(a) What are migmatites? Describe the important types of migmatites and the processes of their formation. (15 marks)

(b) Draw a neat labelled sketch for the albite-anorthite phase diagram (1 atm, dry). Trace the course of crystallization of an initi

Question

(a) What are migmatites? Describe the important types of migmatites and the processes of their formation. (15 marks)

(b) Draw a neat labelled sketch for the albite-anorthite phase diagram (1 atm, dry). Trace the course of crystallization of an initial melt Ab₂₀-An₈₀ within this system. How can you interpret zoning in plagioclase with this system? (15 marks)

(c) (i) Briefly write on the characteristics of I, S, M and A type granites. Elucidate the petrographic and petrogenetic attributes for each type. (12 marks)

(ii) Write a brief account on the Deccan Flood Basalt Volcanism in India. (8 marks)

UPSC Answer Check · Accepted Answer

(a) Migmatites 
Migmatites are common in high-grade metamorphic belts and are mixed rocks produced when a metamorphic rock is partially melted and the melt remains partly segregated. The light felsic melt is leucosome; the dark mafic residue is melanosome; together they form neosome, while the unmelted metamorphic restite is paleosome. The important types are metatexites, coherent rocks in which leucosome forms veins, bands or layers within paleosome, and diatexites or nebulitic migmatites, in which high melt fraction gives a granitic texture and paleosome occurs as enclaves. Formation involves partial melting of metapelites, paragneisses or amphibolites at high temperature and pressure, metamorphic differentiation by melt segregation, and metasomatism or fluid-assisted melting that changes residual composition. The leucosome is commonly granitic to tonalitic, while the melanosome may contain biotite, garnet, amphibole or mafic restite. Indian examples include the Aravalli–Bhilwara and Dharwar cratonic terrains of Rajasthan and Karnataka, where migmatites record crustal anatexis and granite genesis. They thus grade from low-melt metatexites to high-melt diatexites, preserving the transition from metamorphism to igneous melt.

(b) Albite–anorthite diagram 
The dry 1 atm albite–anorthite diagram is a continuous solid-solution diagram. Sketch: x-axis composition from Ab100 (An0) on the left to An100 on the right; Ab20-An80 is plotted at 80% An, close to the An end. y-axis temperature: Ab100 melts at 1118°C and An100 at 1553°C. The upper curve is the liquidus, the lower curve the solidus, and a low-temperature invariant, often labelled a eutectic point, near An50 (about 1350°C) is marked. For an initial melt Ab20-An80, crystallization begins at the liquidus with plagioclase richer in anorthite than the melt, about An90–An95. On cooling, the crystal follows the solidus toward lower An while the residual melt follows the liquidus toward higher Ab. Under equilibrium, crystals re-equilibrate and the final plagioclase approaches the bulk Ab20-An80 composition. Under fractional crystallization, removed crystals make the residual melt move faster toward albite, producing more strongly zoned crystals. The lever rule, fraction solid = (C0-CL)/(CS-CL), is applied using a tie-line between liquidus and solidus at any temperature. Normal zoning, An-rich cores and Ab-rich rims, records continuous cooling and melt evolution; reverse zoning, Ab-rich cores and An-rich rims, implies late addition of anorthite-rich melt, assimilation, volatile effects, or crystal reaction. If diffusion is slow, zoning is preserved; if fast, it is erased.

(c) Granites and Deccan 
(i) I-type granites are igneous-source, usually metaluminous, with hornblende, biotite, quartz and plagioclase, and little excess aluminium; they form by melting of mafic to intermediate crust or mantle-derived material, typical of Cordilleran arcs. I-type may contain mafic enclaves. S-type granites are sedimentary-source, peraluminous, with muscovite, garnet, cordierite or K-feldspar, and form by anatexis of metasediments, as in Himalayan leucogranites. S-type may contain xenocrysts. M-type granites are mantle-derived, oceanic, high in Mg and Fe, with olivine, pyroxene and spinel, and form from partial melting of mantle or ultramafic crust. M-type may be associated with ophiolites. A-type granites are anorogenic, alkaline, often high in incompatible elements, with K-feldspar, Na-plagioclase, biotite, hornblende or pyroxene, and form in rift or post-collisional extensional settings. A-type may be quartz-poor. (ii) The Deccan Traps are Cretaceous–Paleogene continental flood basalts, erupted near 66 Ma, linked to the Reunion mantle plume. The sequence is among the largest flood-basalt provinces on Earth. They are tholeiitic, layered in formations such as Jawhar, Igatpuri and Wai, with thick lava flows, sills and volcaniclastic beds, and their massive volcanism is linked to environmental stress and mass-extinction effects at the K–Pg boundary. CO2 and H2S emissions are linked to climate change and ocean acidification.

Thus, migmatites preserve crustal melting stages that feed granites; the albite–anorthite diagram quantifies plagioclase evolution and zoning; and Indian examples tie global petrogenetic models to local geology.

(a) What are migmatites? Describe the important types of migmatites and the processes of their formation. (15 marks) (b) Draw a…

Model answer

What "Describe" is asking you to do

Structure that answers it

Where marks are lost

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Practice this exact question

More from Geology 2025 Paper II