Chap 7: Igneous Rock Forming Minerals

Mafic Mineral Conditions

Low SiO2 weight % (50~)

High temp of 1200-1300

With Basalt (Gabbro)

Intermediate Mineral Conditions

Middle SiO2 Weight % (60~)

Mid Temp

With Andesite (Diorite)

Felsic Mineral Conditions

High SiO2 weight % (70~)

Low temp ~600

With Rhyolite (granite)

mafic minerals

olivine

pyroxene

plagioclase

intermediate minerals

quartz

amphibole

pyroxene

plagioclase

felsic minerals

A-feldspar

plagioclase

quartz

biotite

muscovite

What is an igneous rock

rock that crystallizes from melt (based on solidus) and magma

solidus

melting line of the mantle where everything under is solid form.

Higher temp than geotherm typically

geotherm

normal geothermal gradient which is ~ earth temperature.

Slope increases after litho - astheno boundary with increased P

rock-forming tectonic settings

• subduction zones

• hot spots

• midocean ridge

none of these have added heat

Midocean ridge mineral formation

• slow seismic velocity

• decompression melting

• brings rock to surface

decompression melting

• heat is constant but the pressure decreases which causes the melt

• at MOR and divergent boundaries

Mantle hot spot mineral formation

• mantle plumes

• hot materials upwell from depth

• form of decompression melting

• steeper geotherm

• hawaii/shield volcanoes

subduction zone mineral formation

• addition of water to mantle allows for melt to occur at lower temperature

• “wet” solidus of mantle peridotite

• from dehydration of hydrous OH in minerals (amphiboles, micas)

how does the addition of water create melt

• dehydration of hydrous minerals enters mantle and lowers temperature to melt.

• most h2O leaves early if from ocean water

where does the subducting slab go?

Releases water and mantle melt,

“breaks off” and sinks to slab graveyard at boundary

How are igneous rocks classified?

Texture and mineral composition.

• ID texture

• ID minerals

• whats the % of mafic minerals

• choose classification diagram (QAP for <90% mafic)

• normalize minerals of interest

phaneritic

coarse, can see the grains with naked eye

• in intrusive and plutonic setting

• Gabbro

aphaneritic

fine graineds, cant see with naked eye

• in extrusive and volcanic setting

• basalt

Naming of Volcanic rocks

• colour as proxy

• chemical analysis and diagrams

• chemical analysis and calculation of what minerals should be there (CIPW norms)

Field relations at MOR

• mafic magma

• basalt and gabbro rock

• olivine, plagioclase, pyroxene

fine grained basalt forms at

rapid cooling at sea floor

coarse grained gabbro forms at

slower cooling ponds in crust

Landforms at MOR

• pillow basalts

• sheeted dike complexes

• gabbro “kinda like bubbles at the opening”

pillow basalt

• underwater lava eruptions

• only basaltic

• squeeze through the MOR opening

Field associations at Hot Spots

• mafic magma

• basalt rock

• olivine, plagioclase, pyroxene

• forms shield volcanoes, yellow stone, hawaii chain, cindercones

landforms at hot spots

• flood basalts (erupts on land in water, larger platforms)

• columnar basalts (hexagonal)

vesicles

• holes from gas bubbles

• common in felsic rck

• only from volcanic rocks

amygdale

• precipitated minerals that filled a vesicle from outside in

• rounded blob

• quartz or calcite

• possibly confused for plag/pyroxene phenocryst

field association for subduction zones

• intermediate magma

• andesite/diorite rock

• plag, pyrox, amphibole, quartz

island arcs

chain of same aged volcanoes at subduction zone at ocean-ocean convergent boundary

• strato volcanoes (more silica, some water, more viscous, explosive)

ocean-continental arc

• intermediate-felsic magma

• andesite, rhyolite, diorite, granite

• plag, amphibole, quartz, afeldspar, biotite, muscovite

O-C arc volcanism

• higher silica

• some water

• more viscous

• explosive

• mafic to felsic rock

hot spot volcanism

• low water

• 50% silica

• flows relatively well

• quiescent eruptions

basalt

• dark, fine grained

• plag, pyrox, olivine

• phenocrysts of olivine

• vesicles/amygdales

• escoria

tuff

• igneous volcanic ask compacted into rock

• phenocrysts common

• felsic magma

• subduction zones

• pyroclastic material

pumice

• from volitile magma

• more water in it

• holy praise be

how do intrusive structures form

• melt forms

• goes up and melts surroundings

• squeezes

• cools as it approaches surface and forms a shape

batholith

magma comes to surface but doesnt erupt

sill

a tabular sheet of igneous rock intruded between and parallel with the existing strata

volcanic neck

solidified remains of volcanoes' conduit and plumbing systems that remain after the rest of a volcano has been eroded away

dike

cuts through and fills cracks, typically planar

laccolith

lens-shaped, that has been intruded between rock strata causing uplift in the shape of a dome.

lopolith

reverse of laccolith, dips in rather than domes out

minerals in mantle

olivine, opx, cpx,

aluminous phase, plag, spinel, garnet

perovskite

main lower mantle mineral forms from olivine

octahedral Si

basaltic magma source

upper mantle

granitic magma source

lower crust

xenolith

fragments of wall rock which were formed at greater depths and brought up

peridotite is common one from the mantle

liquidus

line which shows conditions where under the rock is totally liquid

rises with increasing pressure

solidus change depth at 20km

plagioclase-bearing peridotite changes to spinel-bearing peridotite

solidus change at 70 km

spinel-bearing peridotite changes to garnet-bearing peridotite

porphyry

rock containing abundant phenocrysts

latent heat of fusion

an “extra” amount of heat needed to convert a solid rock to liquid. This does not actually heat it, only used for the conversion

how does the latent heat of fusion limit the amount of melting

prevent the temperature from rising too much above the solidus so rocks only partially melt

stock

igneous intrusion with surface exposure of <100km^2

larger form of a batholith

caldera

large kinda circular part of earth that collapsed into a magma chamber

minerals used in IUGD classification of intrusive igneous rocks

Quartz, Alkali feldspar, plagioclase