Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Granulite (mafic)

Granulite: Granulite is a high-grade metamorphic rock in which Fe-Mg-silicates are dominantly hydroxyl-free; the presence of feldspar and the absence of primary muscovite are critical, cordierite may also be present. The mineral composition is to be indicated by prefixing the major constituents. The rocks with >30% mafic minerals (dominantly pyroxene) may be called mafic granulites, those with <30% mafic minerals (dominantly pyroxene) may be called felsic granulites. Granulite may exhibit a crude gneissic structure due to the parallelism of flat lenses of quartz and/or feldspar. The texture is typically granoblastic. (IUGS Subcommission on the Systematics of Metamorphic Rocks, 2007)

The type location for granulite is the Granulitgebirge in Saxony, eastern Germany. It was here that the term "granulite" first appears in the literature (Weiss, 1803). But the word "granulite" is older, it was invented by the German writer Johann Wolfgang von Goethe (1785) from Latin granulum, "a little grain": "Strange rock type form the Bodetal opposite the Susenburg [in the Harz mountains], which I do not dare to classify either as granite or as porphyry and for which I propose the name granulite because of its content of round quartz grain".

The name granulite is burdened by many ambiguities and was used with different meanings in different countries:

(i) In France it was applied to fine-grained granitic rocks (Michel-Lévy, 1874; Cogné, 1961), but this use did not find common acceptance.
(ii) In Scotland and England, the term granulite was applied to high grade metamorphic products of psammitic rocks. Most widespread is the use of the term granulite for light-colored, quartzo-feldspathic, high-grade metamorphic rocks.

Metamorphic rocks of the granulite grade represent exhumed sections of the lower portions of the earths’ crust. Their study is hence important in understanding deep crustal and crust-mantle interaction processes. Although granulite facies rocks are sporadically exposed in younger terrains, largely by tectonic uplift along fault zones, their abundance in Precambrian shield areas indicates that the lower continental crust is predominantly of granulite grade.

Granulite facies

Granulite facies (Fig.1) was introduced by Eskola (1939) to define the highest grade of regional metamorphic rocks that contain pyroxene in place of normal hydrous ferromagnesian minerals. Mineral assemblages and thermobarometry indicate granulite assemblages equilibrate over a broad range of temperatures generally from about 650 to 900°C but to as much as 1050°C and pressures of generally 5 kbar to as much as 12 kbar, or depths of 20-45 km (Harley, 1989, 1998). Granulite facies minerals are predominantly anhydrous, due to dehydration reactions at high temperatures. Hydrous minerals hornblende and biotite, but not muscovite, can occur in the lower part of the granulite facies. The upper part of the granulite facies is characterized entirely by anhydrous minerals.

Amphibole minerals (tremolite, anthophyllite, hornblende) dehydrate to pyroxene minerals (enstatite, diopside, hypersthene), and phyllosilicate minerals (such as muscovite) dehydrate to anhydrous minerals (orthoclase) in response to high temperatures. Formation of granulite-facies rock, nominally from lower-grade, H₂O-richer, amphibolite facies rock, with a granitic to mafic igneous protolith, is a common, local to regional, dehydration process in the lower crust involving the conversion of OH-bearing biotite and amphibole to ortho- and clinopyroxene. The process by which this occurs can either take the form of partial melting or be induced to occur due to the infiltration of low H₂O-activity, CO₂-rich fluids into the amphibolite system in a process known as solid-state dehydration.

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.

Petrogenesis of granulites

A problem in dealing with the petrogenesis of granulites in a concise way is that granulite terrains vary widely in character, and granulite-facies rocks present many different sorts of phenomena to be explained. The basic petrological problem is that typical granulite-facies assemblages require PH₂O << Ptotal for their stability, and debate has centered around how this low PH2O is achieved. There are three possible models of granulites petrogenesis:

1) Melting and melt extraction: The original reference given for this model is Fyfe (1973), who suggested that granulites could be regarded as restites after removal of relatively large amounts of partial melt. A substantial boost for the model came from the realization that dehydration melting, i.e. vapour-absent incongruent melting of assemblages containing hydrous phases (muscovite, biotite, hornblende) is likely to be an important process in high-grade metamorphism.

2) Influx of CO₂-rich vapor: This model has been championed by R.C. Newton. The type locality for this model is in south India, at classic exposures in stone quarries, e.g. at Kabbaldurga in the Late Archaean amphibolite-granulite transition zone. Solid-state, high-grade dehydration of hornblende- and biotite-bearing rock, by low H₂O-activity fluids, to orthopyroxene clinopyroxene-bearing rocks on a localized scale (cm to m) is a widespread phenomenon. Also referred to, more generally, as "incipient" or "arrested charnockitisation", localized, solid-state dehydration zones occur worldwide in metabasites, tonalites, and granitoids.

3) Metamorphism of dry precursors: In certain granulitic terrains the precursor rocks were already rather dry (e.g. water deficient magmatic rocks and their dehydrated thermal aureoles) so that their subsequent high-grade metamorphism does not require a special mechanism to generate granulite-facies assemblages. The type area for this model is the Adirondack Highlands of upstate New York, where much of the terrain consisted of pre-orogenic pyroxene-bearing igneous rocks and their high-grade thermal aureoles, subsequently metamorphosed to give Grenville age (1000 Ma) granulite-facies assemblages.

Granulite-facies mineral associations

Mafic granulites
Mafic granulites are characterized by Cpx- and/or Hbl-bearing Opx + Pl ± Grt ± Bt ± Kfs ± Qtz mineral assemblages and are broadly basaltic in composition. The general reaction that first introduce Opx to Hbl + Pl ± Qtz ± Cpx ± Grt-bearing mafic amphibolites below 10 Kbar is:

Hbl + Qtz ± Grt = Opx + Cpx ± Pl + L

At pressure above 10 Kbar, an analogous reaction introduces Grt + Cpx without Opx:

Hbl + Pl + Qtz = Grt + Cpx + L

Intermediate granulites
Intermediate granulites are characterized by Cpx- and Hbl-free Opx + Pl ± Grt ± Bt ± Kfs ± Qtz mineral assemblage and are broadly representative of metamorphosed psammites, semipelites and intermediate-felsic rocks in which the major hydrous mineral is biotite. The general reaction that first introduces Opx to these rocks is:

Bt + Qtz ± Pl = Opx + L ± Grt ± Crd ± Kfs

Aluminous granulite
Aluminous granulite represents metamorphosed pelitic bulk composition and may contain combination of the aluminous minerals Grt, Crd, Al2SiO5 (sillimanite, kyanite or andalusite), sapphirine, corundum and osmulite, in addition to Opx and/or Bt. Some workers consider any rock with stable Kfs + Al2SiO5 to be representative of granulite facies. The first appearance of this association is by the general reaction:

Ms + Qtz ± Pl = Al₂SiO₅ + L ± Kfs

The first appearance of Kfs + Al₂SiO₅ occurs 100 ± 150 °C below the first appearance of Opx in mafic and intermediate compositions. In regional and contact metamorphic sequences in which the first development of Kfs + Al₂SiO₅ in metapelites and the first development of Opx in mafic and intermediate composition can both be mapped, the former is invariably significantly down-grade of the latter. Many geologists, therefore, prefer to define the amphibolite-granulite transition in metapelites by mineral associations that develop closer (both spatially in the field and, by extension, in pressure and temperature) to the first appearance of Opx in mafic and intermediate compositions, namely, Grt + Crd + Kfs (at pressures below 9 kbar) or Opx + Al₂SiO₅ (at pressures above 9 kbar). The lowest-grade reactions by which these assemblages develop are, respectively:

Bt + Sil + Qtz ± Pl = Grt + Crd + L ± Kfs

and

Bt + Grt + Qtz ± Pl = Opx + Al₂SiO₅ + L ± Kfs

Higher-grade reactions in aluminous granulites involve the minerals or mineral associations sapphirine (Spr), osumilite (Os) and spinel (Spl) + Qtz, with Spr + Qtz indicating particularly high temperatures in excess of 1000 °C.

Obsolete granulite terminology

Several rock names are associated with the nomenclature of granulite rocks:

♦ Hälleflinta: Obsolete term, used mainly in Sweden and Finland, for a fine.grained compact quartzo-feldspathic rock of horny aspect, which may be banded and/or blastoporphyritic.
♦ Leptynite: Name created by Haüy, initially applied to a fine-grained granulite-facies rock, predominantly consisting of alkali feldspar, containing minor quartz, white mica, garnet and tourmaline, and with a planar gneissose structure.
♦ Leptite: Old term used by Swedish geologists for fine-grained gneissose to granulose metamorphic rocks of sedimentary origin mainly composed of feldspar and quartz with subordinate mafic minerals.
♦ Namiester Stein: An obsolete name for granulite from the locality Námiest in the Bohemian Massif, from where this rock was initially described.
♦ Pyribolite: According to the original definition, a high-grade metamorphic rock composed of plagioclase, hornblende, clinopyroxene, orthopyroxene + garnet. The presence of orthopyroxene is essential, according to the original definition, hornblende and pyroxene being present in approximately equal amounts.
♦ Pyriclasite: High-grade metamorphic rock consisting mainly of feldspar (plagioclase) and pyroxene (Cpx and/or Opx) with or without garnet. The presence of Opx is essential according to the original definition.
♦ Pyrigarnite: Initially defined by Vogel (1967) as a high-grade metamorphic rock composed of pyroxenes and garnet, in which the presence of plagioclase may be expressed by a prefix (plagio-pyrigarnite). This definition was modified by Mehnert (1972), and plagioclase was added to the characteristic constituents of pyrigarnite. According to this revised definition pyrigarnite is composed of plagioclase, garnet and pyroxenes (Cpx and/or Opx), the contents of mafic constituents being higher than 30% (vol).
♦ Stronalite: Regional term (from Strona valley in Italy) for high-grade metamorphic rock mainly composed of garnet, feldspar and quartz. Biotite and cordierite may be present, as well as kyanite and sillimanite.
♦ Trappgranulite: Obsolete term for mafic granulites ("pyroxene granulites") mostly of basaltic composition and consisting of plagioclase, quartz, pyroxene (originally described as a micaceous mineral), pyrrhotine and garnet.
♦ Weisstein: Obsolete term for a granulite from the earliest descriptions of these rocks in Saxony.

granulite2020(1).jpg

Kashmir White "Granite" (garnetiferous granulite). Eastern Ghats Orogenic Belt, India. From James St. John.

Granulite from Dalesice, Czech republic. From Petr Hyks.

Granulite from Ivrea Verbano zone, Italy. Own picture.

Felsic (pelitic) granulite with bright blue cordierite layers. Kottavattom, India. From Eleanore Blereau.

Incipient charnockitisation (charnockite patches in a granulite gneiss; from granulite to charnockite the modality of garnet decreases with the complete removal of biotite). From Eleanore Blereau.

Bibliography

• Harley, S. L. (1989). The origins of granulites: a metamorphic perspective. Geological Magazine, 126(3), 215-247.
• Harlov, D. E. (2012). The potential role of fluids during regional granulite-facies dehydration in the lower crust. Geoscience Frontiers, 3(6), 813-827.
• Heier, K. S. (1973). A Discussion on the evolution of the Precambrian crust-Geochemistry of granulite facies rocks and problems of their origin. Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences, 273(1235), 429-442.
• O’Brien, P. J. (2006). Type-locality granulites: high-pressure rocks formed at eclogite-facies conditions. Mineralogy and Petrology, 86(3-4), 161-175.
• Pattison, D. R., Chacko, T., Farquhar, J., & McFarlane, C. R. (2003). Temperatures of granulite-facies metamorphism: constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange. Journal of Petrology, 44(5), 867-900.
• Santosh, M. (1991). Role of CO₂ in granulite petrogenesis: evidence from fluid inclusions. Jour. Geosci., Osaka City Univ, 34, 1-53.

Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)
Clinopyroxene, orthopyroxene and plagioclase. Hartmannsdorf (Germany). XPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)
Clinopyroxene, orthopyroxene and plagioclase. Hartmannsdorf (Germany). XPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)
Clinopyroxene (green), orthopyroxene (pink) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 10x (Field of view = 2mm)	Clinopyroxene, Orthopyroxene and plagioclase. Hartmannsdorf (Germany). XPL image, 10x (Field of view = 2mm)	Clinopyroxene, Orthopyroxene and plagioclase. Hartmannsdorf (Germany). XPL image, 10x (Field of view = 2mm)
Clinopyroxene (green), orthopyroxene (pink), Garnet (labeled as Grt) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene, orthopyroxene, Garnet (labeled as Grt) and plagioclase. Hartmannsdorf (Germany). XPL image, 2x (Field of view = 7mm)	Clinopyroxene (green), orthopyroxene (pink), Garnet (labeled as Grt) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)
Clinopyroxene (green), orthopyroxene (pink), Garnet (labeled as Grt) and plagioclase (colorless). Hartmannsdorf (Germany). PPL image, 2x (Field of view = 7mm)	Clinopyroxene, orthopyroxene, Garnet (labeled as Grt) and plagioclase. Hartmannsdorf (Germany). XPL image, 2x (Field of view = 7mm)	Garnet. Hartmannsdorf (Germany). XPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (I order gray) pyroxene and hornblende crystals. Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase (I order gray) pyroxene and hornblende crystals. Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (I order gray) pyroxene and hornblende crystals. Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (I order gray) pyroxene and hornblende crystals. Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (I order gray) pyroxene and hornblende crystals. Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) pyroxene (high relief) and hornblende (brown) crystals. Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase (colorless) and orthopyroxene (beige). Leucocratic Granulite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase and orthopyroxene. Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) and orthopyroxene (beige). Leucocratic Granulite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)
Plagioclase and orthopyroxene. Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Plagioclase (colorless) and orthopyroxene (beige). Leucocratic Granulite, Ivrea Verbano, Italy. PPL image, 2x (Field of view = 7mm)	Plagioclase and orthopyroxene. Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)
Antiperthite unmixing in Plagioclase (the darker spots). Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Antiperthite unmixing in Plagioclase (the darker spots). Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)	Antiperthite unmixing in Plagioclase (the darker spots). Leucocratic Granulite, Ivrea Verbano, Italy. XPL image, 2x (Field of view = 7mm)