Granulite Terrains of India | Geology Optional Notes for UPSC PDF Download

Granulites Overview

• Granulite is a type of high-grade metamorphic rock characterized by the dominance of hydroxyl-free Fe-Mg-silicates.
• The presence of feldspar is crucial in granulites, while primary muscovite is typically absent. Cordierite may also be found in some instances.
• Major constituents in granulites are indicated by prefixing their names.

Mafic Granulites

• Rocks with over 30% mafic minerals, predominantly pyroxene, are termed mafic granulites.
• These rocks exhibit specific mineral compositions that distinguish them within the granulite category.

Felsic Granulites

• Felsic granulites contain less than 30% mafic minerals, mainly pyroxene.
• They possess distinct characteristics that differentiate them from mafic granulites.

Examples

• An example of a mafic granulite could be a rock rich in pyroxene and other mafic minerals, with a high metamorphic grade.
• Conversely, a felsic granulite might have a lower proportion of mafic minerals, displaying different mineralogical properties compared to mafic granulites.

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Factors controlling petrogenesis of granulites

• Facies: Granulites are part of the granulite facies, characterized by high temperatures (around 700 degrees Celsius as the lower limit) and moderate to high pressure (3 - 15 kbar, approximately 10 - 50 km depth) mineral assemblages. These conditions typically align with the upper sections of the Barrovian sillimanite zone and, at even higher temperatures, the cordierite-garnet zone. Lowering pressure can lead to the transformation of granulite facies into the pyroxene and sanidinite hornfels facies. In orogenic belts, increasing temperature conditions can give rise to migmatites of the granulite facies.
• Mineral Assemblages: In granulite facies, the minerals are mainly anhydrous due to dehydration reactions occurring at high temperatures. While hydrous minerals like hornblende and biotite may be present in the lower part (referred to as granulite I), the upper part (granulite II) is characterized by entirely anhydrous minerals. For instance, amphibole minerals dehydrate into pyroxene minerals, and phyllosilicate minerals transform into anhydrous minerals in response to elevated temperatures. Common mineral assemblages in these facies include plagioclase, K-feldspar, and aluminum-rich pyroxenes.
• Tectonic Setting: Granulite facies metamorphism typically occurs in regions of high-temperature dynamothermal metamorphism, such as convergent plate boundaries, the base of thick continental crust, and the upper mantle. Some basic granulites may signify the residual rock material left after partial melting at the base of the continental lithosphere. These rocks are commonly found in Precambrian shields and associated anorthosite complexes, where prolonged erosion has brought deep-seated rocks to the surface. Granulite facies rocks are prevalent in ancient deep to middle continental crust that has undergone significant dehydration.

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Petrogenesis of Granulites

• Overview: Granulite terrains exhibit significant variability, presenting a range of features that require explanation in petrology.
• Key Petrological Challenge: The primary issue involves the stability of typical granulite-facies assemblages, which demand very low PH2O compared to the total pressure (Ptotal).
• Models of Petrogenesis:
  • Model 1: Melting phenomena occur where water from intergranular fluid and hydrous phases is absorbed into the melt. Subsequently, the melt, carrying the water, can be eliminated.
  • Model 2: CO2-rich vapor originating externally permeates the rocks, inducing dehydration reactions and dispersing the resulting aqueous fluid.
  • Model 3: In specific terrains, precursor rocks were inherently dry, such as water-deficient magmatic rocks and their dehydrated thermal aureoles. Consequently, their subsequent high-grade metamorphism does not necessitate a distinct mechanism to produce granulite-facies assemblages.

Granulite Terrains Overview

Depletion in Large-Ion Lithophile Elements (LILE)

• In the 1970s, a significant concern was the depletion in LILE in granulite terrains, particularly in the Lewisian gneisses. This depletion was initially attributed to either melt extraction or deep-seated fluid metasomatism.
• It is now understood that the Lewisian gneisses are unique in their exceptional depletion of LILE, while many other granulite terrains exhibit smaller depletions.

CO2-Rich Fluid Inclusions

• A seminal study in southern Norway by Touret revealed a transition from water-rich to CO2-rich fluid inclusions coinciding with the regional orthopyroxene isograd. This pattern is generally observed in granulite terrains, which vary in the abundance and prevalence of fluid inclusions.

Arrested Charnockitisation

• Initially observed in South India and Sri Lanka, arrested charnockitisation is characterized by granulite-facies assemblages (charnockite) occurring in vein-like structures over amphibolite-facies gneisses.
• Similar features, although less prominent, have been identified in various other terrains. These occurrences are often linked to CO2 streaming but may have alternative explanations, emphasizing that the amphibolite-granulite transition is not solely governed by changes in temperature or pressure.

Petrogenetic Models of Granulites

• Model 1: Melting and Melt Extraction
  • The first model, proposed by Fyfe in 1973, suggests that granulites are residual rocks left behind after the extraction of partial melt.
  • Dehydration melting, which involves the incongruent melting of mineral assemblages with hydrous phases like Ms, Bt, and Hbl, is considered a significant process in high-grade metamorphism.
  • Examples of this model include occurrences in Broken Hill, Australia, New England, and Namaqualand, South Africa.
  • Recent studies do not always imply the removal of large amounts of melt in this model.
• Model 2: Influx of CO2-Rich Vapour
  • This model, supported by R.C. Newton at Chicago, suggests that granulites in south India were formed due to the infiltration of CO2-rich vapor.
  • Key locations for this model include stone quarries in south India and the Kerala "khondalite belt" with occurrences possibly dating back to the Pan-African age.
• Model 3: Metamorphism of Dry Precursors
  • This model, observed in the Adirondack Highlands of upstate New York, involves the metamorphism of pre-existing pyroxene-bearing igneous rocks and their thermal aureoles, resulting in granulite-facies assemblages of Grenville age.