Metamorphic Minerals

• Actinolite
• Alurgite
• Andalusite
• Andalusite (chiastolite)
• Antigorite
• Anthophyllite-Gedrite
• Biotite
• Brucite
• Calcite
• Cassiterite
• Charoite
• Chlorite
• Chloritoid
• Chrysocolla
• Cordierite
• Corundum
• Corundum (ruby)
• Kyanite
• Kornerupine
• Kyanite (reticite)
• Deerite
• Dumortierite
• Epidotes
Allanite
Pistacite
Piemontite
Zoisite

• Fassaite
• Fluorite
• Glaucophane
• Garnet
• Hercynite
• Hedenbergite
• Hydrogrossular
• Howieite
• Humites
• Jadeite
• Lawsonite
• Lazulite
• Lazurite
• Malachite
• Minnesotaite
• Muscovite
• Olivine
• Omphacite
• Omphacite Cr
• Pargasite
• Piemontite (Praborna)
• Pyrite
• Prehnite
• Pumpellyite
• Quartz
• Rhodonite
• Rutile
• Sapphirine
• Scapolite
• Serpentine
• Sillimanite
• Sillimanite (fibrolite)
• Staurolite
• Stilpnomelane
• Sugilite
• Talc
• Taramellite
• Thulite
• Tinaksite
• Tinzenite
• Titanite
• Tremolite
• Tourmaline
• Vesuvianite
• Violan
• Wollastonite
• Yoderite
• Zussmanite

Texture and Microstructure

• Augen
• Book shelf sliding
• Boudinage
• Corona
• Decussate
• Deformation lamellae
• Deformation twins
• Crenulation cleavage
• Fringes
• Granoblastic
• Kelyphite
• Kink Bands
• Lepidoblastic
• Mesh
• Mica fish
• Mylonitic
• Nematoblastic
• Oblique foliations
• Polygonal
• Porphyroblastic
• Porphyroclastic
• Pseudomorph
• Ribbon
• S-C fabric
• Skeletal garnet
• Symplectite
• Veins

Metamorphic Rocks

• Albite greenshist
• Amphibolite (felsic)
• Amphibolite (mafic)
• Amphibolite (mylonitic)
• Amphibolite with epidote
• Andalusite schist
• Anyolite
• Augen gneiss
• Blueschist
• Blueschist (Ile de Groix)
• Blueschist (retrograde)
• Buchite
• Calc-schists
• Carbonated peridotite
• Cataclasite
• Charoitite
• Chlorite schist
• Chloritoid Schist (Ile de Groix)
• Cordierite slate
• Corundum-fuchsite rock
• Deformed oolite
• Dunite (mylonitic)
• Eclogite
• Eclogite Porto Ottiolu
• Epidosite
• Gabbro (mylonitic)
• Garnet micaschist
• Glaucophane granofels
• Granite (mylonitic)
• Granulite (felsic)
• Granulite (mafic)
• Granulite (retrograde)
• Granulite (sapphirine-bearing)
• Greenschist
• Hornfels
• Hornfels (albite-epidote)
• Hornfels (cordierite, plagioclase)
• Hornfels (corundum, plagioclase)
• Hornfels (hydrogarnet)
• Jadeite metagranite
• Jadeitite
• Kinzigite
• Lapis lazuli
• Marble
• Marble (calc-silicate)
• Marble (diopside)
• Marble (forsterite)
• Marble (mylonitic)
• Marble (tremolite)
• Mylonite
• Ocean jasper
• Ophicalcite
• Peridotite (mylonitic)
• Phyllite
• Prasinite
• Pseudotachylyte
• Pseudotachylyte (Ivrea-Verbano)
• Quartzite
• Quartzite (tourmaline)
• Rhyolite (mylonitic)
• Rodingite
• Schist
• Seafloor basalt
• Serpentinite
• Serpentinized lherzolite
• Skarn
• Skiddaw Metamorphic Aureole
Skiddaw
1) Spotted slate
2) Andalusite slate
3) Cordierite slate

• Slate
• Slate (ottrelite)
• Talc schist
• Tourmalinite
• Ultramylonite (granitic)
• Ultramylonite (mafic)

Blueschist

Blueschist is a regional metamorphic rock formed under high-pressure (HP) low-temperature (LT) conditions. It is formed in the subduction zone environment with low geothermal gradients (4-14°C km^-1) and is characterized by the presence of HP/LT index minerals like glaucophane, lawsonite, aragonite, jadeite, and deerite (Fig.1). In general, blueschist-facies rocks are stable in subduction zones at depths of 30-60 km and transform to eclogite-facies rocks at greater depths. The preservation of blueschists requires a fast exhumation rate.

Blueschist has been discovered in orogens for a long time. From a scientific point of view, Switzerland geologist Horace Bénédict de Saussure first described the glaucophanites from Saint-Marcel (Aosta Valley, Italian Alps) in 1792 (Godard, 2001). A half-century later, blueschists were also discovered in Greece by the German geologist Hofrath Hausmann, who created the name glaucophane (Hausmann, 1845). Thereafter, glaucophane-bearing rocks have been continuously reported in Greece, New Caledonia, Italy, France, Japan, Indonesia, and American California (Godard, 2001). During the early 20th century, Victor Moritz Goldschmidt and Pentti Eskola applied the concept of "chemical equilibrium" to metamorphic geology in order to relate mineral paragenesis in a metamorphic rock to its bulk composition.

Eskola first realized that particular rocks usually contain virtually identical mineral assemblages, which helped him to develop the concept of metamorphic facies (Eskola, 1920, 1922). Based on mineral assemblages that developed in mafic rocks, Eskola proposed several metamorphic facies, each of which represented a specific range of temperature and pressure. He advocated an independent "glaucophane schist-facies" to describe glaucophane-bearing schists and associated rocks with > 5% proportion of glaucophane. But the genetic and tectonic significance of this facies was not clear. The concept of "blueschist" was introduced into the world of metamorphic geology by E. Bailey in 1962 and was progressively adopted by petrologists.

Fig.1: Schematic diagram of a subduction zone and locations of different metamorphic facies. Modified after Stern (2002).

P-T stability of blueschist-facies minerals

In the metamorphic facies series of mafic rocks (Fig.2), the blueschist-facies locates under HP (P > 0.6 GPa) and LT (T < 550°C) conditions. The blueschist-facies is bounded by the zeolite-, subgreenschist- (pumpellyite-actinolite), and greenschist-facies at lower pressures, by the (epidote) amphibolite-facies on the high-temperature (HT) side and the eclogite facies at higher pressures and temperatures. When the temperature increases, glaucophane breaks down to form chlorite, lawsonite to epidote, and glaucophane reacts with lawsonite to form actinolite; therefore, blueschist-facies is replaced by greenschist-facies. When the pressure increases, glaucophane reacts with albite to form omphacite, and chlorite breaks down to form garnet; therefore, blueschist-facies is transformed to eclogite-facies.

Fig.2: The common metamorphic facies. The boundaries between the facies are depicted as wide bands because they are gradational and approximate. P-P = Prehnite-Pumpellyite facies.

Due to various protoliths and bulk compositions, different rock types contain different mineral assemblages under blueschist-facies conditions (Turner, 1981; Evans, 1990; Guiraud et al., 1990; Frey et al., 1991; Winter, 2010). Diagnostic minerals and mineral parageneses of this facies vary as a function of bulk chemistry. Nevertheless, common blueschist-facies minerals not only are confined to mafic or pelitic compositions such as glaucophane, jadeite, or lawsonite, but also include other typical minerals like carpholite, talc, or chloritoid. Characteristic mineral parageneses for various rock types under blueschist facies conditions are:

Mafic protolith (basalt, andesite, gabbro, diorite): Alkali-amphibole (mostly glaucophane), lawsonite, epidote, jadeite, phengite, chlorite, garnet, quartz.
Ultramafic protolith (peridotite, serpentinite): Serpentine group, mica group, talc, epidote, iron oxides.
Pelitic protolith (Shale, mudstone): Alkali-amphibole, lawsonite, epidote, jadeite, carpholite, chloritoid, talc, muscovite, chlorite, garnet, albite, aragonite, quartz.
Calcareous protolith (Limestone, dolomite, marls): Aragonite/calcite, dolomite, muscovite.
Quartzo-feldsparthic protolith (Sandstone, rhyolite, granite, chert): Jadeite, lawsonite, muscovite, chlorite, kyanite, garnet, aragonite, feldspar, quartz.

Blueschist classifications

Maruyama et al. (1996) reviewed more than 250 recognized HP/LT metamorphic belts worldwide and classified them into Type-A and Type-B according to their protoliths and tectonic backgrounds. Protoliths of Type-A (collision-type) blueschists refer to passive-margin rocks characterized by platform-type carbonates, bimodal volcanic, and peraluminous sediments, whereas protoliths of Type-B (Cordilleran-type) blueschists to active continental-margin rocks in an accretionary complex including bedded chert, MORB and ocean-island basalts, reef limestones, and graywackes (Maruyama et al., 1996). Based on this classification, about 20% of HP/LT metamorphic belts belong to the Type-A and the rest to the Type-B (Maruyama et al., 1996):

Type-A: most of the Type-A blueschist belts lie in Europe and the Tethyan orogen, including these HP-UHP terranes in the Alps, Greece, and Himalaya (Fig.3). The pressure of some Type-A UHP metamorphic terranes reaches up to 4.5 GPa, whereas the highest pressure of blueschists are up to 2.5 GPa.
Type-B: Type-B blueschist belts mainly occur in the circum-Pacific orogenic belts and intracontinental orogens in Asia, of which the highest pressure of blueschists usually are below 2.3 GPa (Fig.3).

Associated peridotites with Type-A blueschists are generally garnet peridotite, but those associated with Type-B blueschists are strongly serpentinized plagioclase- or spinel peridotite.

Fig.3: Spatial distribution of global blueschist (-eclogite) belts. Modified after Maruyama et al. (1996).

Implications of blueschist-eclogite transition for arc magmatism

Blueschists contain large amounts of hydrous minerals like glaucophane (ca. 2.5 wt.% H2O) and lawsonite (ca. 11.5 wt.% H2O) Schmidt and Poli, 1998. As the increasing P-T conditions with depth, dehydration reactions during the blueschist-eclogite transition cause significant dewatering from the subducting slab. Traditional viewpoint believes that slab fluids derived from blueschist-eclogite transition will ascend and metasomatize overlying mantle wedge, trigger partial melting, and finally cause arc magmatism (Peacock, 1993). However, studies in recent decades reveal that arc magmatism is closely related to thermal structures of the subduction zone. In hot subduction zones, high thermal gradients resulted in significant dehydration of subducted oceanic crust at shallow rather than sub-arc depths. Blueschist-eclogite transition happens at about 50-60 km, and oceanic slab-top melting may occur at sub-arc depths (Hernandez-Uribe et al., 2019). In contrast, the blueschist-eclogite transition occurs at about 60–80 km (forearc depths) in the warm subduction zones, and at about 80–100 km (sub-arc depths) in the cold subduction zones.

Blueschist-to-eclogite dehydration causing arc magmatism may only applicable in hot subduction zones. Even in the cold subduction zones, however, blueschist-to-eclogite water release may only cause hydration of the overlying mantle wedge, but does not trigger the volcanic arc magmatism immediately (Zheng et al., 2016; Zheng, 2019), since the mantle wedge was cooled significantly during the cold subduction and temperature is below the wet solidus of hydrated peridotites. Dehydration melting of the hydrous peridotites for arc magmatism happens only after reheating, as a result of the lateral filling of the asthenospheric mantle by mantle asthenosphere convection, or slab rollback. Therefore, aqueous fluids derived from the blueschist-eclogite transition, and UHP fluids or supercritical fluids released at greater depths, facilitate the pre-enrichment of water and trace-elements in the hydrated mantle wedge, which finally contributes to the subsequent arc magmatism and its geochemical signatures.

Blueschist from Franciscan Fm. in N. California, Usa. From Reddit.

Blueschist with garnet (red) crystals. Jenner, California, Usa. From Speaking metamorphically.

Glaucophane lawsonite schist on Syros in Greece. Lawsonite crystals (light coloured) are porphyroblasts in a groundmass of blue glaucophane crystals. From Graeme Churchard.

Lawsonite in blueschists. From Nigel Harris.

Garnet blueschists. Tinos island, Greece, Aegean sea. From Virtual microscope.

Blueschists facies metagabbro with green jadeitic pyroxene veins. The magmatic pyroxene is partly transformed into blue glaucophane. Western Alps, France. From High P.

Bibliography

• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Li, J. (2020). Blueschist: A window into high-pressure/low-temperature metamorphism and subduction zone dynamics. Science China Earth Sciences, 1-16.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.

Photo

Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 2x (Field of view = 7mm)
Relict pyroxene crystal bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystals bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystals bordered by glaucophane (blue), chlorite (green) and titanite layers (brown). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystals bordered by glaucophane and titanite layers. XPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane and Chlorite. XPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane and Chlorite. XPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). The rectangular, colorless crystal is Lawsonite. PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane and Chlorite. The rectangular, yellow crystal is Lawsonite. XPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue) and Chlorite (green). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane and Chlorite. PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane. XPL image, 10x (Field of view = 2mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)	Relict pyroxene crystal bordered by glaucophane (blue). PPL image, 10x (Field of view = 2mm)
Relict pyroxene crystal bordered by glaucophane (blue) and green chlorite. PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystal bordered by glaucophane (blue) and green chlorite. PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystal bordered by glaucophane and chlorite. XPL image, 2x (Field of view = 7mm)
Relict pyroxene crystal bordered by glaucophane (blue) and green chlorite. PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystal bordered by glaucophane and chlorite. XPL image, 2x (Field of view = 7mm)	Relict pyroxene crystal bordered by glaucophane (blue) and green chlorite. PPL image, 2x (Field of view = 7mm)
Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane. XPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)
Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)
Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)	Relict pyroxene crystals bordered by glaucophane (blue). PPL image, 2x (Field of view = 7mm)
Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 2x (Field of view = 7mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. XPL image, 2x (Field of view = 7mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 2x (Field of view = 7mm)
Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. XPL image, 2x (Field of view = 7mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 2x (Field of view = 7mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 10x (Field of view = 2mm)
Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 10x (Field of view = 2mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 10x (Field of view = 2mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. XPL image, 10x (Field of view = 2mm)
Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 10x (Field of view = 2mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. PPL image, 10x (Field of view = 2mm)	Fine-grained blueschist. Franciscan Formation, Laytonville Quarry, California. XPL image, 10x (Field of view = 2mm)
Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). XPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). XPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). XPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals in a blueschist from Ghinivert (Alpi del Monginevro; Alpi Cozie). PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)
Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)
Glaucophane crystals and epidote in a blueschist from Jenner, California, USA. XPL image, 2x (Field of view = 7mm)	Glaucophane (blue) crystals and epidote (pale yellow) in a blueschist from Jenner, California, USA. PPL image, 2x (Field of view = 7mm)	Glaucophane crystals and epidote in a blueschist from Jenner, California, USA. XPL image, 2x (Field of view = 7mm)