Epidote-bearing amphibolite

The name amphibolite has been used in geological literature for nearly 200 years, since Brongniart (1813). Amphibolite is a gneissose or granofelsic metamorphic rock mainly consisting of green, brown or black amphibole and plagioclase (including albite), which combined form ≥75% of the rock and both of which are present as major constituents; the amphibole constitutes ≥50% of the total mafic constituents and is present in an amount of ≥30%. Other common minerals include quartz, clinopyroxene, garnet, epidote-group minerals, biotite, titanite and scapolite.

The Amphibolite classification is based on the following statements:

1) The modal compositions of amphibolites show that most of them contain more than 50% of amphibole, but those with 50 to 30% are not unusual. The content of amphibole and plagioclase together is mostly higher than 90%, and may be as low as 75%.

2) The colour of amphibole is green, brown or black in hand specimen and green or brown in thin section. The common varieties are tschermakitic and magnesio- and ferro-hornblende.

3) Plagioclase is the prevalent light-coloured constituent, the quantity of quartz or epidote or scapolite should be lower than that of plagioclase.

4) Clinopyroxene, where present, should be less abundant than amphibole (hornblende). When pyroxene prevails, the rock should be named hornblende-pyroxene rock or calc-silicate rock, depending on its composition and on the composition of the clinopyroxene.

5) The presence of other major mineral constituents (>5%) is expressed by the corresponding prefix according to general SCMR rules (e.g. garnet amphibolite, pyroxene amphibolite, quartz amphibolite, etc.).

6) The amphibolite is characterised by the presence of hydroxyl-bearing minerals (amphibole, biotite), which prevail over the hydroxyl-free ones (garnet, diopside). The boundary with the higher grade, granulite-facies metamorphic rocks, is determined by the appearance of orthopyroxene.

Amphibolite facies

Amphibolites define a particular set of temperature and pressure conditions known as the amphibolite facies, with temperature of 500 to 750 °C and pressures of 8-7 kbar. Changes in mineralogy depends very much on protolith, however, production of abundant garnet and hornblende are most characteristic. Sodic feldspars are oligoclase rather than the albite that dominates at lower T. Biotite and muscovite are both abundant in pelitic rocks of amphibolite facies. Kyanite and sillimanite are often produced by reaction of muscovite and quartz.

Typical assemblages for different protoliths include:

• Mafic protolith: hornblende + oligoclase ± epidote ± almandine garnet ± titanite ± quartz ± chlorite ± biotite.
• Pelitic protolith: biotite ± muscovite ± oligoclase ± almandine garnet ± cordierite (low-P) ± andalusite (low-P) ± kyanite (high-P) ± sillimanite (moderate-P, and/or high-T) ± staurolite (high -T) ± graphite ± titanite.
• Quartz-feldspathic protolith: oligoclase + alkali feldspar + muscovite + biotite ± hornblende.
• Carbonatic protolith: calcite, dolomite, quartz, diopside, tremolite, forsterite, grossular garnet, hornblende, clinozoisite.

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.

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.
• 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
Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 2x (Field of view = 7mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 2x (Field of view = 7mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 2x (Field of view = 7mm)
Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 2x (Field of view = 7mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 2x (Field of view = 7mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 2x (Field of view = 7mm)
Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 2x (Field of view = 7mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 2x (Field of view = 7mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 2x (Field of view = 7mm)
Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 2x (Field of view = 7mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 10x (Field of view = 2mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 10x (Field of view = 2mm)
Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 10x (Field of view = 2mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 10x (Field of view = 2mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 10x (Field of view = 2mm)
Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 10x (Field of view = 2mm)	Hornblende (green), plagioclase (colorless) and epidote (with high relief and beige) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). PPL image, 10x (Field of view = 2mm)	Hornblende, plagioclase and epidote (strong birefringence) in a amphibolite. Lanzada-scermendone (valmalenco), central Alps (Italy). XPL image, 10x (Field of view = 2mm)
Hornblende (green) and epidote (beige) in a amphibolite. PPL image, 2x (Field of view = 7mm)	Hornblende and epidote in a amphibolite. XPL image, 2x (Field of view = 7mm)	Hornblende (green) and epidote (beige) in a amphibolite. PPL image, 2x (Field of view = 7mm)
Hornblende and epidote in a amphibolite. XPL image, 2x (Field of view = 7mm)	Hornblende (green) and epidote (beige) in a amphibolite. PPL image, 2x (Field of view = 7mm)	Hornblende (green) and epidote (beige) in a amphibolite. PPL image, 2x (Field of view = 7mm)