Tectono-metamorphic evolution of high-P/T and low-P/T metamorphic rocks in the Tia Complex, southern New England Fold Belt, eastern Australia: Insights from K–Ar chronology

The Tia Complex in the southern New England Fold Belt is a poly-metamorphosed Late Paleozoic accretionary complex. It consists mainly of high-P/low-T type pumpellyite–actinolite facies (rare blueschist facies) schists, phyllite and serpentinite (T = 300 °C and P = 5 kbar), and low-P/high-T type amphibolite facies schist and gneiss (T = 600 °C and P < 5 kbar) associated with granodioritic plutons (Tia granodiorite). White mica and biotite K–Ar ages distinguish Carboniferous subduction zone metamorphism and Permian granitic intrusions, respectively. The systematic K–Ar age mapping along a N–S traverse of the Tia Complex exhibits a gradual change. The white mica ages become younger from the lowest-grade zone (339 Ma) to the highest-grade zone (259 Ma). In contrast, Si content of muscovite changes drastically only in the highest-grade zone. The regional changes of white mica K–Ar ages and chemical compositions of micas indicate argon depletion from precursor high-P/low-T type phengitic white mica during the thermal overprinting and recrystallization by granitoids intrusions. Our new K–Ar ages and available geological data postulate a model of the eastward rollback of a subduction zone in Early Permian. The eastward shift of a subduction zone system and subsequent magmatic activities of high-Mg andesite and adakite might explain formation of S-type granitoids (Hillgrove suite) and coeval low-P/high-T type metamorphism in the Tia Complex.

► K–Ar age mapping along a N–S traverse of the Tia Complex exhibits a gradual change.
► White mica ages become younger from the lowest-grade zone to the highest-grade zone.
► Biotite ages distinguish Permian granite from Carboniferous blueschist metamorphism.
► Results indicate Ar depletion from precursor phengites during a thermal overprinting.
► New data postulate the eastward rollback of a subduction zone in Early Permian.