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

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.

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