Greenschist and Greenschist Facies

Greenschist is a foliated, fine- to medium-grained regional metamorphic rock derived from the low-to-medium-grade hydration and recrystallization of mafic igneous protoliths—such as mid-ocean ridge or island-arc basalt, diabase, and gabbro—or immature volcanogenic greywackes. Its characteristic green color and penetrative planar schistosity arise from the parallel alignment of abundant green sheet and chain silicates, specifically chlorite [(Mg,Fe)₅Al(AlSi₃O₁₀)(OH)₈], actinolite [Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂], and epidote [Ca₂(Al,Fe³⁺)₃(SiO₄)₃(OH)], intergrown with sodic albite (NaAlSi₃O₈, An_0–7), quartz (SiO₂), and accessory titanite (CaTiSiO₅), magnetite, or calcite (CaCO₃). When a metabasite of identical mineral assemblage lacks penetrative tectonic foliation—as occurs during static seafloor hydrothermal alteration or in the massive interiors of thick basaltic lava flows—it is termed a greenstone rather than a greenschist.

In metamorphic petrology, the mineral assemblage of mafic greenschist serves as the pressure–temperature anchor for the greenschist facies, one of the eight canonical metamorphic facies introduced by Finnish petrologist Pentti Eskola in 1920. Spanning temperatures of 300 °C to 500 °C and lithostatic pressures of 0.2 to 0.8 GPa (corresponding to crustal depths of approximately 8 to 30 km along medium-pressure orogenic geotherms of 15–30 °C/km), the greenschist facies occupies the central low-grade domain of regional Barrovian belts and Archean–Proterozoic greenstone terranes. Within this same P–T window, interbedded pelitic mudstones recrystallize into slate, phyllite, and lower-grade mica schists belonging to the chlorite, biotite, and almandine–garnet zones.

Hydration and Carbonation of Mafic Protoliths

Anhydrous primary igneous minerals in basalt and gabbro—specifically calcic plagioclase (labradorite–bytownite, An_50–80), augite clinopyroxene [(Ca,Mg,Fe)₂Si₂O₆], olivine [(Mg,Fe)₂SiO₄], and ilmenite (FeTiO₃)—are thermodynamically unstable when infiltrated by H₂O- and CO₂-bearing metamorphic fluids at 300–450 °C. During burial or tectonic imbrication, hydration reactions break down calcic plagioclase and augite into a diagnostic four-phase greenschist assemblage (chlorite-actinolite schist):

Calcic Plagioclase (An₆₀) + Augite + H₂O → Albite (An_0–7) + Chlorite + Actinolite + Epidote/Clinozoisite + Quartz

Because low-grade plagioclase cannot accommodate significant calcium below the peristerite solvus (~450–500 °C), the anorthite component (CaAl₂Si₂O₈) of igneous labradorite is expelled into epidote (or iron-free clinozoisite) and calcic amphibole (actinolite), leaving behind nearly pure sodic albite (An_0–7). Simultaneously, primary igneous ilmenite reacts with calcium and silica liberated during plagioclase breakdown to form wedge-shaped crystals of titanite (sphene, CaTiSiO₅).

If the infiltrating metamorphic fluid possesses a high mole fraction of carbon dioxide (X(CO₂) > 0.05–0.10), calcium and magnesium are partitioned into carbonate minerals—calcite (CaCO₃), dolomite [CaMg(CO₃)₂], or ankerite [Ca(Fe,Mg)(CO₃)₂]—destabilizing actinolite and epidote via carbonation reactions:

3 Actinolite + 2 Epidote + 10 CO₂ + 8 H₂O ⇌ 3 Chlorite + 10 Calcite + 21 SiO₂ (Quartz)

This chlorite + albite + carbonate + quartz assemblage is pervasive in carbonated shear zones hosting orogenic gold deposits across Archean greenstone belts.

Low-Grade Onset: From Prehnite–Pumpellyite to Greenschist

At the low-temperature boundary (~280–320 °C at 0.2–0.5 GPa), the greenschist facies succeeds the sub-greenschist zeolite and prehnite–pumpellyite facies. As temperature rises across ~300 °C, the hydrous Ca–Al silicates prehnite [Ca₂Al(AlSi₃O₁₀)(OH)₂] and pumpellyite [Ca₂MgAl₂(SiO₄)(Si₂O₇)(OH)₂·H₂O] react with chlorite and quartz to nucleate the canonical greenschist index assemblage of actinolite + epidote + H₂O:

Pumpellyite + Chlorite + Quartz → Actinolite + Epidote + H₂O

This dehydration reaction marks the lower thermal boundary ("actinolite-in" isograd) of true greenschist-facies metamorphism.

Prograde Transition to the Amphibolite Facies

At the upper thermal boundary (475 °C to 525 °C at 0.4–0.8 GPa), greenschists transform through the transitional epidote–amphibolite facies into the medium-to-high-grade amphibolite facies via two coupled mineralogical steps:

1. The Plagioclase (Peristerite) Jump: Above ~475–500 °C, epidote and chlorite break down to supply calcium and aluminum to plagioclase. Sodic albite (An_0–7) crosses the thermodynamic miscibility gap known as the peristerite solvus and recrystallizes into calcic oligoclase or andesine (An_17–45).
2. Actinolite-to-Hornblende Transition: Simultaneously, Al-poor pale green actinolite incorporates tetrahedral aluminum (Al^IV substituting for Si⁴⁺), octahedral Al^VI and Ti⁴⁺, and A-site Na⁺ via tschermakitic and edenitic coupled substitutions, converting into dark green to black aluminous hornblende:

Chlorite + Actinolite + Epidote + Albite + Quartz → Hornblende + Oligoclase/Andesine (An_>17) + H₂O

The simultaneous disappearance of chlorite and appearance of hornblende + calcic plagioclase (An_>17) defines the boundary between greenschist and amphibolite.

High-Pressure Transition to the Blueschist Facies

Along cold subduction-zone geotherms (<10–12 °C/km), increasing pressure above 0.6 to 0.8 GPa at 300–450 °C drives greenschist across a pressure-sensitive reaction boundary into the blueschist facies (and at still greater depths >1.2–1.5 GPa into the plagioclase-free eclogite facies). Across the greenschist-to-blueschist transition, sodic albite and calcic actinolite + chlorite react to form the sodic amphibole glaucophane [Na₂(Mg,Fe)₃Al₂Si₈O₂₂(OH)₂] and the high-pressure Ca–Al silicate lawsonite [CaAl₂Si₂O₇(OH)₂·H₂O] or epidote:

Albite + Chlorite + Actinolite → Glaucophane + Epidote/Lawsonite + Quartz + H₂O

Conversely, when subducted blueschists and eclogites are exhumed along a clockwise P–T path and isothermally decompressed into the middle crust (0.3–0.6 GPa at 400 °C), they routinely undergo greenschist-facies overprinting, forming retrograde albite + actinolite + chlorite symplectites around relict glaucophane and garnet cores.

Greenschist-facies rocks occur in three principal tectonic settings: (1) Collisional Orogenic Belts (such as the Scottish Dalradian, the Appalachian Piedmont, the Alpine Helvetic nappes, and the Otago Schist of New Zealand), where folded volcano-sedimentary wedges experience Barrovian burial; (2) Mid-Ocean Ridge Seafloor Hydrothermal Systems, where seawater convecting to 1.5–3.0 km depth beneath oceanic spreading centers alters sheeted dike complexes and upper gabbros into unfoliated greenstones at 300–450 °C; and (3) Archean and Proterozoic Greenstone Belts (such as the Abitibi belt of Canada, the Yilgarn and Pilbara cratons of Western Australia, and the Barberton belt of South Africa). During the prograde greenschist-to-amphibolite devolatilization of thick mafic–ultramafic volcanic sequences (~450–520 °C), the breakdown of chlorite and pyrite releases low-salinity H₂O–CO₂–H₂S metamorphic fluids carrying gold as bisulfide complexes [Au(HS)₂⁻], which migrate upward into brittle–ductile greenschist shear zones and precipitate orogenic lode gold alongside quartz–carbonate–albite veins.

What is the difference between a greenschist and a greenstone?

Both rocks share the identical greenschist-facies mineral assemblage of chlorite + actinolite + epidote + albite, derived from basaltic or gabbroic protoliths. A greenschist was recrystallized under directed non-hydrostatic tectonic stress (differential stress σ₁ > σ₃) in a regional fold-and-thrust belt or shear zone, imparting a penetrative planar foliation (schistosity) defined by aligned chlorite and actinolite crystals. A greenstone recrystallized under static, nearly hydrostatic conditions—such as ocean-floor hydrothermal alteration or contact/burial metamorphism—and therefore retains an isotropic, unfoliated massive or relict igneous texture.

Why are plagioclase feldspars in the greenschist facies restricted to nearly pure albite?

Below approximately 475–500 °C, the plagioclase solid solution between albite (NaAlSi₃O₈) and anorthite (CaAl₂Si₂O₈) exhibits a thermodynamic miscibility gap known as the peristerite solvus (spanning ~An_3–7 to An_20–25) Driven by Al–Si ordering in the tetrahedral framework. Under greenschist-facies conditions, calcium and aluminum are preferentially incorporated into hydrous epidote and actinolite, restricting the stable feldspar phase to nearly pure sodic albite (An_0–7).

How does the greenschist-to-amphibolite transition drive orogenic gold mineralization?

Mafic greenschists contain ~3 to 5 wt% structurally bound H₂O (primarily in chlorite, which holds ~12 wt% H₂O) plus carbonate and disseminated iron sulfides (pyrite). When crustal thickening heats these rocks across the greenschist–amphibolite boundary (~475–525 °C), chlorite dehydrates to form hornblende (only ~2 wt% H₂O) while pyrite desulfidates to pyrrhotite (Fe_1−xS). This narrow thermal window liberates vast volumes of buoyant H₂O–CO₂–H₂S fluid that scavenges trace gold as Au(HS)₂⁻ and deposits it in overlying greenschist-facies fault networks.

1. Eskola, P. (1920). "The mineral facies of rocks." Norsk Geologisk Tidsskrift, 6, 143–194. https://njg.geologi.no/images/NJG_articles/NGT_06_2_143-194.pdf
2. Bucher, K., & Grapes, R. (2011). Petrogenesis of Metamorphic Rocks (8th ed.). Springer. https://doi.org/10.1007/978-3-540-74169-5
3. Maruyama, S., Suzuki, K., & Liou, J. G. (1983). "Greenschist-amphibolite transition equilibria at low pressures." Journal of Petrology, 24(4), 583–604. https://doi.org/10.1093/petrology/24.4.583
4. Phillips, G. N., & Powell, R. (2010). "Formation of gold deposits: a metamorphic devolatilization model." Journal of Metamorphic Geology, 28(6), 689–718. https://doi.org/10.1111/j.1525-1314.2010.00887.x