metamorphic petrology Flashcards
metamorphic rocks
metamorphism
Metamorphic rocks: form through transformation of pre-existing igneous, sedimentary, and metamorphic rocks brought on by changes in the prevailing intensive variables (P and T) and fluids.
- characterize mountain belts, ubiquitous in the mantle (e.g., peridotites) - processes generally not visible at the Earth’s surface (except hydrothermal) - transformation not always complete (allowing reconstruction of past history)
Metamorphim: process or set of processes that affect rocks in such a way as to produce textural changes, mineralogical changes, or both under conditions in the Earth between those of weathering and diagenesis (i.e. lower limit) and melting (upper limit).
- textural changes without mineralogical changes: 2 types
a) recrystallization (e.g., limestone to marble, sandstone to quartzite)
b) cataclasis: crushing/breaking of mineral grains, associated with deformation
- Mineralogical changes:
Neocrystallization: formation of new minerals involving chemical reactions
Evidence of metamorphism:
1) different textures (foliation, cleavage, etc.)
2) new minerals
agents of metamorphism
Physical: temperature, pressure, directed stress
Chemical: chemically active fluids (H2O; CO2, CH4, NaCl, S, etc.)
Temperature:
Lower limit (vs. diagenesis): not well defined: 100-200 °C
Upper limit (vs. anatexis/partial melting): variable: 750-800 °C
melting dependent on the composition of the protolith, presence or absence, types and quantity of fluid phases, and pressure: e.g., wet granites: as low as 600 °C, dry ultrabasic rocks, up to 2000 °C.
Most common temperature range: 150-750 °C (extreme ones do exist)
temperature agent
Temperature invariably increases with depth: rate – geothermal gradient:
closely related to heat flow through the crust usual range: 15-30 °C/km, but 5-60 °C/kmMain sources of heat:
1) heat flowing into the base of the crust from the mantle
2) radioactive decay within the crust (U, K, higher in the continental crust)
3) migrating magmas
4) minor/local sources: friction, fluids, etc
. NB: geothermal gradient varies with depth
steeper near the surface (why)
pressure/directed stress agent
Stress: defined as force per unit area
Pressure: uniform stress confronts the rock equally in all directions
Pressure created by the load of the overlying rocks is called to as Pload (or lithostatic P):
Pload = ρgh where ρ is density; g, acceleration due to gravity; h, depth)
Generally, in the crust increases by 0.1 GPa (giga-pascal) for every 3.3 km of burial
or 0.26 to 0.32 GPa per 10 km depending on compositions (i.e. density)
very high pressures in the mantle and core
Most metamorphic rocks from the crust or the uppermost mantle: 0.1 to 1.5 GPa
Ultra-high-pressure metamorphic rocks: containing diamond, coesite, stishovite
Pressure created by trapped fluid phases is called Pfluid (hydrostatic pressure): PH2O, PCO2
Lithostatic pressure versus hydrostatic pressure: work against each other
hydraulic fracturing occurs when Pfluid > Pload
chemically active fluid agents
Most rocks contain a volatile phase (called fluid; because usually in a supercritical state)
Solid-state reactions versus fluid-assisted reactions
Evidence for a fluid phase:
1) fluid inclusions
2) formation of hydrous or carbonate minerals
3) whole-rock analysis (LOI)
4) isotopic studies
5) presence of veins in metamorphic rocks
6) active metamorphism in modern geothermal fluids (Yellowstone NP, black smokes)
7) common dehydration and de-carbonation reactions
Fluid: usually dominated by H2O, but CO2, CH4, N2, Cl, S, B, F, Na, K, Ca
Fluids produce chemical potential gradients: transfer ions, carrying heat, participate in reactions
classification of metamorphism
Area or volume of rocks affected: regional vs contact
Chemical nature: metasomatism (alteration) produced by fluids
Dominant agent: pyrometamorphism, dynamic metamorphism
Many other names:
Burial metamorphism, impact metamorphism
Prograde metamorphism: relative to T or P
Retrograde metamorphism: from high Ts to low Ts
naming and classification of metamorphic rocks
NO IUGS classification
Four main criteria:
- Nature of the protoliths:
- metasedimentary rocks or more specific: marble, quartzite, etc.
- metavolcanic rocks: metabasalts, metadacites, etc.
Original materials argillaceous or clay-rich sediment arenaceous or sandy sediment clay-sand mixture quartz-sand marl limestone basalt
Metamorphic rock type (adjective) pelite(pelitic) psammite(psammitic/quartzofeldspathic) semi-pelite quartzite calc-silicate(calcareous) marble metabasite(mafic metavolcanic)
- mineralogy
Essentially monomineralic rocks named after their dominant minerals: quartzite, serpentinite, hornblendite, etc.
Names of particularly abundant or diagnostic minerals often used as qualifiers: garnet mica schists, diopside marble, etc.
Two conventions: order of abundance or specific conditions
- Sillimanite muscovite schists
- Sillimanite K-feldspar schists
- rock textures
Textural terms indicate whether or not oriented fabric elements are present to dominate the rock’s appearance (and the scale on which they are developed)
Preferred orientation: best developed in pelites and semi-pelites by alignment of phyllosilicates (e.g., micas, chlorite, etc.), aligned perpendicular to the maximum compression direction giving rise to a Planar fabric (called foliation)
Some names according to grain sizes:
Slates: strongly cleaved rocks in which the cleavage planes are pervasive;y developed throughout the rocks, due to orientation of very fine-grained phyllosilicates. The individual aligned grains are too small to be seen with naked eyes, and the rock has a dull appearance on fresh surfaces
Phyllites: similar to slates but slightly coarser phyllosilicate grains are sometimes discernible in hand specimens and give a silky appearance to the cleaved surfaces. Often the cleavage surfaces are less perfectly planar than in slates.
Schists: characterized by parrallel alignment of moderately to coarse phyllosilicate grains, usually clearly visible with naked eyes (the fabric is called schistosity)
Gneisses: coarse-grained and foliated (sort of planar fabric or compositional layering) rocks:
orthogneisses: gneisses of igneous parentage
paragneisses: gneisses derived form sedimentary rocks
Mylonites: fine-grained rocks produced in zones of intense deformation: protomylonite, mylonite, ultramylonite, pseudotachylyte: thin seams of glassy rocks formed from melting
Hornfels: contact metamorphism in the absence of deformation gives rise to a random fabric of interlocking grains - special names
Greenschists: green foliated metabasites, usually predominantly of chlorite, epidote, actinolite (green in color, greenschist-facies conditions)
Blueschists: dark, lilac-grey foliated metabasites, owing its color to the presence of sodic amphiboles (glaucophane or crossite), seldom truly “blue” in hand specimens
Amphibolites: essentially bimineralic rocks made up of hornblende and plagioclase (orthoamphiblite versus paraamphibolite)
Serpentinites: green, black or reddish rocks composed predominantly of serpentine-group minerals, formed by hydration of igenous or metamorphic peridotites (olivine-rich ultrabasic rocks)
Eclogites: essentially bimineralic rocks of garnet and omphacitic clinopyroxene without plagioclase. Common accessories: quartz, kyanite, amphiboles, zoisite, rutile, diamond, coesite, etc.
Granulites: rocks characterized both a texture and mineralogy indicative of very high temperature metamorphism. Charnockite, distinct variety of K-feldspar and Opx bearing granulite (granitic in composition)
Migmatites: “mixed rocks” composed of a schistose or gneissic portion intimately mixed with veins of apparently igneous quartzo-feldspathic material (leucosome)
Skarns: rocks composed predominantly of calc-silicate minerals
textures of regional metamorphism
Tectonite- a deformed rock with a texture that records the deformation
Fabric- the complete spatial and geometric configuration of textural elements
Foliation- planar textural element
Lineation- linear textural element
types of foliation
refer to picture in slide
useful criteria to describe spaced foliation
refer to picture in slide
continuous schistosity
developed by dynamic recrystallization of biotite, muscovite, and quartz. a. Plane-polarized light, width of field 1 mm.
crenulation cleavage
refer to notes
stages in development in crenutation cleavage
. Stages in the development of crenulation cleavage as a function of temperature and intensity of the second deformation
refer to notes
before and after deformation
refer to notes
pre kinetic crystals
notes
