geology of plutonic rocks. igneous plutonic rocks formed – –900 degree c –50 km depth uplift...

Geology of Plutonic Rocks

Igneous plutonic rocks

• Formed – – 900 degree C – 50 km depth

• Uplift to earth surface

• Enormous decrease in confining pressure

Extrusive

Intrusive orplutonic

Shield regions

• Sweden is an example• roots of former mountain ranges, • stable interior, • resembles granite but • complex history • often formed by extreme

metamorphism rather than by solidification from a melt. Fig 6.1

Mountains – complex Mountains – complex folding folding

Mountains Mountains worn to worn to flat landflat land

• By the Precambria

n –

MagmaMagma molten rock within the earth LavaLava on the earth

Geothermal gradient

• varies • crust thicker in

continental areas– normal rise in

temperature with depth of between 10 to 50 C per km

• crust thinner in oceanic areas

increased tempurature dueto igneous intrusion

normal rise in temperature with depth of between 10 to 50 C per km

Question

• Where does magma form?

• In the crust and upper mantle NOT in the center of the earth

subduction relation

• crustal rockscrustal rocks subducted melt at a lower temperature than do oceanic oceanic rocksrocks– two magma producing events

1. subduction - water rich ocean plate

• the rise of the moisture through the overlying rocks lowers their melting point and initiates melting

2. subduction - heat increases with depth

• the crustal rocks begin to melt and mixes with the magma derived from the mantle

Forms of igneous intrusions

• sheets – layer of intrusion• pluton – irregular body• dikes – vertical sheet

intrusions• sills – horizontal sheet

intrusion• laccoliths – lens shaped • ring dikes, cone sheets –

a cone shaped intrusion• dike swarm – several• pipe of neck – source of

nourishment of a volcano • batholiths – largest body

of an intrusion

• stocks – smaller intrusive body• xenoliths – country rock mass

surrounded by intrusive rocks• roof pendants – inliers of

metamorphic rocks• pegmatites – coarse grained

intrusions• aplites – fine grained intrusions• stratiform complexes –

layered• flow bedding – segregation of

layers• lopolith and cone sill –

mineral deposits


• pluton – irregular body• dikes – vertical sheet


intrusion• laccoliths – lens shaped • ring dikes, cone sheets –

a cone shaped intrusion• dike swarm – several• pipe of neck – source of

nourishment of a volcano • batholiths – largest body

of an intrusion


• pluton – irregular body• dikes – vertical sheet


intrusion• ring dikes, cone

sheets – a cone shaped intrusion

• dike swarm – several• pipe of neck – source of

nourishment of a volcano • batholiths – largest

body of an intrusion


• xenoliths – country rock mass surrounded by intrusive rocks


• pegmatites – coarse grained intrusions• aplites – fine grained intrusions

Forms of igneous intrusions• stratiform complexes – layered

• flow bedding – segregation of layersid• lopolith and cone sill – mineral deposits

Classification of plutonic rocks Fig 6.6

• Few common minerals – their abundance is the basis for classification

• Basic or Mafic rocks – contain minerals with a high melting point and silica content of ca 43 – 50%

• Acidic or Felsic rocks – contain minerals with low melting point and silica content of 65 – 72%

• Intermediate – have silica contents of 50 to 65%

Texture

Textures – normal slow cooling produces sand size interlocking crystalline grains

• Phenocrysts – coarser grains• Porphyry – contains numerous coarse grains

in an otherwise fine grained mass• Coarse crystalline – grains > 2mm• Medium crystalline – grains 0.06-2mm• Fine crystalline – grains < 0.06 mm• Aphanitic – crystals not visible • Phaneritic –visible grains

Texture

• Phenocrysts – coarser grains• Porphyry – contains numerous coarse

grains (phenocrysts) in an otherwise fine grained mass

Rock names Fig 6.6!!!• Granite• Diorite• Gabbro• Peridotite (ultra basic)

• Dunite (untra basic)

•Rhyolite•Andesite•Basalt

•Granodiorite

•Diabas or dolerite

•Anorthosite

•Syenite

•Tonolite•Monzonite•Porfyr

intrusiveintrusive

extrusiveextrusive

OTHERS?OTHERS?

The three components, Q (quartz) + A (alkali (Na-K) feldspar) + P (plagioclase)

Phaneritic – visible grains

Serpentinite

• an altered ultra basic, peridotite (olivine) has been an altered ultra basic, peridotite (olivine) has been replaced by the mineral serpentinereplaced by the mineral serpentine

• this is a chemical weathering process which is associated this is a chemical weathering process which is associated with a with a 70% volume increase70% volume increase

• this increase in volume results often in the internal this increase in volume results often in the internal deformationdeformation of the rock; fracturing and shearing of the rock; fracturing and shearing

jointing in granitic rocks

• arise from general crustal strain, cooling, and unloading

Sheet joints

• typical for igneous rocks, called also exfoliation joints or lift joint

• no sheet joints below 60 m• Sheet joints conform to the

topography, fig 6.12a, 6.10a • slopes steeper than the angle of

friction, ca 35 degrees, tensile fractures develop and wall arch, an overhang

• sheet jointing is well developed in igneous rocks, but not exclusive, it also occurs in soils and other rocks to some extent

• Formed – – 900 degree C – 50 km depth

• Uplift to earth surface

• Enormous decrease in confining pressure

Sheet weathering due to unconfinement

Joints due to relaxation

two to thee preferred directions of joints two to thee preferred directions of joints is common, is common, joint setjoint set

Question

• ??Why is sheet jointing more prominent in igneous rocks than other rocks?

• Unloading is one of the main reasons. • Igneous rocks are formed at up to 50 km

depth. With 27Mpa/Km times 50 km = 1350 MPa pressure at the time of formation; uni directional!! Upon uplift this pressure is reduced and the rocks relax, with a vertical unload stress of 27 MPa.

unloading

unloading in tunnels – different names for different rocks – for igneous rocks it is called:

• Popping rock - is a term used in underground operations where the rock pops off the rock face. This can be very violent and is due to the unloading due to the underground excavation

weathering in plutonic rocks

• physical weathering – mechanical breakdown of earth material at the earth surface. Ex. Heating/cooling, wetting/drying, plants and animals including man.

• chemical weathering – chemical decomposition due to a chemical reaction changing the composition of the earth material, ex carbonic acid replacing silicate minerals, feldspar changing to kaolin, mica changing to limonite and kaolin.

chemical weathering –

• acts on igneous minerals in the order of solidification

• Bowen’s reaction series (fig 6.6)

• high temperature minerals are more rapidly affected

• low temperature minerals more stable

chemical weathering –

• Basic and ultrabasic – form montmorillonite clays

• Grainitic rocks – form kaolinites

Weathering profiles

• form relative rapidly in granitic rocks

• a layer of clay minerals forms at the surface

• by the continuous downward percolation of water and carbon dioxide

• in the vadose zone above the water table

Spheroidal weathering

• common in jointed igneous rocks where the

• percolation of water is concentrated to the joints

• the fresh rock delineated by the fractures is slowly effected but

• the corners are more rapidly effected thus spherical shapes are formed

Joints enhance weatheringJoints enhance weathering

Paleozoic – Sweden was near the equator

• Rounded rock mass due to weathering

Exfoliation – is formed in the spheres by chemical expansion in the weathering granite

•Rounded blocks due to chemical weathering

•Open joints

It is clear that this is “granite” by the way it weathers

Saprolite• decomposed

granite, residual material formed from weathering resulting in a residual soil

Description of a residual soil is “fuzzy”

two variables• I. the degree of weathering of the

rock

• II. the abundance of altered minerals

Classes of weathering of igneous rocks

• Several different classification systems

• Different authors

All contain several classes

in this case 6 classes

I – fresh (f)II – slightly weathered (sw)III – moderately weathered (mw)IV – highly weathered (hw)V – completely weathered (cw)

VI – residual soil (rs) Hong Kong – zones of weathering p. 225, zones A (residual soil), B, C, D and Fresh rockProfile development in Hong Kong – figures 6.18 1-4, 6.19 a-f!


in this case 6 classes

I – fresh (f)II – slightly weathered (sw)III – moderately weathered (mw)IV – highly weathered (hw)V – completely weathered (cw)

VI – residual soil (rs)

Chemically weathered

granite


Granite weathers to a sandy soil

Rock Quality – some tests

Index tests – give information about the Index tests – give information about the rockrock – fresh or weathered and to what – fresh or weathered and to what degreedegree

• Porosity• Bulk density• Compressibility• Tensile strength• Elastic constants• Point load test


Fluid adsorption, classes 1-4Almost impermeableSlightly permeableModerately permeableHighly permeable


Slake behaviorSlake behavior - degree of disintegration of 40 to 50 grams of specimen after 5-min immersion in water

Class 1 – no changeClass 2 – less than halfClass 3 – more than halfClass 4 – total disintegration

Effect of climate and rock type on weathering

Precipitation/evaporation ratio is important • Weinert - N valueWeinert - N value is a weathering index

N<5, chemical weathering is favored over mechanical – decomposition is the predominate process

N>5, mechanical weathering is favored over chemical – decomposition is predominate

Weathering of basic and ultrabasic rocks

• N > 2, montmorillonite• N between 1-2, kaolinite


• Extreme – tropical climates laterite soilslaterite soils are produced

• where all silica is removed and • some clay minerals replaced by

iron, aluminum, and magnesioum oxided and hydroxides


Engineering properties

plutonic rocks

exploration

• weather profile nature: extent of rock and soil cover

• hazards of boulders • hazard of soil flow• slides of serpentine• sheet slides• rock falls

excavation

• core stones– size

• drilling can divert along joints

foundations

• hardness and soundness• core stones – differential support• driving piles difficult in weathered

material• collapsing residual soil• disposal of water in weathered

terrain, erosion susceptible

dams

• earth fill dams can be placed on soil profiles of I-IV possible V

• concrete dams can be placed on sound rock and possible zones I and II

• Permeability a problem in weathered zones

• Permeability between sheets common• Serpentine is not suitable for any dam

construction

underground works

• weathering down to 60 m (500 m)• variable hardness difficult• popping rock danger• diabase dikes act often as

subsurface dams – water can be a problem upon penetration

• serpentine dangerous

ground water

• fault zones• weathered granite

case histories

mammoth pool dam – sheeted granodiorite

San Joaquin River, California


San Joaquin River, California

biotite granodioriteweathering depth – 30

msaprolite used as

aggregate for a 100 m high dam – without clay core


• surface covered with core stones

• largest was a sheet of granite, 5000 m3,

• valley filled with alluvial sediments with maximum depth of 30 m


• bedrock contained numerous jointsbedrock contained numerous joints• open or partly filled with alluvial open or partly filled with alluvial

sand and weathered debrissand and weathered debris• bedrock grouted downward 5 m – to bedrock grouted downward 5 m – to

reduce compressibility of the open reduce compressibility of the open fissures and jointsfissures and joints

• grout curtain down to 15 m below grout curtain down to 15 m below the foundation and 12 m into the the foundation and 12 m into the abutmentsabutments


• grouting– must go slow– at low pressures– some sheets are bolted prior to

grouting– otherwise uplift of sheet joints


• grouting• estimated 5 000 sacks• required 42 000 sacks

• why – aperture of joints very large – one as wide as 40 cm!

• NOTE: apertures of 100 cm not uncommon in Sweden


• rock bolts installed to stabilize sheets• drainage holes were made to insure that low

water pressures would be maintained between sheets after the dam was filled– 15 m, 5º from horizontal, into the sheets to intercept all

possible open sheet joints

Malaysian granite hydroelectric project

• Porphyritic granite with 35% quartz and 5% biotite• hairline fractures• occasional shear zone healed with calcite, chlorite or

quartz


• Shear zones and mylonite and brecciated granite


• surface outcrops minimal due to jungle vegetation• Lineaments visible on aerial photographs suggested

faults and shear zones• 67 drill holes


• Tunneling was the biggest problem with weathered zones and faults

• weathering average 30 m • but also in the tunnel at 300 m• residual soil was up to 6 m thick


• grade VI material in weathered profile had a clay content of 20%

• grade V was sand with less than 10% clay

• Grade V1 material used to form a core

• Grade V formed the shells


• shear zones at 250 m depth contained 7 to 22 cm thick layers of grade IV and V weathered grainite

• at 450 m depth in the tunnel slabbing occurred in the walls

• erosion was a problem in weathered granite

• divert the tunnel to a different direction to avid problem zones and faults zones

Question

• Can decomposed granite furnish satisfactory materials for concrete aggregate?

Question

• How can it be determined that a borehole through soil and saprolite extending into unweathered rock has not actually bottomed in a core stone?

Question

• A granitic pluton is not bedded in the sense that a sedimentary rock is bedded. How then could a conspicuous fracture be identified definitively as a fault?

Question

• Granitic core stones are well developed in Hong Kong whereas granitic rocks of Korea generally lack then. How is this possible?

geology of plutonic rocks. igneous plutonic rocks formed – –900 degree c –50 km depth uplift...

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