How Metamorphic Facies, Barrovian Zones, P-T Paths & UHP Eclogites Unlock Mountain Building
Direct Petrological & Geochemical Summary: When sedimentary or igneous rocks are subjected to rising temperatures and pressures during tectonic plate collision or subduction, their original mineral assemblages become unstable and recrystallize into new solid-state mineral assemblages known as Metamorphic Facies. As metamorphic grade increases in regional orogenic belts (the classic Barrovian Sequence named after George Barrow from the Scottish Highlands), pelitic shales undergo progressive dehydration reactions—producing characteristic aluminum-silicate index minerals: Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite. Under cold subduction conditions (<25°C/km geotherm), rocks transform into high-density Blueschist and ultra-high pressure (UHP) Eclogite, where standard quartz collapses into dense Coesite or microdiamond at depths exceeding 70 to 100 km before exhuming back to Earth's surface.
🔬 Lithostatic Pressure (P), Geothermal Gradient & Al₂SiO₅ Phase Equations
- Lithostatic Overburden Pressure (P in GPa from Depth Z in km & Rock Density ρ ≈ 2.85 g/cm³):
P (GPa) ≈ ρ · g · Z ≈ 0.027 · Z (km)|1 GPa = 10 kbar ≈ 37 km Crustal Depth - Aluminosilicate Polymorph Triple Point (Andalusite – Kyanite – Sillimanite):
T_triple ≈ 500 °C, P_triple ≈ 0.38 GPa (~14 km depth)|Kyanite (High-P) → Sillimanite (High-T) - Eclogite Density & Seismic P-Wave Velocity Discontinuity (Mohr-Continental Moho):
Vp = a + b·(ρ - 2.5)→ Eclogiteρ ≈ 3.50 g/cm³drives subducting slab pull.
Complete Reference Chart: 8 Metamorphic Facies, Barrovian Zones & Mineral Assemblages
| Metamorphic Facies & Regime | Typical P & T Range | Barrovian Index Minerals | Geological Setting & P-T Path Significance |
|---|---|---|---|
Ultra-High Pressure EclogitePyrope Garnet + Omphacite | P > 2.2 GPa (>75 km) T = 550 – 1000 °C | Coesite, Microdiamond, Pyrope, Omphacite, Kyanite | Subducted continental crust reaching mantle depths; coesite/diamond indicators |
Barrovian Regional SequencePelitic Shale Protolith | 0.3 – 1.0 GPa (10–35 km) T = 350 – 650 °C | Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite | Classic medium-pressure orogenic collision belt (Scottish Highlands) |
Glaucophane BlueschistNa-Amphibole Facies | 0.6 – 1.6 GPa (20–55 km) T = 200 – 500 °C | Glaucophane, Lawsonite, Jadeite, Aragonite, Pumpellyite | Cold subduction zone geothermal gradient (<15°C/km); rapid trench burial |
Almandine AmphiboliteMedium to High Grade | 0.4 – 1.2 GPa (15–40 km) T = 550 – 750 °C | Hornblende, Almandine Garnet, Plagioclase, Staurolite | Widespread regional dynamothermal metamorphism in root zones of orogens |
High-T Granulite FaciesLower Crustal Melting | 0.5 – 1.2 GPa (18–40 km) T = 700 – 1100 °C | Orthopyroxene, Clinopyroxene, Garnet, Sillimanite, K-Feldspar | Dry lower continental crust; anhydrous melting and granulite-facies terrains |
Greenschist FaciesLow-Grade Regional | 0.2 – 0.6 GPa (6–20 km) T = 300 – 500 °C | Chlorite, Actinolite, Epidote, Albite, Sericite (Muscovite) | Hydrous low-grade regional metamorphism of basaltic and pelitic protoliths |
Buchan Low-Pressure FaciesAndalusite Series | 0.1 – 0.4 GPa (3–14 km) T = 450 – 700 °C | Andalusite, Cordierite, Sillimanite, Biotite, Plagioclase | High-temperature, low-pressure regional metamorphism (reactive granitic plutons) |
Contact Pyroxene HornfelsThermal Aureole | 0.05 – 0.3 GPa (1–10 km) T = 400 – 900 °C | Cordierite, Andalusite, Diopside, Vesuvianite, Grossular | Thermal baking around shallow epizonal granitic magma intrusions |
Frequently Asked Questions
How do geologists distinguish between Barrovian (Medium-P) and Buchan (Low-P) metamorphic series?
The distinction lies in the geothermal gradient ($\Delta T / \Delta Z$). In a Barrovian sequence (medium pressure, ~30°C/km), aluminum-silicate polymorphs progress sequentially from Chlorite → Biotite → Garnet → Staurolite → Kyanite → Sillimanite. In a Buchan sequence (low pressure, high temperature gradient >50°C/km found in rift zones or thermal aureoles), pressure is too low for the dense Kyanite stability field, so rocks pass directly through Andalusite before converting to Sillimanite at higher temperatures.
Why are eclogites so dense and why do they drive subducting tectonic plates?
When basaltic ocean crust subducts into the mantle, low-density hydrous minerals (plagioclase feldspar and pyroxene) react at depths exceeding 40–50 km to form anhydrous Eclogite—consisting of high-density pyrope-almandine garnet ($\rho \approx 3.8\text{ g/cm}^3$) and sodic omphacite ($\rho \approx 3.3\text{ g/cm}^3$). Because eclogite has a significantly higher density than the surrounding asthenospheric mantle ($3.4\text{ vs. }3.3\text{ g/cm}^3$), the negative buoyancy of the eclogitized slab creates a powerful downward gravitational force known as slab pull, driving plate tectonics.
Can you share your metamorphic P-T path calculation on X (Twitter)?
Yes! Click the "𝕏 Post on X" button at the top of the lab to share your live Barrovian P-T simulation with pre-populated geochronology hashtags, or click "🖼️ Save 1200×630 Social Card" to export a high-definition 1200×630 preview card optimized for X, LinkedIn, and Discord link previews.