TIMING OF GARNET GROWTH AND BREAKDOWN USING MONAZITE GEOCHRONOLOGY IN THE SCANDIAN (TACONIC) OROGENIC WEDGE, NW SCOTLAND

The middle to late Silurian (c. 435-415 Ma) orogenic wedge preserved in the northwest Scottish Highlands consists of a stack of thrust nappes composed of Neoproterozoic age Moine Supergroup metasedimentary rocks. There is a continuous metamorphic gradient across the wedge ranging from anchizonal in the most foreland thrusts to upper amphibolite facies (migmatite grade) in the most hinterland positions. Electron microprobe-based monazite geochronology and geochemistry from across northwest Scotland provide timing constraints on the metamorphic and deformation history at various structural positions. A sample from the immediate hanging-wall of the more hinterland-positioned Sgurr-Beag thrust yields only Precambrian monazite (580-780 Ma), highlighting the polyphase metamorphism of the Moine Supergroup and raising questions about the age of preserved mineral assemblages in that area. A sample from the immediate footwall of the more foreland-positioned Ben Hope thrust (~70 km north along orogenic strike) yields 420-400 Ma monazite grains that texturally appear to record part of the retrograde path, but are difficult to interpret. A sample of upper amphibolite facies schist from the immediate hanging-wall of the Naver thrust (intermediate foreland-hinterland position) hosts a large population of compositionally complex monazite, that show a distinct correlation between Yttrium concentration and time. In monazite domains dated 445-440 Ma, Y concentrations are about 0.5 wt%, and one monazite (443±2 Ma) is included in garnet. 440-435 Ma domains record a sharp increase in Y to ~1.8wt%. Post 435 Ma domains are characterized by lower and more variable Y concentrations at 0.5-1.2 wt%. These data appear to constrain garnet growth to >440 Ma and record Y release from garnet breakdown at 440-435 Ma. Thus, garnet retrogression and exhumation on the Naver thrust may have begun by 440 Ma, which is earlier than previously interpreted. These results imply that the overall evolution of the Scandian orogenic wedge may have taken place over a longer period than is typically recognized, and accord with recent garnet dates by Bird et al. (2013) of 450-445 Ma. However, it is possible that xenotime has played an important role in controlling Y concentration in monazite, which will be explored with further geochronology.