Garnet morphology distribution in the northern part of the Moine Supergroup, Scottish Caledonides

Abstract

Garnet is a versatile and useful indicator mineral exploited in numerous geological studies. Despite its utility in providing thermobarometry and geochronology constraints, many difficulties remain in making meaningful interpretations of such data. In this paper, we characterize garnet grains from over 140 garnet-bearing metasedimentary rock samples collected from the northern part of the Moine Supergroup (Scottish Caledonides). Large, euhedral garnet grains are interpreted to be indicative of prograde metamorphic growth during the most recent (Scandian, c. 430 Ma) phase of orogenesis. Anhedral garnet is largely restricted to the relatively low-grade (greenschistlower amphibolite facies) Moine thrust sheet, with an abrupt change in morphology and grain size when traced across the overlying Ben Hope and Sgurr Beag thrusts into the higher grade, more hinterland-positioned thrust sheets. Our results suggest that caution should be exercised in using anhedral garnet in the Moine thrust sheet to estimate peak P-T conditions associated with low temperature (<~500°C) Scandian metamorphism, because in at least some cases garnet growth may have occurred during an earlier metamorphic event. However, chemical and isotopic data from this structurally lower anhedral garnet may still be useful for deconvolving a possible polymetamorphic history for this thrust sheet. In the immediately overlying Ben Hope thrust sheet, garnet has prograde euhedral (Scandian?) rims, indicating that the Ben Hope thrust must represent a significant thermal and/or chemical break. Inclusion distribution and mineral assemblages in garnet have been used to gain further insight on garnet growth conditions and to distinguish garnet that likely contains multiple generations of growth. Although our results are specific to the Caledonides of northern Scotland, this work highlights the general necessity of a comprehensive petrographic assessment in advance of interpreting large suites of garnet-derived thermodynamic and geochronological data. Fig. 3. Detailed geological map and cross-section of the Sgurr Beag thrust sheet and underlying Moine thrust sheet exposed within the Fannich Mountains. A klippe of the thrust sheet is preserved in the NW part of the area. Garnet morphologies are shown, with anhedral (rounded) garnet typically located within the Moine thrust sheet and euhedral (or subhedral) garnet typically within the Sgurr Beag thrust sheet. After British Geological Survey (2011).

The Deep Structure and Rheology of a Plate Boundary-Scale Shear Zone: Constraints from an Exhumed Caledonian Shear Zone, NW Scotland

“…Plagioclase is present as relict sedimentary porphyroclasts and smaller grains incorporated into the matrix, whereas potassium feldspar is chiefly incorporated into the matrix but occurs as porphyroclasts at lower structural levels. Garnets, where present, display varying morphologies from the euhedral to anhedral [33] and skeletal. Meter-scale thick lenses of amphibolite (hornblende + quartz + feldspar ± garnet ± white mica ± biotite ± titanite ± epidote group minerals) crop out at the base of the Ben Hope Nappe and as rare inliers within the Moine schist.…”

Interplay Between Crustal‐Scale Thrusting, High Metamorphic Heating Rates, and the Development of Inverted Thermal‐Metamorphic Gradients: Numerical Models and Examples From the Caledonides of Northern Scotland

“…This steady increase in temperature is also recorded in metamorphic assemblages, as peak temperatures determined from garnet‐biotite thermometry yield similar temperatures to the quartz fabrics at equivalent structural positions. The only notable exceptions to this include small anhedral garnets preserved in the structurally lowest parts of the Moine thrust sheet (Ashley et al., 2017) and larger euhedral garnets from the Sgurr Beag thrust sheet. The latter two examples are currently interpreted to preserve older Knoydartian age metamorphic conditions (Mazza et al., 2018; With these exceptions noted, metamorphic assemblages and pervasive deformation fabrics preserved in the northern nappe stack largely record Scandian prograde heating and deformation‐related modification of these earlier tectonometamorphic events (Ashley et al., 2015; Dallmeyer et al., 2001; Freeman et al., 1998; Kinny et al., 2003; Mako et al., 2019; Thigpen et al., 2013).…”

“…The latter two examples are currently interpreted to preserve older Knoydartian age metamorphic conditions (Mazza et al., 2018; With these exceptions noted, metamorphic assemblages and pervasive deformation fabrics preserved in the northern nappe stack largely record Scandian prograde heating and deformation‐related modification of these earlier tectonometamorphic events (Ashley et al., 2015; Dallmeyer et al., 2001; Freeman et al., 1998; Kinny et al., 2003; Mako et al., 2019; Thigpen et al., 2013). These data have led multiple studies (Ashley et al., 2015, 2017; Mako et al., 2019; Thigpen et al., 2013) to infer that the thermal sequence preserved in northern Scotland is largely intact and is not a composite sequence resulting from superposition of multiple metamorphic events (e.g., Barr et al., 1986).…”

Tectonic Transport Directions, Shear Senses and Deformation Temperatures Indicated by Quartz c-Axis Fabrics and Microstructures in a NW-SE Transect across the Moine and Sgurr Beag Thrust Sheets, Caledonian Orogen of Northern Scotland

“…Biotite remains fresh and highly pleochroic with fox-red colour in most samples but is chloritized in SB-1 located immediately above the Sgurr Beag thrust in a lens-shaped unit of sheared Lewisian gneiss [120,121]. Anhedral garnet is present in more pelitic samples from the uppermost part of the Moine thrust sheet (MT-42, MT-43), while euhedral garnet is present in the Sgurr Beag thrust sheet (SB-PET-1, SB-2 to SB-6); see Ashley et al, [75], their Figure 4.…”

“…Biotite remains fresh and highly pleochroic with fox-red colour in some samples but is chloritized in other samples and particularly on the vertical fold limbs. Garnet is present in the more pelitic samples from the western part of Area 2 (MT-29 to MT-34, M-PET-1) and individual grains are euhedral in all samples (Ashley et al, [75], their Figures 3 and 4).…”