Four Basic Agents of Metamorphism: 18-Geol-A1 Mineralogy and Petrology, May 2016, Q5

Question 5: Four Basic Agents of Metamorphism (Part 1 – 10 marks)

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The four basic agents of metamorphism
Agent	What it does	Geological example
Heat (temperature)	Drives mineral reactions and recrystallization toward assemblages stable at higher T; the primary control on metamorphic grade.	The thermal aureole surrounding a shallow granitic pluton, producing a zoned sequence of hornfels grades outward from the contact.
Pressure (confining/lithostatic)	Uniform, all-sided pressure from the weight of overlying rock; increases density and favours mineral assemblages with smaller molar volume, without imposing a preferred orientation.	Burial metamorphism of a thick sedimentary basin pile (e.g. zeolite-facies burial diagenesis/metamorphism at the base of a deep sedimentary basin).
Differential stress (directed pressure)	Unequal stress in different directions; drives dynamic recrystallization, grain flattening/elongation and preferred mineral alignment, producing foliation and lineation.	Development of schistosity/mylonitic fabric in a ductile shear zone or at a convergent-margin fold-and-thrust belt (regional dynamothermal metamorphism).
Chemically active fluids	Aqueous/carbonic fluids (magmatic, connate or meteoric) transport heat and dissolved species, catalyzing reactions and enabling metasomatic (allochemical) mass transfer that dry solid-state diffusion could not achieve on a human timescale.	Skarn formation (metasomatism) at the contact between an intrusion and a limestone country rock, where fluid-borne Si, Fe, Al and Mg are exchanged for country-rock Ca.

Topic. The four fundamental agents of metamorphism.

Key relations. Metamorphic grade tracks T (and, secondarily, P) via index-mineral isograds; foliation intensity tracks the magnitude of differential stress; metasomatic mineral zoning tracks fluid flux and chemical gradient, independent of T/P alone.

Why this works. Distinguishing these four agents matters because each imprints a different, diagnostic textural/mineralogical signature on the resulting rock — heat-and-pressure alone (no differential stress, no fluid) gives an isotropic, isochemical hornfels; differential stress gives a foliated fabric; fluid activity gives allochemical (metasomatic) compositional zoning — so identifying which agent(s) dominated at a given outcrop is the basis for interpreting its tectonic setting.

Common pitfall. Treating "pressure" as a single agent without distinguishing confining (isotropic) pressure from differential (directed) stress — only the latter produces foliation.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §21 (metamorphic agents, textures and fabrics).

Question 5 of 13: Four Basic Agents of Metamorphism

Question 5: Four Basic Agents of Metamorphism (Part 1 – 10 marks)