Laxfordian metamorphic conditions of the Palaeoproterozoic Loch Maree Group, Lewisian Complex, NW Scotland (Published in Scottish Journal of Geology, 35(1), 1999, pp. 31–50)

Abstract

write: Droop et al. (1999) make a welcome addition to the Lewisian literature in providing the first rigorous published estimates of FT conditions at the Laxfordian metamorphic peak in the Loch Maree Group (530-630°C, 6.5 kbar, using various methods including calcite-dolomite and garnet-biotite geothermometers and garnet-plagioclase geobarometer). In their discussion of the implications of the relatively high pressure-temperature estimates, they note an apparent contradiction with the tectonic model for the early Laxfordian deformation presented by Coward and Park (1987) and Park et al. (1987). The 1987 tectonic model for the Gairloch area resulted from the recognition that the strong early Laxfordian deformation was associated with a WNW-ESE movement direction implying oblique slip on gently inclined planes, with both strike-slip and dip-slip components. The reason for choosing the dextral/normal-slip option for these movements then was because of the observation that certain of the more prominent shear zones exhibited dextral, NE-down movement-sense criteria. However subsequent more detailed work on the Gairloch shear zones (Lei and Park 1993) showed that the movement history on the shear zones was more complex than first thought, that both sinistral/normal and dextral/reverse movement criteria existed for the early shear zones, but that the main early movements involved overthrusting to WNW, i.e. compressional, in agreement with Droop et a/.'s conclusions.Droop et a/.'s estimates of the metamorphic conditions provide timely additional evidence in support of new work by ourselves, together with J.N. Connelly, recently accepted for publication (Park et al. in press), in which we reinterpret the Loch Maree Group (LMG) as an accretionary complex consisting of a stack of slices of oceanic material, together with continentally derived elastics, underthrusting continental basement to the ESE. In this model, the lower parts of the complex would be subjected to the higher pressures, and it is interesting to note that, while Droop et a/.'s assertion that the Gairloch outcrops of the LMG present lower-pressure assemblages is true for the main part of the outcrop, it is not true for the southwestern parts, where garnet coexists with hornblende and andesine in the amphibolites (although the assemblages are commonly retrogressed). Furthermore, staurolite is found in the thin band of schists NE of the Aundrary amphibolite and the marble bands 1.5 km further to the NE again contain diopside rather than tremolite as is the case within the main Gairloch outcrop. It would appear therefore that the Gairloch outcrop juxtaposes different metamorphic assemblages; as is very commonly observed in subduction-accretion complexes (Barr et al. 1999).Our model envisages the earliest movements being compressional, i.e. overthrusting to the WNW (underthrusting to ESE), followed by localized extension, accompanied by retrogression, on certain discrete shear zones in response to tectonic overthickening of the crust. It would be interesting to...