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18-Geol-A1 Mineralogy and Petrology · May 2017

Question 4 of 13: Four Basic Agents of Metamorphism

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Notes on this paper

EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-May. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2 lists eight questions with instructions to answer "5 of the 7" (a source discrepancy noted on the exam page itself).

Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry/formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation and mixing, metamorphic agents/facies, volcanic processes, phase equilibria and AFM projections, magma viscosity, layered intrusions, tectonic melting mechanisms).

Question 4: Four Basic Agents of Metamorphism (Part 1 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

The four basic agents of metamorphism
AgentWhat it doesGeological example
Heat (temperature)Drives mineral reactions and recrystallization toward higher-T stable assemblages; the primary control on metamorphic grade.Thermal aureole around a shallow granitic pluton, producing a zoned sequence of hornfels facies outward from the contact.
Pressure (confining/lithostatic)Uniform, all-sided pressure from overlying rock load; increases density and favours smaller-molar-volume assemblages without imposing preferred orientation.Burial metamorphism at the base of a thick sedimentary basin (zeolite/prehnite–pumpellyite-facies burial diagenesis).
Differential stressUnequal stress in different directions; drives dynamic recrystallization and preferred mineral alignment, producing foliation/lineation.Schistose fabric developed in a ductile shear zone at a convergent-margin fold-and-thrust belt (regional dynamothermal metamorphism).
Chemically active fluidsAqueous/carbonic fluids transport heat and dissolved species, catalyzing reaction rates and enabling metasomatic (allochemical) mass transfer.Skarn formation at an intrusion–limestone contact, where fluid-borne Si, Fe, Al and Mg are exchanged for country-rock Ca.