Deformation-assisted melt migration and melt-rock interaction during the intracontinental Alice Springs Orogeny, central Australia
Aqueous fluids play an important tectonic role in terrain retrogression and crustal softening of deep to mid crustal rocks during metamorphism. Moreover, the role of a wide compositional spectrum of fluids is essential in mineral system science as a medium of mass transfer. High strain zones, shear zones and faults are Earth’s primary crustal-scale conduits of fluid (sensu lato) migration and mass transfer. However, aqueous fluids become richer in solute and give way to silicate melts at higher temperatures, deeper in the crust. The role of silicate melts in hydration, retrogression, rheological weakening, and mass transfer are less studied and understood compared to aqueous fluids. In some studies, the type of fluid is not stated or is assumed to have been an aqueous fluid, and in others, aqueous fluid and silicate melt are conflated. The broad theme of this thesis is melt-rock interaction in high strain zones and how to strengthen our interpretations when inferring melt-present deformation, a fundamental process in the evolution of high-temperature rock systems. An anastomosing shear zone network exposed in central Australia formed during the last major tectonic event in the region, the intracontinental Alice Springs Orogeny (ASO; 450–300 Ma). The deepest exhumed core of the orogen exposes high-temperature shear zones in the Strangways Range and these are explored in this thesis to characterise melt-rock interaction in mid to deep crustal rocks. The study uses field work and microstructural and microchemical methods to: i) distinguish aqueous fluid-rock interaction from melt-rock interaction; ii) explore the regional-scale rheological consequences of syntectonic melt migration through shear zones; iii) investigate the mechanism that enriches rocks in oxide minerals in both continental and oceanic settings, and iv) examine melt-rock interaction and the formation of reaction instability fingers at mineral grain boundaries that may dissolve a precursor mineral and precipitate other minerals to form epigenetic inclusions.
Most amphibolite facies shear zones in the Strangways Range, central Australia preserve lenses of undeformed granite with faceted feldspar grains. Selvedges around the lenses are rich in biotite and high-temperature minerals including sillimanite and garnet. Microstructures indicative of the former presence of melt are pervasive in these shear zones, consistent with previous deformation-assisted melt migration and melt-rock interaction. In contrast, greenschist facies shear zones formed in the presence of meteoric water in the Reynolds-Anmatjira ranges to the northwest lack leucosome and instead present quartz veins that run parallel to shear zone. These lower-temperature shear zones also show recrystallised grains and low temperature minerals including chlorite and muscovite. A comparison of microstructures shows that the meteoric water-present shear zones preserve undulose extinction in quartz, bent mica, a bimodal grain size of quartz, core and mantle structures, recrystallised plagioclase and ragged edges of felspar grains against mica – all features generally lacking in rocks from the melt-present shear zones of the Strangways Range. The analysis of rare earth element patterns in whole rocks and minerals is found to be the most reliable way to distinguish the two fluid. This is due to the rare earth elements being more mobile in silicate melts compared to aqueous fluids. As such, detailed analysis of samples from the melt-present shear zones shows chemical trends alongside progressive modification of the precursor rock, suggesting melt buffering of the system. Variation of CL response internal to grains and rare earth element chemistry are used to infer multiple events of open system melt migration. In combination with quantitative orientation analysis, the geochemical patterns are used to propose a model for shear zone reactivation and widening.
A depletion of quartz and feldspar minerals is observed with increased mode of ilmenite or magnetite in the core of some central Australian shear zones. A spatial relationship with melt-microstructures and reaction replacement microstructures suggests oxide enrichment during melt migration through the shear zones. Incorporating oxide gabbros from an oceanic setting, a comparative approach deciphers the ilmenite enrichment mechanism by melt-rock interaction in both tectonic settings. Detailed microstructural characterisation reveals interstitial habits for oxide minerals, forming a skeletal network, in reaction replacement textures. The quantitative orientation analysis of ilmenite and associated grains support the idea of crystallisation of minerals in the presence of melt. Additionally, the microchemistry of minerals points to an open system. Synthesising models of gabbroic melt with the structural and chemical patterns of the natural samples indicates that a fractionated gabbroic melt led to the replacement-enrichment of oxide minerals by process of reactive porous flow in the oceanic setting.
Understanding reactions is an important step in conceptualising mass transfer models and the processes that take place at the grain scale. Garnet porphyroblasts examined from a shear zone in central Australia reveal corroded rims of garnet that are partially replaced by biotite ± magnetite. Inclusions of magnetite and biotite have elongate, highly irregular shapes with finger-like protrusions. Quantitative orientation analysis of nearby oxide inclusions shows that neighbourhoods of inclusions and adjacent matrix grains share the same crystallographic orientation. This is consistent with the three-dimensional connectivity of these grains. Micro-computed tomographic scans reveal millimeter-scale networks of magnetite inclusions that form highly irregular, tubular, branching structures within garnet grains. The inclusions may also physically connect with grains in the matrix, leading to a conclusion that the inclusions are epigenetic and formed during melt-rock interaction and replacement of garnet. The former presence of melt is supported by the mineral chemistry of magnetite grains which indicate a magmatic origin. Therefore, the thesis demonstrates that shear zones are an important conduit for melt migration and that melt-present deformation can be recognised by key mineral assemblages, microchemical signatures and microstructural characteristics.