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.
An index mineral is a mineral whose first appearance, for a given protolith bulk composition, marks the crossing of a specific isograd — a mappable line of constant metamorphic grade. In the classic Barrovian sequence developed on a pelitic protolith, the index minerals appear in the order chlorite → biotite → garnet → staurolite → kyanite → sillimanite with increasing grade.
A metamorphic facies is a set of mineral assemblages (across a range of protolith bulk compositions, not just one) that recur together and record a characteristic range of pressure and temperature, regardless of where in the world or in what protolith they occur — e.g. greenschist facies (chlorite + actinolite + albite ± epidote, low P/T), amphibolite facies (hornblende + plagioclase, moderate P/T), granulite facies (anhydrous pyroxene-bearing assemblages, high T), blueschist facies (glaucophane + lawsonite, high P/low T) and eclogite facies (garnet + omphacite, very high P).
A facies series is the characteristic sequence of facies encountered as grade increases along one particular P/T gradient (i.e. one particular geothermal setting), because different tectonic environments impose systematically different P/T ratios. Three canonical series are recognized: the Barrovian (medium P/T) series typical of regional continent–continent collision, the Buchan (low P/T) series typical of high-heat-flow settings such as volcanic arcs or contact-dominated terranes, and the high-P/T (blueschist–eclogite) series typical of subduction zones, where cold slab material is buried rapidly with too little time to heat up.
Index minerals are the field tool used to subdivide grade within one facies series (e.g., the chlorite/biotite/garnet/staurolite/kyanite/sillimanite zones are all internal subdivisions of the Barrovian series, spanning from greenschist through amphibolite facies); the facies present at a given outcrop identify the P–T conditions reached; and the particular sequence of facies encountered across a metamorphic terrane identifies which facies series (and hence which tectonic setting/P-T-t path) that terrane experienced. A classic example: the Barrovian zones of the Scottish Highlands were the type locality that defined both the index-mineral concept and the medium-P/T facies series that bears the same name.
