U–Pb geochronology and microstructural analysis of apatite within a Proterozoic crustal-scale shear zone, Outer Hebrides, NW Scotland

Abstract: U–Pb apatite geochronology is increasingly recognised as a valuable tool for constraining the age of mid-crustal ductile shear zones. The crustal-scale Outer Hebrides Fault Zone (OHFZ) within the Laurentian foreland of the Scottish Caledonides has long been of uncertain age and tectonic significance. Earliest deformation within the OHFZ was associated with top-to-the-NW ductile thrusting that formed a belt of greenschist facies mylonites within host Archaean–Palaeoproterozoic basement gneisses. Previous estimates for the timing of thrusting vary between c. 1600 Ma and c. 430 Ma. The mylonitic fabrics are defined by a recrystallised assemblage of quartz + albite/oligoclase + sericite + actinolite + epidote + apatite ± calcite, consistent with deformation temperatures of 400-500 °C and within the range of reported closure temperatures for Pb diffusion in apatite. U–Pb (LA–ICP–MS) dating of two texturally distinct apatite grain types within the mylonites has yielded ages mostly in the range c. 1100–900 Ma. The OHFZ is therefore interpreted as a Grenville–Sveconorwegian structure that formed during the tripartite collision of Laurentia, Baltica, and Amazonia and the assembly of Rodinia.