Contact metamorphism and granitic emplacement in the Tamworth area

The Moonbi Adamellite is a Permo-Triassic 'I'-type pluton which has intruded into the upper crust in the New England Orogen, N.S.W. The wall rocks covered in this study make up part of the Tamworth Terrane. They consist of argillites, arenites, conglomerates and limestones. The limestones have been separated into autochthonous or allochthonous deposits. The mafic igneous rocks are considered to be intrusive. They include an as yet unrecognised intrusion: the Oaky Creek andesite, which is found to the north of the Tamworth area. -- Contact metamorphism has been recognized within an aureole 3-4 kms wide. Rocks of the upper hornblende hornfels fades are found at the highest grades along the contact. The biotite isograd was seen as the boundary between the contact metamorphic rocks and the regional Tamworth Terrane. The regional geology in the Tamworth Terrane indicates a confining pressure of approximately 1 kbar at the time of emplacement of the adamellite. Prograde metamorphic assemblages indicate that the contact metamorphic fluids were water rich (XcO2<0.25). The source of the fluids is from prograde reactions and from meteoric water. The precursor magma rose through the crust as a water under-saturated magma. The magma is considered to have absorbed fluids from the contact rocks through fractures and permeable units. The water is hypothesised to have been then released through pegmatite dykes as the magma crystallised. -- A model of transport and emplacement of the Moonbi Adamellite involves a twostage mechanism. Initial space was made through roof lifting along faults and fractures in the cold country rocks. Then as the rocks became hot and infiltrated by fluids they became susceptible to deformation as increasing magma joined the intrusion through a feeder dyke. This caused weak syn-intrusive deformation in the wall rocks.