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

(b) Write the mineralogy and texture of basalt. How does basaltic magma form in deep earth ? (15 marks)

(c) Discuss

Question

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

(b) Write the mineralogy and texture of basalt. How does basaltic magma form in deep earth ? (15 marks)

(c) Discuss the process of magma generation in the Earth's interior and its causes. (20 marks)

UPSC Answer Check · Accepted Answer

The directive 'discuss' in part (c) demands a comprehensive, analytical treatment of magma generation processes, while parts (a) and (b) require descriptive depth. Allocate approximately 30% time/words to (a) on prograde reactions with clear P-T diagrams, 30% to (b) on basalt mineralogy and magma genesis, and 40% to (c) discussing melting mechanisms, heat sources, and tectonic settings. Structure with brief introductions for each part, systematic body coverage, and a concluding synthesis linking metamorphism, basaltic volcanism, and global magma generation.
- Part (a): Progressive mineral reactions in argillaceous rocks — kaolinite → pyrophyllite → kyanite/sillimanite; chlorite → biotite → garnet; muscovite → K-feldspar; with P-T conditions for each isograd
- Part (a): AFM and ACF diagrams showing mineral stability fields across Barrovian and Buchan metamorphic series
- Part (b): Basalt mineralogy — plagioclase (labradorite-bytownite), clinopyroxene (augite), olivine, glass/groundmass; textures — ophitic, subophitic, intergranular, intersertal, vesicular, amygdaloidal, porphyritic
- Part (b): Basaltic magma formation — decompression melting at mid-ocean ridges, adiabatic ascent of peridotite; potential temperature and solidus relationships
- Part (c): Magma generation mechanisms — decompression melting, flux melting (addition of H₂O/CO₂), heat transfer melting; causes including mantle plumes, subduction zone processes, continental rifting
- Part (c): Depth-temperature constraints — asthenosphere melting at 1300-1400°C, presence of garnet lherzolite vs. spinel lherzolite residues; degree of partial melting and melt extraction
- Integrated understanding: Link between metamorphic grade, geothermal gradient, and igneous activity; contrast shallow crustal metamorphism with deep mantle melting processes

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Accurately describes all mineral reactions with correct chemical formulas and P-T conditions for (a); precisely identifies basalt minerals and textures with correct genetic interpretation for (b); comprehensively explains three melting mechanisms with accurate depth-temperature constraints and mantle source characteristics for (c)	Identifies major mineral changes but confuses reaction sequences or P-T ranges; lists basalt minerals with minor errors and describes textures without genetic link; mentions melting mechanisms but conflates causes or lacks depth specificity	Incorrect mineral reactions or wrong metamorphic grade assignments; fundamentally wrong basalt mineralogy or confused texture terminology; misunderstands magma generation mechanisms or attributes melting to wrong sources
Diagram / cross-section	20%	10	Draws clear P-T-t paths with labeled isograds and reaction curves for (a); includes AFM/ACF compatibility diagrams; sketches accurate basalt thin-section textures for (b); presents cross-section of mantle melting zones with decompression paths for (c)	Provides basic P-T diagram with some labels; attempts texture sketches but lacks detail; shows generic melting diagram without specific tectonic context	Missing diagrams for (a); unlabeled or incorrect diagrams; no visual representation of textures or melting processes; diagrams contradict text description
Field evidence	15%	7.5	Cites Barrovian zones in Scottish Highlands or equivalent for (a); references Deccan Traps basalt characteristics, pillow lava structures, or amygdale fillings for (b); mentions ophiolite complexes, xenolith studies, or seismic tomography evidence for mantle melting for (c)	General reference to metamorphic terrains without specific locality; mentions basalt occurrences without Indian context; vague reference to volcanic activity without linking to deep processes	No field examples cited; hypothetical or invented localities; confuses field relationships between metamorphic and igneous rocks
Quantitative reasoning	20%	10	Provides specific P-T values (e.g., 300-800°C, 3-10 kb for Barrovian); gives precise melting temperatures and depths (e.g., 1300°C at 60-80 km for dry peridotite); mentions degrees of partial melting (5-20% for MORB); includes geothermal gradient calculations	Rough temperature ranges without pressure correlation; approximate melting depths; mentions 'high temperature' without values; qualitative treatment of partial melting	No quantitative data; grossly incorrect values; confuses pressure and temperature units; treats all melting as same temperature regardless of mechanism
Indian / economic relevance	20%	10	Links argillaceous metamorphism to Eastern Ghats mobile belt or Kerala khondalites for (a); discusses Deccan Traps basalt resources, building stones, or groundwater in vesicular basalts for (b); connects magma generation to Indian hotspot (Réunion), Laccadives-Chagos ridge, or economic mineralization associated with mafic magmatism for (c)	Brief mention of Deccan Traps without elaboration; general statement about metamorphic belts in India; superficial connection between magma and mineral resources	No Indian examples; irrelevant economic points; misses opportunity to cite significant Indian geological features despite their global importance

Q3

Directive word: Discuss

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Approach

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