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

Metamorphism, facies and pelite 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 at low T and high P, producing blueschist or eclogite 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, represented on ACF/AKF diagrams by univariant reaction curves, 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, not UHT >900°C) stabilizes sillimanite, garnet, K-feldspar and often orthopyroxene, while kyanite and staurolite are no longer stable.

Zoning 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 pressure rises, stabilizing 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.

Anorthosites: characters and petrogenesis. Anorthosites are coarse-grained rocks with >90% plagioclase, usually An-rich. Archean massif-type anorthosites are massive, cumulate, calcic (An>90), with minor clinopyroxene, magnetite/ilmenite and sometimes Fe–Ti oxide ores. Proterozoic anorthosite–mangerite–charnockite–rapakivi suite anorthosites show antiperthitic plagioclase, rapakivi textures, mangerite and charnockite associations, and Fe–Ti oxide mineralization. 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 AMCR-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 Archean Sittampundi and Proterozoic Bolangir anorthosites. Together, metamorphic facies, plagioclase zoning and anorthosite textures record P–T–t paths, magma evolution and crustal differentiation.