Geology 2025 Paper II 50 marks Describe

Q3

(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)

हिंदी में प्रश्न पढ़ें

(a) मिग्मेटाइट क्या होते हैं? मिग्मेटाइट के महत्त्वपूर्ण प्रकारों तथा उनके बनने की प्रक्रियाओं का वर्णन कीजिए। (15 अंक) (b) स्वच्छ नामांकित आरेख की सहायता से एल्बाइट-एनॉर्थाइट का कल्पित चित्र (1 वायुमंडलीय दाब, शुष्क) दर्शाइए। एक प्रारंभिक गलित जिसमें Ab₂₀-An₈₀ है, उसके क्रिस्टलीभवन पथ का इस पद्धति में अनुरेखण कीजिए। इस पद्धति में प्लेजियोक्लेज के मंडलन की व्याख्या कैसे की जा सकती है? (15 अंक) (c) (i) I, S, M और A प्रकार के ग्रेनाइट की विशेषताओं को संक्षेप में लिखिए। प्रत्येक की पेट्रोग्राफिक व पेट्रोजेनेटिक विशेषताओं पर प्रकाश डालिए। (12 अंक) (ii) भारत के डेक्कन फ्लड बेसाल्ट ज्वालामुखी-क्रिया का संक्षिप्त विवरण लिखिए। (8 अंक)

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Directive word: Describe

This question asks you to describe. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

See our UPSC directive words guide for a full breakdown of how to respond to each command word.

How this answer will be evaluated

Approach

This multi-part descriptive question requires systematic coverage of four distinct topics. Allocate approximately 30% time/words to part (a) on migmatites, 30% to part (b) on phase diagrams with emphasis on the sketch, 25% to part (c)(i) on granite typology, and 15% to part (c)(ii) on Deccan volcanism. Begin each part with clear definitions, develop with process explanations and examples, and conclude with petrogenetic significance where applicable.

Key points expected

  • Part (a): Definition of migmatites as mixed rocks with neosome (leucosome + melanosome) and paleosome; classification into metatexis (coherent) and diatexis (nebulitic); formation processes including partial melting, metamorphic differentiation, and metasomatism; examples from Indian Precambrian terrains like Rajasthan or Karnataka
  • Part (b): Correctly drawn binary phase diagram with liquidus and solidus curves, eutectic point; crystallization path for Ab₂₀-An₈₀ showing equilibrium and fractional crystallization; lever rule application; normal and reverse zoning in plagioclase explained through compositional changes during cooling
  • Part (c)(i): I-type (igneous source, metaluminous, Cordilleran), S-type (sedimentary source, peraluminous, Himalayan), M-type (mantle-derived, oceanic), A-type (anorogenic, alkaline, rift settings); petrographic features like presence of muscovite/garnet in S-type, hornblende in I-type; petrogenetic models for each
  • Part (c)(ii): Deccan Traps as continental flood basalts; timing (Cretaceous-Paleogene boundary ~66 Ma); mantle plume model (Reunion hotspot); tholeiitic chemistry; Deccan stratigraphy with formations like Jawhar, Igatpuri, Wai; environmental and biological impacts including mass extinction linkages
  • Integration: Synthesis showing how migmatites represent crustal melting stages leading to granite genesis, with phase diagrams quantifying plagioclase evolution, and Indian examples tying global petrogenetic models to local geology

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precise definitions across all parts: migmatite structure (neosome/paleosome distinction), accurate phase diagram topology with correct invariant points, granite typology criteria (source, aluminum saturation, tectonic setting), and Deccan volcanism chronostratigraphy; uses correct terminology like 'metatexis', 'eutectic', 'peraluminous', 'tholeiitic'Generally correct definitions with minor errors; conflates metatexis/diatexis or misplaces granite types; phase diagram essentially correct but missing labels; Deccan description lacks specific formationsFundamental misconceptions: treats migmatites as simple metamorphic rocks, draws wrong phase diagram topology, confuses I/S/M/A types, or gives incorrect Deccan age/setting
Diagram / cross-section15%7.5Neat, fully labelled Ab-An binary diagram with temperature-composition axes, liquidus and solidus curves, eutectic point marked, tie lines shown; crystallization path for Ab₂₀-An₈₀ clearly traced with arrows; may include supplementary sketch of zoned plagioclase crystalDiagram present but incomplete: missing some labels, curves approximately correct, crystallization path indicated but not clearly explainedNo diagram or unrecognizable sketch; wrong system drawn (e.g., ternary instead of binary); no crystallization path indicated
Field evidence20%10Specific Indian/local examples: migmatites from Aravalli or Dharwar craton; granite types illustrated with Himalayan S-type (High Himalayan Leucogranite) vs Peninsular I-type (Charnockite series); field relationships of Deccan Traps with Western Ghats escarpment and dyke swarms; mentions mesoscopic structures like stromatic, nebulitic, schlierenGeneric field descriptions without specific Indian localities; mentions 'Precambrian shields' or 'Himalayas' without precision; Deccan description lacks structural detailNo field evidence cited; purely theoretical treatment; incorrect or invented localities
Quantitative reasoning20%10Lever rule application to determine melt/crystal proportions at given temperature; quantitative estimation of plagioclase composition changes during fractional crystallization; mentions An-content ranges for different granite types; Deccan volume estimates (~1.5 million km³) and eruption ratesQualitative discussion of crystallization without calculations; mentions 'proportions change' without quantification; some numerical data for Deccan but incompleteNo quantitative treatment; ignores lever rule entirely; no numerical data anywhere
Indian / economic relevance20%10Economic significance: migmatite-associated rare-element mineralization (Be, Li in leucosomes); granite-hosted Sn-W deposits (Rajasthan) vs porphyry Cu-Mo (Himalayan I-type); Deccan basalt as building stone, aquifer, and potential geothermal resource; links Deccan volcanism to K-Pg extinction with Indian stratigraphic evidence (Anjar, Jhilmili intertrappeans)Mentions economic importance generally without specifics; notes Deccan basalt utility but misses mineralization links; weak connection to Indian geological heritageNo economic or societal relevance discussed; misses opportunity to highlight Indian geological significance

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