Allt Dherue
Geological Conservation Review site | GCR #2974 | Structural and Metamorphic Geology | Moine
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Summary
Within this site intensely strained schists, largely derived from the Lewisian basement and including an unusual garnet-staurolite-kyanite pelite, contain metagabbroic and meta-ultrabasic Caledonian intrusives. The platy feldspathic, micaceous and hornblendic Meadie Schists of this area are strongly folded and rodded with abundant quartz veins and white mica. Garnet-staurolite-kyanite Meadie pelite occurs [NC540446]. Geochemical evidence suggests that the Meadie schists largely represent Lewisian basement lithologies intensely strained and reworked in a zone of Caledonian ductile shear. The Meadie pelite with its unusual mineralogy and complex fabric may also be derived from the Lewisian basement and not, as was previously thought, from part of the Moine sedimentary cover succession. A large metagabbroic intrusion occurs within highly strained Meadie schists [NC535451]. This body contains relict igneous textures and minerals and is one of a suite of mildly alkaline Moine intrusives, the Loch a'Mhoid metagabbro/dolerite amphibolite suite. They are believed to be Caledonian in age, syn-D2 to pre-D3. Further east [NC538451] two prominent lensoid bodies of serpentinite have given rise to an ice-carried train of boulders, which is marked on Geological Survey maps as a large intrusion. It has been argued on geochemical grounds that this serpentinite originated as olivine cumulate from Loch a'Mhoid metagabbro magma. The interest at this site is multifaceted. First there are the Meadie schists and Meadie pelite, unusual lithologies probably derived from the Lewisian basement, which contains a wealth of tectonic and petrological evidence concerning Moine orogenesis and reworking of the basement. If the Meadie pelite is Lewisian in origin its use as an indicator of kyanite grade Moine metamorphism is highly questionable. Then there are the metagabbroic and meta-ultrabasic bodies, of somewhat controversial origin, which appear to represent a differentiated suite of Caledonian minor intrusives.
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