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

Abstract

SynopsisThe 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 thermobarometric 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.

Palaeoproterozoic terrane assembly in the Lewisian Gneiss Complex on the Scottish mainland, south of Gruinard Bay: SHRIMP U–Pb zircon evidence

“…Since the 1877  13 Ma granulite facies metamorphism of the Ialltaig gneiss is younger than this event the tectonic intercalations that took place in this area can only be post-1877 Ma. The evolutionary model of tectonic thickening suggested by Droop et al (1999) is perfectly acceptable for the "early Laxfordian" structures described because of the extremely well-constrained metamorphic data. However, this event cannot be that responsible for the metamorphism of the Ialltaig gneisses as it is too early.…”

“…There is no debate that the metamorphic grade of the Loch Maree Group has never exceeded low amphibolite facies conditions (e.g. Droop et al, 1999). The Ard gneiss constrains the peak metamorphism to have occurred before ca.…”

“…The intrusion of the Ard gneiss protolith into the Loch Maree Group is considered to have occurred during peak metamorphism, due to its lack of strong D 1 /D 2 fabrics and the inclusions of minor amphibolite and marble bands (Park et al 2001). This was modelled from the preserved assemblages by Droop et al (1999) as 530  20C at 6.5  1.5 kbar. Alternatively, a limited development of D 1 /D 2 fabrics in the Ard gneiss may be explained by the relative competency contrast between a competent granodiorite sheet and incompetent, already foliated supracrustal rocks, with the latter accommodating more D 1 /D 2 strain.…”

Polymetamorphism in the mainland Lewisian complex, NW Scotland – phase equilibria and geochronological constraints from the Cnoc an t’Sidhean suite

“…The intrusion of the Scourie dyke swarm is constrained to have occurred in the interval between c. 2.4 and c. 2.0 Ga (Chapman, 1979;Heaman & Tarney, 1989;Waters et al, 1990). Absolute ages for subsequent Laxfordian metamorphism are constrained to c. 1.75-1.70 Ga (Waters et al, 1990;Corfu et al, 1994) and P-T conditions have been estimated at 6.5 ± 1.5 kbar and 630 ± 30°C within the Loch Maree Group in the southern region (Droop et al, 1999). Laxfordian fabrics are closely related to Inverian structures due to transposition of Inverian fabrics under Laxfordian strain .…”

“…, 1990; Corfu et al. , 1994) and P–T conditions have been estimated at 6.5 ± 1.5 kbar and 630 ± 30 °C within the Loch Maree Group in the southern region (Droop et al. , 1999).…”