SYNDEFORMATIONAL FLUID FLOW IN METABASITE ROCKS ALONG THE SOUTHERN IBERIAN SHEAR ZONE

MORB-derived metabasites affected by the Southern Iberian Shear Zone in southwestern Spain underwent syndeformational D₂ retrograde metamorphism from upper amphibolite to greenschist facies, which required the input of water. Prior to D₂, the metabasites existed as strongly layered high grade amphibolites. D₂ folded and then sheared the layers, producing mylonitic fabrics. Fluid assisted microstructures occur throughout the shear zone, including veins and dissolution seams creating schistose foliation. Progressively towards the shear zone, increased strain intensity corresponds with decreased metamorphic grade, and rock mineralogy changed from plagioclase + hornblende ± diopside ± quartz to plagioclase + hornblende + actinolite ± quartz ± epidote ± chlorite. This fluid-assisted breakdown of hornblende weakened the rock, as the structure of hornblende is stronger than the structures of chlorite and epidote. Bulk major element geochemistry was analyzed via XRF. Volume loss estimated from multi-element isocons is approximately 2.4% at the amphibolite facies shear zone boundary with an additional 7.1% at the mafic schist boundary. Ca and Al enrichments begin at the shear zone boundary and amplify with increasing strain. Minor Si depletion occurs at the amphibolite to mafic schist boundary. K is enriched throughout the zone to widely varying degrees. Substitution of elements within amphiboles, chlorite, and epidote likely accommodated the chemical changes. Ca, Al, and Si geochemical changes occur in tandem with metamorphic phase changes and deformation intensity, demonstrating channelized fluid flow along the shear zone. Conversely, localized K enrichment is found regardless of metamorphic phase changes and strain; it likely occurred via fracturing and sporadic fluid influx along localized pathways across the shear zone, allowing K-rich fluids from metapelitic rocks to cross the shear zone into the metabasites. The results are consistent with a conduit-barrier fluid flow model accompanied by episodic fracturing that allowed flow crossing the shear zone. This example demonstrates a positive feedback between deformation, fluid flow, and metamorphism which promotes softening and strain localization.