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Aqueous fluid advection: a plausible mechanism for heating the shallow crust to form HTLP metamorphic belts
thesisposted on 2022-03-28, 21:54 authored by Catherine A. Stuart
The Wongwibinda Metamorphic Complex is a High-Temperature Low-Pressure metamorphic belt in the Southern New England Fold Belt composed of Carboniferous meta-pelitic schists, characterised by a steep (<55 °C km-1) metamorphic field gradient. While the regional metamorphism is well documented in literature, presently there is no suitable mechanism to explain the elevated temperatures of the crust. A thermal anomaly is recognised in the centre of the complex, evident through the distribution of cordierite-bearing rock around folded quartzite units. Petrographic analysis reveals a vein network of complex mineralogy through the schists of the aureole. From field relationships, the overall structure of the quartzite appears analogous to a large quartz vein sequence grown during regional deformation. EBSD mapping of banding in the quartzite reveals only slight similarities compared to the microstructure of a quartz vein in the schists. Using XRF data, attempts to classify the quartzite based on protolith were inconclusive. Whole rock chemistry analysis revealed metasomatism through the aureole, characterised by Fe, Mn, K and heavy REE enrichment and Ca depletion compared to regional values. Similar enrichment and depletion values are observed in the quartzite chemistry. Comparing mineral chemistry between the schists, veins in the schists and the quartzite allowed estimations of fluid-rock interaction which agreed with metasomatic changes in the schists. Following the evidence for metasomatism in the schists, the quartzite is established as a site of high fluid flux, with fluids driving the metamorphic evolution of this site. An aqueous fluid advection model is proposed, whereby pulses of hot fluids migrate upwards through the crust and are focussed through the WMC. Upon reaching the complex, they are diverted and slowed, and phases begin to crystallise out of the fluid. Transported by the fluids, additional heat enters the surrounding meta-sedimentary sequences, driving metamorphism coeval with metasomatism driven by the pulses of fluid. Whilst this is model best fits evidence found in this study it is by no means confirmed, and further research is suggested into both the thermal and metasomatic history of these rocks to further constrain the mechanisms driving HTLP metamorphism.