18-Geol-A1 Mineralogy and Petrology · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
EGBC National Exam — Geological Engineering, 04-Geol-A1 Mineralogy and Petrology, 2017-Dec. Closed book; no calculator permitted. Part 1 requires all five 10-mark short-answer questions (50 marks); Part 2's page-1 header says "5 of the 8" while the page-3 instructions say "5 of the 7" and list exactly 7 questions (a source discrepancy noted on the exam page itself).
Reference texts: Klein & Dutrow, Manual of Mineral Science, 23rd ed. (silicate/sulfide/carbonate structural classification, mineral chemistry and formulas); Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed. (magmatic differentiation, metamorphic agents/facies, volcanic processes, layered intrusions and cumulates, partial melting, ophiolites); 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.
An ophiolite is a slice of oceanic lithosphere — mantle peridotite through crustal gabbro, sheeted dikes, pillow basalt and its pelagic sediment cap — that has been tectonically emplaced onto continental or arc crust rather than being subducted.
Ophiolites form at a spreading centre exactly as normal oceanic crust does: partial melting of upwelling mantle feeds a crustal magma chamber, which crystallizes cumulates at its base, feeds a sheeted dike swarm, and erupts pillow basalt at the ridge axis, all overlain in time by a pelagic sediment drape. The oceanic lithosphere is then obducted — thrust up and over the edge of a continental margin or arc — during closure of the ocean or back-arc basin that hosted it, typically where a buoyant continental or arc block enters a subduction zone and jams it: the dense oceanic lithosphere itself cannot follow the rest of the slab down, so a slice is scraped off and thrust onto the overriding plate instead.
Ophiolites survive in the rock record for several reinforcing reasons. First, obduction places the slice structurally above buoyant continental or arc crust at a collisional suture, removing it from the subduction conveyor that would otherwise recycle it back into the mantle — unlike the vast majority of oceanic lithosphere, which is subducted and destroyed within <200 Myr of formation. Second, the mantle-peridotite base is commonly serpentinized during and after emplacement, which lowers its density and increases its resistance to further metamorphic/metasomatic alteration, helping stabilize the sequence. Third, post-obduction burial beneath younger sedimentary or volcanic cover can protect all or part of a sequence from surface erosion until later uplift re-exposes it. Because obduction is intrinsically tied to continent–continent or arc–continent collision, surviving ophiolite belts mark ancient suture zones (e.g. the Bay of Islands ophiolite, Newfoundland; the Troodos ophiolite, Cyprus; the Semail ophiolite, Oman) and are used as first-order evidence for reconstructing the position and timing of former ocean-basin closures in the geological record.
