The microchemical and microstructural evolution of fluid and melt transfer in deep crystal shear zones
thesisposted on 28.03.2022, 10:10 authored by David Barbosa da Silva
Fluids and melt are increasingly recognised as key agents of weakening, promoting crustal deformation in mountain building systems. Zones of deformation within these systems, tens of kilometres deep in the crust, are considered principal conduits formelt or aqueous fluid flow (i.e. mass transfer) and are sites of melt-rock or fluid-rock reaction. In the high-temperature middle and lower crust, melt-present deformation and melt-rock interaction in high-strain zones enhances orogenesis due to (1) hydration reaction softening and (2) the intrinsic weak competence of melt-present deformation. Consequently, melt-present and hydrous high-strain zones act as powerful strain accommodators, facilitating the exhumation of deep crustal basement rocks. This combination of geological processes is interpreted as essential for having enabled the Palaeozoic Alice Springs Orogeny, an intracontinental orogenic event that formed compressional mountain belts within the interior of the Australian plate. As aqueous fluids have classically been invoked for fluid-driven reaction weakening in the middle and upper-crust, the role of melt in rheological weakening during orogenesis has somewhat been overlooked, particularly in intracontinental orogens. The aim of this thesis is to fill that knowledge gap by recognizing high-strain zones of intense melt-rock interaction and studying the microstructural and microchemical characteristics of the melt pathways. The overarching theme of the thesis is an attempt to determine what role weak rheological zones played in the deformation of the Australian plate during the intracontinental Alice Springs Orogeny. The study presents tectonic-scale geodynamic modelling in addition to a detailed analysis of exhumed ancient metamorphic rocks that are interpreted to have formed during melt-present deformation in the Gough Dam shear zone (GDSZ), Central Australia. During compressive tectonic periods, primary features such as the geometry of the orogenic system, pre-existing crustal structures and transmitted stresses are highly relevant in the accommodation and concentration of differential stress within continental plates, with enhancement of intracontinental orogenesis if principal transmitted stresses are oriented at a low angle to the weak intraplate zone. Secondary stresses to the principal N-S tectonic activity on the Australian continent, contributed to marked periodicity during the Alice Springs Orogeny, as indicated by a strong temporal link between tectonic activity in Eastern and Central Australia. Deep-seated, km-scale shear zones comprising low to high vol% of biotite-rich glimmerite schist and felsic components are exposed in the core of the Alice Spring Orogeny, crosscutting ancient granulite protoliths. The glimmerite schist components are interpreted to have replaced the precursor granulites during melt-rock interaction, increasing the temperature and the proportion of biotite in the high-strain zones with calculated increase of time-integrated melt flux. Within the GDSZ, (1) Granite dykes, (2) granite lenses and(3) faceted K-feldspar phenocrysts hosted in K-feldspar-quartz-rich glimmerite schist all contain undeformed K-feldspar and quartz components, implying channelized melt flux within the high-strain zone and posterior collapse and entrapment of crystalline phases from the fluxing melt. These relationships are consistent with melt-present high-strain deformation combined with melt-induced reaction softening. The episodic nature of tectonic activity may be controlled by melt-production at depth, periods of melt pressure and/or stress build-up and release enhanced by periods of plate tectonic reconfigurations changing the nature of far-field stresses -- abstract.