Rb-Sr and U-Pb geochronology of neoblasts and porphyroclasts constrain movements along reactivated segments of the Outer Hebrides Fault Zone, NW Scotland

The crustal-scale Outer Hebrides Fault Zone (OHFZ) (NW Scotland) was initiated as a Stenian-Tonian (1040-950 Ma) ductile thrust, as determined by published U-Pb dating of syn-kinematic apatite. Thrust-related mylonites were then reactivated to form phyllonite belts during successive sinistral strike-slip and extensional displacements. Within the phyllonites, syn-kinematic white mica was formed at temperatures of 150-370°C. New early Devonian (408-396 Ma) Rb-Sr white mica ages from sinistrally deformed phyllonites and an extensionally deformed phyllonite overlap, consistent with broadly coeval development during sinistral transtension. Early Devonian phyllonitisation was coeval with sinistral strike-slip faulting along the Great Glen Fault Zone and transtensional development of the Orcadian and Moray Firth Old Red Sandstone basins on and adjacent to the Scottish mainland. Apatites occur as cataclastically deformed porphyroclasts within phyllonites along the eastern margin of the OHFZ, yielding U-Pb ages of 1020-950 Ma. This age signature suggests that the early ductile thrust zone seen onshore in the Northern Segment is located offshore to the east in the Southern Segment and extends the entire length of the Outer Hebrides. Our study highlights the importance of carrying out microstructural analysis of mineral phases prior to isotopic analysis.