Canisp Shear Zone

Rocks from the Canisp Shear Zone at Achmelvich, Assynt field course sample set. This is an example of a deep crustal shear zone in which the rocks have been modified by static recrystallization after they experienced heterogeneous high-strain deformation.
S99-6 Amphibolitised Scourie Dyke  Sample S99-6, plane-polarised light, field of view 4 mm. Scourie dyke from the headland N of the main beach, outside the shear zone. Originally a gabbronorite, now with a wholly metamorphic amphibolite-facies assemblage, but not deformed or recrystallized. S99-6 Amphibolitised Scourie Dyke  Sample S99-6, scanned slide, field of view 1.5 cm. At this scale you can see stronger hints of the igneous texture with hornblende replacing pyroxene, sphene and ilmenite replacing igneous Fe-Ti oxide, and patches of recrystallized sodic feldspar with epidote granules replacing igneous plagioclase. S99-7 Sheared Scourie dyke  Sample S99-7, whole-slide scan, field of view almost 3 cm. This sample shows heterogeneous deformation, being a section across the edge of one of the set of anastomosing shear zones that accommodate the strain in metagabbroic rocks in the shear zone - many of which are Scourie dykes. S99-7 Sheared Scourie dyke  Sample S99-7, plane polarised light, field of view 3mm. This view is from the coarser domain in the section. In the lower part of the field of view, spongy blue-green hornblende patches alternate with elongate feldspar-rich domains, mimicking the texture of the original gabbro. A couple of foliated domains traverse the upper part of the view. S99-7 Sheared Scourie dyke  Sample S99-7, plane polarised light, field of view 3mm. From the opposite end of the same thin section as the previous slide, i.e. about 2cm away. The rock is fine grained, strongly foliated, and thinly banded with alternation of hornblende-rich and feldspar-rich layers. Note the tendency of c-axis-aligned hornblende (long bluish prisms) to be arranged in discrete narrow bands.
S99-7 Sheared Scourie dyke  Sample S99-7, crossed polars, field of view 3mm. Same as previous view - strongly foliated amphibolite from the high-strain part of the specimen. S99-8 Tonalitic gneiss  Sample S99-8, plane-polarised light, field of view 4 mm. Coarse poikiloblastic blue-green hornblende prisms grow over and at varying angles to the finer-grained shear fabric in this tonalitic gneiss. Note the very strongly aligned biotite flakes, and the quartz stringer at top of photo. S99-8 Tonalitic gneiss  Sample S99-8, crossed polars, field of view 4 mm. Same view, showing the fine-grained granoblastic-polygonal matrix dominated by plagioclase. S99-8 Tonalitic gneiss  Sample S99-8, scanned slide, field of view 1.5 cm. An overview of the rock texture. S05-1 Tonalitic gneiss  Sample S05-1, plane-polarised light, field of view 4 mm. View from another tonalitic gneiss, showing hornblende poikiloblasts arranged in bands but showing little crystallographic preferred orientation (note the variety of pleochroic colours shown).  In hand specimen the amphiboles are arranged more-or-less randomly in the foliation: this is sometimes called 'garben' texture or 'garbenschiefer' in German.
S99-8 Tonalitic gneiss  Sample S99-8, plane-polarised light, field of view 4 mm. Alternating bands of different minerals: plagioclase-rich, quartz ribbons, hornblende-rich layers. S99-8 Tonalitic gneiss  Sample S99-8, crossed polars, field of view 4 mm.Same view as previous photo. This shows clearly the granoblastic-polygonal texture resulting from static grain growth after deformation produced the strongly banded fabric. Also note the much larger grain size in the quartz ribbons compared to the plagioclase bands. S99-9 Tonalitic gneiss  Sample S99-9, plane-polarised light, field of view 4 mm. This shows one of the typical fabrics of blastomylonites - high-strain rocks that have experienced static grain growth after deformation. Passchier & Trouw refer to such textures as striped gneiss. Quartz ribbons alternate with felspathic bands. In outcrop this rock is almost schistose. S99-9 Tonalitic gneiss  Sample S99-9, crossed polars, field of view 4 mm. Note again the coarser grain size in quartz ribbons. This is partly due to the greater grain boundary mobility of quartz at any given temperature, and partly to pinning of plagioclase grains by fine-grained dispersed mica and epidote granules. S05-1 Tonalitic gneiss  Sample S05-1, plane-polarised light, field of view 4 mm. Another specimen showing the ribbon texture, here with biotite, amphibole and opaque oxide in the narrow feldspathic layers.
S05-1 Tonalitic gneiss  Sample S05-1, crossed polars and sensitive tint plate, field of view 4 mm. In this view you can detect from the contrasting and varied interference colours that there does not appear to be any significant crystallographic preferred orientation resulting from the recrystallization in either quartz or plagioclase domains. S99-10 Trondhjemitic gneiss  Sample S99-10, slide scan, field of view 1.5 cm. This is a trondhjemitic gneiss, coarser and more leucocratic, but still shows the ribbon texture of alternating quartz and feldspar bands. S99-10 Trondhjemitic gneiss  Sample S99-10, plane-polarised light, field of view 4 mm. This is a closer view of the ribbon texture of alternating quartz and feldspar bands. S99-10 Trondhjemitic gneiss  Sample S99-10, crossed polars, field of view 4 mm. Again it shows the characteristic features of granoblastic-polygonal texture with coarser grain size in quartz ribbons. S05-2 Ultramafic mylonite  Sample S05-2, crossed-polars slide scan, field of view 1.5 cm. This is a mylonitised ultrabasic gneiss composed of almost 100% actinolite. Undeformed equivalents outside the shear zone are dominated by coarse, randomly-oriented actinolite prisms.
S05-2 Ultramafic mylonite  Sample S05-2, plane-polarised light, field of view 4 mm. Here we can see larger  crystals with dusty cores set in a matrix of very strongly aligned fine actinolite prisms. S05-2 Ultramafic mylonite  Sample S05-2, crossed polars, field of view 4 mm. In this view we see that the largest actinolite crystal occupies the centre of a lensoid microlithon as if it was a porphyroclast, but it has composite pointed terminations that cut across the wrapping foliation: there has been significant static growth on the porphyroclast core, which is most likely marked by the dusty region in its interior. S05-2 Ultramafic mylonite  Sample S05-2, plane-polarised light, field of view 4 mm. Another area of the slide showing three dusty actinolite cores that represent porphyroclasts formed during deformation, with clearer rim outlines that cut across the foliation. S05-2 Ultramafic mylonite  Sample S05-2, crossed polars, field of view 4 mm. This view confirms that these twinned actinolite crystals now overgrow the matrix fabric which wrapped their original cores. S05-2 Ultramafic mylonite  Sample S05-2, crossed polars, field of view 4 mm. Elsewhere, more complex microlithon structures are revealed in the ultrabasic mylonite.