(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. A P–T diagram would show chlorite, biotite, garnet, staurolite, kyanite and sillimanite isograds trending to higher temperature with increasing pressure. 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 + 3 quartz → pyrophyllite; chlorite + muscovite + quartz → biotite + Al-rich chlorite + H₂O; chlorite + muscovite + quartz → almandine garnet + biotite + H₂O; chlorite + muscovite + quartz → staurolite + biotite + H₂O; muscovite + quartz → kyanite + K-feldspar + H₂O; and muscovite + quartz → sillimanite + K-feldspar + H₂O. In the low-pressure Buchan series, staurolite and kyanite are replaced by cordierite, and sillimanite appears at lower pressure. The AFM diagram would show the Barrovian and Buchan stability fields; ACF 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 dominates; intergranular and intersertal, where groundmass fills interstices; vesicular and amygdaloidal, from gas bubbles and their infill; and porphyritic, with phenocrysts in a fine matrix. 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.

(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. 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 basalt, higher degrees more mafic or ultramafic melts, and extraction leaves depleted peridotite. These deep mantle processes differ from shallow crustal metamorphism, where prograde argillaceous reactions record lower pressures and temperatures. Yet they are linked: igneous intrusions raise the geothermal gradient and can drive regional metamorphism, while metamorphic dehydration and decarbonation supply volatiles that modify later melting. The way forward is to interpret every magma body by its pressure–temperature path, volatile content, residue, and tectonic setting, thereby connecting metamorphic grade, basaltic volcanism and global magma generation.