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

Abstract

Synopsis The supracrustal rocks of the Loch Maree Group (LMG), which form a Palaeoproterozoic cover to the Scourian basement of the Lewisian complex, underwent amphibolite-facies regional metamorphism during the Laxfordian orogeny. Micro-structural studies indicate that the peak of metamorphism in both the Loch Maree and Gairloch outcrops of the LMG was preceded by at least one major phase of ductile deformation, recorded by annealed lineations and foliations and by inclusion trails in minerals. At Loch Maree, this involved intense WNW–ESE stretching. The metamorphic peak occurred statically and was followed by (i) intense top-to-NW shearing, recorded by mylonitic fabrics particularly associated with the Loch Maree Thrust and (ii) several generations 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 experienced relatively little retrogressive deformation. A garnetiferous calc-silicate rock, an ironstone and two dolomitic marbles from the Gairloch outcrop yielded Laxfordian peak-metamorphic conditions of 530 ± 20°C and 6.5 ± 1.5 kbar consistent with the epidote–amphibolite–facies assemblages. In general, the mylonites of Loch Maree outcrop proved unsuitable for thermo-barometric work, but a protomylonitic garnet–staurolite–mica schist yielded peak-metamorphic conditions of 630 ± 30°C and 6.5 ± 1.5 kbar. Temperatures during mylonitization were still high enough to stabilize garnet (i.e. > 470°C) which recrystallized as tiny neoblasts in the mylonites. The peak-metamorphic pressures imply burial of the supracrustal LMG to c. 24 ± 5 km, requiring substantial early Laxfordian tectonic thickening of the crust. Thus the main phase of basement–cover interleaving occurred before the development of the mylonitic fabric marked by the Loch Maree Thrust. Geothermal gradients at the metamorphic peak were moderate ( c. 20–30°C km −1 ), consistent with metamorphism in tectonically thickened crust. Existing tectonic models for the Laxfordian fail to account for the tectonic burial of the LMG to mid-crustal depths.