Geology 2023 Paper II 50 marks Describe

Q3

(a) Describe the changes in crystallized solid composition in albite-anorthite system at 1 atm pressure during cooling of a liquid of An₅₀ composition from 1500 °C temperature. (20 marks) (b) How is a granite defined? Discuss petrogenesis of a calc-alkaline peraluminous granite. (15 marks) (c) What are the assumptions involved for plotting quartz-bearing metamorphic rocks of basaltic composition in an ACF triangular diagram? (15 marks)

हिंदी में प्रश्न पढ़ें

(a) 1 वातावरण दबाव (1 atm प्रेशर) में An₅₀ संयोजन के द्रव में 1500 °C तापमान से ठंडे हो रहे एल्बाइट-एनोर्थाइट तंत्र के अंदर क्रिस्टलीकृत ठोस संयोजनात्मक बदलाव का वर्णन कीजिए। (20 अंक) (b) ग्रेनाइट को कैसे परिभाषित किया जाता है? एक कैल्क-एल्कलाइन परएल्यूमिनस ग्रेनाइट की शैलोत्पत्ति (पेट्रोजेनेसिस) पर चर्चा कीजिए। (15 अंक) (c) बेसाल्टिक संयोजन वाली क्वार्ट्ज-युक्त कायांतरित चट्टानों को एक ए० सी० एफ० त्रिकोणीय चित्र में दर्शाने के लिए कौन-सी मान्यताएँ शामिल हैं? (15 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

The directive 'describe' demands systematic, detailed exposition with visual support. Structure: brief introduction on phase equilibria relevance → Part (a): ~40% time/words on binary phase diagram with cooling path, lever rule calculations, and solid composition evolution → Part (b): ~30% on IUGS definition, then petrogenesis via crustal melting/magma mixing with Indian examples → Part (c): ~30% on ACF diagram assumptions, projecting from Qtz-saturated basaltic compositions. Conclude with synthesis on how phase diagrams unify igneous and metamorphic studies.

Key points expected

  • Part (a): Binary phase diagram of albite-anorthite at 1 atm; initial liquid composition An₅₀; liquidus and solidus temperatures; progressive crystallization with cooling path showing changing solid composition from An-rich to bulk composition; lever rule application for solid/liquid proportions at key temperatures (1400°C, 1300°C, etc.)
  • Part (a): Final solid composition reaching An₅₀ at eutectic completion; continuous solid solution behavior; no thermal arrest except at beginning and end of crystallization
  • Part (b): IUGS definition of granite (Q+Or+Pl > 20% Q, Or > Pl by volume); mineralogical and chemical criteria distinguishing from granodiorite/syenite
  • Part (b): Peraluminous characteristics (A/CNK > 1, corundum-normative, muscovite/garnet common); calc-alkaline affinity; petrogenetic models including S-type granite formation via pelitic sediment melting, crustal assimilation, or magma mixing; Indian examples from Rajasthan (Malani suite), Bundelkhand craton, or Himalayan leucogranites
  • Part (c): ACF diagram projection from quartz-saturated plane; assumption of excess SiO₂ making quartz invisible component; Fe-Mg combined as F component; projection of 4-component AFMQ system onto ACF plane; basaltic composition plotting in C-rich apex region
  • Part (c): Specific assumptions: all Fe as FeO, projection from muscovite/paragonite for pelites or from appropriate phase for mafics; limitations regarding Fe³⁺/Fe²⁺ ratio and Mn neglect; applicability to metabasites showing granulite/amphibolite facies assemblages

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%12.5For (a): correctly identifies An₅₀ liquidus (~1450°C) and solidus (~1380°C), tracks solid composition evolution from ~An₈₀ to An₅₀, applies lever rule accurately. For (b): precise IUGS definition, distinguishes peraluminous (A/CNK>1) from metaluminous, explains crustal melting model with residual mineral controls. For (c): correctly states projection from Qtz-orthoclase join or equivalent, understands Fe-Mg combining rationale.Basic phase diagram features correct but lever rule application vague; granite definition partially correct (confuses Q-Or proportions); ACF assumptions listed superficially without projection geometry understanding.Major errors: confuses eutectic with peritectic, misidentifies An₅₀ as eutectic composition, defines granite by silica content only, treats ACF as true 3-component system without projection.
Diagram / cross-section25%12.5For (a): hand-drawn binary T-X diagram with accurate liquidus/solidus curves, tie-lines at multiple temperatures, cooling path arrows, labeled fields (L, L+S, S). For (c): ACF triangle with A=Al₂O₃-(Na₂O+K₂O), C=CaO, F=FeO+MgO apices, Qtz projection indicated, basaltic composition point plotted. Clean, labeled, thermodynamically correct.Rough sketches with correct topology but missing temperature labels or tie-line demonstrations; ACF diagram drawn but apices incorrectly defined or projection method unclear.No diagrams or seriously flawed ones (e.g., wrong axes, confused with ternary eutectic, ACF as ABC triangle); diagrams described in text only.
Field evidence15%7.5For (b): cites specific Indian peraluminous granite occurrences—Himalayan leucogranites (Manaslu, Gangotri), Chhotanagpur Gneissic Complex, or Malani magmatism; links mineralogy (muscovite, garnet, tourmaline) to field settings of crustal anatexis; mentions economic significance (Sn-W mineralization with peraluminous granites).Generic mention of 'Himalayan granites' without specificity; no Indian examples or confused global references.No field evidence; or irrelevant examples (oceanic basalts, kimberlites) showing fundamental misunderstanding of granite tectonic settings.
Quantitative reasoning20%10For (a): lever rule calculations at 2-3 temperatures showing solid/liquid proportions and compositions; quantitative tracking of bulk system evolution. For (c): understands projection mathematics (recalculating analyses to ACF coordinates), recognizes mole basis for components.Qualitative description of 'more plagioclase crystallizes' without proportions; mentions lever rule but doesn't demonstrate; ACF plotting described without calculation basis.No quantitative treatment; confuses weight and mole percentages; arbitrary numerical claims without phase diagram basis.
Indian / economic relevance15%7.5For (b): connects peraluminous granite petrogenesis to Indian crustal evolution—Proterozoic mobile belts, Gondwana assembly; economic deposits (Rajasthan Sn-W belts, Jharkhand rare earths linked to peraluminous magmatism). For (c): mentions Indian granulite terrains (Eastern Ghats, Southern Granulite Terrain) where ACF diagrams decipher metamorphic history.Brief mention of Indian geology without integration; economic relevance stated generically.No Indian context; or irrelevant economic claims (petroleum, coal in granite context).

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