EFFECTS OF VARIATIONS IN MINERALOGY AND LINEAR MICA CONTINUITY ON DUCTILE DEFORMATION IN METAPELITIC ROCKS

The Southern Iberian Shear Zone (SISZ) in Andalucía, Spain is a Variscan ductile shear zone that marks the suture between the Ossa Morena and South Portuguese Zones and follows the rheologic boundary between the Beja-Acebuches Metabasites and the Pulo do Lobo (PdL) formations. The PdL contains quartzites and metapelites that were metamorphosed by a pervasive low-grade event (D1) followed by localized greenschist-lower amphibolite facies metamorphism, attributed to a second phase of deformation related to motion along the SISZ (D2). Our goal is to better understand the role that mineral concentrations and arrangements play in strain localization along the SISZ within the PdL. Mineralogy and microstructures are studied microscopically using a conventional petrographic microscope and ImageJ, image analysis software that is used to isolate grains based on their relative colors. The PdL samples are primarily composed of two minerals—quartz and biotite, which display different colors in plane polarized light. By setting a color threshold within ImageJ, particle analysis can be conducted for the entire sample or along a single line; the former provides bulk composition, while the latter provides Linear Mica Continuity (LMC) data. LMC is the percentage of mica intersected along a linear traverse, which quantifies the extent to which micas are clustered. Eight single line (1 pixel width) LMC measurements are calculated parallel to the main foliation for each thin section, and the maximum LMC value (LMC_max) is used to represent the degree of mica clustering in each sample. LMC is an important deformation parameter, as measured LMC_max values are related to composition and distance from the shear zone. LMC_max increases with higher mica concentrations and greater distance from the SISZ. Decreases in LMC_max are potentially related to increases in strain accumulation; low LMC_max implies greater strength in the PdL close to the metabasite contact. We hypothesize that D1 created relatively continuous mica clusters forming slaty cleavage, whereas D2 changed the rocks to schists which generated more complex S-C fabrics with larger grain sizes, reducing LMC_max. Thus, the effects of prior rock fabrics and metamorphic changes may result in a nonintuitive relationship between mica connectivity and strain accumulation.