The Wongwibinda Complex, New England fold belt: a tilted low-P high-T terrane?

The Wongwibinda Complex is a rare exposure of deep low-pressure, high-temperature (low-P, high-T) New England Fold Belt near Armidale, NSW. The west to east thermal gradient in the Complex, which ranges from virtually unmetamorphosed sediments to amphibolite facies schists and migmatites producing a 20 km regional aureole, is well documented in the literature. However models which adequately explain its formation are limited. Landenberger et al. (1995) proposed that large scale, W-over-E reverse faulting along major crustal shear zones has exhumed this high-grade metamorphic terrane and tilted the section of crust. Through a combination of petrology, geothermobarometry and gravity modelling this study tested the tilted terrane model and found it inadequate to completely explain the thermal gradient in the Wongwibinda Complex. Geothermobarometry showed that peak P-T conditions occurred during the first major deformation event, D1. The P-T assessment, in combination with mineral chemistry analysis of garnet, has identified an early retrograde event, during D2, in the Wongwibinda Complex. This study provides the first documentation of retrograde metamorphic conditions under amphibolite facies. Geothermobarometry results found peak P-T conditions of T = 695 ± 22º C and P= 4.0 ± 0.54 kbar and retrograde P-Tconditions of T= 635 to 655º C and P 3.0 to 4.7 kbar. Gravity modelling in the central part of the Wongwibinda Complex, east-west through the thermal gradient, provides the first 2D interpretation of the subsurface structure. Of importance, is the lack of large hidden plutons in the model which indicates such plutons do not provide the heat source for the observed thermal gradient. Geothermobarometry found no definitive pressure gradient and determined that the maximum resolvable tilt is approximately 10º. W-over-E reverse movement on the Wongwibinda Fault is responsible for the exhumation of the Wongwibinda Complex. Tilting during exhumation, of less than 10º, is likely but the tilted terrane model is not effective in explaining the thermal gradient. Advective heat transfer during episodic granitoid emplacement via a magma transfer zone is proposed as an alternative model to explain the low-P, high-T metamorphic conditions of the Wongwibinda Complex.