Metamorphic Mineral Assemblages and Pressure-Temperature Conditions

Core Idea

Metamorphic rocks form under elevated pressure and temperature; specific mineral assemblages (facies) such as greenschist, amphibolite, and granulite define the P-T conditions during metamorphism. These assemblages preserve a record of deep crustal or mantle processes and plate convergence.

Explainer

From your study of mineral crystal systems and metamorphic rocks, you know that metamorphism transforms existing rocks under elevated temperature and pressure, producing new minerals stable under those conditions. The key insight of metamorphic petrology is that the specific combination of minerals in a rock — its mineral assemblage — is not random. It is controlled by the pressure-temperature (P-T) conditions during metamorphism and the bulk chemical composition of the original rock. Two rocks with the same starting chemistry subjected to the same P-T conditions will develop the same mineral assemblage, regardless of where on Earth they are found. This predictability is what makes mineral assemblages powerful diagnostic tools.

The concept that organizes this relationship is the metamorphic facies — a set of P-T conditions defined by characteristic mineral assemblages in rocks of common compositions. The major facies form a map across pressure-temperature space. Greenschist facies (~300–500°C, moderate pressure) is named for its green minerals: chlorite, epidote, and actinolite, which give the rock a distinctive green color. Amphibolite facies (~500–700°C, moderate to high pressure) is dominated by hornblende amphibole and plagioclase. Granulite facies (~700–900°C, moderate pressure) represents the highest-temperature regional metamorphism, where hydrous minerals break down and anhydrous minerals like pyroxene and garnet dominate. At high pressure but relatively low temperature, blueschist facies produces the striking blue amphibole glaucophane — diagnostic of subduction zones where cold oceanic crust is driven to great depths. Even higher pressure yields eclogite facies, with its distinctive garnet-plus-green-pyroxene (omphacite) assemblage, recording conditions deep in subduction channels.

The reason these assemblages are so informative is that metamorphic minerals reach chemical equilibrium at the peak conditions and are then preserved as the rock is brought back to the surface. Consider a basalt dragged down in a subduction zone: at shallow depth it contains zeolites and clay minerals (zeolite facies). As it descends to ~30 km depth and temperatures of 300–400°C, those minerals become unstable and are replaced by chlorite and actinolite (greenschist facies). Driven deeper to 50–70 km and pressures exceeding 1 GPa, glaucophane replaces actinolite and the rock becomes a blueschist. Each transition is a chemical reaction driven by changing stability — minerals that were stable at one set of conditions decompose and reform as new phases at another. If the rock is then exhumed rapidly enough, the high-pressure minerals are preserved rather than reverting to lower-pressure equivalents, giving geologists a direct window into conditions tens of kilometers below the surface.

Reading metamorphic assemblages in the field is therefore a form of geological forensics. By identifying the minerals present, plotting them on a P-T diagram, and noting what is absent (the absence of certain minerals can be as diagnostic as their presence), a petrologist reconstructs the P-T path — the trajectory the rock followed through pressure-temperature space during burial, peak metamorphism, and exhumation. These paths reveal the tectonic history of entire mountain belts: clockwise P-T paths (heating during burial, then cooling during uplift) are characteristic of collision zones, while counterclockwise paths suggest contact metamorphism or unusual tectonic settings. Every metamorphic assemblage is a frozen thermometer and barometer, recording conditions that no human could ever directly observe.