Deformation, metamorphism and migmatite genesis in the Wongwibinda Metamorphic Complex, Eastern Australia

Farrell, T. R.

Title: Deformation, metamorphism and migmatite genesis in the Wongwibinda Metamorphic Complex, Eastern Australia
Creator: Farrell, T. R.
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 1992
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: The Wongwibinda Metamorphic Complex, situated in the southern New England Fold Belt of eastern Australia, is a small area of high grade schists and migmatites associated with the Abroi Granodiorite, a member of the S-type Hillgrove Plutonic Suite. The sedimentary protoliths are inferred to be a succession of turbidites, and less abundant intercalated basalt and chert, that accumulated in the Middle-Late Palaezoic accretionary complex that eventually evolved into the New England Fold Belt. Substantial crustal heating followed that cessation of subduction at about 310Ma, causing almost total recystallisation of existing accretionary fabrics and initiating a prolonged period of intense deformation. The first stage of deformation (D₁) was contemporaneous with amphibolite facies metamorphism and commenced after the sequence had reached temperatures appropriate to the formation of K-feldspar and cordierite in meta-pelites via the reaction Qtz + Bt + Ms = Crd + Kfs + H₂O. The P-T conditions were in the range 2.6-3.0kbar at T < 630°C for low grade schists and pelitic hornfelses, and 660-675°C at 2.8-3.8kbar for high grade rocks. D₁ resulted in the development of steeply plunging, open to very tight folds with a pervasive, steeply dipping, N-S trending axial plane foliation. Intense rotational strain during D₁, resulted in the formation of a variety of asymmetric structures in calc-silicate and psammitic layers. The second stage of deformation (D₂) took place at approximately 300Ma and was coeval with the emplacement of the Abroi Granodiorite and localised partial melting of the sequence. At this stage the maximum temperatures in migmatites were in the range 700-710°C, appropriate for the commencement of melting in graphitic pelites. The transition between D₁ and D₂ was marked by the large scale rotation of the sequence until structures were in a suitable orientation (~E-W) for the refolding of F₁ folds and the buckling and transposition of S₁ into S₂. D₂ structures are concentrated in migmatites and adjacent to granitoids, suggesting that deformation was thermally controlled. Retrogression of cordierite + K-feldspar to Bt + Sil + Qtz assemblages at pressures > 3.2kbar, and to Bt + Ms + Qtz assemblages at lower pressure, was synchronous with D₂ in unmigmatised schists, and post-D₂ in two-mica granites and pelitic migmatites. The formation of fibrolite-bearing assemblages suggests that there was a pressure increase during cooling, which is consistent with some degree of crustal thickening and is supported by the substantial shortening indicated by the development of compressional structures in D₂. The final phase of deformation was a ductile shearing event (D₃) that occurred after crystallisation of the Abroi Granodiorite and the migmatites. Shearing probably commenced at temperatures of 600-650°C and a pressure of 3.5-4kbar in migmatites and slightly lower pressure in unmigmatised schists. It resulted in the development of several major shear zones (Wongwibinda, Glen Mohr and Glen Bluff Shear Zones) as well as numerous subsidiary structures. Kinematic indicators, such as S-C fabrics, asymmetric porphyroclass, biotite "fish", and mineral stretching lineations, indicate west over east, predmoninantly dip-slip, reverse movement on all the major shear zones. Minimum displacements of 12km on the Wongwibinda Shear Zone, as indicated by the strain analysis of S-C mylonites, and comparable displacements on the other shear zones, resulted in substantial crustal shortening as the complex was uplifted and thrust over the Permian rocks to the east. Shearing had largely ceased by the time the complex had cooled to 400°C at about 260Ma. Migmatites in the Wongwibinda Complex contain two generations of structurally, chemically and genetically distinct leucosomes. The first generations of structurally, chemically and genetically distinct leucosomes. The first generation of leucosomes (D₁) are conformable with D₁ structures, and formed under subsolidus conditions through the incongruent dissolution of biotite and plagioclase, by infiltrating fluids, to produce the quartz. The modal mineralogy and bulk chemistry of the D₁ leucosomes is a reflection of the fluid a_k+/a_Na+ ratio and the degree of fluid/rock interaction. Thin vein-like quartz-feldspar±biotite leucosomes formed as a result of progressive fluid infiltration along grain boundaries and microcracks parallel to S₁. Fluid/rock ratios were relatively low and the dissolution reactions did not go to completion. Vein-like leucosomes are characterised by high SiO₂, variable K₂O, Ba and REE, and low Fe₂O₃, MnO, TiO₂, Rb, Zr and MgO contents. The major variability in their compositions is dependent on whether they contain K-feldspar and/or biotite. In contrast, thicker, quartz-rich lenticular leucosomes formed along hydraulic fractures by more extreme dissolution at higher fluid/rock ratios. They have high SiO₂, and very low K₂O, Na₂O, CaO, Fe₂O₃, MgO, MnO, REE, Hf, Rb, Zr and Th, and flat depleted REE patterns compared to the mesosome. Second generation (D₂) leucosomes are anatectic in origin and crosscut the D₁ subsolidus leucosomes. They are divided into biotite-granite and leucogranite/pegmatite leucosomes, principally on the basis of their contrasting textures and mineralogy. The former typically occur in irregular patches adjacent to shear zones or as larger, rounded pods. They characteristically display diffuse margins and ghost structures conformable with S₁ in the mesosome. The leucogranite/pegmatite leucosomes commonly occur in shear zones, where they have sharp intrusive contacts with the mesosome, or as irregular, texturally heterogenous pods. The biotite-rich leucosomes formed by partial melting promoted by the influx of fluids along shear zones and other D₂ structures. These locally generated melts were not able to segregate effectively from their source, either because the degree of melting was too low, due to the limited supply of water, or they were not tectonically segregated. They typically have cotectic compositions and display broadly similar major and trace element contents to the mesosome, apart from small increases in SiO₂, K₂O, Ba and slightly lower Fe₂O₃, and MgO contents. in contrast, the leucogranite/pegmatite leucosomes formed by deformation-controlled disequilibrium melting and the forceful emplacement of the melts into active shear zones. They display high SiO₂, K₂O and low Fe₂O₃, MgO, MnO, TiO₂ and LREE contents, and a characteristic positive Eu anomaly, consistent with partial melting involving quartz and feldspars, an subsequent plagioclase fractionation. The Wongwibinda Metamorphic Complex followed an anti-clockwise P-T-time path controlled by the intrusion of granitoid magmas. The sequence initially underwent near isobaric heating, followed by shortening and crustal thickening as the rocks were thermally weakened at temperatures close the the metamorphic peak. This resulted in an increase in pressure and an initially isobaric or slightly up-pressure cooling path. The anomalously high metamorphic grade of the Wongwibinda Complex, compared to other areas in the former accretionary prism, can be accounted for by the exhumation of the complex from deeper crustal levels during uplift in D₃.
Subject: Wongwibinda Metamorphic Complex; Eastern Australia; schists; migmatites; Abroi Granodiorite; geoscience; geology
Identifier: http://hdl.handle.net/1959.13/1312497
Identifier: uon:22409
Language: eng

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