Geology 2022 Paper II 50 marks Discuss

Q3

(a) What are different types of metamorphism and what are their controlling factors ? State characteristic mineral assemblages which appear under different facies during regional metamorphism of pelitic rocks. (20 marks) (b) Define different types of zoning observed in minerals. Discuss processes of formation of different types of zoning in plagioclase with the help of Albite-Anorthite system. (15 marks) (c) State the petrographic characters of different types of anorthosites. Write a note on petrogenesis of anorthosites. (15 marks)

हिंदी में प्रश्न पढ़ें

(a) विभिन्न प्रकार के कायांतरण क्या होते हैं और इनके नियंत्रक कारक क्या हैं ? पैलिटिक शैलों के क्षेत्रीय कायांतरण के समय अलग-अलग संलक्षणी में दिखाई देने वाले विशिष्ट खनिज संयोजनों का वर्णन कीजिये । (20 अंक) (b) खनिजों में अवलोकित विभिन्न प्रकार के मंडलन को परिभाषित करें । एल्बाइट-एनार्थाइट समूह की सहायता से प्लेजियोक्लेज में विभिन्न प्रकार के मंडलन के गठन की प्रक्रिया पर चर्चा कीजिये । (15 अंक) (c) विभिन्न प्रकार के एनाथोसाइट्स के शैलवर्णीय लक्षण बताएं । एनाथोसाइट के शैलजनन पर टिप्पणी लिखिये । (15 अंक)

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Directive word: Discuss

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

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How this answer will be evaluated

Approach

The directive 'discuss' demands a comprehensive, analytical treatment with balanced coverage across all three sub-parts. Allocate approximately 40% of time/words to part (a) given its 20 marks, and 30% each to parts (b) and (c). Structure as: brief introduction on metamorphism and mineral evolution → systematic treatment of (a) types/factors/facies with ACF/AKF diagrams → (b) zoning types with Albite-Anorthite binary phase diagram → (c) anorthosite classification and petrogenesis → concluding synthesis on crustal evolution significance.

Key points expected

  • Part (a): Distinguish contact, regional, dynamic, hydrothermal and ocean-floor metamorphism; identify P-T-t (pressure-temperature-time) and fluid composition as controlling factors; enumerate Barrovian zones (chlorite → biotite → garnet → staurolite → kyanite → sillimanite) with diagnostic mineral assemblages for pelites
  • Part (a): Correctly place greenschist, amphibolite and granulite facies on P-T grid; cite index minerals like chlorite, biotite, almandine, staurolite, kyanite, sillimanite; mention aluminous pelite reactions including muscovite breakdown
  • Part (b): Define normal, reverse, oscillatory and sector zoning; explain plagioclase zoning via fractional crystallization (Bowen's reaction series), magma mixing, and pressure changes; illustrate with Albite-Anorthite binary diagram showing liquidus-solidus relationships and tie-lines
  • Part (b): Correlate An-content variation with cooling rate and diffusion rates; mention resorption textures and sieve textures as evidence of zoning modification
  • Part (c): Classify anorthosites into Archean (massif-type), Proterozoic (anorthosite-mangerite-charnockite-rapakivi suite), and layered intrusion types; describe characteristic textures including cumulate, antiperthitic feldspars, and Fe-Ti oxide ores
  • Part (c): Evaluate petrogenetic models including crystal accumulation from basaltic magma, flotation of plagioclase, and emplacement as crystal mushes; cite Bushveld and Stillwater complexes and Indian examples like Sittampundi and Bolangir anorthosites

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5Precisely defines all metamorphic types with correct P-T ranges; accurately describes Barrovian zones and facies series; correctly distinguishes zoning types and their crystallization controls; properly classifies anorthosite types with accurate petrogenetic mechanisms citing magma chamber processesIdentifies major metamorphic types and facies but confuses some zone boundaries or omits key index minerals; describes zoning generally but misinterprets formation mechanisms; lists anorthosite types but conflates petrogenetic models or omits key processesFundamental errors in metamorphic classification (e.g., confuses facies with zones); incorrect zoning definitions; major misconceptions about anorthosite origins (e.g., calling them purely magmatic without cumulate processes)
Diagram / cross-section20%10Draws accurate P-T-t diagram for metamorphic facies with geothermal gradients; precise ACF/AKF triangular diagrams for pelitic assemblages; clear Albite-Anorthite binary phase diagram with liquidus-solidus curves, tie-lines and zoning profiles; schematic anorthosite layered intrusion cross-sectionsBasic P-T diagram present but lacks precision in facies boundaries; attempts phase diagrams but with errors in invariant points or field labels; simple sketches without proper annotation or scaleMissing critical diagrams; grossly inaccurate phase diagrams; diagrams copied without understanding (e.g., wrong axes, mislabeled fields); no attempt at Albite-Anorthite binary system
Field evidence15%7.5Cites specific Indian occurrences: Sittampundi anorthosite complex (Tamil Nadu) for layered anorthosites; Bolangir and Keonjhar massifs (Odisha); Chilka Lake area examples; mentions Himalayan inverted metamorphism and Kerala khondalite belt for regional metamorphism field relationsMentions generic field settings (e.g., 'Himalayan rocks') without specific localities; vague references to Indian anorthosites without names; describes field criteria generally without linking to specific examplesNo Indian examples; purely theoretical treatment; confuses geographic locations or cites non-existent occurrences
Quantitative reasoning20%10Provides approximate P-T values for facies boundaries (e.g., greenschist-amphibolite ~500°C, 4-8 kbar); quantitative An-content ranges for plagioclase zoning (An₀-An₁₀₀); calculates approximate depths from pressure estimates; mentions diffusion coefficients and cooling rate effects on zoning widthQualitative P-T descriptions without values; mentions An-content ranges approximately; understands depth-pressure relationship but doesn't calculateNo quantitative treatment; purely descriptive; incorrect orders of magnitude for P-T conditions; confuses pressure and temperature effects
Indian / economic relevance20%10Discusses economic significance: anorthosites as Ti-Fe-V ore sources (Sittampundi: ilmenite-magnetite deposits); potential for rare earth elements; dimension stone potential of anorthosites; metamorphic belts in India hosting mineral deposits (garnet, kyanite, sillimanite in khondalite belts); strategic importance of Indian anorthosite-hosted oresMentions that anorthosites contain iron ores generally; notes some metamorphic minerals have industrial use without specifics; recognizes Indian occurrences have economic value without elaborationNo economic discussion; misses ore potential entirely; no connection to Indian resource base or strategic minerals

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