Mineralogical changes with increased strain; insights from the shear zones in Cap de Creus, NE Spain

Abstract

The Cap the Creus peninsula at the northeastern end of the Pyrenees consist of variscan LP- HT metasediments affected by a large network of greenschist shear zones. The age of these shear zones is still debated. These shear zones provide excellent exposure for studying their petrology and structural characteristics. This research investigates the relation between petrology, geochemistry and strain in mylonites and adjacent country rock with samples from two localities at the Cap the Creus peninsula, named i) Cala Sardina, a larger-scale zone, and ii) Tudela, a smaller-scale zone. Samples were collected at varying distances from the shear zone to compare low- to high- strain mylonites. Optical microscopy, X-ray fluorescence, Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) in combination with Automated Mineralogy (AM) software were used to determine mineral assemblages and bulk rock compositions (BRC). Proximal samples showed lower-grade, more hydrated mineral assemblages compared to distal ones, despite similar BRC. Through optical microscopy and SEM-phase mapping it is also observed that most of the retrograde overprinting of the mineral assemblage took place after shearing deformation. Thermodynamic modelling with Perple_X allowed for estimation of metamorphic conditions. Both localities showed higher grade metamorphic conditions overprinted by greenschist-facies conditions where the Tudela locality (at least partially) deformed under undersaturated conditions. Laser Ablation Inducted Coupled Plasma Mass Spectrometry (LA-ICP-MS) provided trace element data to study fluid interactions between shear zones and country rock. Mass transport values of - 9.69% and +11.1% were observed over ~20 cm in Tudela samples, while Cala Sardina samples showed +8.14% mass transport over 100 cm. It is concluded that water input through the shear zones appears essential for the retrograde overprinting of previous high-grade mineral assemblages, and that developed shear zones are likely good conduits of water to the surrounding country rock. This explains lower grade assemblages closer to the shear zone and the bulk of the retrograde overprinting happening after shearing deformation. Here the scale of the shear zone, rather than distance or strain, appears to influence the water throughput. A final remark can be made about the age of the Cap de Creus shear zones; As retrograde overprinting appears to happen after the shearing deformation starts, the dating of retrograde white mica by Vissers et al. (2017) likely reflects a later stage of shearing or reactivation during this time, and not necessarily the initiation of these shear zones. This could indicate that the hypothesis of Druguet (2001) and Carreras (2001), which states that these shear zones originated during the later stages of the Variscan orogeny, is correct.