18-Geol-A1 Mineralogy and Petrology · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2016-May. Closed book; no calculator permitted. Part 1 requires all six 10-mark short-answer questions (60 marks); Part 2 instructs "four of the seven ten-mark questions" (40 marks).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (mineral/silicate structural classification, ore mineralogy); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, cumulates, metamorphic reactions and facies, AFM projections, volcanic processes, oceanic crust petrogenesis); Nesse, Introduction to Optical Mineralogy, 4th ed. (index-mineral optics); Boggs, Petrology of Sedimentary Rocks, 2nd ed. (sedimentary/pyroclastic textural context).
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.
Contact metamorphism is localized, thermally-driven metamorphism of the country rock immediately surrounding a hot igneous intrusion (a thermal aureole). Heat — conducted (and, near the contact, sometimes assisted by magmatic fluids) outward from the pluton — is the dominant agent; confining pressure at the shallow-to-moderate emplacement depths typical of most intrusions is modest, and, critically, differential stress is negligible because the country rock is simply being baked in place, not tectonically deformed.
Mineral assemblages are zoned outward from the contact, from the highest-grade pyroxene-hornfels facies immediately adjacent to the intrusion, through hornblende-hornfels facies, to lowest-grade albite-epidote-hornfels facies at the aureole's outer edge, defining a sequence of index minerals appropriate to the protolith — for a pelitic protolith this includes andalusite and cordierite (low-P, high-T index minerals diagnostic of contact rather than regional metamorphism, since andalusite is the low-pressure $\text{Al}_2\text{SiO}_5$ polymorph); for a siliceous limestone protolith, decarbonation produces wollastonite ($\text{CaSiO}_3$ from calcite + quartz).
Because there is no differential stress, contact metamorphic rocks (hornfels) are characteristically non-foliated: fine-grained and hard, with a granoblastic (equant, polygonal, roughly equidimensional grains meeting at triple junctions) or decussate (randomly-oriented acicular/platy grains, e.g. andalusite or cordierite porphyroblasts, with no preferred orientation) fabric. A distinctive spotted texture is common at lower grade, where incipient porphyroblasts or clots of new metamorphic minerals appear as darker spots against a finer matrix retaining relict sedimentary texture.
