18-Geol-A1 Mineralogy and Petrology · December 2019
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2019-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate structural classification, mineral chemistry and substitution, crystal systems, sulfide/carbonate ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, tectonic settings of magmatism, metamorphic/metasomatic processes, volcanic and pyroclastic processes, plate-tectonic cycle).
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.
Dynamic metamorphism (also called cataclastic or dislocation metamorphism) is metamorphism localized along fault and shear zones, where differential stress is the dominant metamorphic agent and heat/confining pressure play only a secondary role — the opposite emphasis from contact metamorphism, where heat dominates. Its style changes sharply across the brittle–ductile transition (roughly 10–15 km depth in quartzo-feldspathic crust), because the underlying deformation mechanism itself changes from fracturing to crystal-plastic flow.
Above the brittle–ductile transition, rock deforms by fracturing and frictional sliding, mechanically crushing and rotating grains with little or no recrystallization. This produces a spectrum of cataclasite textures — fault breccia (coarse angular clasts) grading through cataclasite proper to unconsolidated, clay-rich fault gouge in the highest-strain core. Mineralogy is largely unchanged from the protolith, since temperature is too low for significant solid-state recrystallization; only minor local sericite/chlorite forms from fluid-assisted alteration.
Below the transition, the same differential stress instead drives crystal-plastic flow: grains deform internally and dynamically recrystallize rather than fracture, producing mylonite — a strongly foliated, lineated rock with markedly reduced grain size relative to its protolith. Diagnostic textures include elongate, dynamically-recrystallized ribbon quartz; rotated, asymmetric porphyroclasts with recrystallized tails recording a consistent shear sense; and, at high strain, S–C fabrics. Mineralogy is typically the protolith assemblage re-textured, not reacted, since dynamic recrystallization needs no P–T change to grow new minerals.
