TECTONIC IMPLICATIONS OF RB-SR BIOTITE AGES FOR THE HILLGROVE PLUTONIC SUITE, NEW-ENGLAND FOLD BELT, NSW, AUSTRALIA

The Late Carboniferous (similar to 300 Ma) Hillgrove Plutonic Suite is a deformed, fault-bound suite of S-type granitoids in the southern Ne w England Fold Belt of eastern Australia. Large-scale, W-over-E, high- angle, reverse faulting along major crustal shear zones has exhumed hi gh-grade metamorphic terranes such as the Tia and Wongwibinda migmatit e complexes, thereby exposing a tilted section of crust. Exhumed lower amphibolite facies mylonite zones within granitoids have been dated b y biotite Rb-Sr using both neocrystallized and relict magmatic biotite . Combined, the analyses define a narrow age range of 266-258 Ma, whic h 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 r apidly, as indicated by identical Rb-Sr ages for relict muscovite and biotite in the adjacent Wongwibinda Complex. Rb-Sr biotite ages for gr anites not directly affected by ductile shearing range from 296 to 256 Ma, the oldest of which is within error of the similar to 300 Ma empl acement ages given by U-Pb zircon dating. Rb-Sr ages obtained from rel ict biotite in lower-grade, greenschist facies mylonites define a wide range of ages (290-271 Ma), similar to the range acquired for the uns heared granites, The data suggest that shearing in the low-grade mylon ites took place at, or just below, the biotite Rb-Sr closure temperatu re. 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 similar to 260 Ma. Biotite Rb-Sr ages of less than 250 Ma are only recorded where samples are affected by local static thermal recrystallization associ ated 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 M a) uplift was the culmination of major E-W compression, which involved large-scale dispersal, oroclinal bending and tilting of crustal block s.