CONSTRAINING THE POLYMETAMORPHIC HISTORY OF THE NORTHERN HIGHLANDS AND GRAMPIAN TERRANES, SCOTLAND: PRELIMINARY RESULTS FROM MONAZITE AND XENOTIME U-PB GEOCHRONOLOGY

The Northern Highlands (NHT) and Grampian (GT) terranes in Scotland are separated by the Great Glen fault zone (GGFZ). In the NHT and GT, deformation and metamorphism related to the Neoproterozoic Knoydartian (~870 Ma) orogeny and the Ordovician Grampian (~470 Ma) and Siluro-Devonian Scandian (~430 Ma) phases of the Caledonian orogeny have been recognized. However, in many parts of these terranes, the spatial distribution, timing, and conditions of these metamorphic events have not been quantified using modern analytical approaches. Additionally, the role that the GGFZ played in the assembly of northern Scotland, which may be one of the most significant challenges for unraveling Caledonian tectonics, remains contested. Specifically, early views held that the GGFZ accommodated only a few hundred kms of Silurian-Devonian age sinistral strike-slip motion; however, a lack of evidence for Scandian deformation and metamorphism in the GT to the southeast of the GGFZ has led some workers to propose that the NHT may have been as much as 700-2000 km to the northeast of the GT during Scandian collision, requiring the considerable displacement on the GGFZ to juxtapose the two terranes by Lower Devonian times. More recently, some studies have proposed considerably smaller displacements (<200 km) on the GGFZ based on interpretations that link metamorphism and magmatic activity across the GGFZ. Although there is widespread evidence for Scandian metamorphism and deformation in the northernmost NHT, there are very few data for constraining the timing and conditions of the various metamorphic events in the southern half of the NHT and the adjacent parts of the GT. This contribution reports preliminary results of laser ablation split-stream (LASS) U-Pb geochronology and trace element analyses of monazite and xenotime from 17 samples collected within the southern NHT and the northwestern GT. Metamorphic mineral and accessory phase assemblages were evaluated to determine kinematic and petrotectonic relationships and in-situ LASS geochronology was used to date multiple growth zones recognized from elemental mapping. These results should allow us to constrain the timing of key pro- and retrograde metamorphic and deformation events, and in doing so they may enable us to develop more detailed P-T-t histories for these terranes.