic Suite, New England Fold Belt, N.S.W., Australia

Abstract

The Late Carboniferous (∼ 300 Ma) Hillgrove Plutonic Suite is a deformed, fault-bound suite of S-type granitoids in the southern New England Fold Belt of eastern Australia. Large-scale, W-over-E, high-angle, reverse faulting along major crustal shear zones has exhumed high-grade metamorphic terranes such as the Tia and Wongwibinda migmatite complexes, thereby exposing a tilted section of crust. Exhumed lower amphibolite facies mylonite zones within granitoids have been dated by biotite RbSr using both neocrystallized and relict magmatic biotite. Combined, the analyses define a narrow age range of 266-258 Ma, which indicates that faulting and uplift occurred during the Late Permian. Although the initial temperature conditions during shearing were well above the biotite RbSr closure temperature, these mylonites cooled rapidly, as indicated by identical RbSr ages for relict muscovite and biotite in the adjacent Wongwibinda Complex. RbSr biotite ages for granites not directly affected by ductile shearing range from 296 to 256 Ma, the oldest of which is within error of the ∼ 300 Ma emplacement ages given by UPb zircon dating. RbSr ages obtained from relict biotite in lower-grade, greenschist facies mylonites define a wide range of ages (290-271 Ma), similar to the range acquired for the unsheared granites. The data suggest that shearing in the low-grade mylonites took place at, or just below, the biotite RbSr closure temperature. This range of biotite ages, which varies systematically across the fault blocks, reflects the combined effects of slow cooling from 300 Ma, followed by rapid uplift and tilting of the terrane at ∼ 260 Ma. Biotite RbSr ages of less than 250 Ma are only recorded where samples are affected by local static thermal recrystallization associated with intrusion of Triassic granites. The biotite RbSr data place important chronological constraints on the tectonic evolution of the southern New England Fold Belt. Up to 12 km of Late Permian (268-256 Ma) uplift was the culmination of major E–W compression, which involved large-scale dispersal, oroclinal bending and tilting of crustal blocks.