(a) What are different types of metamorphism and what are their controlling factors ? State characteristic mineral assemblages…

(a) Types of metamorphism, controlling factors and pelitic assemblages

Metamorphism is the solid-state transformation of rocks by heat, pressure, stress and fluids. The main types are contact, regional, dynamic, hydrothermal, ocean-floor, burial and impact/shock metamorphism. Contact metamorphism is dominated by heat from an intrusion; regional metamorphism reflects large-scale P–T–t changes in orogens; dynamic metamorphism is controlled by shear and faulting; hydrothermal metamorphism by fluid-rock reaction; ocean-floor metamorphism occurs under high heat flow and low P near mid-ocean ridges, producing zeolite to greenschist (spilitic) assemblages; burial metamorphism reflects increasing lithostatic pressure; impact/shock metamorphism records sudden pressure and temperature spikes. The controlling factors are pressure, temperature, time, differential stress, fluid composition and rock composition. Fluids lower the solidus and promote reaction. In regional pelites, the Barrovian sequence of index-mineral zones, displayed on AFM (Thompson) projections, records progressive metamorphism. The chlorite zone (greenschist facies, ~300–450°C) contains chlorite, muscovite, plagioclase and quartz; the biotite zone adds biotite; the garnet zone adds almandine garnet; the staurolite zone adds staurolite; the kyanite zone adds kyanite. These assemblages pass from greenschist to amphibolite facies. Sillimanite first appears by the polymorphic transition from kyanite or andalusite at the sillimanite isograd; at higher T, about 650–700°C, the second sillimanite isograd is marked by muscovite + quartz → sillimanite + K-feldspar + H2O, i.e. muscovite breakdown. In granulite facies pelites, high T (typically ~700–900°C) stabilizes sillimanite, garnet, K-feldspar, cordierite and often orthopyroxene, while staurolite and muscovite are no longer stable. In India, Barrovian zones are seen in the Lesser Himalaya of Sikkim and Garhwal (inverted across the Main Central Thrust), and garnet-sillimanite khondalites of granulite facies in the Eastern Ghats and the Kerala khondalite belt, with sapphirine-bearing ultra-high-temperature rocks locally in the Eastern Ghats.

(b) Zoning in minerals and in plagioclase

Zoning is compositional variation within a crystal. Normal zoning has a more An-rich core and Ab-rich rim; reverse zoning has an An-rich rim; oscillatory zoning shows repeated compositional bands; sector zoning has different compositions in different crystal sectors. In the Albite–Anorthite system, plagioclase forms a continuous solid solution; the liquidus and solidus converge at the end-members, and tie-lines connect coexisting melt and crystal. During fractional crystallization along Bowen’s reaction series, an early An-rich core forms, and the residual melt becomes Ab-rich, producing normal zoning.

Reverse zoning may form when a more mafic, An-rich melt is mixed in, or when the water content of the melt rises, which stabilizes more An-rich plagioclase. Oscillatory zoning records repeated changes in melt composition, temperature, pressure or growth rate, often due to magma mixing or convection. Sector zoning in plagioclase forms when different crystal faces or sectors grow under locally different melt conditions, or when rapid growth and variable diffusion cause different faces to incorporate different An or trace elements; it can also be produced by local fluid or melt pockets at the crystal surface. Slow cooling and fast diffusion tend to homogenize compositions, whereas rapid cooling preserves sharp zoning. Resorption and sieve textures show that crystals can be partially dissolved or modified by later melts.

(c) Petrographic types and petrogenesis of anorthosites

Anorthosites are coarse-grained rocks with >90% plagioclase, usually An-rich. Archaean (megacrystic) anorthosites are layered, calcic (about An80–95), with equant plagioclase megacrysts, minor clinopyroxene and hornblende, and characteristic chromitite layers. Proterozoic massif-type anorthosites of the anorthosite–mangerite–charnockite–granite (AMCG) suite are andesine–labradorite (about An40–65), coarse to megacrystic, often with protoclastic (crushed) margins, high-Al orthopyroxene megacrysts and Fe–Ti oxide ores, and are associated with mangerite, charnockite and rapakivi granite. Layered-intrusion anorthosites occur as cumulate layers with ophitic or intercumulate textures, Fe–Ti oxide layers, and are associated with mafic–ultramafic sequences. Petrogenetically, Archean calcic anorthosites formed by crystal accumulation and flotation of plagioclase from basaltic or tholeiitic magmas, emplaced as crystal mushes in Archean crust. Proterozoic AMCG-suite anorthosites are linked to post-orogenic extension and plagioclase flotation/mush emplacement. Layered-intrusion anorthosites formed in situ by crystal settling, fractionation and magma recharge in layered mafic intrusions, as in the Bushveld and Stillwater complexes, where anorthosite layers record plagioclase cumulate formation. Indian examples include the Archaean Sittampundi layered complex in Tamil Nadu with its chromitites, and the Proterozoic massif anorthosites of Oddanchatram (Tamil Nadu) and Chilka Lake and Bolangir in the Eastern Ghats.