Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland

The supracrustal rocks of the Loch Maree Group (LMG), which form a Palaeopr oterozoic cover to the Scourian basement of the Lewisian complex, underwent amphibolite-facies regional metamorphism during the Laxfordian orogeny. Mi cro-structural studies indicate that the peak of metamorphism in both the L och Maree and Gairloch outcrops of the LMG was preceded by at least one maj or phase of ductile deformation, recorded by annealed lineations and foliat ions and by inclusion trails in minerals. At Loch Maree, this involved inte nse WNW-ESE stretching. The metamorphic peak occurred statically and was fo llowed by (i) intense top-to-NW shearing, recorded by mylonitic fabrics par ticularly associated with the Loch Maree Thrust and (ii) several generation s of open folding. These retrograde fabrics and folds reflect tectonically- induced uplift and exhumation during late Laxfordian times. Thermobarometric studies were undertaken on low-variance rocks that had exp erienced relatively little retrogressive deformation. A garnetiferous calc- silicate rock, an ironstone and two dolomitic marbles from the Gairloch out crop yielded Laxfordian peak-metamorphic conditions of 530 +/- 20 degrees C and 6.5 +/- 1.5 kbar consistent with the epidote-amphibolite-facies assemb lages. In general, the mylonites of Loch Maree outcrop proved unsuitable fo r thermobarometric work, but a protomylonitic garnet-staurolite-mica schist yielded peak-metamorphic conditions of 630 +/- 30 degrees C and 6.5 +/- 1. 5 kbar. Temperatures during mylonitization were still high enough to stabil ize garnet (i.e. > 470 degrees C) which recrystallized as tiny neoblasts in the mylonites. The peak-metamorphic pressures imply burial of the supracrustal LMG to c. 2 4 +/- 5 km, requiring substantial early Laxfordian tectonic thickening of t he crust. Thus the main phase of basement-cover interleaving occurred befor e the development of the mylonitic fabric marked by the Loch Maree Thrust. Geothermal gradients at the metamorphic peak were moderate (c. 20-30 degree s C km(-1)), consistent with metamorphism in tectonically thickened crust. Existing tectonic models for the Laxfordian fail to account for the tectoni c burial of the LMG to mid-crustal depths.