Characterizing the Geochemical Changes Across a Strain Gradient in the Beja-Acebuches Metabasites Due to Retrograde Metamorphism and Fluid Flow Along the Southern Iberian Shear Zone

The Southern Iberian Shear Zone (SISZ), SW Spain, provides an ideal location to study the effects of syntectonic fluid on rock chemistry, deformation, and metamorphism. The SISZ is a 60km long, 300-500m wide shear zone, striking 115 and dipping approximately 50 degrees to the NNE, that follows the contact between metamorphosed MORB-derived Acebuches metabasites (AB) of the Ossa-Morena zone and the metamorphosed Pulo do Lobo (PdL) accretionary prism. Both units underwent two deformational events: 1) a HT/LP event that reached amphibolite facies and significantly dehydrated the rocks, and 2) a retrograde metamorphic event from amphibolite-greenschist facies that required the addition of H2O. Major element geochemical characterization of the AB was undertaken using x-ray fluorescence to enable a better understanding of fluid-rock interaction. These geochemical data from the shear zone indicate enrichments in K2O, CaO, and Al2O3 with a depletion in SiO2. Element vs distance from shear zone plots demonstrate that the movement of CaO, Al2O3, and SiO2 is related to the retrograde metamorphic event because the enrichments (CaO, Al2O3) and depletion (SiO2) is greater with increasing retrograde metamorphism. However, sporadic and variable K2O enrichment is documented throughout the shear zone, even in lithologies furthest from the shear zone. The movement of CaO, Al2O3, and SiO2 within the lithologies closest to the shear zone support the Conduit-Barrier Fluid Flow model for parallel fluid flow. The sporadic enrichment of K2O in samples away from the shear zone suggests that fluid migrated outward in localized pathways through brittle fractures