Chapter 7: Metamorphic Rocks
Changes in mineralogy occur from regions of low-grade metamorphism to regions of high-grade metamorphism:
- Low grade: Slight: Chlorite - Medium grade: medium: Garnet -High grade: intense: Sillimanite
Index minerals and Metamorphic Grade
-different T and P conditions produce different minerals -certain minerals, called index minerals, are good indicators of the metamorphic conditions (P,T) in which they form -metamorphic grade is a measure of the degree of metamorphic alteration -T is the more important factor in grade changes
Contact/Thermal Metamorphism:
-driven by rise in T within the host rock occurs due to a rise in T when magma invades a host rock -a zone of alteration called an aureole forms in the rock surrounding the magma -most easily recognized when it occurs at the surface, or in a near-surface environment
protoliths
parent rock
The class of foliated rocks includes:
slate metasandstone metaconglomerate phyllite schist amphibolite gneiss migmatite
Large-scale metamorphism occurs along. . .
- subduction zones at convergent boundaries -several metamorphic environments exist here -important site of magma generation -mountainous terrains along subduction zones exhibit distinct linear belts of metamorphic rocks > high P, low T zones nearest the trench > high T, low P zones further inland in the region of igneous activity
Metamorphic Rock Classification
-Foliated: has a through going planar fabric >subjected to differential stress > has a significant component of inequant minerals > classified by composition, grain size and foliation type -Non-Foliated: no planar fabric evident > crystallized without differential stress > comprised of equant minerals only > classified by mineral composition
Foliation: parallel alignment of platy and/or elongated minerals:
-Recrystallization of minerals in the direction of preferred orientation -compositional banding forms when light minerals separate from dark minerals during recrystallization -preferred mineral orientation develops where differential stress causes the squashing and shearing of rock, of that its inequant grains align parallel with each other
Metamorphic environments/Settings:
-Thermal/Contact metamorphism: heating by a plutonic intrusion -metamorphism along fault zones: Dynamic- shearing in a fault zone -regional/dynamothermal: heat and pressure alteration due to orogenenies -Burial meta.: increases T and P by deep burial in a basin -hydrothermal meta.: alteration by hot water leaching -Impact/Shock meta.: extreme high P attending a bolide impact -subduction: surprising high P, Low P alteration
Regional/Dynamothermal Metamorphism
-associated with mountain building when rocks are buried deeply because of crustal thickening -produces the greatest quantity of metamorphic rock -rocks usually display zones of contact and/or hydrothermal metamorphism
Burial Metamorphism
-associated with very thick sedimentary strata -non-foliated -required depth varies from one location to another depending on the prevailing geothermal gradient
metamorphic
-changed from original "parent" meta=change morph=form or shape -metamorphism can occur to any protolith
Hydrothermal Metamorphism
-chemical alterations from hot, ion-rich water called hydrothermal thermal solutions -it circulates through fissures and cracks that develop in rock -metasomatism: the process by which the overall composition of the rocks changes during metamorphism due to reaction with hot ionized water -most widespread along the axis of the mid-ocean ridge system
Non-Foliated Rocks: Marble (may be foliated)
-coarse, crystalline, monomineral -parent rock was limestone or dolostone -composed essentially of calcite or dolomite crystals -used as a decorative and monument stone -exhibits a variety of colors
Foliated textures: Gneissic Banding
-during higher grades of metamorphism, ion migration results in the segregation of minerals -gneissic rocks exhibit a distinctive banded appearance (compositional or gneissic banding)
Hydrothermal Fluids:
-enhance migration of ions by dissolving minerals -aid in recrystallization of existing minerals -accelerate chemical reactions -change composition of rock: >elements from dissolved minerals may be leached out >dissolved ions moved by fluid flow along fractures
Common metamorphic rocks- Phyllite
-fine grained, fine mica-rich rock -a gradation in the degree of metamorphism between slate and schist -platy minerals not large enough to be identified with the unaided eye -low-medium grade metamorphism -clay minerals neocrystallize into tiny micas -the fine micas reflect light yielding a satiny sheen -glossy (silky) sheen and wavy surfaces -exhibits rock cleavage -composed mainly of fine crystalls of muscovite and/or chlorite
Common metamorphic rocks: Schist
-fine to coarse grained rock with large micas -medium to high grade metamorphism -platy minerals predominante -schist often has other minerals due to recrystallization (Quartz, feldspar, Kyanite, Garnet, Staurolite, Sillimanite) -large, non-mica minerals are called porphyroblasts -to indicate composition, mineral names are used > ex: mica schist, garnet schist
Migmatites
-highest grade of metamorphism that is transitional to igneous rocks -contain light bands of igneous components along with areas of unmelted metamorphic rock -mineralogy controls behavior -when a gneiss is heated, felsic minerals melt first, mafic minerals tend to retain their metamorphic character
Hydrothermal Fluids cont.:
-hydrothermal alteration is called metasomatism -fluids transmit to: >pore spaces of sedimentary rocks > fractures in igneous or metamorphic rocks
Pressure (stress)
-increases with depth -confining (lithostatic) pressure applies forces equally in all directions (it is isotropic) resulting from burial -rocks may also be subjected to differential stress which is unequal in different directions (anisotropic) and the rock is consequently distorted; it tends to shorten rocks in the direction of pressure and lengthen them in the direction perpendicular to that force
grade of metamorphism
-is reflected in the texture and mineralogy of metamorphic rocks -texture refers to the size, shape, and arrangement of grains within a rock
Non-Foliated Metamorphic Rocks
-lack foliation -develop in environments where deformation is minimal -typically composed of minerals that exhibit equidimensional crystals -marble (P.R. limeston); quartzite (P.R. quartz sandstone), amphibolite (P.R. basalt or gabbro), and hornfels (created by heating)
Agents of Metamorphism: Hydrothermal Fluids
-mainly water with other volatile components from 3 sources: >trapped in the pore spaces of sedimentary rocks >within magma >the dehydration of water bearing minerals, such as clays, amphiboles, and micas
Common metamorphic rocks: Gneiss
-medium to coarse grained -banded appearance -high-grade metamorphism -often composed of white or light-colored quartz and feldspar-rich layers with bands of dark ferromag. minerals such as biotite, amphibole, pyroxene
Dynamic Metamorphism
-metamorphism along fault zones -occurs at depth and high temperatures -pre-existing minerals deform by ductile flow -differential stress -shear stress -mylonite forms in a shear zone
Foliation
-metamorphism leads to the development of metamorphic mineral assemblages and metamorphic foliation -during regional metamorphism rocks typically display a preferred orientation called foliation -foliation= any planar (nearly flat) arrangement (preferred orientation) of inequant (platy or elongate) mineral grains or structural features within a rock
Metamorphism and Plate Tectonics:
-most metamorphism occurs along convergent plate boundaries -compressional stresses deform the edges of the plate -formation of the earth's major mountain belts including the Alps, Himalayas, and Appalachians
Composition of the parent rock:
-no new elements or chemical compounds are added (except water) -mineral content of the metamorphic rock is controlled by the chemical composition of the parent rock
Non-Foliated Rocks: Quartzite
-parent rock= quartz-rich sandstone -quartz grains are recrystallized and fused together -quartzite is hard, glassy, and resistant
Shock Metamorphism
-periodically, earth is struck by hight speed projectiles called meteorites > comets > asteroid -bolide impacts generate a compressional shock wave > extremely high P > heat that vaporizes or melts large masses of rock > products are called impacites - these conditions generate high P minerals: > coseite > stishovite
Other metamorphic textures:
-porphyroblastic textures -large grains (porphyroblasts) surrounded by a fine-grained matrix of other minerals -typically garnet, staurolite, and/or andalusite
Foliated textures: Slaty cleavage
-rock cleavage is a type of foliation in which rocks split cleanly into thin slabs along surfaces where platy minerals are aligned -slaty cleavage where rocks can be easily split into thin, tabular sheets -closely spaced planar surfaces along which rocks split
Foliated textures: Schistosity
-schistosity is a type of foliation defined by the parallel alignment of medium to coarse-grained platy minerals (mica flakes) -platy minerals are discernible with the unaided eye and exhibit a planar or layered structure -rocks having this texture are referred to as schist
Subduction Metamorphism
-subduction zones create a unique metamorphic facies called blueschist (can only form in subduction zones) -trenches and accretionary prisms are characterized by. . . > a low geothermal gradient; low T > high P -these conditions favor glaucophane, a blue amphibole mineral
Metamorphic Grade
-systematic variations in the mineralogy, foliation and often the texture of metamorphic rocks are related to the variations in the degree of metamorphism -refers to the series of T and P regimes under which metamorphism takes place
Heat
-the most important agent -provides the energy to drive chemical reactions that results in the: recrystallization results in new stable minerals -two sources of heat: heat from magma and hot groundwater (leads to contact metamorphism) an increase in temperature with depth due to the geothermal gradient
Slate
-very fine-grained, clay-rich metamorphosed shale -foliated, has excellent rock cleavage-slaty cleavage -most often generated from low-grade metamorphism of shale, mudstone, or siltstone
metamorphism does NOT include:
-weathering -diagenesis -melting
Progression from low grade to high grade metamorphism:
Foliated: - Shale - Slate -Phyllite - Schist - Gneiss - Migmatite Non-Foliated: - Quartzite - Marble -as the grade increases, grain-size increases)
due to change in physical or chemical conditions:
burial tectonic stresses heating by magma fluid alteration
the class of non-foliated rocks includes:
hornfelse marble quartzite
Protoliths undergo pronounced changes in. . .
texture mineralogy
During metamorphism. . .
the roc must remain essentially solid
