Metamorphic Facies and Mineral Assemblages Contact and Regional

Metamorphic Facies and Mineral Assemblages Contact and Regional Metamorphism 1

Metamorphic Grade • One of the primary goals of metamorphic petrology is to interpret P-T conditions under which a rock (or set of rocks) formed • Metamorphic grade – relative temperature and pressure conditions under which metamorphic rocks form • Low-grade metamorphism – T ~200 to 320ºC, relatively low pressure – abundant hydrous minerals • High-grade metamorphism – T >320ºC and relatively high pressure – Dehydration; less hydrous minerals common • Prograde metamorphism – T and/or P, grade of metamorphism increases • Retrograde metamorphism – T and/or P, grade of metamorphism decreases 2

Metamorphic Facies • = set of rocks characterized by equilibrium mineral assemblage that reflect specific range of metamorphic (TP) conditions • Mineral assemblage present depends on protolith composition and P-T conditions – Ex: marble, metabasalt and schist all in amphibolite facies 3

Metamorphic Series • 3. 2. • 1. 2. 3. Progression or sequence of facies across a metamorphic terrain Identified by field and experimental work Contact Series – Hi T, low P = contact metamorphism Dynamic Regional Series – Mod T & P = Barrovian Static Regional Series – Lo T, hi P = Blueschist 4

Metamorphic Phase Assemblages • Specific minerals present within a facies (P-T conditions) depends largely on protolith composition • Equilibrium assemblages can be shown on ternary phase diagrams (with some assumptions and simplifications) • Hypothetical A-B-C – 7 possible minerals can occur at this temp & pressure – Tie lines show pairs of minerals in equilibrium – Smaller triangles show regions where sets of 3 minerals are stable together 5

Metamorphic Phase Assemblages • Changing the P-T conditions changes – Location of the tie lines – Regions of coexisting stable mineral assemblages • Due to chemical reactions between minerals Change in conditions AB + A 2 C => 2 A + ABC 6

Metamorphic Phase Assemblages • Actual metamorphic rocks typically contain ~10 common elements (Si, Al, Mg, Fe, Ca, Na, K, H 2 O, CO 2) • Simplify to plot on ternary diagram – Ignore components (assume present in rock) – Combine components (e. g. , Fe and Mg) – Limit diagram to specific rock type (e. g. , metabasalt, pelite) 7

Metamorphic Phase Assemblages • Useful for plotting most common metamorphic rocks • Limitations: – Assumes quartz present – Assumes Mg & Fe freely substitute • ACF – A = Al 2 O 3 – C = Ca. O – F = Fe. O + Mg. O 8

Metamorphic Phase Assemblages • AFM – A = Al 2 O 3 – F = Fe. O – M = Mg. O • Useful for plotting mafic (metabasalt) rocks, pelites, and some metasandstones • Discriminates between Fe and Mg and allows for solid solution (tie lines) • Limitations: – Assumes quartz present 9

Metamorphic Phase Assemblages • Minerals present reflect equilibrium P-T conditions for different protoliths • As P-T conditions increase (prograde metamorphism) minerals react with each other and with fluids – Some minerals no longer stable; new minerals appear 10

Metamorphic Reactions and Critical Minerals • For a given rock composition, some reactions occur under specific P-T conditions • Critical minerals – = minerals with limited P-T stability – Presence indicates certain metamorphic conditions – Ex: • • Kyanite Andalusite Sillimanite Polymorphs of Al 2 Si. O 5 Al 2 Si 4 O 10(OH)2 <=> Al 2 Si. O 5 + 3 Si. O 2 + H 2 O Pyrophyllite Ky or Andal Qtz fluid 11

Metamorphic Isograds • Reactions and appearance/disappearance of critical minerals can be used to determine P-T conditions • Isograd = “line” to indicate same grade of metamorphism – First appearance of mineral of increasing metamorphic grade 12

Assemblages, Facies, and Series • For a given parent rock composition, mineral assemblage reflects P-T conditions • A variety of different rocks metamorphosed under the same conditions indicate facies • Regional changes between facies indicate series (and tectonic setting) 13

Contact Metamorphism • Result of high geothermal gradient produced locally around intruding magma • Restricted to relatively shallow depths (low pressure) • Rocks generally not foliated • Produce fine grained rocks called hornfels • Contact aureole = surrounding rocks metamorphosed by heat of intrusion • Size/shape of aureole depends on: – Size, temp, cooling history of the igneous intrusion – Properties of the country rock (conductivity, presence of water) 14

Contact Metamorphism: Hornfels • Mineral assemblage characterized by hornfels facies series 1. Pyroxene hornfels – Albite-epidote hfls – Hornblende hfls – Pyroxene hfls – Sanidinite 15

Contact Metamorphism: Skarn • Contact metamorphism of carbonate or siliceous carbonate • Usually involves significant metasomatism (chemical exchange between magmatic fluids and rock) • Can be hosts of Au, Cu, Fe, Mo, Sn, W, Zn-Pb Wollastonite-garnet skarn 16

Regional Metamorphism 4. 3. 2. 1. • Affects large areas of earth’s crust • Produced during orogenic (mountainbuilding) event • Foliated rocks developed under med-high T and P: – 1. Buchan 2. Barrovian • Rocks developed under low-med T and high P: – 3. Sanbagawa 4. Franciscan 17

Regional Metamorphism: Barrovian • “Normal” metamorphic series recognized in mountain belts world-wide • Medium to high T; low to high P • Moderate to high geothermal gradient • Critical mineral sequence: – Kaolinite => pyrophyllite => kyanite => sillimanite • Textural sequence (in pelitic rocks): – Slate => phyllite => schist => gneiss • Facies sequence: – Zeolite => prehnite-pumpellyite => greenschist => amphibolite => 18 granulite

Regional Metamorphism: Franciscan • Low T; high P • Low geothermal gradient • Little directed stress (little deformation) • Distinguished by presence of blue, sodic amphiboles • Facies sequence: – Zeolite => prehnitepumpellyite => blueschist => eclogite 19

Regional Metamorphism and Tectonics • Barrovian (mod-high T and P): – Develop in orogenic belts at convergent margins – Regional heat flow increased by upward movement of magma and migration of fluids – Thrust faulting thickens crust, increasing pressure and directed stress – Geothermal gradient of ~20 -40ºC/km • “Blueschist” (Franciscan) (low T and high P): – Occur world-wide in orogenic belts of Phanerozoic age – Associated with subduction zones • rapid subduction of oceanic crust depresses isotherms – Geothermal gradient of ~10 -20ºC/km 20

Regional Metamorphism and Tectonics • Paired metamorphic belts – specific to subduction zones • Barrovian – Formed in rocks beneath and around the arc – Heat due to magmatic activity • Blueschist – formed in the mélange of the trench 21

Collaborative Activity • Begin working on maps and metamorphic phase diagrams for the homework! 22

Collaborative Activity 23

Collaborative Activity 24