Earth Mat Flashcards - Ch 17
equal diameters in all directions and assume forms approximated by spheres or cubes.
Equant
one direction in which the grain diameter is not equal to the other grain diameters
Inequant
A1 < A2,A3
What shape
Tabular
A1 > A2=/A3
What shape
Bladed
A1 > A2=A3
What shape
Acicular
What divides aphanitic and phaneritic
Aphanitic < 1mm > Phaneritic
are large relict grains from the protolith that have experienced deformation but have retained their original composition.
Porphyroclasts
In gneiss porhyroclasts, Felds and Qtz are
Augen and Flaser
are large grains that have experienced neocrystallization and growth in response to favorable temperature and pressure conditions during metamorphism.
Porphyroblasts
no orientation of inequant grain = no foliation
Random grain orientation
sub-parrallel inequant grain = foliations and lineations
Preferred grain orientation
On the basis of grain orientation, metamorphic rocks can be divided into two major groups:
Rocks with non - foliated textures
Rocks with foliated textures
commonly produced by dynamic metamorphism, contact metamorphism or burial metamorphism
Non-foliated metamorphic rocks
texture is a fine-grained (<1.0 mm diameter), non-foliated fabric that develops by contact metamorphism, producing a rock called hornfels
Hornfelsic
Hornfels develops in
metamorphic aureoles, adjacent to igneous intrusions.
textures are characterized by large (> 1.0 mm diameter) equant grains or large inequant crystals that lack preferred orientation
Granoblastic
textures occur in granulites, associated with
deep burial (high PT)
common rocks with granoblastic textures
Metaquartzite, marbles
contain > 90% quartz and are derived from quartz - rich sandstone or chert protoliths
Metaquartzites
are granoblastic metamorphic rocks rich in calcite and/or dolomite.
Marbles
recrystallization of limestone or dolostone protoliths via
dynamothermal, deep burial or contact metamorphism
are granoblastic calc - silicate rocks formed by contact metamorphism of carbonate country rocks such as limestone or dolostone.
Skarns , also known as tactites,
Metamorphic rocks with ____________are composed of fractured, angular particles that form in response to the brittle crushing of grains during deformation in upper crustal fault zones
cataclastic textures
Cataclastic rocks that lack cohesions are either _____(<2mm) or _____(>2mm)
Breccia, Gouge
are derived from metamorphism of sedimentary or igneous breccias
Metabreccias
10-50% matrix cataclasite
Protocataclasite
50-90% matrix cataclasite
Cataclasite
90-100% matrix cataclasite
Ultracataclasite
Fault reactivations can be caused by
cataclasites rupturing more easily than the host rock
are glassy rocks produced by high strain rates generating localized melting in fault zones
Pseudotachylites
are high strain rate cataclastic rocks created by the tremendous short - term stresses associated with extraterrestrial rock bodies impacting Earth
Impactites
that form as rocks locally melted from impact and showers down like droplets as it cools
Glassy spherules called tektites
that form due to the intense stresses that deform crystal structures
Deformation lamellae
Coesite first discovered at
Meteor crater, AZ
are non-crystalline, high grade coals that form by heating, compressing and chemically altering bituminous coal.
Anthracites
are derived from conglomerate protoliths and contain sub - rounded to rounded relict clasts with diameters > 2 mm.
Metaconglomerates
form by the metamorphism of conglomerates and/or breccias in response to strong non - uniform stress during dynamothermal or dynamic metamorphism.
Stretched pebble metaconglomerates
Metamophosed pebbles/cobbles are
short/flattened at Z strain, elongated at X strain direction
are serpentine-rich metamorphic rocks that occur in non - foliated or foliated forms.
Serpentinites
Serpentinization is the where
ol and px are hydrated to form serpentines
are fine - grained rocks that form through the alteration of ultrabasic rocks, or magnesium - rich sedimentary rocks such as dolostone, by low temperature and low pressure hydrothermal fluids.
Soapstones
are green - colored rocks rich in silicate minerals that commonly include chlorite, epidote, prehnite, pumpellyite, talc, serpentine, actinolite and albite.
Greenstones
Many __________(sodium - rich basalt) and _____________(sodium - rich andesite) occur in greenstones.
spilites; keratophyres
Greenstone sequence (basal to surface)
Metavolcanics (komatiites) and metabasalts
-> inter/silicic metavolcaninc and metavolcaniclastic
-> graywackes and chert.
Greenstone belts commonly parallel _____________ containing rocks of granitic to dioritic composition metamorphosed at high temperatures and pressures
granulite belts c
Greenstone belts; Extensive km wide, Archean and Proterozoic cratons
Ex.
Barberton Belt, S. Africa
Eastern Goldfields, W. Aus
Superior & Slave Cratons, NA
Sao Francisco Craton, Brazil
Greenstone belt formed by
Formed from H2O + CO2 metasomatism of B and UB rocks
Why are greenstone belts uncommon in the Phanerozoic Eon?
related to lower geothermal gradients in the Phanerozoic as well as changes in plate tectonics
Precambrian metamorphism = high grade in shallow depths; prevention of deep subduction
are dark - colored rocks composed largely of amphiboles, such as hornblende, and plagioclase
Amphibolites
Amphibolite from basic Igni protolith
Ortho-Amphibolites
Amphibolite from sedimentary protolith
Para-amphibolites
metamorphism in granulites
(>800°C) and high pressure (> 10 kbar; ~ 33 km depth) metamorphism
High density of ___________ may drive subduction (slab-pull effect)
eclogite
are very high pressure, high temperaturerocks that develop principally from basalt/gabbro protoliths.
Eclogites
are fine - grained, aluminum - rich, pelitic rocks that possess fl at, planar cleavage.
Slates
________ ° C, slates are transformed to Phyllite
250
In the case of slates, which minerals are stable in low temps? in high temps
kao, smec, zeo;
illite, chl, mica
In anoxic envi: oragnics recrystallize to
graphite
display phyllitic cleavage, characterized by larger crystals and more wavy surfaces than slaty cleavage
Phyllites
At 250 - 300°C smec and illite transforms to
seri, musc, talc, chl
_____ phyllites are transformed gradationally into schist
~300 ° C,
is a very common foliation defined by the sub - parallel arrangement of macroscopic platy minerals such as phyllosilicates in closely spaced metamorphic layers.
Schistosity
Schists are produced by _metamorphism at convergent plate boundaries with temperatures >300 ° C
dynamothermal
are characterized by gneissic layering, a foliation characterized by the arrangement of minerals into distinct color bands.
Gneisses
Gneisses that develop from an igneous protoliths are called
orthogneiss
gneiss from sedimentary protoliths are called
paragneiss
which refers to the thin, sill - like intrusion of magma into parallel country rock layers
“ Lit par lit intrusion ” ,
Gneisses form in dynamothermal settings at temperatures that commonly exceed
~ 600 ° C.
processes have been suggested for the development of gneissic banding:
Originate by layer transposition.
Lit Par Lit
Anatexis
Layer transposition process
Transposition results from the pulling apart of earlier folded layers resulting in the separation of hinges and limbs
Insoluble phyllosilicates in fold limbs and soluble qts/felds in hinge, tightly compressed forming alternating bands
are “ mixed ” rocks that possess textural and structural characteristics of both igneous and metamorphic rocks
Migmatites
are silica - and iron - rich rocks that formed primarily in the Early Proterozoic and Archean.
Ironstones
are metaquartzites that contain 20 – 30% iron and are also commonly banded
Taconites; found largely in lake superior
Metamorphic ______ are pervasively deformed rocks so that their original composition and texture are largely obliterated.
tectonites
tectonites are Classified by solid state flow fabric from deformation
what are they
S tectonites = Foliated
L tectonites = lineation, no foliation
L-S tectonites = Foliation, Lineation
are rock structures or textural elements that provide information concerning the relative sense of displacement of rock components within fault zones or shear zone
Shear sense indicators
indicate the sense of displacement in shear zones and may also provide information on displacement distance.
Offset markers
allows displacement distance to be measured; intersections by linear features and planar faults/shear zones
Piercing Points
are asymmetrical porphyroclasts or poryphyroblasts with mineral tails that “ point ” in the direction of shear.
Grain tail complexes
Mineral tails may form by a combination of:
Plastic flattening of pre-existing grains
Pressure soln from grain center
Dynamic. recryst. at the rim of the grain
Neocryst in pressure shadows around grain
____________ grain tail complexes consist of wedge - shaped tails that do not cross the reference plane of shear
slow grain rotation; fast tail growth
Tail growth at minimum compressive stress (45° to ref plane) during rotation
Sigma (σ)
_________ tail complexes are produced by relatively rapid grain rotation relative to tail growth rate
Fast grain rotation = bending of inner portion of the tail, crosses reference plane
Delta (δ) grain
fractures
oriented at shear direction
<45° inclination from foilation
Synthetic Fractures
fractures
oriented against shear direction
>45° inclination from foilation
Anti-synthetic Fractures
develop in mylonitic, schistose and gneissic rocks subjected to ductile shear.
S - C foliations
in S-C
The letter S represents __________ and the letter C is for ____________, a French term for shear direction, which lies in the C plane
Schistosity; cisaillement
