ESSC 110- Lecture 11 Metamorphism and Metamorphic Rocks

How do metamorphic rock form

How do metamorphic rock form

when a pre-existing rock, protolith, undergoes a solid-state change in response to the modification of its environment

Metamophism

can alter any photolith through a slow solid-state change in texture and mineralogy

Metamorphic changes are due to variation in:

temperature, pressure, tectonic stress, and amount of reactive water

Example of metamorphism changing mineralogy

-red shale(protolith)-quartz, clay, and iron oxide -Gneiss (metamorphic rock)-quartz, feldspar, biotite, and garnet

how do metamorphic rocks change mineralogy

they contain new minerals produced through metamorphism, make up metamorphic assemblage

Metamorphism changes texture

unique texture, intergrown and interlocking grains manifested by metamorphic foliation

Foliation

texture defined by alignment of platy minerals(mics) or creation of alternating light/dark bands

what are the 5 metamorphic processes

1-recrystalization 2-phase change 3-metamorphic reaction 4-pressure solution 5- plastic deformation

Recrystallization

change in shape and size of grains whithout changing the identity of the minerals making up the grains ex) limestone-marble

Phase change

transforms one mineral into another mineral with the same composition but another crystal structure

Neocrystallization

metamorphic reaction, new minerals form from old. The initial minerals become unstable, change to new minerals, and the original protolith minerals are digested in reactions. Elements restructures to form new mineral assemlage ex) shale-> garnet mica schist

Pressure Solution

wet rock is squeezed more strongly in one direction and mineral grains dissolve where their surface meets another grain producing ions, precipitation of the ion water in less pressure

Plastic deformation

mineral grains soften and deform, requires elevated temp and pressure, rock is squeezed or sheared, minerals change shape without breaking and therefore behave plastically

Causes of metamorphism

the agents of metamorphism are heat, pressure, compression and shear, and hot water. not all these agents are required but often co-occur and rocks may be overprinted by multiple events

Characteristics of metamorphism due to heat

-occurs between 250 degC and 850 degC -heat energy breaks and reforms atomic bonds -the sources of heat are the geothermal gradient, magmatic intrustions or compression 0the depth to metamorphic T varies with tectonic setting

Characteristics of metamorphism due to pressure

P increases with depth in the crust, metamorphosism occurs mostly in 2-13km with 270-300 bars per km -increase in pressure packs atoms more tightly together, creating denser minerals, involves phase change or neocrystalization

Characteristics of metamorphism due to pressure and temperature

Mineral stability is dependent on P and T changes in T and P lead to changes in minerals, illustrated by the three polymorphs Al2SiO5-

Differential Stress

stress greater in one directio -different than pressure, which is equal in all directions -differential stress is a common result of tectonic forces

orogenesis

creates horizontal compression (push)

rifting

creates horizontal extension (pull apart)

two kinds of differential stress

normal and shear

Normal stress

operates perpendicular to a surface -tension (pull apart) -compression (push together normal stress)

Shear stress

moves one part of a material sideways, causes material to be smeared out, rolling dough

Compression and shear stress combine with elevated T and P causes

rocks to change shape without breaking, internal textures of deforming rocks can also change (minerals rotate into preferred orientations, and grow relative to stretching)

Preferred orientation

differential stress causes mineral shapes to align in preferred orientation of inequant minerals is a common feature of metamorphic rocks

How does preferred mineral orientation develop

1-pressure solution (occurs in wet rocks at low T) -minerals dissolve at compressed faces, grow where compression is less, grains become shorter, parallel to compression) 2-plastic deformations (occurs at higher temp) -existing grains flatten by deforming internally 3-shear rotation and flattening (flattens and rotates into alignment)

Hydrothermal fluid metamorphism

Hot water with dissolved ions and volatiles is hydrothermal fluid that facilitates metamorphism by accelerating chemical reactions and alter rocks by adding or subtracting elements, hydrothermal alteration is called metasomatism

What is foliation and what is it due to

parallel planar surfaces or layers in metamorphic rock, gives the rock a streaked or striped look, foliated rocks often break along foliation planes, these are due to preferred inequant mineral orientation, and compositional banding

What are the types of metamorphic rock

foliated and non-foliated

Foliated metamorphic rocks

have though going planar fabric, subjected to differential stress, have a significant component of platy minerals, classified by composition, grain size, and foliation type

Nonfoliated metamorphic rocks

no planar fabric evident, minerals recrystalized without compression or shear, comprised of equant minerals only, classified by mineral composition

Types of foliated metamorphic rocks

1-slate 2-phylite 3-metaconglomerate 4-schist 5-gneiss

Slate

fine grained, low grade metamorphic shale. -has a distinct foliation called slaty cleavage -develops by parallel alignment of platy clay minerals, develops clevage perpendicular to the compression, and breaks along foliation, creating sheets used for roofing

Phyllite

fine grained mica rich rock, formed by low to medium grade alteration of slate. Clay minerals neocrystalize into tiny micas. Has a silky sheen called phyllitic luster

Metaconglomerate

metamorphed conglomerate, where grvel clasts are flattened by pressure solution, plastic deformation. foliation is defined by the flattened gravel clasts

schist

fine to coarse rock with larger micas, mediup high grade metamorphism, and has a distinct foliation from large micas called schistosity, schist has mineral assemblage reflecting the protolith

Gneiss

distinct compositional bands, often contorted, light bands of felsic minerals (quartz and feldspars) and dark bands (mafic minerals like biotite and amphibole)

How does compositional banding develop

original layering in the protolith, extensive high T shearing and metamorphic differentiation -solid state differentiation, chemical reactions segregate light and dark layers

Migmatite

is a partially melted gneiss, features from igneous and metamorphic rocks, the felsic bands melt and recrystallize in the gneiss

Characteristics of nonfloated metamorphic rocks

lack a planar fabric, becuase the rock is not subjected to differential stress, it does have a dominance of equant stress, and has an absence of platy minerals like clays or micas

Types of nonfoliated metamorphic rocks

hornfels, amphibolite, quartzite, marble

Hornfels

alteration by heating, associated with plutonic intrusions, finely crystalline

Amphibolite

dominated by dark amphibole minerals, they have a basalt or gabbro protolith, usually not well foliated, quartz and feldspar poor

Quartzite

almost pure quartz in composition, forms by alternation of quartz sandstone. Sand grains in the protolith recrystallize and fuse. It is hard, glassy, resistant, breaks by conchoidal fracture

marble

coarsely crystalline calcite or dolostone protolith. Completely changed rock, many colors, used for sculptures

Index mineral maps

define metamorphic zones, where boundaries are isograds

Metamorphic facies

mineral assemblages from specific protolith at specific P and T conditions, create rocks that are similar, and named for dominant mineral

Metamorphic environments

different setting yeild different effects via the variation in geothermal gradient, changing gradients of differential stress, and variability in the nature of hydrothermal fluids, and are all governed by plate tectonics

types of meta morphism

 Thermal—heating by a plutonic intrusion  Burial—increases in P and T by deep burial in a basin  Dynamic—shearing in a fault zone  Regional—P and T alteration due to orogenesis  Hydrothermal—alteration by hot water leaching  Subduction—high P, low T alteration  Shock—extreme high P attending a bolide impact

How do metamorphic rocks return to the surface

exhumation is due to the uplift, collapse and erosion

where to find metamorphics

large regions of ancient high-grade rocks, called shields, are exposed in continental interiors

Sheilds

are eroded remnants or orogenic belts, sheild rocks form the basement under sedimentary cover