Tectonic Transport Directions and Timing of Metamorphism in the southern part of the Northern Highlands Terrane, Scotland

Abstract

The metasedimentary rocks of the Northern Highlands Terrane (NHT) in NW Scotland were deformed and metamorphosed during a series of orogenic events which occurred from the Neoproterozoic (Knoydartian orogeny) to the Ordovician-early Devonian (Caledonian orogeny). Previous kinematic studies in the northern and central parts of the NHT indicate that tectonic transport changed from top to the N in the core of the orogen at 450-440 Ma to top to the W in the foreland margin at ~437-430 Ma. Here we extend these studies to the southern part of the NHT using microstructural, quartz crystal fabric, and isotopic analysis on oriented samples collected from four E-W transects across the Sgurr Beag thrust (SBT) in the Steep Belt and from multiple sites in the Flat Belt. These samples record at least two Caledonian metamorphic and deformational events with a nebulous early high temperature event and a later lower temperature event, although it is not clear if these are distinct events or parts of continuous metamorphism and deformation during early progressive cooling of the orogen. Early high temperatures of mineral crystallization and deformation (< c.675 °C) are indicated by Zr-in-titanite thermometry and microstructural evidence for both quartz and feldspar grains dynamically recrystallizing by grain boundary migration processes. These early high temperature microstructures are partially-completely overprinted by lower temperature (490-650 °C) D2-D3 microstructures and quartz crystal fabrics associated with the macroscopic foliation and mineral stretching lineation present in each sample. In the Flat Belt the D2 lineation plunges gently to the SSE, but in the Steep Belt the composite D2-D3 lineation plunges more steeply (to almost vertical) towards the SE/SSE on the SE dipping foliation. Quartz c-axis fabric indicate top to the NNE shearing at deformation temperatures of 550-650 °C in the Flat Belt, top down to the SW shearing at 540-590 °C in the Steep Belt transect across the SBT at Loch Monar, and top down to the SE/SSE shearing at ~490-650 °C in more southerly positioned Steep Belt transects at Glen Shiel, Kinloch Hourn, and Lochailort-Loch Eilt. U-Pb isotopic ages collected from titanite, monazite, and xenotime indicate that mineral growth occurred during the Knoydartian - late Neoproterozoic (765-638 Ma), the Grampian II (460–437 Ma), and late/post-Scandian events (418–412 Ma). Some of the older Caledonian (Grampian-Scandian) ages are compatible with geologic constraints for cessation of D3 shearing (Steep Belt) and buckle folding (Flat Belt) at c. 432-425 Ma and mineral growth may have occurred during D2 and/or D3 deformation. Younger Scandian mineral ages are problematic and could, for example, be explained by periods of fluid flow and associated mineral growth (perhaps associated with strike-slip faulting) that post-dates D2-D3 deformation within the NHT.