From Protolith to Product

The protolith is the pre-existing rock from which a metamorphic rock formed. Knowing the protolith is essential because it explains the bulk chemistry and therefore which minerals can form. A silica-rich sandstone and an alumina-rich shale, metamorphosed at the same temperature and pressure, will produce completely different metamorphic rocks.

Geologists often name metamorphic rocks by combining the protolith name with a metamorphic suffix — for example, metabasalt or metapelite — when the parent is still recognisable. When the parent is unknown or fully transformed, the rock gets a standard metamorphic name such as schist or gneiss.

The most famous metamorphic progression starts with shale, a clay-rich sedimentary rock. Because clay minerals are platy and aluminium-rich, shale produces a spectacular sequence of foliated rocks:

• Slate (low grade) — clay recrystallises into tiny chlorite and muscovite micas; slaty cleavage.
• Phyllite (low-medium grade) — micas grow larger, giving a silky sheen; cleavage is wavy.
• Schist (medium grade) — abundant visible mica flakes; porphyroblasts such as garnet may appear.
• Gneiss (high grade) — segregation into light (quartz, feldspar) and dark (biotite, amphibole) bands.

At the highest grades, partial melting begins and migmatite forms — a mixed rock with metamorphic host and light granitic veins.

Marble is the metamorphic product of limestone or dolostone. Because calcite and dolomite are equidimensional carbonate minerals, marble is typically non-foliated and massive. It recrystallises into a sparkling, interlocking mosaic of calcite crystals that can be polished to a high shine.

The purity of the protolith determines the colour: pure limestone makes white marble (like Carrara), while impurities such as clay, graphite, or iron oxide create grey, green, or pink varieties. Because marble is composed of calcite, it reacts vigorously with dilute acid — a quick field test.

Quartzite forms when quartz sandstone is metamorphosed. At low grade, the quartz grains simply recrystallise and grow larger, but the original rounded grain shapes may still be visible. At higher grade, the quartz grains interlock so thoroughly that the original sedimentary texture is erased.

True quartzite is extremely hard and durable. When it fractures, the break cuts through quartz grains, not around them — a key distinction from sandstone, where fractures follow grain boundaries. Quartzite is non-foliated unless later deformed in shear zones.

Mafic igneous rocks such as basalt and gabbro transform along different paths depending on the geothermal gradient — the rate at which temperature rises with depth:

Regional metamorphism (moderate P, moderate T — mountain belts):

• Greenschist (low-medium grade) — chlorite, epidote, and actinolite replace pyroxene and plagioclase; green colour from chlorite.
• Amphibolite (medium-high grade) — hornblende and plagioclase dominate; typically dark with visible crystals.
• Granulite (very high grade) — pyroxene and plagioclase indicate dry, hot conditions deep in the thickened crust.

Subduction-zone metamorphism (high P, low T — cold descending slab):

• Blueschist — glaucophane and lawsonite form under high pressure and relatively low temperature, distinctive of subduction zones.
• Eclogite (very high pressure) — omphacite (a pyroxene) and pyrope-rich garnet; dense, often striking red-and-green banding. This is the highest-pressure product of basaltic protoliths.

The regional path is driven by heat in thickened continental crust; the subduction path is driven by pressure in a cold oceanic slab dragged into the mantle.