Download Classification of Igneous Rocks

Classification of Igneous Rocks
General Introduction to Igneous Rocks
Igneous rocks form by direct crystallization of minerals from a magma melt. Intrusive (plutonic)
rocks crystallize at depth, whereas extrusive (volcanic and pyroclastic rocks) rocks crystallize after the
magma reaches the earth's surface. In general, extrusive rocks have a finer grained texture than
intrusive rocks.
Magmatic activity occurs at sea-floor spreading ridges and other rift zones, volcanic arcs
(subduction zones) and hot spots (intraplate volcanism). The chemistry and mineralogy of igneous
rocks which form in different tectonic settings differ markedly from one another... a concept explored
we explore in detail in this course.
Igneous rocks can be classified according to the percentage of SiO2. The table below is a general
guide to igneous rock classification. You will explore these boundaries in more detail in this course.
IGNEOUS COMPOSITIONAL NAMES AND MAGMA TYPES
SiO2 (wt. %)
<45
45 -52
52 – 57
57 - 63
63 - 68
>68
Compositional or
Chemical
Equivalent
Ultrabasi
c
basic
basic to
intermediate
intermediate
intermediate to
acidic or silicic
acidic or
silicic
Magma Type
ultramafi
c
mafic
mafic to
intermediate
intermediate
intermediate to
felsic
felsic
Extrusive Rock
Name
komatiite
basalt
basaltic andesite
andesite
dacite
rhyolite
Intrusive Rock
Name
peridotite
gabbr
o
Diorite
diorite or quartz
diorite
granodiorite
granite
NOTES
1.
Magma type refers to colour of extrusive rocks (light to dark) with increasing SiO2
%.
2.
The terms acidic and basic, when used in this context, have NOTHING to do with
pH.
3.
This table does NOT contain all possible igneous rock types; it is a general guide to
help you equate SiO2% with common rock names.
Classification of volcanic rocks
Classification of metamorphic rocks
METAMORPHIC SETTINGS
Metamorphism takes place in a variety of geologic settings. A general outline of metamorphic
settings is shown below. It should be noted that with the exception of impact metamorphism, the
general types grade into one another.
General metamorphic settings (after Yardley et al., 1990)
Type
Description
Contact
(thermal)
Heating of country rocks during igneous
intrusion or beneath thick flows
large scale metamorphism characteristic of
mountain belts and shild areas as a result
of tectonism
Regional
(burial)
dynamic
hydrothermal
impact
Common effects
(assume that there will be new mineral
growth with all types)
Growth of new metamorphic minerals in
random orientations
Example rock
types
hornfels
Involves burial to produce elevated
Large variety of rock
pressures and temperatures controlled by
types, including
the depth attained in the crust or mantle
slates, phyllites,
and deformation to produce tectonic
schists, gneisses
fabrics
"Subset" of regional metamorphism;
involves the postdiagenetic, progressive
changes occuring to sedimentary rocks
during burial
response to intense strain and is commonly
Oriented frabrics, brecciation,
of localised occurrence
granularization
chemical reactions as a result of cirulation
Metasomatism (change in chemical
fluids. Common at sea floor spreading
composition)
centres
imact of large, high velocity meteorites Shock effects producing dense minerals at
mylonites, fault
breccias
skarns
Shatter cones,
earths surface which normally occur at
mantle depths
Classification by bulk composition and nature of source rocks
Parent Material
Argillaceous/clay-rich sediments (lutites)
arenaceous (predominately sand-size) sediments
clay-sand mixtures
quart-sand (quartz areenites)
marl (lime muds)
limestone or dolostone
basalt
shocked quartz
Rock type
pelites
psammites
semi-pelite
quartzite
calc-silicate/calcareous
marble
metabasite (metamafic)
Classification by Texture
Fine
Poorly Foliated
hornfels
Well Foliated
Well Foliated and Sheared
slate, phyllite
mylonite
Description of rocks based on texture
Name
Slate
Phyllite
Schist
Gneiss
Mylonite
Hornfels
fels
Grain Size
Medium
granofels
marble
quartzite
amphibolite
schist
mylonite schist
Coarse
granofels
marble
quartzite
amphibolite
gneiss
augen gneiss
Description
-strongly cleaved rock
-cleavage planes pervasively developed through rock due to orientation of fine
phyllosilcate grains
-indeividual grains too fine to be visible with naked eye
-overall dull appearance
-similar to slate, but slightly coarser phyllosilicate grains
-grains sometimes discernible in hand specimen, giving silk appearance to cleavage
surfaces
-often cleavage planes less perfectly planar than slates
-parallel alignment of moderately coarse grains (fabric=schistocity)
-grains usually cle4arly visible by eye
-grains composed of phyllosilicates and other minerals such as hornblende, actinolite,
kyanite
-course grained rock (grain size several millimetres) and
-foliated (planar fabric: either schistosity or compositional layering)
-tendency for different minerals to segregate into layers parallel to foliation (gneissic
layering):
typically quartz and feldspar rich layers tend to separate from micaceous layers.
Varieties:
--Orthogneiss: igneous parentage
-- paragneiss: metasedimentary gneisses
-fine grained rock produced in intense ductile deformation
-pre-existing grains are deformed and re-crystallized as finer grains
-contact metamorphic rock
tough rock with a random fabric of interlocking grains
-poor in sheet silicates, texture not obviously schistose, but not hornfels
-most have equigranular texture, -e.g., granofels
Special Metamorphic rock names
Name
Description
-green, foliated metabasite
-usually composed predominaately of chlorite, epidote, and actinolite
-dark, lilac-grey foliate metabasite, seldom truly blue in hand speciment
Blueschist
-colour due to presence of abundant sodic amphibole (glaucophane, crossite)
-essentially bimineralic (hornblende and plagioclase) dark green rock
-wide range of accesory minerals
Amphibolite
-can be metabasites (ortho-amphibolite) or
-______metasediments (para-amphibolites)
-green, black or reddish rock composed of predominately serpentine
Serpentinite
-formed by hydration of igneous or metamorphic peridotites
-metabasite composed of garnet and omphacitic pyroxene with no plagioclase
Eclogite
feldspar. Common accesories: quartz, kyanite, amphiboles, zoisite, rutile, minor
sulphides
-characterized by both a +/- equidimensional, straight-sided (polygonal) grains for all
mineral species, and a very high-temperature mineralogy (closely reated to clacGranulite
alkaline basic to moderately acid plutonic rocks - feldspar, pyroxene, amphibole)
-charnokite suite: distinct variety of K-feldspar and hypersthene-bearing granulites.
-a mixed rock of schistose or gneissic portion intimately mixed with veins of
Migmatite
apparently quartzofeldspathic material (known as leucosomes)
-result of metasomatism where large amounts of Si, Al, Fe, and Mg introduced during
intrusion.
Skarn
-impure marble containing crystals of calc-silicate minerals such as garnet, epidote,
etc.
-created by compositional changes in country rock at igneous contacts
Reference: after information in Yardely et al., 1989; Ehlers and Blatt, 1982
Greenschist
Classification of metamorphic facies
One of the primary goals of metamorphic petrology is to interpret the pressures (P) and temperatures
(T) at which a rock formed, or, in regional geology, the P-T history of a set of rocks. Eskola (1915)
devised a facies scheme which identified assemblages of minerals which correspond to particular P-T
conditions. Hence, we talk about the zeolite facies, as low temperature-low pressure conditions typified
by the growth of zeolites. A modern version of this scheme is illustrated here; the diagram includes the
P-T stability fields of the important aluminosilcate polymorphs (kyanite, sillimanite, and andalusite),
and the experimentally determined melting curve for granite. after Yardley (1989)