Briefly write on the characteristics of I, S, M and A type granites. Elucidate the petrographic and petrogenetic attributes for each type.

Classification of Granites: I, S, M, and A Types

The I, S, M, and A-type classification categorizes granites based on their inferred genesis, reflecting distinct source rocks, magmatic processes, and tectonic environments.

1. I-type Granites (Igneous Protolith)

I-type granites are believed to originate from the partial melting of igneous source rocks, such as metaigneous rocks or mafic to intermediate igneous crustal rocks, that have not undergone surface weathering processes. They are often associated with active continental margins and subduction zones.

• Petrographic Attributes:
  • Mineralogy: Typically metaluminous to weakly peraluminous. They contain hornblende and biotite as dominant mafic minerals. Accessory minerals include sphene (titanite), allanite, magnetite, zircon, and apatite. Muscovite is generally absent, or if present, is secondary.
  • Texture: Can be aphanitic to phaneritic, often porphyritic with feldspar phenocrysts. Plagioclase often shows zoning and albite twinning. Quartz and potassium feldspar rarely exhibit granophyric textures. Mafic igneous xenoliths are common.
  • Color: Usually lighter in color, reflecting lower mafic mineral content compared to S-types.
• Petrogenetic Attributes:
  • Source: Derived from the partial melting of an igneous protolith (metaigneous rocks) in the lower continental crust or from fractional crystallization of mafic magmas.
  • Geochemistry: Characterized by relatively high sodium (Na₂O > 3.2% in felsic varieties), calcium, and a relatively low aluminium saturation index (ASI = Al₂O₃ / (CaO + Na₂O + K₂O) < 1.1). They are typically metaluminous to weakly peraluminous, meaning Al₂O₃ < (Na₂O + K₂O + CaO). Initial ⁸⁷Sr/⁸⁶Sr ratios are low (typically < 0.708). They are generally more oxidized.
  • Tectonic Setting: Commonly found in continental arcs and active continental margins related to subduction zones (e.g., Sierra Nevada batholith, USA).

2. S-type Granites (Sedimentary Protolith)

S-type granites are interpreted to have formed from the partial melting of supracrustal (sedimentary or metasedimentary) source rocks that have undergone one or more cycles of weathering and erosion, leading to enrichment in alumina.

• Petrographic Attributes:
  • Mineralogy: Strongly peraluminous, characterized by the presence of primary muscovite, biotite, cordierite, garnet, sillimanite, and tourmaline. K-feldspar is often white. Accessory minerals include monazite, ilmenite, and abundant zircon. Hornblende is typically absent.
  • Texture: Variable crystal size, often porphyritic. May contain metamorphic xenoliths/enclaves of the sedimentary source rock, and large, restite milky quartz inclusions.
  • Color: Often light-colored due to high felsic content, but can vary.
• Petrogenetic Attributes:
  • Source: Derived from the partial melting of sedimentary or metasedimentary rocks (e.g., shales, greywackes) in the middle to lower continental crust.
  • Geochemistry: Characterized by relatively low sodium (Na₂O < 3.2% in felsic rocks) and elevated aluminium, with an ASI > 1.1 (strongly peraluminous), and often contain normative corundum (>1%). Initial ⁸⁷Sr/⁸⁶Sr ratios are typically higher (> 0.708 to 0.715), reflecting the long crustal residence time of the sedimentary source. They are generally more reduced.
  • Tectonic Setting: Frequently associated with continental collision zones and deeply eroded cores of fold-thrust mountain belts (e.g., Himalayas, Lachlan Fold Belt, Australia).

3. M-type Granites (Mantle-derived Protolith)

M-type granites are rare and thought to originate from the fractional crystallization of mafic magmas derived directly from the mantle, representing the most primitive end-member of granitic rocks.

• Petrographic Attributes:
  • Mineralogy: Typically metaluminous. Mineral assemblages often resemble those of I-type granites, with biotite and hornblende. Pyroxenes might be present.
  • Texture: Often fine-grained, reflecting rapid cooling or a greater influence of mafic magmatic processes.
• Petrogenetic Attributes:
  • Source: Formed by extensive fractional crystallization of mantle-derived basaltic magmas.
  • Geochemistry: Similar to island arc volcanic rocks, with chemical and isotopic compositions reflecting a mantle source. They are generally rich in MgO and FeO.
  • Tectonic Setting: Primarily found in oceanic island arcs and active continental margins where mafic magmas are common. They are less common in continental settings due to the difficulty of producing granite through fractional crystallization of basalt.

4. A-type Granites (Anorogenic/Alkaline/Anhydrous)

A-type granites are characterized by their anorogenic (non-orogenic) tectonic setting and alkaline or anhydrous nature. They are often associated with extensional environments, such as continental rifts or hotspots.

• Petrographic Attributes:
  • Mineralogy: Can range from metaluminous to peralkaline, and sometimes peraluminous. Distinctive for their iron-rich mafic silicates like fayalite (Fe-olivine), ferrohedenbergite (Fe-pyroxene), annite (Fe-biotite), and ferrohastingsite (Fe-amphibole). In peralkaline varieties, sodic amphiboles (riebeckite, arfvedsonite) and sodic pyroxenes (aegirine) are common. They are relatively anhydrous.
  • Texture: Often coarse-grained and massive (lacking internal structures). Perthitic feldspars are common.
• Petrogenetic Attributes:
  • Source: Believed to form from the partial melting of dry, refractory (residue) granulitic lower continental crust, often after the extraction of orogenic granites, or from fractionation of mantle-derived mafic melts in an extensional setting. High heat flow is crucial, often from upwelling asthenosphere.
  • Geochemistry: High in silica (SiO₂), total alkalis (Na₂O + K₂O), Fe/Mg ratio, and high field strength elements (HFSE) like Zr, Nb, Ga, Y, and REE (especially light REE). They have low CaO, MgO, Sr, and Ba. Characteristically high Ga/Al ratios. Zircon saturation temperatures are often high (900-1000°C).
  • Tectonic Setting: Predominantly found in continental rifts, hotspots, post-collisional, or intraplate extensional environments (e.g., Yellowstone Caldera rhyolites, Basin & Range Province, USA).

Comparative Summary of Granite Types

The table below summarizes the key attributes of I, S, M, and A-type granites:

Feature	I-type (Igneous)	S-type (Sedimentary)	M-type (Mantle)	A-type (Anorogenic/Alkaline)
Protolith/Source	Metaigneous rocks of crustal origin (e.g., basalt, amphibolite)	Metasedimentary rocks (e.g., shales, greywackes)	Fractional crystallization of mantle-derived basaltic magma	Dry, refractory lower crustal granulites; fractionated mafic melts
Alumina Saturation Index (ASI)	< 1.1 (Metaluminous to weakly peraluminous)	> 1.1 (Strongly peraluminous)	~ 1 (Metaluminous)	Variable: metaluminous to peralkaline, sometimes peraluminous
Key Mafic Minerals	Biotite, Hornblende	Biotite, Muscovite, Cordierite, Garnet, Sillimanite, Tourmaline	Biotite, Hornblende (similar to I-type)	Fe-rich biotite, amphibole, pyroxene, fayalite; sodic amphiboles/pyroxenes (peralkaline)
Accessory Minerals	Sphene, Allanite, Magnetite, Zircon, Apatite	Monazite, Ilmenite, Zircon, Apatite, Sulfides	Similar to I-type (e.g., Magnetite, Ilmenite)	Fluorite, Zircon, Nb-minerals, REE-minerals
Xenoliths	Mafic igneous xenoliths	Metasedimentary xenoliths (restite)	Rare, often absent	Rare, often absent
Na2O Content	High (>3.2% in felsic types)	Low (<3.2% in felsic types)	Moderate to high	High (Total alkalis Na2O+K2O > Al2O3 in peralkaline)
87Sr/86Sr Initial Ratio	Low (< 0.708)	High (> 0.708)	Low (Mantle-like)	Variable, often low to intermediate
Redox State	More oxidized	More reduced	Oxidized	Variable (can be anhydrous)
Tectonic Setting	Continental arcs, active continental margins (subduction zones)	Continental collision zones, orogenic belts	Oceanic island arcs, active continental margins	Continental rifts, hotspots, post-collisional, intraplate extensional