18-Geol-A1 Mineralogy and Petrology · December 2017
Question 8 of 12: Metamorphism vs. Metasomatism — Exoskarn, Endoskarn and Hornfels
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
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 8: Metamorphism vs. Metasomatism — Exoskarn, Endoskarn and Hornfels (Part 2 – 10 marks)
Metamorphism is isochemical recrystallization: bulk rock composition is essentially conserved while mineralogy and texture re-equilibrate to changed P–T conditions. Metasomatism is allochemical: a fluid phase actively adds and/or removes chemical components, changing the bulk composition, and is typically superimposed on ordinary metamorphic recrystallization.
Exoskarn, endoskarn and hornfels
A skarn is the calc-silicate rock produced by metasomatism at (most commonly) an igneous–carbonate contact:
Exoskarn develops on the country-rock (carbonate) side of the contact: Si, Fe, Al and Mg diffuse outward from the intrusion into the Ca-rich carbonate, producing garnet–pyroxene–wollastonite assemblages.
Endoskarn develops within the margin of the intrusion itself, where Ca (and $\text{CO}_2$) diffusing inward from the carbonate wall rock metasomatizes the pluton's outer edge.
Hornfels is the purely thermal, isochemical contact-metamorphic product — either farther from the fluid-dominated contact zone, or where the country rock is not a reactive carbonate (e.g. a pelitic hornfels), so no significant mass transfer occurs.
All three form in the same contact-aureole setting around an intrusion. Hornfels marks the isochemical, heat-only end of the spectrum, while exoskarn and endoskarn are the allochemical, fluid-mass-transfer-dominated near-contact zones that develop specifically where a reactive carbonate protolith is present.
