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 in the AB coincident with lower grade metamorphism and folding in the PdL, and 2) a retrograde metamorphic event accompanied by the development of the SISZ that produced mylonitic fabrics, which extend broadly into the AB and only locally into the PdL at the shear zone boundary. The amphibolite to greenschist facies retrograde metamorphism in the AB required addition of H₂O, as the metabasites were significantly dehydrated during the first HT/LP event. This research tests the hypothesis that fluid-rock interaction in the Southern Iberian Shear zone (SISZ) led to major element geochemical alteration and strain softening.

Six transects were densely sampled across the strain gradient, starting from the AB outside the shear boundary through the contact and slightly into the PdL. Within each transect, there are four AB domains starting from outside the shear zone at the structural upper boundary: coarse-grained, banded amphibolites with pyroxene; medium-grained, banded amphibolites without pyroxene; folded amphibolites that represent the outermost shear boundary; and sheared chloritic mafic schists close to the shear contact.

Major element geochemical characterization of the AB was undertaken using x-ray fluorescence to enable a better understanding of fluid-rock interaction. Preliminary geochemical data from two transects indicate decreases in Ti and Fe corresponding with an Al increase at the retrograde transition from actinolite to chlorite. These trends suggest that fluid infiltration along the shear zone enhanced metamorphic retrograde reactions. We interpret a positive feedback between fluid flow and retrograde metamorphism that enabled the rheological weakening, possibly responsible for the sharp shear zone contact along its lower boundary.