MONAZITE AND TITANITE CONSTRAINTS ON THE PRECAMBRIAN METAMORPHIC EVOLUTION OF THE NW HIGHLANDS TERRANE, SCOTLAND

The NW Highlands Terrane of Scotland experienced at least three major phases of metamorphism known as the Scandian (445-425 Ma), Grampian (470-460 Ma) and Knoydartian (850-725 Ma) orogenies. We present constraints on the conditions, duration and spatial extent of these metamorphic episodes using monazite, xenotime and titanite geochronology, coupled with metamorphic petrology. The central part of the NW Highlands Terrane (south of Assynt and north of Inverness) is composed of two thrust sheets: the structurally lower Moine nappe and the structurally higher Sgurr Beag nappe. Geochronologic evidence for the Knoydartian event is restricted to the Sgurr Beag nappe. Garnets in the Sgurr Beag nappe have a consistent paragenesis with monazite and xenotime that suggests their growth was coeval. Monazite and xenotime give upper and lower intercept ages of c. 800 Ma and c. 550 Ma, respectively. Precambrian monazite also shows a regional trend of heavy rare earth element depletion, confirming its co-crystallization with garnet. Our P-T constraints suggest that garnet-bearing metamorphic assemblages formed at 600-650°C and 6-7 kbar. Titanite from an amphibolite gneiss at the highest structural position in the Sgurr Beag nappe and from the Fannichs klippe toward the foreland are Grampian in age.

Geochronologic evidence for the Scandian phase of metamorphism is relatively restricted in the central NW Highlands terrane. Titanite from several samples in the metasediments of the Moine Nappe show no evidence of resetting to Scandian, Grampian or Knoydartian ages. One sample has a Grenville-age titanite population (1007±23 Ma), which in the Moine Supergroup (deposited <1000 Ma) suggests a detrital origin. However, quartz fabric thermometry in the Moine nappe shows evidence of ~600°C deformation and thermodynamic modeling suggests peak temperatures of 600±50°C. These data suggest that the Moine nappe spent a relatively short time, on the order of 1-10 Ma, above 600°C during Scandian, Grampian and Knoydartian metamorphism. This contrasts with previous data in the Moine nappe 130 km SSW, where c. 740 Ma metamorphic titanite has been documented (Tanner and Evans, 2003). Our data also suggest that the Moine and Sgurr Beag nappes in the central NW Highlands terrane had very distinct metamorphic histories prior to Scandian orogenesis.