Formation of Metamorphic Rocks - Study Notes

Grading Breakdown: Exam-1 ( $15\%$ ), Exam-2 ( $15\%$ ), Exam-3 ( $15\%$ ), Final Exam ( $30\%$ ), Laboratory ( $25\%$ ).
Exam Logistics: In-person in SEC_A235 from 10:30 to 11:30 am.

Etymology: Derived from Greek meta (change) and morphe (form).
Definition: The process by which pre-existing rocks undergo changes in mineral content, chemical composition, or physical texture in a solid state due to elevated temperatures, pressures, or chemically active fluids.
Metamorphic Grade: Reflects the degree to which the parent rock changes during metamorphism (low-, medium-, to high-grade).
Governing Factors: Temperature, pressure, chemically active fluid availability, and parent rock (protolith) composition.

Thermal Gradient: Temperature increases with depth into Earth; requires approximately $12\,\text{km}$ of burial to reach minimum metamorphic conditions (e.g., tectonic displacement changing rock burial depth from $4\,\text{km}$ at $100\,^\circ\text{C}$ to $10\,\text{km}$ at $260\,^\circ\text{C}$ ).
Heating Mechanisms: Sediment burial, tectonic burial (thrusting of one crustal block over another), and magma intrusion.
Thermal Reactions:
- Dehydration: Loss of water under heat, e.g., $KAl_3Si_3O_{10}(OH)_2 + SiO_2 \rightarrow Al_2SiO_5 + KAlSi_3O_8 + H_2O$ (muscovite + quartz \rightarrow sillimanite + K-feldspar + water).
- Degassing: Loss of gas under heat, e.g., $CaMg(CO_3)_2 \rightarrow CaCO_3 + MgO + CO_2$ (dolomite \rightarrow calcite + periclase + carbon dioxide).

Recrystallization: Pressure increases density and recrystallizes minerals into new sizes and shapes without altering overall bulk chemical composition.
Stress & Strain: Under normal stress, minerals dissolve along the maximum stress axis and crystallize along the minimum stress axis, causing grains to elongate and flatten perpendicular to maximum stress.
Foliation: Any planar arrangement of mineral grains or structural features resulting from stress, including parallel alignment of platy/elongated minerals, flattened pebbles, slaty cleavage, or compositional banding.

Low-grade vs High-grade metamorphism

Functions: Accelerates reaction rates, lowers required metamorphic temperatures, and transports dissolved ions into or out of the rock.
Hydration Example: $2\,Mg_2SiO_4 + 2\,H_2O + CO_2 \rightarrow Mg_3Si_2O_5(OH)_4 + MgCO_3$ (Mg-olivine + water + carbon dioxide \rightarrow serpentine + magnesite).
Fluid Sources: Pore spaces, fractures, and grain boundaries in parent rock, igneous intrusions, or fluids released via high-temperature dehydration/degassing reactions.

Stability Fields: Minerals remain stable within defined temperature and pressure limits, breaking down outside those ranges to form new minerals.
$Al_2SiO_5$ Polymorphs: Andalusite, kyanite, and sillimanite share the composition $Al_2SiO_5$ and act as key indicator minerals for specific pressure-temperature ranges.
Index Mineral Utility: Index minerals (e.g., chlorite, biotite, garnet, staurolite, kyanite, sillimanite) define specific metamorphic conditions, whereas quartz and feldspars are stable across wide ranges and are non-diagnostic.

Foliated Rocks (classified primarily by texture and degree of foliation):
- Slate: Fine-grained with slaty cleavage, smooth dull surface; formed under low-grade metamorphism (parent rock: shale, mudstone, or siltstone).
- Phyllite: Fine-grained with a glossy sheen and wavy surfaces (parent rock: shale, mudstone, or siltstone).
- Schist: Medium- to coarse-grained with scaly foliation, dominated by micas (parent rock: shale, mudstone, siltstone, or igneous rocks).
- Gneiss: Coarse-grained with light and dark compositional banding; formed under high-grade metamorphism (parent rock: shale, granite, or volcanic rocks).
Non-Foliated Rocks (classified primarily by mineral composition):
- Marble: Interlocking calcite or dolomite grains, relatively soft ( $3$ on Mohs scale) (parent rock: limestone, dolostone).
- Quartzite: Fused, interlocking quartz grains, massive and very hard (parent rock: quartz sandstone).
- Hornfels: Very fine-grained, dark, exceedingly tough (parent rock: shale or variable).

Uplift Mechanisms: Metamorphic rocks formed at depth are exposed at Earth's surface through mountain-building uplift and subsequent erosion.
Non-Reversibility: Metamorphic rocks do not revert to original parent minerals at the surface because temperature, pressure, fluid availability, and time are insufficient for reverse reactions, especially after gases and water have been driven off.

Contact Metamorphism: Localized thermal transformation adjacent to igneous intrusions or below lava flows ( $< 1\,\text{m}$ to $20\,\text{km}$ scale); driven by heat; common product is hornfels.
Hydrothermal Metamorphism: Large-volume interaction of hot geothermal fluids with rock; prominent along mid-ocean ridges and divergent plate boundaries; yields economic sulfide ore deposits (copper, zinc, nickel).
Regional Metamorphism: Widespread transformation covering tens of thousands of square kilometers along convergent plate boundaries; driven by high tectonic stress and geothermal gradients; forms mountain ranges and paired metamorphic belts (e.g., Japan: low-T/high-P near trench, low-P/high-T near volcanic arc).
Fault Zone Metamorphism: Near-surface brittle fracture produces fault breccia/gouge; deep ductile flow produces mylonites.
Impact Metamorphism: High-velocity meteorite impacts generate fused fragmented rock and glass-rich ejecta called impactites.