18-Geol-A1 Mineralogy and Petrology · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 18-Geol-A1 Mineralogy and Petrology, 2018-Dec. Closed book; no calculator permitted.
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/oxide structural classification, mineral chemistry and formulas, crystal systems); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, Bowen's reaction series, metamorphic agents/facies, volcanic and pyroclastic processes, partial melting); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (carbonate mineral diagnostics).
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 overwhelmingly the dominant agent and heat/confining pressure play only a secondary role — the opposite emphasis from contact metamorphism, where heat dominates and differential stress is negligible. Its character changes sharply with depth, because the deformation mechanism itself changes from brittle to ductile across the brittle–ductile transition (roughly 10–15 km in quartzo-feldspathic crust).
Above the brittle–ductile transition, rock deforms by fracturing and frictional sliding. Grains are mechanically crushed and rotated with little or no recrystallization, producing a spectrum of cataclasite textures: fault breccia (coarse, angular clasts) grading through cataclasite proper to ultracataclasite and unconsolidated, clay-rich fault gouge in the highest-strain core. Mineralogically these rocks are largely unchanged from the protolith — new mineral growth is minor (local sericite/chlorite from fluid-assisted alteration along the fracture network) because temperature is too low to drive significant solid-state recrystallization.
Below the brittle–ductile transition, the same differential stress instead drives crystal-plastic flow. Grains deform internally and dynamically recrystallize rather than fracture, producing mylonite: a strongly foliated, often lineated rock with a markedly reduced grain size relative to the protolith. Diagnostic textures include elongate, dynamically-recrystallized ribbon quartz; rotated, asymmetric porphyroclasts with recrystallized "tails" that record a consistent shear sense; and, at high strain, S–C fabrics (two intersecting foliations recording the finite and incremental strain). Mineralogy is typically the same assemblage as the protolith, re-textured rather than reacted, because dynamic recrystallization does not require the P–T change needed for new mineral growth.
