Retrograde metamorphism of the Wongwibinda Complex, New England Fold Belt and the implications of 2.5D subsurface geophysical structure for the metamorphic history

Abstract

Garnet-bearing schists and migmatites sampled from the high-T, low-P Wongwibinda Complex in the New England Fold Belt, northern New South Wales, contain S₁ and S₂ assemblages that are inferred to have formed within error of each other at T = 700 and 650°C, respectively, and P = 400 and 380 MPa, respectively. Garnet grains commonly display a zoning profile that includes a flat unzoned interior with narrow (<350 μm) rims of variable composition. We interpret the unzoned cores as resulting from elemental homogenisation at peak D₁ metamorphic conditions and the narrow rims (with increased Mn) as resorbed grain edges that formed during retrograde conditions (D₂ and thereafter). The retrograde overprint is nearly pervasive across the complex and is most notable nearer to shear zones and intrusive rocks that cut S₁, including the Hillgrove Plutonic Suite. A gravity traverse across the complex determined the Wongwibinda Fault is best modelled with a dip of 65° towards the west but did not identify any substantial concealed mafic plutons, suggesting that the heat source for the shallow crustal thermal perturbation is not imaged beneath the complex today.

Key Words:

• garnet–cordierite
• gravity surveys
• regional aureoles
• retrograde metamorphism
• Wongwibinda Complex

Acknowledgements

Macquarie University funding to NRD (MURDG) provided financial support to conduct this research. This work began with the honours project of CRD. CRD thanks Norman Pearson for his assistance with the geochemical analyses and Craig O'Neill for help with the thermal modelling. Critical reviews by Dick Flood and Robin Offler improved an earlier version of this manuscript. The analytical data were obtained using instrumentation funded by ARC LIEF and DEST, Systemic Infrastructure Grants, industry partners and Macquarie University. This is contribution 629 from the Australian Research Council National Key Centre for Geochemical Evolution and Metallogeny of Continents (URL: <http://www.gemoc.mq.edu.au>).