Metamorphic conditions of the Cap de Creus shear zones (NE Spain) - Insights from an integrated petrological, electron microscopy and numerical approach
Melick, Hans van
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Recent advances in the field of scanning electron microscopy (SEM) allow the simultaneous, gridwise, collection of energy dispersive X-ray spectra (EDS) and the capture of back-scattered electrons (BSE). Phase maps of sample surfaces are created by comparing pre- (or post) defined phase classifications to relative elemental abundances obtained by automated deconvolution of the EDS signal. The construction of phase maps using this method is named ’SEM-EDS automated mineralogy’, or SEM-AM. In this work an SEM-AM-based workflow is evaluated in a case study on pegmatites and high-grade metasediments that have been strongly deformed into shear zones under greenschist facies conditions. While the structural features of these Cap de Creus area (NE Spain) greenschist facies shear zones have been studied in detail, only few PT constraints are available. Bulk rock chemistry (BRC) of the samples is calculated by applying a Python-based script to SEM-AM derived data integrated with phase classifications constrained by EDS-SEM and wavelength dispersive X-ray spectroscopy (WDS) microprobe spot measurements. This calculated BRC is compared to BRC obtained using X-ray fluorescence (XRF) on the corresponding samples, and subsequently used as input for numerical modeling with Perple_X. The resulting equilibrium mineral assemblages are compared to the mineral assemblages observed in the samples in order to constrain deformation conditions. To obtain additional constraints on deformation temperatures, quartz microstructures are analyzed and the Ti-in-Quartz geothermometer is applied to selected quartz grains. Trace element chemistry is studied to quantify the effect of fluid influx. This thesis demonstrates how the aforementioned SEM-AM workflow can deliver high-quality whole-rock chemistry results that are consistent with traditional methods, providing that mineral chemistry can be adequately constrained. The SEM-AM false color phase maps are a great asset because they visualize the relation between mineral associations, structural features and mineral reactions. Integration of the SEM-AM results with a numerical approach provides new PT estimates for the greenschist shear zones, ranging from 400-430 °C at 2.5 kbar to 410-450 °C at 3.5 kbar. Uncertainties in Ti-activity result in a large T uncertainty of results obtained using the Ti-in-Quartz geothermometer, but results suggest pressure may be as low as 2 kbar, associated with temperatures between 390-420 °C. For an undeformed sample PT conditions are estimated at 650±30 °C at pressures of >3.9 kbar, which is in accordance with earlier estimates. From trace element chemistry a mass increase of ±54% is computed as a result of element transport by Si- and presumably Ca-rich fluids, and thermodynamic modeling highlights the effects Fe speciation has on mineral stabilities. The here presented PT results, quartz microstructures, and observed Al2SiO5 polymorph behavior, indicate deformation under progressively decreasing metamorphic conditions consistent with the retrograde stages of the Variscan orogeny.