Petrology and geochemistry of the Tynong Province Granitoids, Victoria, Australia

This thesis investigates the origin, evolution and emplacement of magmas parental to the Tynong Province granitoids, located in the Melbourne Zone, Lachlan Orogen, Australia. The Province comprises at least five plutons: Baw Baw, Tanjil Bren, Toorongo, Tynong and Lysterfield. This thesis uses a number of techniques to build on field observations and petrography to investigate these plutons. This thesis combined regional field observations and petrography, with a more detailed investigation of the Tynong North quarry, to investigate magma hybridization processes. The Tynong North quarry, within the Tynong pluton, exposes abundant evidence for mixing and mingling between contrasted magmas. This evidence includes neighbouring outcrops of coarse-grained felsic and finer-grained more mafic rocks, mafic microgranular enclaves (MME), rapakivi and antirapakivi textures, plagioclase and quartz ocelli, pseudomorphs after orthopyroxene, and acicular apatite in hybrid quartz diorite. In addition to field, mineralogical and textural evidence there is also strong geochemical evidence for magma mixing based on whole rock major and trace element compositions (chapter 5), whole rock Rb-Sr and Sm-Nd isotopic compositions and the Lu-Hf isotopic compositions of zircons. The complex relationship that exists between whole rock εNd and silica content of these rocks indicates that they went through multiple stages of hybridization (chapter 6). The mantle-like Lu-Hf isotopic compositions of zircons from the Tynong and Toorongo plutons show that there have been only relatively minor contributions of crustal material in the evolution of these granitoids. The ranges of mantle-like εHf of zircons in both plutons further indicate that magma mixing played a major role in the origin of these granitoids, supporting field and other geochemical evidence. In order to test the applicability of a new magma mixing model for the Tynong Province granitoids,the chemical signatures of chaotic mixing of contrasted magma as established by numerical modelling and physical mixing experiments (Perugini, et al. 2006; De Campos, et al. 2011) have been compared with mixed rock sequences of the Tynong pluton and the Askja 1875 A.D. basalt-rhyolite eruption, and also rock sequences apparently dominated by crystal fractionation such as the Leh and Gyamsa granitic plutons, Ladakh, India (chapter 7). As predicted by Perugini et al. (2006) elements with similar diffusivities vary across related rocks in a strongly co-related manner, as measured by correlation coefficients, whilst those with different diffusivities are only weakly correlated. However, it also became clear that the Perugini et al. (2006) findings cannot distinguish fractionated magma systems from mixed magma systems. The complexity of the partial hybridization during chaotic flow determined by De Campos et al. (2011) was tested using the partially mixed glasses of the Askja flow. It was found that direct comparison do experimental results was impossible. It was concluded from this that the complexities of the mixing process investigated by De Campos et al. (2011) are specific to the nature of the two melts undergoing hybridization, suggesting that incomplete chaotic mixing may lead to unpredictable Petrology and Geochemistry of the Tynong Province Granitoids, Victoria, Australia mixing curves, depending on the nature of the two melts, the duration of the mixing event, and the nature of the flow leading to mixing. The temperature and pressure of pluton emplacement have been investigated by using THERMOCALC phase analysis and the recently calibrated TitaniQ thermobarometer. A P-T pseudosection of the Toorongo contact aureole cordierite-bearing migmatitic hornfels using THERMOCALC indicates that peak metamorphism in the aureole 690 ºC to 750 ºC at P between 1 and 3 kbar (chapter 9) which is also supported by the presence of cordierite in the hornfels. Temperature estimations for the Tynong hybrid quartz diorite, Toorongo granodiorite and Toorongo pluton contact aureole migmatitic hornfels (same sample as used to construct the pseudosection) using TitaniQ resulted in sub-magmatic temperatures varying between 550 and 650 °C (chapter 8).These temperature estimates indicate that the quartz grains of these samples either record temperature re-equilibration during cooling or that the thermobarometer is not applicable at such low pressure. In summary, this thesis has applied a number of new methodological approaches to study an old problem, that of magma hybridization. Results point to multi-stage hybridization as the main underlying process in the genesis of the magmatic rocks of the Tynong Province. Unlike other applications of the TitaniQ method to igneous rocks, this thesis has demonstrated that samples of the Tynong Province yield essentially sub-magmatic temperatures, indicating either an issue with using the geothermobarometer in low pressure systems, or a re-setting of the Ti-content in quartz at low temperatures. Application of Perugini et al.’s (2006) approach to investigate the geochemical signature of magma mixing, resulting from chaotic mixing processes, failed to distinguish mixing from fractionation. Furthermore, the complex signature of incomplete mixing caused by different element diffusivities derived by De Campos et al. (2011) could not be directly applied to the Tynong magma compositions or to the hybridized volcanic glass of the Askja volcano. This suggests the possibility that their results are specific to the compositions used in their experiments, and that mixing of magmas of other compositions leads to different mixing curves that cannot be predicted from either the simple, traditional mixing line, or their chaotic mixing experiments. If so magma mixing processes may be chemically more complex than even De Campos et al. (2011) envisaged. The overall conclusion is that whilst there have been considerable methodological advances to investigate the genesis and emplacement of magmas, such as TitaniQ, chaotic mixing models and experiments to explain geochemical mixing trends, improvements on the THERMOCALC method for application to anatectic rocks, and the use of Hf in zircons, their direct application to magmatic systems still needs further advances to enable their widespread use and strengthen their predictive power. References De Campos, et al. 2011 (Conts. Min. and Petrol., v. 161: 863-881). Perugini, et al. 2006 (Earth Planet. Sci. Lett., v. 243: 669-680).