Granulite (retrograde)

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.

Retrograde metamorphism: is the mineralogical adjustment of relatively high-grade metamorphic rocks to temperatures lower than those of their initial metamorphism, in which minerals characteristic of a lower metamorphic grade developed at the expense of minerals formed at a higher metamorphic grade. Retrograde metamorphism is also known as diaphthoresis (from the Greek "to degrade"); the term was first applied by Becke (1909) to a phyllonite containing relict high-grade metamorphic minerals.

In general, the changes in mineral assemblage and mineral composition that occur during burial and heating are referred to as prograde metamorphism, whereas those that occur during uplift and cooling of a rock represent retrograde metamorphism. If thermodynamic equilibrium were always maintained, one might expect all the reactions that occur during prograde metamorphism to be reversed during subsequent uplift of the rocks and re-exposure at Earth's surface; in this case, metamorphic rocks would never be seen in outcrop. Two factors mitigate against complete retrogression of metamorphic rocks during their return to Earth's surface:

(1) The efficient removal of the water and carbon dioxide released during prograde devolatilization reactions by upward migration of the fluid along grain boundaries and through fractures. Because almost all the water released during heating by reactions (such as when chlorite reacts with quartz to yield garnet and water) is removed from the site of reaction, the reaction cannot be reversed during cooling unless water is subsequently added to the rock. Thus, garnet can be preserved at Earth's surface even though it is thermodynamically unstable at such low temperatures and pressures.

(2) Metamorphic reactions do not typically operate in reverse during cooling is that reaction rates are increased by rising temperatures.

During cooling, reaction kinetics become sluggish, and metastable mineral assemblages and compositions can be preserved well outside their normal stability fields. Thus, prograde reactions are generally more efficient than retrograde reactions. It is common, however, to find at least some signs of retrogression in most metamorphic rocks. For example, garnets are often rimmed by small amounts of chlorite and quartz, indicating that limited quantities of water were available for the reverse of the reaction during cooling.

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
Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 2x (Field of view = 7mm)	Kelifitic mixtrure after garnet (the fibrous, gray material), biotite and clinopyroxene crystals. PPL image, 2x (Field of view = 7mm)	Kelifitic mixtrure after garnet (the fibrous, gray material), biotite and clinopyroxene crystals. XPL image, 2x (Field of view = 7mm)
Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 10x (Field of view = 2mm)	Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 10x (Field of view = 2mm)	Kelifitic mixtrure after garnet (the fibrous, gray material) and irregular clinopyroxene crystals. PPL image, 10x (Field of view = 2mm)
Kelifitic mixtrure after garnet (the fibrous, gray material) and irregular clinopyroxene crystals. PPL image, 10x (Field of view = 2mm)	Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 2x (Field of view = 7mm)	Garnet (isotropic) with kelifitic corona and biotite crystals. XPL image, 2x (Field of view = 7mm)
Garnet (isotropic) with kelifitic corona, clinopyroxene and biotite crystals. XPL image, 2x (Field of view = 7mm)	Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 2x (Field of view = 7mm)	Garnet (pale beige) with kelifitic corona and biotite crystals. PPL image, 2x (Field of view = 7mm)