TITANITE AND MONAZITE CONSTRAINTS ON THE TIMING OF METAMORPHISM AND PENETRATIVE DEFORMATION IN THE SOUTHERN PART OF THE NORTHERN HIGHLANDS TERRANE, SCOTLAND

The Northern Highlands Terrane (NHT) of northern Scotland has experienced multiple tectonic events, including the Knoydartian (870-720 Ma) and Caledonian orogenies, the latter divided into Grampian (~475-460 Ma) and Scandian (~435-420 Ma) events. Here we present constraints on the timing of metamorphism in the southern part of the NHT with preliminary new U-Pb isotopic ages from titanite, monazite, and xenotime.

The southern part of the NHT is divided into two structural regions by the NE-SW trending Loch Quoich Line: the western Steep Belt, which includes the Sgurr Beag thrust (SBT), and the eastern Flat Belt. Previous isotopic dating at isolated locations within the Sgurr Beag thrust sheet suggests that the SBT was emplaced during the late Knoydartian orogeny and then metamorphosed and deformed again during the Grampian orogeny. However, this timeline is uncertain due to the lack of isotopic dating on samples along the SBT itself and more generally in the southern part of the NHT.

We collected samples from four E-W transects crossing the SBT in the Steep Belt: Loch Monar, Glen Sheil, Kinloch Hourn, and Loch Ailort. U-Pb and trace element data were collected at the Arizona LaserChron Center on a Thermo Scientific Element2 mass spectrometer. Preliminary analysis of titanite data collected for samples from Glen Sheil, Lochailort, Loch Monar, and in the Flat Belt to the east, indicate multiple age populations of grains. Many titanites are complexly zoned in yttrium, with cores ranging from 750 to 690 Ma and rims ranging from 450 to 415Ma. Grains with no zoning, such as those from Lochailort and Loch Monar, have ages between 450-415 Ma. Most titanite grains, both with and without zoning, are aligned parallel to the foliation of the host rock. These ages suggest an original phase of Knoydatrian metamorphism which was then succeeded by Grampian - Scandian phases of metamorphism and penetrative deformation.