EARTH MATERIALS VIII The Rock Cycle: Metamorphic Rocks Professor Peter Doyle P.doyle@imperial.ac.uk Profdoyle@btinternet.com 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 occurs when large areas of the crust are subjected to high temperatures and/or pressures • Contact metamorphism occurs adjacent to igneous intrusions causing mainly an increase in temperature but also pressure in some instances METAMORPHIC FACIES: temperature & pressure regimes Low Pressure High Pressure Low temperature High Temperature Assemblage of metamorphic rocks formed under a similar range of pressure and temperature conditions METAMORPHIC FACIES: typical settings Intraplate: normal range Contact: High temperature Arc: increased temperature Subduction zones: high pressure Increasing Temperature METAMORPHIC FACIES: mineral assemblages Original chemistry of parent rock controls the mineral assemblage formed during metamorphism Metamorphic Index minerals • INDEX MINERALS are diagnostic minerals formed by metamorphic reactions • They are a function of P, T and rock & fluid compositions (X) at time of the reaction • Used to define metamorphic zones • Demonstrates progress of metamorphism REGIONAL METAMORPHISM OF SHALES LOW GRADE HIGH GRADE CHLORITE BIOTITE GARNET STAUROLITE KYANITE 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 Record of sets of P-T-X conditions in the metamorphic reaction Minerals function of temperature, pressure & parent rock Increasing Grade & relevant facies Index minerals REGIONAL METAMORPHISM OF SHALES CONTACT METAMORPHISM: heat alone SANDSTONES & SHALES: (siliciclastic source) LIMESTONES: (carbonate source) Minerals produced a function of temperature & parent rock TEXTURES: INDICATORS OF METAMORPHIC PROCESS Pressurized Heated Foliated rocks: pressurised rocks • Pressure induces re-organisation and growth of new minerals • Large percentages of platey minerals • Increasing grade indicated by level of foliation: Slate-Phyllite-Schist-Gneiss • Slate: splitting surfaces at an angle to bedding • Phyllite: Phyllite similar, with platy mineral growth • Schist: segregation of platey minerals in preferred orientation • Gneiss: Coarser grade foliation, distinct banding FOLIATED ROCKS phyllite FOLIATION: INTENSITY INCREASES WITH INCREASING PRESSURE DEVELOPMENT OF FOLIATION Platy minerals oriented normal to compressive forces Compressive stress GRAIN ORIENTATION Schist SLATEY CLEAVAGE: planar foliation (not bedding) Little new mineral growth SCHISTOSITY: wavy foliation Porphyroblasts: new crystal growth in matrix SCHIST WITH GARNET PORPHYROBLASTS GLAUCOPHANE BLUE SCHIST High Pressure metamorphism GNEISSOSE FOLIATION, SEGREGATION OF MINERALS (micas, feldspar, quartz, amphibole, etc) Non-foliated metamorphic rocks I: Contact metamorphism • Formed from contact heating or regional metamorphism • Hornfels: formed by high temperature/low pressure contact metamorphism, uniform grain size with no preferred orientation • FINE GRAINED GRANULAR (GRANOBLASTIC) • 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 Spotted hornfels: porphyroblasts Hornfels: uniformly granular Non-foliated metamorphic rocks II: Regional metamorphism • Quartzites: metamorphosed quartzrich sandstones • metamorphism produces recrystallised, recrystallised uniform-sized, uniform-sized quartz crystals with no foliation • Marbles: metamorphosed limestones • metamorphism produces uniform, uniform equigranular, equigranular interlocking recrystallised calcite crystals with no foliation Regional metamorphic rocks Marble Quartzite [...]... METAMORPHISM: heat alone SANDSTONES & SHALES: (siliciclastic source) LIMESTONES: (carbonate source) Minerals produced a function of temperature & parent rock TEXTURES: INDICATORS OF METAMORPHIC PROCESS Pressurized Heated Foliated rocks: pressurised rocks • Pressure induces re-organisation and growth of new minerals • Large percentages of platey minerals • Increasing grade indicated by level of foliation:... with small irregular porphyroblasts of minerals such as cordierite & andalusite • Minerals form during metamorphic heating at low pressures Contact metamorphic rocks Spotted hornfels: porphyroblasts Hornfels: uniformly granular Non-foliated metamorphic rocks II: Regional metamorphism • Quartzites: metamorphosed quartzrich sandstones • metamorphism produces recrystallised, recrystallised uniform-sized,... Non-foliated metamorphic rocks I: Contact metamorphism • Formed from contact heating or regional metamorphism • Hornfels: formed by high temperature/low pressure contact metamorphism, uniform grain size with no preferred orientation • FINE GRAINED GRANULAR (GRANOBLASTIC) • Spotted Hornfels: similar, but with small irregular porphyroblasts of minerals such as cordierite & andalusite • Minerals form during metamorphic. .. with no foliation • Marbles: metamorphosed limestones • metamorphism produces uniform, uniform equigranular, equigranular interlocking recrystallised calcite crystals with no foliation Regional metamorphic rocks Marble Quartzite ... angle to bedding • Phyllite: Phyllite similar, with platy mineral growth • Schist: segregation of platey minerals in preferred orientation • Gneiss: Coarser grade foliation, distinct banding FOLIATED ROCKS phyllite FOLIATION: INTENSITY INCREASES WITH INCREASING PRESSURE DEVELOPMENT OF FOLIATION Platy minerals oriented normal to compressive forces Compressive stress GRAIN ORIENTATION Schist SLATEY CLEAVAGE: