(a) Discuss briefly the processes of magma generation in the Earth's interior. How is grain size of an igneous rock related to…

(a) Magma generation, grain size and differentiation

Magma forms when rock crosses its solidus through decompression, added volatiles or added heat. Decompression melting occurs where hot peridotite upwells beneath mid-ocean ridges, hotspots (the Réunion plume behind the Deccan Traps) and rifts: pressure falls faster than temperature, giving basaltic melt. Flux melting occurs above subducting slabs: water released from the slab lowers the solidus of the mantle wedge, giving hydrous, calc-alkaline magma. Heat transfer occurs when mantle-derived magma ponds at the base of thickened crust, or radiogenic heating warms it, so that crust melts at about 650–800 °C to give granitic melt.

Grain size is inversely related to cooling rate. Slow cooling in plutons gives few nuclei and time for growth, so crystals are coarse (phaneritic granite, gabbro). Rapid cooling in lava gives many nuclei and little growth, so crystals are fine (aphanitic basalt) or absent (obsidian glass). Two-stage cooling, slow at depth and then fast on eruption, gives porphyritic texture with phenocrysts in a fine groundmass.

Fractional crystallization removes early-formed olivine, pyroxene and Ca-rich plagioclase (Bowen's reaction series) by settling or filter pressing. The residual melt becomes richer in silica, alkalis and volatiles and evolves from basalt through andesite to rhyolite. Assimilation is a different process: the magma dissolves country rock, which uses up heat and so promotes crystallization, while adding crustal components to the melt. Together (AFC) they explain a range of compositions from one parent magma; some Deccan Trap flows carry a crustal contamination signature.

(b) Structures and textures of metamorphic rocks

Four common types are foliation, lineation, porphyroblastic texture and granoblastic texture.

Foliation is planar alignment of platy minerals (mica, chlorite) by recrystallization under directed stress, with platy minerals growing perpendicular to the maximum compressive stress. It grades from slaty cleavage through schistosity to gneissic banding as grade rises, as in the banded Chotanagpur gneisses. Sketch: parallel mica flakes in schist and alternating light and dark bands in gneiss.

Lineation is linear alignment of elongate minerals (hornblende, sillimanite) or stretched grains, formed by shear or stretching along the extension direction. Sketch: aligned needles on a foliation plane.

Porphyroblastic texture has large new crystals (garnet, staurolite) growing in a finer matrix during metamorphism; porphyroblasts may rotate during shear, leaving curved inclusion trails. Sketch: garnet with spiral inclusions in a schist matrix.

Granoblastic texture is equigranular, interlocking polygonal grains without preferred orientation, formed by high-temperature recrystallization under little differential stress, as in Eastern Ghats khondalites and charnockites and in quartzite or marble. Sketch: polygonal grains meeting at about 120° triple junctions.

(c) Migmatite and granite origin

A migmatite is a mixed rock of unmelted metamorphic rock (melanosome or paleosome) and light, granite-like leucosome, formed by partial melting (anatexis) of crust at high grade. If melt is scarce and the older fabric survives, it is a metatexite; if melt is abundant and structures are disrupted, it is a diatexite, which grades into granite.

Migmatites show that granitic melt can be generated from crustal rock by anatexis, segregated and moved, giving S-type granites, in contrast to granites from mantle-derived magmas. The Higher Himalayan Crystallines grade into migmatites that are linked to Himalayan leucogranites such as Manaslu and Gangotri, and migmatitic gneisses of the Chotanagpur belt show the same link.