PETROLOGY from usep lecture

PETROLOGY

Branch of geology dealing with the ORIGIN, OCCURRENCE,

STRUCTURE AND HISTORY OF ROCKS.

PETROGRAPHY

Deals with descriptive part of rock science involving textural,

mineralogical and chemical examinations through microscope (thin

sections)

Igneous rocks

rocks are formed by the crystallization of molten material (magmaor lava) that comes from within the earth

Basaltic Magma

Types of magma

SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Na

Andesitic Magma

SiO2 55-65 wt%, intermediate. in Fe, Mg,Ca, Na, K

Rhyolitic Magma

SiO2 65-75%, low in Fe, Mg, Ca, high in K,Na

Aphanitic texture

Fine grained because it cooled quickly at or near the surface.

Phaneritic texture

Coarse crystals cooled slowly at great depth

Porphyritic texture

(bi-modal)

Pegmatitic

very coarse

Sedimentary rocks

are formed from materials derived from pre-existing rock through mechanical,

chemical or organic processes.

Weathering, erosion, transportation, deposition,diagenesis

The formation of a clastic sediment and sedimentary rocks involves five

processes:

Weathering

causes rock to break down (mechanical and chemical)

Erosion

movement of sediment or soil from one location to another by means of water, ice or wind. Erosion overlaps with transportation.

Transportation

Sediment can be transported by sliding down slopes, being picked up by the wind, or by being carried by running water in streams, rivers,or ocean currents.

Deposition

Sediment is deposited when the energy of the transporting medium becomes too low to continue the transport process. In other words, if the velocity of the transporting medium becomes too low to transport sediment, the sediment will fall

out and become deposited. The final sediment thus reflects the energy of the transporting medium.

Diagenesis

processes involved in the reconstruction of sediments between the time of deposition and the time at which lithification takes place.

Clastic/Detrital

–formed from mechanical weathering debris from pre-existing rocks

Chemical

formed when dissolved materials precipitate from solutions

Biological/organic

formed from accumulation of plant or animal debris

Mud cracks

drying on top of bed.

Ripple marks

wave action, form on top of beds.

Fossils

traces of plants or animals preserved in sedimentary rocks.

Metamorphic rocks

solid state recrystallization of pre-existing rocks due to physical and chemical conditions primarily caused by

1. heat

2. pressure

3. chemically active fluids.

Foliation

parallel arrangement of minerals in a metamorphic rock.

Schistosity

A preferred orientation of inequaint mineral grains or grain aggregates produced by

metamorphic processes.

Gneissose

Either a poorly-developed schistosity or segregated into layers by metamorphic processes

Shale, phyllite, schist,gneiss

Foliated Metamorphic rocks

Slate

compact, very fine-

grained, metamorphic rock

with a well-developed

cleavage. Freshly cleaved

surfaces are dull

Phyllite

a rock with a

schistosity in which very fine

phyllosilicates

(sericite/phengite and/or

chlorite), although rarely

coarse enough to see unaided,

impart a silky sheen to the

foliation surface.With both a foliation and

lineation are very common.

Schist

are

restricted to those

metamorphic rocks in which

the foliated minerals are

coarse enough to see easily in

hand specimen.

Gneiss

may also be lineated,

but must also show

segregations of felsic-mineral-

rich and dark-mineral-rich

concentrations.

Granofels, hornfels, marble, quartzite

Non-Foliated Metamorphic Rocks

Granofels

a comprehensive term for any isotropic rock (a

rock with no preferred orientation).

Hornfels

is a type of granofels that is typically very fine-

grained and compact, and occurs in contact aureoles.

Hornfelses are tough, and tend to splinter when broken.

Marble

a metamorphic rock

composed predominantly of

calcite or dolomite. The protolith is

typically limestone or dolostone.

Quartzite

a metamorphic rock

composed predominantly of

quartz. The protolith is typically

sandstone. Some confusion may

result from the use of this term in

sedimentary petrology for a pure

quartz sandstone.

Greenschist/Greenstone:

a low-grade

metamorphic rock that typically contains

chlorite, actinolite, epidote, and albite. Note

that the first three minerals are green, which

imparts the color to the rock. Such a rock is

called greenschist if foliated, and greenstone

if not. The protolith is either a mafic igneous

rock or graywacke.

Amphibolite:

a metamorphic rock dominated by

hornblende + plagioclase. Amphibolites may

be foliated or non-foliated. The protolith is

either a mafic igneous rock or graywacke.

Serpentinite:

an ultramafic rock metamorphosed at low grade, so that it

contains mostly serpentine.

Blueschist:

a blue amphibole-bearing metamorphosed mafic igneous rock

or mafic graywacke. This term is so commonly applied to such rocks that it

is even applied to non-schistose rocks.

Eclogite:

a green and red metamorphic rock that contains clinopyroxene and

garnet (omphacite + pyrope). The protolith is typically basaltic.

Skarn:

a contact metamorphosed and

silica metasomatized carbonate rock

containing calc-silicate minerals, such

as grossular, epidote, tremolite,

vesuvianite, etc. Tactite is a synonym.

Granulite:

a high grade rock of pelitic, mafic,

or quartzo-feldspathic parentage that is

predominantly composed of OH-free

minerals. Muscovite is absent and plagioclase

and orthopyroxene are common.

Migmatite:

a composite silicate rock that is

heterogeneous on the 1-10 cm scale, commonly

having a dark gneissic matrix (melanosome) and

lighter felsic portions (leucosome). Migmatites may

appear layered, or the leucosomes may occur as

pods or form a network of cross-cutting veins.