EARTH MATERIALS VIII The Rock Cycle: Metamorphic Rocks Professor Peter Doyle

THE ROCK CYCLE

Metamorphic rocks: typically complex Pressure indicated by fabric (foliation) Heat indicated by growth of new minerals/crystals

Metamorphic Rocks Two major types of metamorphism Regional metamorphism high temperatures and/or pressuresRegional metamorphism occurs when large areas of the crust are subjected to high temperatures and/or pressures Contact metamorphism increase in temperaturepressureContact metamorphism occurs adjacent to igneous intrusions causing mainly an increase in temperature but also pressure in some instances

METAMORPHIC FACIES: temperature & pressure regimes metamorphic rocksa similar range of pressure and temperature conditions Assemblage of metamorphic rocks formed under a similar range of pressure and temperature conditions Low Pressure High Temperature Low temperature High Pressure

METAMORPHIC FACIES: typical settings Subduction zones: high pressure Intraplate: normal range Arc: increased temperature Contact: High temperature

METAMORPHIC FACIES: mineral assemblages Original chemistry of parent rock controls the mineral assemblage formed during metamorphism Increasing Temperature

Metamorphic Index minerals INDEX MINERALSdiagnostic minerals metamorphic reactionsINDEX MINERALS are diagnostic minerals formed by metamorphic reactions PTrock & fluid compositions (X)They are a function of P, T and rock & fluid compositions (X) at time of the reaction metamorphic zonesUsed to define metamorphic zones progress of metamorphismDemonstrates progress of metamorphism REGIONAL METAMORPHISM OF SHALES LOW GRADE CHLORITE BIOTITE GARNET STAUROLITE KYANITE HIGH GRADE SILLIMANITE Index Minerals formed: controlled by original rock (X), P, and T.

ISOGRADS: lines on a geological map showing locations of first appearance of index minerals Increasing grade P-T-X Record of sets of P-T-X conditions in the metamorphic reaction

REGIONAL METAMORPHISM OF SHALES Increasing Grade & relevant facies Index minerals Minerals function of temperature, pressure & parent rock

REGIONAL METAMORPHISM OF BASALTS Increasing Grade & relevant facies Index minerals

SANDSTONES & SHALES: (siliciclastic source) LIMESTONES: (carbonate source) CONTACT METAMORPHISM: heat alone temperature parent rock Minerals produced a function of temperature & parent rock

TEXTURES: INDICATORS OF METAMORPHIC PROCESS Pressurized Heated

Foliated rocks: pressurised rocks re-organisationgrowth new mineralsPressure induces re-organisation and growth of new minerals plateyLarge percentages of platey minerals Slate-Phyllite-Schist-GneissIncreasing grade indicated by level of foliation: Slate-Phyllite-Schist-Gneiss Slate:Slate: splitting surfaces at an angle to bedding PhyllitePhyllite: similar, with platy mineral growth Schist:Schist: segregation of platey minerals in preferred orientation Gneiss: Coarser grade foliation, distinct banding

phyllite FOLIATION: INTENSITY INCREASES WITH INCREASING PRESSURE FOLIATED ROCKS

DEVELOPMENT OF FOLIATION Platy minerals oriented normal to compressive forces

GRAIN ORIENTATION Schist Compressive stress

SLATEY CLEAVAGE: planar foliation (not bedding) Little new mineral growth

SCHISTOSITY: wavy foliation

SCHIST WITH GARNET PORPHYROBLASTS Porphyroblasts: new crystal growth in matrix

GLAUCOPHANE BLUE SCHIST High Pressure metamorphism

GNEISSOSE FOLIATION, SEGREGATION OF MINERALS (micas, feldspar, quartz, amphibole, etc)

Non-foliated metamorphic rocks I: Contact metamorphism contact heatingregional metamorphismFormed from contact heating or regional metamorphism Hornfels:Hornfels: formed by high temperature/low pressure contact metamorphism, uniform grain size with no preferred orientation FINE GRAINED GRANULAR (GRANOBLASTIC)FINE GRAINED GRANULAR (GRANOBLASTIC) Spotted Hornfels:Spotted Hornfels: similar, but with small irregular porphyroblasts of minerals such as cordierite & andalusite Minerals form during metamorphic heating at low pressures

Contact metamorphic rocks Hornfels: uniformly granular Spotted hornfels: porphyroblasts

Non-foliated metamorphic rocks II: Regional metamorphism Quartzites:Quartzites: metamorphosed quartz- rich sandstones recrystallised uniform-sizedmetamorphism produces recrystallised, uniform-sized, quartz crystals with no foliation Marbles:Marbles: metamorphosed limestones uniform equigranularmetamorphism produces uniform, equigranular, interlocking recrystallised calcite crystals with no foliation.

Regional metamorphic rocks Marble Quartzite