Regional-high

High-grade regional metamorphic rocks, including migmatites and granulites from both Phanerozoic orogenic belts and Precambrian metamorphic terrains.
  • 84g-22 Granulite facies metapelitic garnet-cordierite gneiss, Namaqualand, South Africa. A typical shale at the highest grade of metamorphism.
  • IMG 4540 Granulite facies metapelitic garnet-cordierite gneiss: red garnet, dark greenish cordierite conspicuous in central band. Other minerals: K-feldspar, plagioclase, a little biotite, sillimanite in some layers. Hand specimen from Baffin Island, Canada.
  • IMG 4535 Granulite facies metapelitic garnet-cordierite gneiss. Folded greensish bands at left contain K-feldspar, and represent the former presence of partial melt. Hand specimen from Baffin Island, Canada.
  • IMG 4615 Granulite facies metapelitic garnet-biotite sillimanite gneiss. Central band contains much garnet and K-feldspar, and marks the former location of partial melting. See later migmatite photos. Hand specimen from Baffin Island, Canada.
  • 113-1341 IMG Hand specimen of gneiss with cordierite (pale) orthopyroxene (large black crystals), phlogopite (dark, in matrix with cordierite) and a little garnet (red). This is not a metapelitic sediment (too much Mg and Fe, too little alkalis) but more likely a mafic volcanic rock affected by pre-metamorphic alteration. From Namaqualand, South Africa.
  • IMG 4552 Granulite facies metaquartzite. Very coarse-grained; banding (bedding or tectonic?) still just distinguishable. Hand specimen from Baffin Island, Canada.
  • IMG 4545 Granulite-facies calc-silicate gneiss: white wollastonite, grey plagioclase, dark green diopside. Hand specimen from Baffin Island, Canada.
  • IMG 4549 Granulite facies marble, coarse-grained calcite, green diopside. Hand specimen from Baffin Island, Canada.
  • IMG 4559 Granulite facies. Forsterite marble, with minor diopside. Dark pyroxene, yelloe-green olivine, white calcite. Hand specimen from Baffin Island, Canada.
  • IMG 0699 Granulite facies trondhjemitic gneiss (plagioclase + quartz). Both minerals are darker coloured than in the equivalent amphibolite-facies rock. Lewisian gneiss complex, Scourie, NW Scotland.
  • 105-0586 IMG Granulite facies tonalitic gneiss, rich in plagioclase, with foliation defined by thin layers of pyroxene (ortho- and clino-, when rock is fresh). Lewisian gneiss complex, Scourie, NW Scotland.
  • 123-2399 IMG Boulder of foliated, banded and folded tonalitic gneiss. Lewisian gneiss complex, Scourie, NW Scotland.
  • IMG 1950 Granulite facies tonalitic gneiss with isoclinally folded compositional layering. Lewisian gneiss complex, Scourie, NW Scotland.
  • IMG 6349 Green charnockitic gneiss. Charnockites are orthopyroxene-bearing rocks of broadly granitic composition. The green colour commonly derives from chloritic or serpentinous alteration of orthopyroxene that permeates fractures and grain boundaries in the rock. From a slab in a stone merchant's yard.
  • 015 Pyroxene granulite: granulite facies metabasic rock with green clinopyroxene, brownish orthopyroxene, pale plagioclase feldspar, and a little relict hornblende. Akia terrain, West Greenland.
  • IMG 0687 Garnet-clinopyroxene granulite. Formed at high pressure from a basic igneous precursor. Lewisian gneiss complex, Scourie, NW Scotland.
  • 99-132 Large garnets with reaction coronas (of plagioclase + orthopyroxene) in garnet-clinopyroxene granulite. Lewisian gneiss complex, Scourie, NW Scotland.
  • s99-2 Garnet-bearing metagabbro formed from coarse-grained dyke rock intruded into the Lewisian gneiss complex, Scourie, NW Scotland.
  • IMG 4406 Retrograde effects in high-grade rocks: access of fluid along a fracture has converted pyroxene (paler green) to hornblende (darker green). Metagabbro dyke in Lewisian gneiss complex, Scourie, NW Scotland.
  • s98-13 Granulite facies ultrabasic gneiss from the Lewisian gneiss complex, Scourie, NW Scotland. Note the contrast between the freshly broken surface (uniformly dark, lower left) and the weathered surface, upper right, on which you can distinguish sefter yellow-weathering olivine and more resistant brown and green pyroxenes.
  • 105-0583 IMG Outcrop surface of granulite facies ultrabasic gneiss, Lewisian gneiss complex, Scourie, NW Scotland. This is a relatively pyroxene-rich composition, dominated by green clinopyroxene.
  • IMG 0694 On this outcrop surface of granulite facies ultrabasic gneiss you can distinguish resistant green pyroxene from softer brown-weathering olivine. Lewisian gneiss complex, Scourie, NW Scotland.
  • IMG 4674 In the presence of aqueous fluid, melting of metapelitic and quartzofeldspathic rocks can take place in the upper amphibolite facies, giving rise to migmatites like this, with light-coloured leucosomes (of granitic or trondhjemitic composition) in a darker schistose matrix. SE British Columbia, Canada.
  • IMG 4675 Lenses of granitic leucosome strung out parallel to foliation in biotite-sillimanite schist. These styles of partially melted rock are called stromatic migmatites. SE British Columbia, Canada.
  • 84g-03 In the absence of aqueous fluid, a hydrous mineral such as biotite must provide the water needed to make granitic melt. Here the process has begun in a granulite-facies metapelitic gneiss, producing melt plus garnet as an anhydrous product. From Namaqualand, South Africa.
  • a3-08 In this rock the melting process has advanced further, and light-coloured patches, containing large garnets, disrupt and cut across the banding of this biotite-cordierite gneiss. Arunta block, Central Australia.
  • IMG 6351 This is the same process, dehydration melting of biotite, in a gneiss from India, and you can have your kitchen counter made of this. The leucosomes are rich in garnet and K-feldspar. From a slab in a stone merchant's yard.
  • 87a-21 A semipelitic gneiss from Namaqualand, South Africa, with the classic granulite-facies migmatitic texture of garnet-bearing leucosomes in a biotite-bearing host gneiss.
  • 87a-27 Detail of melt-rich leucosome above with a garnet-rich layer against the granulite facies cordierite-biotite-sillimanite gneiss host rock below. From Namaqualand, South Africa.
  • IMG 4616 Hand specimen showing a garnet-bearing leucosome lens wrapped by biotite-sillimanite gneiss, from Baffin Island, Canada.
  • a2-14 A metapelitic gneiss in which partial melt has migrated into a network of shear bands, from which it may leave the rock. Mount Stafford area, Arunta block, Central Australia.
  • 88a-11 Hornblende-biotite gneiss cut across by a vein containing the granulite facies assemblage orthopyroxene - K-feldspar - plagioclase - quartz. Some authors attribute this to an influx of carbonic fluid promoting dehydraton of the amphibolite-facies host rock, but it could also be related to dehydration melting. Karnataka, southern India.