Prolonged high-temperature Caledonian metamorphism in the southern part of the Northern Highlands terrane, Scotland, indicated by integrated monazite and xenotime laser ablation split-stream petrochronology and thermometry

The Northern Highlands terrane (NHT) of Scotland preserves a complex Neoproterozoic–Paleozoic tectono-metamorphic evolution. Work over the past three decades has helped constrain the timing and conditions of metamorphism in the northern NHT, yet parts of the southern NHT remain less well-constrained. New petrochronological analyses (accessory phase geochronology, monazite–xenotime (Mnz–Xtm) and garnet–biotite (Grt–Bt) thermometry, microstructural analysis) of the high-grade Sgurr Beag Thrust sheet in the southern NHT provide significant constraints on the thermal and deformational history. Monazite inclusions in garnet cores and inner rims yield two age populations: an ∼750–720 Ma population correlated with the Knoydartian (840–720 Ma) orogeny and an ∼680–600 Ma population provisionally linked to a regional hydrothermal event. A third event ranging from ∼465 to 435 Ma, peaking at ∼450 Ma, is recorded by monazite and xenotime in garnet inner rims and the matrix. Paragenetic fabric relationships between garnet, monazite and xenotime indicate at least two distinct phases of post-450 Ma garnet growth, and associated Mnz–Xtm and Grt–Bt thermometry yields 640–680°C conditions. These data suggest a continuous and prolonged high-temperature orogenic event that contrasts with traditional interpretations of distinct Grampian and Scandian events or simple Scandian re-working of older Grampian and/or Knoydartian metamorphic assemblages.