Regional-medium

Medium-grade regional metamorphic rocks including the Barrovian index minerals in metapelites and amphibolite-facies calcareous sediments and meta-igneous rocks from collisional metamorphic belts.
  • met4b Biotite schist, Moine Supergroup, Isle of Mull, Scotland. Biotite (black), plagioclase (white), quartz (glassy), with a little muscovite and (red) garnet.
  • met3b Garnet-mica schist, Eastern Alps. View looking down onto foliation surface. The foliation defined by muscovite and a little biotite wraps around the top of 1 cm garnets. Rusty staining in schists is commonly caused by weathering of small amounts of pyrite in the rock.
  • almandine1 Aluminous gneiss with a weak banding, from Namaqualand, S Africa. Almandine garnet (red) and light blue-grey blades of kyanite lie in a dark, biotite-rich matrix.
  • staurolite2 Aluminous gneiss from Namaqualand, S Africa. Almandine garnet (red), staurolite (brownish-black prisms) and kyanite (prominent light grey blades) lie in a matrix of biotite (dark), cordierite and plagioclase.
  • IMG 4595 Metapelitic andalusite schist from Baffin Island, Canada. Regional metamorphism at low pressure. Dark prisms of andalusite are aligned in the foliation surface. Matrix consistes of biotite, muscovite and quartz. Width of view 12 cm.
  • IMG 0174 Metapelitic staurolite-andalusite schist, NE Scotland. Cut surface of hand specimen. Staurolite forms small blocky poikiloblasts, darker where packed with fine inclusions. Darker patches are large poikiloblasts of andalusite. Irregular wavy schistosity defined by biotite. Other matrix minerals are quartz, plagioclase and muscovite. Width of view ca. 6 cm.
  • IMG 4783 Andalusite schist, SE British Columbia, Canada. Stumpy grey cm-sized andalusite prisms in muscovite-rich schist.
  • IMG 4562 Sillimanite schist, SE British Columbia, Canada. Sillimanite is not often easy to see in hand specimen, but here it forms creamy-coloured aligned prisms (left and right of centre. Note also the cluster of coarse muscovite flakes in the centre - this may be a pseudomorph after a previous Al-rich mineral such as andalusite.
  • IMG 4614 Nodular sillimanite schist, Baffin Island, Canada. The oval cream-coloured patches are made of clusters of sillimanite needles set in quartz. The matrix consists of feldspars and quartz, and the original rock was most likely a rhyolitic volcanic horizon that had undergone some leaching and alteration before metamorphism.
  • IMG 4633 Nodular sillimanite schist, Baffin Island, Canada; cut surface normal to foliation. The light-coloured lenticles consist of sillimanite needles set in quartz. The matrix consists of feldspars and quartz, and the original rock was most likely a rhyolitic volcanic horizon that had undergone some leaching and alteration before metamorphism.
  • IMG 4713 Sillimanite schist, SE British Columbia, Canada. Fibrous aggregates of sillimanite together with quartz form flattened oval nodules in this metapelitic biotite schist.
  • IMG 4714 Sillimanite schist, SE British Columbia, Canada. Fibrous aggregates of sillimanite together with quartz form flattened oval nodules in this metapelitic biotite schist.
  • IMG 4760 Metapelitic schist, SE British Columbia, Canada. Outcrop view, normal to foliation. Garnet porphyroblasts, wavy foliation, alternating mica-rich and quartz-feldspar-rich domains. Field of view ca. 15 cm across.
  • IMG 4762 Metapelitic garnet-kyanite schist, SE British Columbia, Canada. Field of view ca. 8 cm across.
  • 94a-09 Metapelitic kyanite schist, Glen Esk, Scotland (Barrovian type locality). Coarse grey-white prisms of kyanite are set in a muscovite-rich matrix. Note also the deformed quartz vein-stringers.
  • kyanite1 Kyanite-quartz segregation in schist, Glen Esk, NE Scotland.
  • IMG 4626 Amphibolite facies calc-silicate rock with very coarse actinolite. Hand specimen from Baffin Island, Canada.
  • 82d-11 Banded marble, Namaqualand Metamorphic Complex, South Africa. A siliceoous dolomite, with bands of brown dolomite, grey calcite, and black ferromagnesian minerals (mainly forsterite and tremolite).
  • met6a White marble with ruby corundum, Karakoram Mountains, Pakistan.
  • RF2 28 Banded quartzofeldspathic gneiss, West Norway. An amphibolite-facies biotite-bearing gneiss with a bulk composition similar to a granodiorite; could have either a sedimentary or an igneous origin. [photo: A L Wain]
  • nws02-133 Lewisian gneiss, north of Loch Laxford, NW Scotland. An amphibolite-facies streaky biotite gneiss with layers and lenses of white and pink granitic material.
  • 106-0649 IMG Lewisian gneiss, north of Loch Laxford, NW Scotland. Grey biotite gneiss of granodioritic composition with diffuse patches of pink alkali feldspar.
  • 106-0652 IMG Lewisian gneiss, north of Loch Laxford, NW Scotland. Age relationships shown by discordant relations between grey biotite gneiss, black biotite-hornblende metabasic rock (interfolded and foliated but contact cuts gneissic foliation at small angle) and pink pegmatitic granite sheet (not foiated, cuts gneissic foliation at high angle).
  • s98-27a Hand specimen of Lewisian trondhjemitic gneiss, NW Scotland (trondhjemites are plagioclase-quartz igneous rocks with minimal mafic mineral content - a leuco-tonalite). This one has a strong foliation and an amphibolite-facies assemblage of white feldspar, glassy quartz and a little green-black hornblende.
  • met5a Biotite-bearing granitic augen-gneiss, cut by an undeformed microgranite sheet.
  • a98-9 Hand specimen of granodioritic biotite gneiss, from Loch Laxford, NW Scotland.
  • IMG 4773 Augen gneiss formed from porphyritic granite, SE British Columbia, Canada.
  • IMG 4778 Detail of augen gneiss formed from porphyritic granite, SE British Columbia, Canada. Augen (eyes) consist of K-feldspar.
  • IMG 5913 Granitic gneiss: the Man o' War gneiss, Lizard Point, Cornwall. Strongly foliated, with minor folds.
  • hornblende1 Gneiss formed from banded volcanic rocks, with coarse-grained hornblende alternating with plagioclase-rich bands in folded layering, lower Orange River area, S Africa.
  • 106-0678 IMG Fold in banded amphibolite-facies mafic gneiss, Achmelvich, NW Scotland. The banding results from varying proportions of plagioclase and hornblende. Note the alignment of hornblende defining a gneissose fabric parallel to the axial surface of the fold.
  • 124-2426 IMG Amphibolite (hornblende + plagioclase) formed by flattening and shear deformation of a metagabbro. The precursor is a 'Scourie Dyke', deformed in a major shear zone at Achmelvich, NW Scotland.
  • s99-7 Hand specimen of amphibolite from a shear zone in the Lewisaian Complex, Achmelvich, NW Scotland. The specimen show a gradation in the grain size and intensity of the foliation, a result of increasing strain towards the bottom of the field of view.
  • 137-3728 IMG Field contact between amphibolite-facies metabasic gneiss (plagioclase + hornblende, below) and ultrabasic gneiss (mostly hornblende, above), Achmelvich, NW Scotland. These rocks probably formed part of a basic-ultrabasic layered igneous intrusion.
  • IMG 0177a Dark hornblende on foliation surface of a somewhat schistose amphibolite from Baffin Island, Canada. Field of view ca. 4 cm across.
  • IMG 5900 Epidote amphibolite, Lizard Point, Cornwall. This foliation surface has a strong lineation defined by 'rods' of greenish-yellow epidote alternating with dark hornblende and white plagioclase.
  • IMG 5193 Amphibolite from the Lizard Complex, Cornwall. Blocky hornblende crystals in a matrix of plagioclase. This type contrasts with the banded epidote amphibolites seen at Lizard Point.
  • IMG 4627 Felsic garnet-amphibolite, SE British Columbia, Canada. Garnet (reddish), hornblende(dark green), plagioclase (white) and quartz (glassy).
  • IMG 4669 Garnet amphibolite, SE British Columbia, Canada. Abundant garnet porphyroblasts scattered in a matrix of hornblende and plagioclase with a weak fine banding.
  • IMG 0691 Amphibolite formed by retrograde replacement of a pyroxene granulite, with dark green amphibole and white plagioclase preserving the shapes of former pyroxene and feldspar grains. Metabasic gneiss adjacent to pegmatite, Scourie, NW Scotland.
  • IMG 5235 Sheared ophiolitic metagabbro, Lizard Complex, Cornwall. Lens-shaped low-strain domains still have the igneous texture of an ophiolitic gabbro, but these are wrapped around by recrystallised amphibolite-facies shear zones.
  • IMG 5238 Sheared ophiolitic metagabbro, Lizard Complex, Cornwall. Green hornblende, creamy-white plagioclase. The larger mafic crystals probably contain relict clinopyroxene. Beware: some rounded white 'crystals' are small barnacles.
  • 137-3747 IMG Rounded ultrabasic pod in tonalitic gneiss, Achmelvich, NW Scotland. The ultrabasic rock has been hydrated and carbonated and now consists of a core of pale dolomite and talc, with a thick rim of inward-projecting actinolite prisms.
  • 137-3749 IMG Detail of ultrabasic pod in gneiss, Achmelvich, NW Scotland. Green actinolite prisms projecting into light-coloured dolomite + talc matrix.
  • IMG 0729 Ultrabasic gneiss, Achmelvich, NW Scotland. Spiky brownish prisms project from a softer matrix of green chlorite with some dolomite and talc (mostly weathered away from this surface. The brown prisms look like an amphibole but are not actinolite: they mostly consist of a lamellar intergrowth of cummingtonite and an amphibole-like triple-chain silicate (clinojimthompsonite).