(a) Describe the mineral reactions in prograde metamorphism of argillaceous sedimentary rocks with appropriate diagrams. (15…

(a) Prograde metamorphism of argillaceous rocks. In prograde Barrovian metamorphism, an argillaceous sediment evolves through predictable mineral isograds as temperature and pressure rise. The accompanying P–T diagram shows the isograds (chlorite, biotite, garnet, staurolite, kyanite and sillimanite) and the contrasting Barrovian (high-P) and Buchan (low-P) paths. Approximate conditions are pyrophyllite at 200–250°C and 2–3 kbar, chlorite at 200–300°C and 2–4 kbar, biotite at 300–400°C and 3–5 kbar, garnet at 400–500°C and 4–6 kbar, staurolite at 500–600°C and 5–7 kbar, kyanite at 550–650°C and 5–8 kbar, and sillimanite above 600–650°C and 4–8 kbar. The key reactions are: kaolinite + 2 quartz → pyrophyllite + H₂O; chlorite + muscovite + quartz → biotite + Al-rich chlorite + H₂O; chlorite + muscovite + quartz → almandine garnet + biotite + H₂O; chlorite + muscovite + quartz → staurolite + biotite + H₂O; staurolite + muscovite + quartz → kyanite + biotite + H₂O; kyanite → sillimanite (polymorphic, at the first sillimanite isograd); and, at the second sillimanite isograd, muscovite + quartz → sillimanite + K-feldspar + H₂O (about 650 °C), beyond which partial melting (migmatite formation) begins. In the low-pressure Buchan series, staurolite and kyanite are replaced by cordierite, and sillimanite appears at lower pressure. The AFM projection (below) tracks the successive pelite assemblages — chlorite + muscovite + biotite → garnet + biotite → staurolite + garnet + biotite → kyanite + staurolite + biotite → sillimanite + biotite + K-feldspar — while cordierite replaces staurolite and kyanite in the low-pressure Buchan type; the ACF diagram is used for Ca-bearing mafic rocks, not for argillaceous rocks, so it is not the primary pelite diagram.

(b) Basalt mineralogy, texture and origin. Basalt is a mafic volcanic rock, commonly composed of labradorite–bytownite plagioclase (An₅₀₋₈₀), clinopyroxene augite, olivine, and a glassy or fine-grained groundmass. Its textures include ophitic, where pyroxene surrounds plagioclase laths; subophitic, where plagioclase laths are only partly enclosed by pyroxene; intergranular and intersertal, where groundmass fills interstices; vesicular and amygdaloidal, from gas bubbles and their infill; and porphyritic, with phenocrysts in a fine matrix; diktytaxitic and glomeroporphyritic varieties also occur. Deccan trap basalts show ophitic and subophitic textures. In India, Deccan trap basalts record plume-related decompression melting. Basaltic magma forms mainly by decompression melting at mid-ocean ridges: hot peridotite rises adiabatically, pressure falls, and the material crosses its solidus. The mantle potential temperature, typically near 1350–1400°C, must exceed the solidus at ridge pressures; a small degree of partial melting yields basaltic melt. At ridges about 10–20% melting of spinel lherzolite leaves a harzburgite residue; beneath plumes, deeper garnet-bearing peridotite melts to give OIB/alkali basalts and large igneous provinces such as the Deccan, where plume-related decompression is supplemented by rifting.

(c) Magma generation in the Earth’s interior. Magma generation is controlled by pressure, temperature, composition and volatiles. Decompression melting occurs where mantle rises, such as at mid-ocean ridges, mantle plumes and continental rifts; no external heat is required, but the solidus is crossed because pressure drops. Flux melting is important above subduction zones, where water released from the subducting slab lowers the solidus of the overlying mantle wedge; CO₂, released mainly by decarbonation of carbonates, can also lower melting temperatures. Heat-transfer melting occurs where hot mantle or crustal rocks conduct or convectively heat adjacent material, as beneath plume heads or in thickened crust. Crustal anatexis — melting of thickened or water-fluxed crust — yields granitic melts and migmatites. The causes are therefore tectonic: ridge upwelling, plume activity (Hawaii, Deccan), subduction-related hydration, and rifting. Mantle convection supplies the long-term heat and motion that sustain these settings, while local tectonics control where melting is focused. Depth–temperature constraints are critical: asthenospheric melting commonly occurs near 1300–1400°C, while garnet lherzolite residues indicate deeper, higher-pressure melting and spinel lherzolite residues indicate shallower melting. The degree of partial melting and melt extraction determine melt composition: low degrees produce alkali basalt, moderate degrees basalt, higher degrees more mafic or ultramafic (picritic/komatiitic) melts, and extraction leaves depleted peridotite.