1. Petrology
A volcanic sand grain seen under the microscope, with plane-polarized light in the upper picture,
and cross polarized light in the lower picture. Scale box is 0.25 mm.
Petrology (from Greek: πέτρα, petra, rock; and λόγος, logos, knowledge) is the branch of
geology that studies rocks, and the conditions in which rocks form.
Lithology was once approximately synonymous with petrography, but in current usage, lithology
focusses on macroscopic hand-sample or outcrop-scale description of rocks, while petrography is
the speciality that deals with microscopic details.
In the oil industry, lithology, or more specifically mud logging, is the graphic representation of
geological formations being drilled through, and drawn on a log called a mud log. As the cuttings
are circulated out of the borehole they are sampled, examined (typically under a 10x microscope)
and tested chemically when needed.
Methodology
Petrology utilizes the classical fields of mineralogy, petrography, optical mineralogy, and
chemical analyses to describe the composition and texture of rocks. Modern petrologists also
include the principles of geochemistry and geophysics through the studies of geochemical trends

2. and cycles and the use of thermodynamic data and experiments to better understand the origins
of rocks.
Branches
There are three branches of petrology, corresponding to the three types of rocks: igneous,
metamorphic, and sedimentary, and another dealing with experimental techniques:
 Igneous petrology focuses on the composition and texture of igneous rocks (rocks such as
granite or basalt which have crystallized from molten rock or magma). Igneous rocks
include volcanic and plutonic rocks.
 Sedimentary petrology focuses on the composition and texture of sedimentary rocks
(rocks such as sandstone, shale, or limestone which consist of pieces or particles derived
from other rocks or biological or chemical deposits, and are usually bound together in a
matrix of finer material).
 Metamorphic petrology focuses on the composition and texture of metamorphic rocks
(rocks such as slate, marble, gneiss, or schist which started out as sedimentary or igneous
rocks but which have undergone chemical, mineralogical or textural changes due to
extremes of pressure, temperature or both)
 Experimental petrology employs high-pressure, high-temperature apparatus to investigate
the geochemistry and phase relations of natural or synthetic materials at elevated
pressures and temperatures. Experiments are particularly useful for investigating rocks of
the lower crust and upper mantle that rarely survive the journey to the surface in pristine
condition. The work of experimental petrologists has laid a foundation on which modern
understanding of igneous and metamorphic processes has been built.
Igneous rock
Geologic provinces of the world (USGS)
Shield Platform Orogen Basin Large igneous Oceanic crust: 0–20 Ma 20–65
province Extended crust Ma >65 Ma

3. Volcanic rock in North America. Plutonic rock in North America.
Igneous rock (derived from the Latin word igneus meaning of fire, from ignis meaning fire) is
one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous
rock is formed through the cooling and solidification of magma or lava. Igneous rock may form
with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the
surface as extrusive (volcanic) rocks. This magma can be derived from partial melts of pre-
existing rocks in either a planet's mantle or crust. Typically, the melting is caused by one or more
of three processes: an increase in temperature, a decrease in pressure, or a change in
composition. Over 700 types of igneous rocks have been described, most of them having formed
beneath the surface of Earth's crust. These have diverse properties, depending on their
composition and how they were formed.

Geological significance
The upper 16 kilometres (10 mi) of Earth's crust is composed of approximately 95% igneous
rocks with only a thin, widespread covering of sedimentary and metamorphic rocks.[1]
Igneous rocks are geologically important because:
 which some igneous rocks are extracted, and the temperature and pressure conditions that
allowed this extraction, and/or of other pre-existing rock that melted;
 their absolute ages can be obtained from various forms of radiometric dating and thus can
be compared to adjacent geological strata, allowing a time sequence of events;
 their features are usually characteristic of a specific tectonic environment, allowing
tectonic reconstitutions (see plate tectonics);
 in some special circumstances they host important mineral deposits (ores): for example,
tungsten, tin, and uranium are commonly associated with granites and diorites, whereas
ores of chromium and platinum are commonly associated with gabbros.
Morphology and setting
In terms of modes of occurrence, igneous rocks can be either intrusive (plutonic), extrusive
(volcanic) or hypabyssal.

4. Intrusive igneous rocks
Close-up of granite (an intrusive igneous rock) exposed in Chennai, India.
Intrusive igneous rocks are formed from magma that cools and solidifies within the crust of a
planet. Surrounded by pre-existing rock (called country rock), the magma cools slowly, and as a
result these rocks are coarse grained. The mineral grains in such rocks can generally be identified
with the naked eye. Intrusive rocks can also be classified according to the shape and size of the
intrusive body and its relation to the other formations into which it intrudes. Typical intrusive
formations are batholiths, stocks, laccoliths, sills and dikes.
The central cores of major mountain ranges consist of intrusive igneous rocks, usually granite.
When exposed by erosion, these cores (called batholiths) may occupy huge areas of the Earth's
surface.
Coarse grained intrusive igneous rocks which form at depth within the crust are termed as
abyssal; intrusive igneous rocks which form near the surface are termed hypabyssal.
Extrusive igneous rocks
Basalt (an extrusive igneous rock in this case); light coloured tracks show the direction of lava
flow.
Extrusive igneous rocks are formed at the crust's surface as a result of the partial melting of rocks
within the mantle and crust. Extrusive Igneous rocks cool and solidify quicker than intrusive
igneous rocks. Since the rocks cool very quickly they are fine grained.

5. The melted rock, with or without suspended crystals and gas bubbles, is called magma. Magma
rises because it is less dense than the rock from which it was created. When it reaches the
surface, magma extruded onto the surface either beneath water or air, is called lava. Eruptions of
volcanoes into air are termed subaerial whereas those occurring underneath the ocean are termed
submarine. Black smokers and mid-ocean ridge basalt are examples of submarine volcanic
activity.
The volume of extrusive rock erupted annually by volcanoes varies with plate tectonic setting.
Extrusive rock is produced in the following proportions: [2]
 divergent boundary: 73%
 convergent boundary (subduction zone): 15%
 hotspot: 12%.
Magma which erupts from a volcano behaves according to its viscosity, determined by
temperature, composition, and crystal content. High-temperature magma, most of which is
basaltic in composition, behaves in a manner similar to thick oil and, as it cools, treacle. Long,
thin basalt flows with pahoehoe surfaces are common. Intermediate composition magma such as
andesite tends to form cinder cones of intermingled ash, tuff and lava, and may have viscosity
similar to thick, cold molasses or even rubber when erupted. Felsic magma such as rhyolite is
usually erupted at low temperature and is up to 10,000 times as viscous as basalt. Volcanoes with
rhyolitic magma commonly erupt explosively, and rhyolitic lava flows typically are of limited
extent and have steep margins, because the magma is so viscous.
Felsic and intermediate magmas that erupt often do so violently, with explosions driven by
release of dissolved gases — typically water but also carbon dioxide. Explosively erupted
pyroclastic material is called tephra and includes tuff, agglomerate and ignimbrite. Fine volcanic
ash is also erupted and forms ash tuff deposits which can often cover vast areas.
Because lava cools and crystallizes rapidly, it is fine grained. If the cooling has been so rapid as
to prevent the formation of even small crystals after extrusion, the resulting rock may be mostly
glass (such as the rock obsidian). If the cooling of the lava happened slowly, the rocks would be
coarse-grained.
Because the minerals are mostly fine-grained, it is much more difficult to distinguish between
the different types of extrusive igneous rocks than between different types of intrusive igneous
rocks. Generally, the mineral constituents of fine-grained extrusive igneous rocks can only be
determined by examination of thin sections of the rock under a microscope, so only an
approximate classification can usually be made in the field.
Hypabyssal igneous rocks
Hypabyssal igneous rocks are formed at a depth in between the plutonic and volcanic rocks.
Hypabyssal rocks are less common than plutonic or volcanic rocks and do often form dikes, sills
or laccoliths.

6. Classification
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical
composition, and the geometry of the igneous body.
The classification of the many types of different igneous rocks can provide us with important
information about the conditions under which they formed. Two important variables used for the
classification of igneous rocks are particle size, which largely depends upon the cooling history,
and the mineral composition of the rock. Feldspars, quartz or feldspathoids, olivines, pyroxenes,
amphiboles, and micas are all important minerals in the formation of almost all igneous rocks,
and they are basic to the classification of these rocks. All other minerals present are regarded as
nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous
rocks with other essential minerals are very rare, and these rare rocks include those with essential
carbonates.
In a simplified classification, igneous rock types are separated on the basis of the type of feldspar
present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of
iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-
oversaturated. Rocks with feldspathoids are silica-undersaturated, because feldspathoids cannot
coexist in a stable association with quartz.
Igneous rocks which have crystals large enough to be seen by the naked eye are called
phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking,
phaneritic implies an intrusive origin; aphanitic an extrusive one.
An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is
termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable
size before the main mass of the magma crystallizes as finer-grained, uniform material.
Texture
Gabbro specimen showing phaneritic texture; Rock Creek Canyon, eastern Sierra Nevada,
California; scale bar is 2.0 cm.
Main article: Rock microstructure

7. Texture is an important criterion for the naming of volcanic rocks. The texture of volcanic rocks,
including the size, shape, orientation, and distribution of mineral grains and the intergrain
relationships, will determine whether the rock is termed a tuff, a pyroclastic lava or a simple
lava.
However, the texture is only a subordinate part of classifying volcanic rocks, as most often there
needs to be chemical information gleaned from rocks with extremely fine-grained groundmass or
from airfall tuffs, which may be formed from volcanic ash.
Textural criteria are less critical in classifying intrusive rocks where the majority of minerals will
be visible to the naked eye or at least using a hand lens, magnifying glass or microscope.
Plutonic rocks tend also to be less texturally varied and less prone to gaining structural fabrics.
Textural terms can be used to differentiate different intrusive phases of large plutons, for
instance porphyritic margins to large intrusive bodies, porphyry stocks and subvolcanic dikes
(apophyses). Mineralogical classification is used most often to classify plutonic rocks. Chemical
classifications are preferred to classify volcanic rocks, with phenocryst species used as a prefix,
e.g. "olivine-bearing picrite" or "orthoclase-phyric rhyolite".
Basic classification scheme for igneous rocks on their mineralogy. If the approximate volume
fractions of minerals in the rock are known the rock name and silica content can be read off the
diagram. This is not an exact method because the classification of igneous rocks also depends on
other components than silica, yet in most cases it is a good first guess.
Chemical classification
Igneous rocks can be classified according to chemical or mineralogical parameters:
Chemical: total alkali-silica content (TAS diagram) for volcanic rock classification used when
modal or mineralogic data is unavailable:
 acid igneous rocks containing a high silica content, greater than 63% SiO 2 (examples
granite and rhyolite)

8.  intermediate igneous rocks containing between 52 - 63% SiO2 (example andesite and
dacite)
 basic igneous rocks have low silica 45 - 52% and typically high iron - magnesium
content (example gabbro and basalt)
 ultrabasic igneous rocks with less than 45% silica. (examples picrite and komatiite)
 alkalic igneous rocks with 5 - 15% alkali (K2O + Na2O) content or with a molar ratio of
alkali to silica greater than 1:6. (examples phonolite and trachyte)
Note: the acid-basic terminology is used more broadly in older (generally British)
geological literature. In current literature felsic-mafic roughly substitutes for acid-basic.
Chemical classification also extends to differentiating rocks which are chemically similar
according to the TAS diagram, for instance;
 Ultrapotassic; rocks containing molar K2O/Na2O >3
 Peralkaline; rocks containing molar (K2O + Na2O)/ Al2O3 >1
 Peraluminous; rocks containing molar (K2O + Na2O)/ Al2O3 <1
An idealized mineralogy (the normative mineralogy) can be calculated from the chemical
composition, and the calculation is useful for rocks too fine-grained or too altered for
identification of minerals that crystallized from the melt. For instance, normative quartz
classifies a rock as silica-oversaturated; an example is rhyolite. A normative feldspathoid
classifies a rock as silica-undersaturated; an example is nephelinite.
History of classification
In 1902 a group of American petrographers proposed that all existing classifications of igneous
rocks should be discarded and replaced by a "quantitative" classification based on chemical
analysis. They showed how vague and often unscientific was much of the existing terminology
and argued that as the chemical composition of an igneous rock was its most fundamental
characteristic it should be elevated to prime position.
Geological occurrence, structure, mineralogical constitution—the hitherto accepted criteria for
the discrimination of rock species—were relegated to the background. The completed rock
analysis is first to be interpreted in terms of the rock-forming minerals which might be expected
to be formed when the magma crystallizes, e.g., quartz feldspars, olivine, akermannite,
feldspathoids, magnetite, corundum and so on, and the rocks are divided into groups strictly
according to the relative proportion of these minerals to one another. [3][4]
Mineralogical classification
For volcanic rocks, mineralogy is important in classifying and naming lavas. The most important
criterion is the phenocryst species, followed by the groundmass mineralogy. Often, where the
groundmass is aphanitic, chemical classification must be used to properly identify a volcanic
rock.

9. Mineralogic contents - felsic versus mafic
 felsic rock, highest content of silicon, with predominance of quartz, alkali feldspar and/or
feldspathoids: the felsic minerals; these rocks (e.g., granite, rhyolite) are usually light
coloured, and have low density.
 mafic rock, lesser content of silicon relative to felsic rocks, with predominance of mafic
minerals pyroxenes, olivines and calcic plagioclase; these rocks (example, basalt, gabbro)
are usually dark coloured, and have a higher density than felsic rocks.
 ultramafic rock, lowest content of silicon, with more than 90% of mafic minerals (e.g.,
dunite).
For intrusive, plutonic and usually phaneritic igneous rocks where all minerals are visible at least
via microscope, the mineralogy is used to classify the rock. This usually occurs on ternary
diagrams, where the relative proportions of three minerals are used to classify the rock.
The following table is a simple subdivision of igneous rocks according both to their composition
and mode of occurrence.
Composition
Mode of occurrence Felsic Intermediate Mafic Ultramafic
Intrusive Granite Diorite Gabbro Peridotite
Extrusive Rhyolite Andesite Basalt Komatiite
Essential rock forming silicates
Felsic Intermediate Mafic Ultramafic
Coarse Grained Granite Diorite Gabbro Peridotite
Medium Grained Diabase
Fine Grained Rhyolite Andesite Basalt Komatiite
Example of classification
Granite is an igneous intrusive rock (crystallized at depth), with felsic composition (rich in silica
and predominately quartz plus potassium-rich feldspar plus sodium-rich plagioclase) and
phaneritic, subeuhedral texture (minerals are visible to the unaided eye and commonly some of
them retain original crystallographic shapes).
Magma origination
The Earth's crust averages about 35 kilometers thick under the continents, but averages only
some 7-10 kilometers beneath the oceans. The continental crust is composed primarily of
sedimentary rocks resting on crystalline basement formed of a great variety of metamorphic and
igneous rocks including granulite and granite. Oceanic crust is composed primarily of basalt and
gabbro. Both continental and oceanic crust rest on peridotite of the mantle.

10. Rocks may melt in response to a decrease in pressure, to a change in composition such as an
addition of water, to an increase in temperature, or to a combination of these processes.
Other mechanisms, such as melting from impact of a meteorite, are less important today, but
impacts during accretion of the Earth led to extensive melting, and the outer several hundred
kilometers of our early Earth probably was an ocean of magma. Impacts of large meteorites in
last few hundred million years have been proposed as one mechanism responsible for the
extensive basalt magmatism of several large igneous provinces.
Decompression
Decompression melting occurs because of a decrease in pressure. [5] The solidus temperatures of
most rocks (the temperatures below which they are completely solid) increase with increasing
pressure in the absence of water. Peridotite at depth in the Earth's mantle may be hotter than its
solidus temperature at some shallower level. If such rock rises during the convection of solid
mantle, it will cool slightly as it expands in an adiabatic process, but the cooling is only about
0.3°C per kilometer. Experimental studies of appropriate peridotite samples document that the
solidus temperatures increase by 3°C to 4°C per kilometer. If the rock rises far enough, it will
begin to melt. Melt droplets can coalesce into larger volumes and be intruded upwards. This
process of melting from upward movement of solid mantle is critical in the evolution of Earth.
Decompression melting creates the ocean crust at mid-ocean ridges. Decompression melting
caused by the rise of mantle plumes is responsible for creating ocean islands like the Hawaiian
islands. Plume-related decompression melting also is the most common explanation for flood
basalts and oceanic plateaus (two types of large igneous provinces), although other causes such
as melting related to meteorite impact have been proposed for some of these huge volumes of
igneous rock.
Effects of water and carbon dioxide
The change of rock composition most responsible for creation of magma is the addition of water.
Water lowers the solidus temperature of rocks at a given pressure. For example, at a depth of
about 100 kilometers, peridotite begins to melt near 800°C in the presence of excess water, but
near or above about 1500°C in the absence of water. [6] Water is driven out of the oceanic
lithosphere in subduction zones, and it causes melting in the overlying mantle. Hydrous magmas
of basalt and andesite composition are produced directly and indirectly as results of dehydration
during the subduction process. Such magmas and those derived from them build up island arcs
such as those in the Pacific ring of fire. These magmas form rocks of the calc-alkaline series, an
important part of continental crust.
The addition of carbon dioxide is relatively a much less important cause of magma formation
than addition of water, but genesis of some silica-undersaturated magmas has been attributed to
the dominance of carbon dioxide over water in their mantle source regions. In the presence of
carbon dioxide, experiments document that the peridotite solidus temperature decreases by about
200°C in a narrow pressure interval at pressures corresponding to a depth of about 70 km. At
greater depths, carbon dioxide can have more effect: at depths to about 200 km, the temperatures

11. of initial melting of a carbonated peridotite composition were determined to be 450°C to 600°C
lower than for the same composition with no carbon dioxide. [7] Magmas of rock types such as
nephelinite, carbonatite, and kimberlite are among those that may be generated following an
influx of carbon dioxide into mantle at depths greater than about 70 km.
Temperature increase
Increase of temperature is the most typical mechanism for formation of magma within
continental crust. Such temperature increases can occur because of the upward intrusion of
magma from the mantle. Temperatures can also exceed the solidus of a crustal rock in
continental crust thickened by compression at a plate boundary. The plate boundary between the
Indian and Asian continental masses provides a well-studied example, as the Tibetan Plateau just
north of the boundary has crust about 80 kilometers thick, roughly twice the thickness of normal
continental crust. Studies of electrical resistivity deduced from magnetotelluric data have
detected a layer that appears to contain silicate melt and that stretches for at least 1000
kilometers within the middle crust along the southern margin of the Tibetan Plateau. [8] Granite
and rhyolite are types of igneous rock commonly interpreted as products of melting of
continental crust because of increases of temperature. Temperature increases also may contribute
to the melting of lithosphere dragged down in a subduction zone.
Magma evolution
Schematic diagrams showing the principles behind fractional crystallisation in a magma. While
cooling, the magma evolves in composition because different minerals crystallize from the melt.
1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclase
crystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock
forms.
Most magmas only entirely melt for small parts of their histories. More typically, they are mixes
of melt and crystals, and sometimes also of gas bubbles. Melt, crystals, and bubbles usually have
different densities, and so they can separate as magmas evolve.
As magma cools, minerals typically crystallize from the melt at different temperatures (fractional
crystallization). As minerals crystallize, the composition of the residual melt typically changes. If
crystals separate from melt, then the residual melt will differ in composition from the parent
magma. For instance, a magma of gabbroic composition can produce a residual melt of granitic

12. composition if early formed crystals are separated from the magma. Gabbro may have a liquidus
temperature near 1200°C, and derivative granite-composition melt may have a liquidus
temperature as low as about 700°C. Incompatible elements are concentrated in the last residues
of magma during fractional crystallization and in the first melts produced during partial melting:
either process can form the magma that crystallizes to pegmatite, a rock type commonly enriched
in incompatible elements. Bowen's reaction series is important for understanding the idealised
sequence of fractional crystallisation of a magma.
Magma composition can be determined by processes other than partial melting and fractional
crystallization. For instance, magmas commonly interact with rocks they intrude, both by
melting those rocks and by reacting with them. Magmas of different compositions can mix with
one another. In rare cases, melts can separate into two immiscible melts of contrasting
compositions.
There are relatively few minerals that are important in the formation of common igneous rocks,
because the magma from which the minerals crystallize is rich in only certain elements: silicon,
oxygen, aluminium, sodium, potassium, calcium, iron, and magnesium. These are the elements
which combine to form the silicate minerals, which account for over ninety percent of all igneous
rocks. The chemistry of igneous rocks is expressed differently for major and minor elements and
for trace elements. Contents of major and minor elements are conventionally expressed as weight
percent oxides (e.g., 51% SiO2, and 1.50% TiO2). Abundances of trace elements are
conventionally expressed as parts per million by weight (e.g., 420 ppm Ni, and 5.1 ppm Sm).
The term "trace element" typically is used for elements present in most rocks at abundances less
than 100 ppm or so, but some trace elements may be present in some rocks at abundances
exceeding 1000 ppm. The diversity of rock compositions has been defined by a huge mass of
analytical data—over 230,000 rock analyses can be accessed on the web through a site sponsored
by the U. S. National Science Foundation (see the External Link to EarthChem).
Etymology
The word "igneous" is derived from the Latin ignis, meaning "of fire". Volcanic rocks are named
after Vulcan, the Roman name for the god of fire.
Intrusive rocks are also called plutonic rocks, named after Pluto, the Roman god of the
underworld.
Bowen's reaction series
Discontinuous Continuous
High
Series Series
Plagioclase
Olivine
(Calcium rich)
Pyroxene

13. Relative
Biotite Plagioclase
Crystallization
(Black Mica) (Sodium rich)
Temperature
Orthoclase
Muscovite
(White Mica)
Quartz
Low
Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L.
Bowen who was able to explain why certain types of minerals tend to be found together while
others are almost never associated with one another. He experimented in the early 1900s with
powdered rock material that was heated until it melted and then allowed to cool to a target
temperature whereupon he observed the types of minerals that formed in the rocks produced. He
repeated this process with progressively cooler temperatures and the results he obtained led him
to formulate his reaction series which is still accepted today as the idealized progression of
minerals produced by cooling magma. Based upon Bowen's work, one can infer from the
minerals present in a rock the relative conditions under which the material had formed.
Description
Olivine weathering to iddingsite within a mantle xenolith, demonstrating the principles of the
Goldich dissolution series
The series is broken into two branches, the continuous and the discontinuous. The branch on the
right is the continuous. The minerals at the top of the illustration (given aside) are first to
crystallize and so the temperature gradient can be read to be from high to low with the high
temperature minerals being on the top and the low temperature ones on the bottom. Since the
surface of the Earth is a low temperature environment compared to the zones of rock formation,
the chart also easily shows the stability of minerals with the ones at bottom being most stable and
the ones at top being quickest to weather, known as the Goldich dissolution series. This is
because minerals are most stable in the conditions closest to those under which they had formed.
Put simply, the high temperature minerals, the first ones to crystallize in a mass of magma, are

14. most unstable at the Earth's surface and quickest to weather because the surface is most different
from the conditions under which they were created while the low temperature minerals are much
more stable because the conditions at the surface are much more similar to the conditions under
which they formed.
Pluton
Plutonic redirects here, for the Australian gold mine see Plutonic Gold Mine
A Jurassic pluton of pink monzonite intruded below and beneath a section of gray
sedimentary rocks and then was subsequently uplifted and exposed, near Notch Peak,
House Range, Utah.
A pluton in geology is an intrusive igneous rock (called a plutonic rock) body that crystallized
from magma slowly cooling below the surface of the Earth. Plutons include batholiths, dikes,
sills, laccoliths, lopoliths, and other igneous bodies. In practice, "pluton" usually refers to a
distinctive mass of igneous rock, typically kilometers in dimension, without a tabular shape like
those of dikes and sills. Batholiths commonly are aggregations of plutons. Examples of plutons
include Cardinal Peak and Mount Kinabalu.
The most common rock types in plutons are granite, granodiorite, tonalite, monzonite, and quartz
diorite. The term granitoid is used for a general, light colored, coarse-grained igneous rock in
which a proper, or more specific name, is not known. Use of granitoid should be restricted to the
field wherever possible.
The term originated from Pluto, the ancient Roman god of the underworld. The use of the name
and concept goes back to the beginnings of the science of geology in the late 18th century and
the then hotly debated theories of plutonism (or vulcanism), and neptunism regarding the origin
of basalt.
Batholith

15. Half Dome, a granite monolith in Yosemite National Park and part of the Sierra Nevada
batholith.
A batholith (from Greek bathos, depth + lithos, rock) is a large emplacement of igneous
intrusive (also called plutonic) rock that forms from cooled magma deep in the earth's crust.
Batholiths are almost always made mostly of felsic or intermediate rock-types, such as granite,
quartz monzonite, or diorite (see also granite dome).

Formation
Although they may appear uniform, batholiths are in fact structures with complex histories and
compositions. They are composed of multiple masses, or plutons, bodies of igneous rock of
irregular dimensions (typically at least several kilometers) that can be distinguished from
adjacent igneous rock by some combination of criteria including age, composition, texture, or
mappable structures. Individual plutons are crystallized from magma that traveled toward the
surface from a zone of partial melting near the base of the Earth's crust.
Traditionally, these plutons have been considered to form by ascent of relatively buoyant magma
in large masses called plutonic diapirs. Because, the diapirs are liquefied and very hot, they tend
to rise through the surrounding country rock, pushing it aside and partially melting it. Most
diapirs do not reach the surface to form volcanoes, but instead slow down, cool and usually
solidify 5 to 30 kilometers underground as plutons (hence the use of the word pluton; in
reference to the Roman god of the underworld Pluto). It has also been proposed[who?] that plutons
commonly are formed not by diapiric ascent of large magma diapirs, but rather by aggregation of
smaller volumes of magma that ascended as dikes. [citation needed]
A batholith is formed when many plutons converge to form a huge expanse of granitic rock.
Some batholiths are mammoth, paralleling past and present subduction zones and other heat
sources for hundreds of kilometers in continental crust. One such batholith is the Sierra Nevada
Batholith, which is a continuous granitic formation that makes up much of the Sierra Nevada in
California. An even larger batholith, the Coast Plutonic Complex is found predominantly in the

16. Coast Mountains of western Canada, and extends for 1,800 kilometers and reaches into
southeastern Alaska.
Surface expression and erosion
A batholith is an exposed area of mostly continuous plutonic rock that covers an area larger than
100 square kilometers. Areas smaller than 100 square kilometers are called stocks. However, the
majority of batholiths visible at the surface (via outcroppings) have areas far greater than 100
square kilometers. These areas are exposed to the surface through the process of erosion
accelerated by continental uplift acting over many tens of millions to hundreds of millions of
years. This process has removed several tens of square kilometers of overlying rock in many
areas, exposing the once deeply buried batholiths.
Batholiths exposed at the surface are subjected to huge pressure differences between their former
homes deep in the earth and their new homes at or near the surface. As a result, their crystal
structure expands slightly and over time. This manifests itself by a form of mass wasting called
exfoliation. This form of erosion causes convex and relatively thin sheets of rock to slough off
the exposed surfaces of batholiths (a process accelerated by frost wedging). The result: fairly
clean and rounded rock faces. A well-known result of this process is Half Dome, located in
Yosemite Valley.
Dike (geology)
Banded gneiss with dike of granite orthogneiss.

17. An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosed
host rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.
A dike or dyke in geology is a type of sheet intrusion referring to any geologic body that cuts
discordantly across
 planar wall rock structures, such as bedding or foliation
 massive rock formations, like igneous/magmatic intrusions and salt diapirs.
Dikes can therefore be either intrusive or sedimentary in origin.
Magmatic dikes
A diabase dike crosscutting horizontal limestone beds in Arizona.
A small dike on the Baranof Cross-Island Trail, Alaska.

18. An intrusive dike is
an igneous body with
a very high aspect
ratio, which means
that its thickness is
usually much smaller
than the other two
dimensions.
Thickness can vary
Dikes in the Black Canyon of the Gunnison National Park, Colorado, USA from sub-centimeter
. scale to many
meters, and the
lateral dimensions can extend over many kilometers. A dike is an intrusion into an opening
cross-cutting fissure, shouldering aside other pre-existing layers or bodies of rock; this implies
that a dike is always younger than the rocks that contain it. Dikes are usually high angle to near
vertical in orientation, but subsequent tectonic deformation may rotate the sequence of strata
through which the dike propagates so that the latter becomes horizontal. Near horizontal, or
conformable intrusions, along bedding planes between strata are called intrusive sills.
Sometimes dikes appear as swarms, consisting of several to hundreds of dikes emplaced more or
less contemporaneously during a single intrusive event. The world's largest dike swarm is the
Mackenzie dike swarm in the Northwest Territories, Canada.[1]
Shiprock, New Mexico, the volcanic neck in the distance, with radiating dike on its south side.
Photo credit: USGS Digital Data Series
Dikes often form as either radial or concentric swarms around plutonic intrusives, volcanic necks
or feeder vents in volcanic cones. The latter are known as ring dikes.
Dikes can vary in texture and their composition can range from diabase or basaltic to granitic or
rhyolitic, but on a global perspective the basaltic composition prevails, manifesting ascent of vast
volumes of mantle-derived magmas through fractured lithosphere throughout Earth history.
Pegmatite dikes are extremely coarse crystalline granitic rocks often associated with late-stage
granite intrusions or metamorphic segregations. Aplite dikes are fine grained or sugary textured
intrusives of granitic composition.

19. Sedimentary dikes
Clastic dike (left of notebook) in the Chinle Formation in the Island In the Sky District of
Canyonlands National Park, Utah.
Sedimentary dikes or clastic dikes are vertical bodies of sedimentary rock that cut off other rock
layers. They can form in two ways:
 When a shallow unconsolidated sediment is composed of alternating coarse grained and
impermeable clay layers the fluid pressure inside the coarser layers may reach a critical
value due to lithostatic overburden. Driven by the fluid pressure the sediment breaks
through overlying layers and forms a dike.
 When a soil is under permafrost conditions the pore water is totally frozen. When cracks
are formed in such rocks, they may fill up with sediments that fall in from above. The
result is a vertical body of sediment that cuts through horizontal layers: a dike.
Magmatic dikes radiating from West Spanish Peak

20. Sill (geology)
Illustration showing the difference between a dike and a sill.
Salisbury Crags in Edinburgh, Scotland, a sill partially exposed during the ice ages
Mid-Carboniferous dolerite sill cutting Lower Carboniferous shales and sandstones, Horton
Bluff, Minas Basin South Shore, Nova Scotia
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of
sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in
metamorphic rock. The term sill is synonymous with concordant intrusive sheet. This means that
the sill does not cut across preexisting rocks, in contrast to dikes which do cut across older rocks.
Sills are always parallel to beds (layers) of the surrounding country rock. Usually they are in a
horizontal orientation, although tectonic processes can cause rotation of sills into near vertical

21. orientations. They can be confused with solidified lava flows; however, there are several
differences between them. Intruded sills will show partial melting and incorporation of the
surrounding country rock. On both the "upper" and "lower" contact surfaces of the country rock
into which the sill has intruded, evidence of heating will be observed (contact metamorphism).
Lava flows will show this evidence only on the lower side of the flow. In addition, lava flows
will typically show evidence of vesicles (bubbles) where gases escaped into the atmosphere.
Because sills generally form at depth (up to many kilometers), the pressure of overlying rock
prevents this from happening much, if at all. Lava flows will also typically show evidence of
weathering on their upper surface, whereas sills, if still covered by country rock, typically do not.
Associated ore deposits
Certain layered intrusions are a variety of sill that often contain important ore deposits.
Precambrian examples include the Bushveld, Insizwa and the Great Dyke complexes of southern
Africa, the Duluth intrusive complex of the Superior District, and the Stillwater igneous complex
of the United States. Phanerozoic examples are usually smaller and include the Rùm peridotite
complex of Scotland and the Skaergaard igneous complex of east Greenland. These intrusions
often contain concentrations of gold, platinum, chromium and other rare elements.
Transgressive sills
Despite their concordant nature, many large sills change stratigraphic level within the intruded
sequence, with each concordant part of the intrusion linked by relatively short dike-like
segments. Such sills are known as transgressive, examples include the Whin Sill and sills within
the Karoo basin.[1][2] The geometry of large sill complexes in sedimentary basins has become
clearer with the availability of 3D seismic reflection data.[3] Such data has shown that many sills
have an overall saucer shape and that many others are at least in part transgressive. [4]
Laccolith
A laccolith is a sheet intrusion (or concordant pluton) that has been injected between two layers
of sedimentary rock. The pressure of the magma is high enough that the overlying strata are
forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.

22. A laccolith intruding into and deforming strata
Laccolith exposed by erosion of overlying strata in Montana
Pink monzonite intrudes within the grey Cambrian and Ordovician strata near Notch Peak, Utah.
Laccoliths tend to form at relatively shallow depths and are typically formed by relatively
viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Cooling
underground takes place slowly, giving time for larger crystals to form in the cooling magma.
The surface rock above laccoliths often erodes away completely, leaving the core mound of
igneous rock. The term was first applied as laccolite by Grove Karl Gilbert after his study of
intrusions of diorite in the Henry Mountains of Utah in about 1875.
It is often difficult to reconstruct shapes of intrusions. For instance, Devils Tower in Wyoming
was thought to be a volcanic neck, but study has revealed it to be an eroded laccolith[1]. The rock
would have had to cool very slowly so as to form the slender pencil-shaped columns of phonolite
porphyry seen today. However, erosion has stripped away the overlying and surrounding rock,
and so it is impossible to reconstruct the original shape of the igneous intrusion; that rock may
not be the remnant of a laccolith. At other localities, such as in the Henry Mountains and other
isolated mountain ranges of the Colorado Plateau, some intrusions demonstrably have shapes of
laccoliths. The small Barber Hill syenite-stock laccolith in Charlotte, Vermont USA, has several
volcanic trachyte dikes associated with it. Molybdenite is also visible in outcrops on this exposed
laccolith.

23. There are many examples of possible laccoliths on the surface of the Moon. [2] These igneous
features may be confused with impact cratering.
Lopolith
Diagram showing the shape of a lopolith (7)
A lopolith is a large igneous intrusion which is lenticular in shape with a depressed central
region. Lopoliths are generally concordant with the intruded strata with dike or funnel-shaped
feeder bodies below the body. The term was first defined and used by Frank Fitch Grout during
the early 1900s in describing the Duluth gabbro complex in northern Minnesota and adjacent
Ontario.
Lopoliths typically consist of large layered intrusions that range in age from Archean to Eocene.
Examples include the Duluth gabbro, the Sudbury Igneous Complex of Ontario, the Bushveld
igneous complex of South Africa, the Skaergaard complex of Greenland and the Humboldt
lopolith of Nevada. The Sudbury and Bushveld occurrences have been attributed to impact
events and associated crustal melting.
Subvolcanic rock
A subvolcanic rock, also known as a hypabyssal rock, is an igneous rock that originates at
medium to shallow depths within the crust and contain intermediate grain size and often
porphyritic texture. They have textures between volcanic and plutonic rocks. Subvolcanic rocks
include diabase and porphyry.
Porphyry (geology)

24. .
A piece of porphyry
Rhyolite porphyry. Scale bar in lower left is 1 cm.
Porphyry is a variety of igneous rock consisting of large-grained crystals, such as feldspar or
quartz, dispersed in a fine-grained feldspathic matrix or groundmass. The larger crystals are
called phenocrysts. In its non-geologic, traditional use, the term "porphyry" refers to the purple-
red form of this stone, valued for its appearance.
The term "porphyry" is from Greek and means "purple". Purple was the color of royalty, and the
"Imperial Porphyry" was a deep purple igneous rock with large crystals of plagioclase. This rock
was prized for various monuments and building projects in Imperial Rome and later.
Subsequently the name was given to igneous rocks with large crystals. Porphyritic now refers to
a texture of igneous rocks. Its chief characteristic is a large difference between the size of the
tiny matrix crystals and other much larger phenocrysts. Porphyries may be aphanites or
phanerites, that is, the groundmass may have invisibly small crystals, like basalt, or the
individual crystals of the groundmass may be easily distinguished with the eye, as in granite.
Most types of igneous rocks may display some degree of porphyritic texture.
Granite
Granite
— Igneous Rock —

25. Granite containing potassium feldspar, plagioclase feldspar,
quartz, and biotite and/or amphibole
Composition
Potassium feldspar, plagioclase feldspar, and quartz;
differing amounts of muscovite, biotite, and hornblende-type
amphiboles.
Granite (pronounced /ˈɡrænɨt/) is a common and widely occurring type of intrusive, felsic,
igneous rock. Granites usually have a medium- to coarse-grained texture. Occasionally some
individual crystals (phenocrysts) are larger than the groundmass, in which case the texture is
known as porphyritic. A granitic rock with a porphyritic texture is sometimes known as a
porphyry. Granites can be pink to gray in color, depending on their chemistry and mineralogy.
By definition, granite has a color index (the percentage of the rock made up of dark minerals) of
less than 25%. Outcrops of granite tend to form tors and rounded massifs. Granites sometimes
occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole
or hornfels. Granite is usually found in the continental plates of the Earth's crust.
Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it
has gained widespread use as a construction stone. The average density of granite is between
2.65[1] and 2.75 g/cm3, its compressive strength usually lies above 200 MPa, and its viscosity at
standard temperature and pressure is 3-6 • 1019 Pa·s.[2]
The word granite comes from the Latin granum, a grain, in reference to the coarse-grained
structure of such a crystalline rock.
Granitoid is a general, descriptive field term for light-colored, coarse-grained igneous rocks.
Petrographic examination is required for identification of specific types of granitoids. [3]
Mineralogy

26. Orbicular granite near the town of Caldera, northern Chile
Granite is classified according to the QAPF diagram for coarse grained plutonic rocks and is
named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline)
and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern
petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid
or nearly devoid of plagioclase, the rock is referred to as alkali granite. When a granitoid
contains less than 10% orthoclase, it is called tonalite; pyroxene and amphibole are common in
tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica
granite. Two-mica granites are typically high in potassium and low in plagioclase, and are
usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite.
Granite has poor primary permeability but strong secondary permeability.
Chemical composition
A worldwide average of the chemical composition of granite, by weight percent: [4]
The Stawamus Chief is a granite monolith in British Columbia
 SiO2 — 72.04%
 Al2O3 — 14.42%
 K2O — 4.12%
 Na2O — 3.69%
 CaO — 1.82%
 FeO — 1.68%

27.  Fe2O3 — 1.22%
 MgO — 0.71%
 TiO2 — 0.30%
 P2O5 — 0.12%
 MnO — 0.05%
Based on 2485 analyses
Occurrence
Granite is currently known only on Earth, where it forms a major part of continental crust.
Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in
batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic
composition called aplites are often associated with the margins of granitic intrusions. In some
locations, very coarse-grained pegmatite masses occur with granite.
Granite has been intruded into the crust of the Earth during all geologic periods, although much
of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust
and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of
the continents.
Origin
Close-up of granite exposed in Chennai, India.
Granite is an igneous rock and is formed from magma. Granitic magma has many potential
origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the
crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust.
The origin of granite is contentious and has led to varied schemes of classification. Classification
schemes are regional and include French, British, and American systems.
Geochemical origins

28. Various granites (cut and polished surfaces)
Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that
have compositions at or near a eutectic point (or a temperature minimum on a cotectic curve).
Magmas will evolve to the eutectic because of igneous differentiation, or because they represent
low degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron,
magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkali
feldspar (rich in potassium) and quartz (SiO2), are two of the defining constituents of granite.
Close-up of granite from Yosemite National Park, valley of the Merced River
This process operates regardless of the origin of the parental magma to the granite, and
regardless of its chemistry. However, the composition and origin of the magma which
differentiates into granite, leaves certain geochemical and mineral evidence as to what the
granite's parental rock was. The final mineralogy, texture and chemical composition of a granite
is often distinctive as to its origin. For instance, a granite which is formed from melted sediments
may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in
plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are
based.
Chappell & White classification system
The letter-based Chappell & White classification system was proposed initially to divide granites
into I-type granite (or igneous protolith) granite and S-type or sedimentary protolith granite.[5]
Both of these types of granite are formed by melting of high grade metamorphic rocks, either
other granite or intrusive mafic rocks, or buried sediment, respectively.
M-type or mantle derived granite was proposed later, to cover those granites which were clearly
sourced from crystallized mafic magmas, generally sourced from the mantle. These are rare,
because it is difficult to turn basalt into granite via fractional crystallisation.

29. A-type or anorogenic granites are formed above volcanic "hot spot" activity and have peculiar
mineralogy and geochemistry. These granites are formed by melting of the lower crust under
conditions that are usually extremely dry. The rhyolites of the Yellowstone caldera are examples
of volcanic equivalents of A-type granite.[6][7]
Granitization
An old, and largely discounted theory, granitization states that granite is formed in place by
extreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g.
calcium to transform the metamorphic rock into a granite. This was supposed to occur across a
migrating front. The production of granite by metamorphic heat is difficult, but is observed to
occur in certain amphibolite and granulite terrains. In-situ granitisation or melting by
metamorphism is difficult to recognise except where leucosome and melanosome textures are
present in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is
a magma, so these rocks are seen as a transitional between the two, but are not technically
granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires
high temperature, and also water or other volatiles which act as a catalyst by lowering the solidus
temperature of the rock.
Ascent and emplacement
Roche Rock, Cornwall
The Cheesewring, a granite tor on the southern edge of Bodmin Moor, Cornwall

30. The ascent and emplacement of large volumes of granite within the upper continental crust is a
source of much debate amongst geologists. There is a lack of field evidence for any proposed
mechanisms, so hypotheses are predominantly based upon experimental data. There are two
major hypotheses for the ascent of magma through the crust:
 Stokes Diapir
 Fracture Propagation
Of these two mechanisms, Stokes diapir was favoured for many years in the absence of a
reasonable alternative. The basic idea is that magma will rise through the crust as a single mass
through buoyancy. As it rises it heats the wall rocks, causing them to behave as a power-law
fluid and thus flow around the pluton allowing it to pass rapidly and without major heat loss. [8]
This is entirely feasible in the warm, ductile lower crust where rocks are easily deformed, but
runs into problems in the upper crust which is far colder and more brittle. Rocks there do not
deform so easily: for magma to rise as a pluton it would expend far too much energy in heating
wall rocks, thus cooling and solidifying before reaching higher levels within the crust.
Nowadays fracture propagation is the mechanism preferred by many geologists as it largely
eliminates the major problems of moving a huge mass of magma through cold brittle crust.
Magma rises instead in small channels along self-propagating dykes which form along new or
pre-existing fault systems and networks of active shear zones (Clemens, 1998). [9] As these
narrow conduits open, the first magma to enter solidifies and provides a form of insulation for
later magma.
Granitic magma must make room for itself or be intruded into other rocks in order to form an
intrusion, and several mechanisms have been proposed to explain how large batholiths have been
emplaced:
 Stoping, where the granite cracks the wall rocks and pushes upwards as it removes blocks
of the overlying crust
 Assimilation, where the granite melts its way up into the crust and removes overlying
material in this way
 Inflation, where the granite body inflates under pressure and is injected into position
Most geologists today accept that a combination of these phenomena can be used to explain
granite intrusions, and that not all granites can be explained entirely by one or another
mechanism.
Granodiorite

31. A sample of granodiorite from Massif Central, France
Photomicrograph of thin section of granodiorite from Slovakia (in crossed polarised light)
Granodiorite (pronounced /ˌɡrænɵˈdaɪ.ɵraɪt/ or /ˌɡreɪnɵˈdaɪ.ɵraɪt/) is an intrusive igneous rock
similar to granite, but containing more plagioclase than potassium feldspar. Officially, it is
defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65%
of the feldspar is plagioclase. It usually contains abundant biotite mica and hornblende, giving it
a darker appearance than true granite. Mica may be present in well-formed hexagonal crystals,
and hornblende may appear as needle-like crystals.

Geology
On average the upper continental crust has the same composition as granodiorite.
Granodiorite is a plutonic igneous rock, formed by an intrusion of silica-rich magma, which
cools in batholiths or stocks below the Earth's surface. It is usually only exposed at the surface
after uplift and erosion have occurred. The volcanic equivalent of granodiorite is dacite.
Syenite
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32. Syenite
leucocratic variety of nepheline syenite from Sweden (särnaite).
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but
with the quartz either absent or present in relatively small amounts (<5%).
The feldspar component of syenite is predominantly alkaline in character (usually orthoclase) .
Plagioclase feldspars may be present in small quantities, less than 10%.
When present, ferromagnesian minerals are usually hornblende amphibole, rarely pyroxene or
biotite. Biotite is rare, because in a syenite magma most aluminium is used in producing feldspar.
Syenites are usually peralkaline and peraluminous, with high proportions of alkali elements and
aluminium.
Syenites are formed from alkaline igneous activity, generally formed in thick continental crustal
areas, or in Cordilleran subduction zones. To produce a syenite, it is necessary to melt a granitic
or igneous protolith to a fairly low degree of partial melting. This is required because potassium
is an incompatible element and tends to enter a melt first, whereas higher degrees of partial
melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite,
adamellite or tonalite.
At very low degrees of partial melting a silica undersaturated melt is produced, forming a
nepheline syenite, where orthoclase is replaced by a feldspathoid such as leucite, nepheline or
analcime.

33. Syenite is not a common rock, some of the more important occurrences being in New England,
Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, and Norway.
Etymology
The term syenite was originally applied to hornblende granite like that of Syene in Egypt, from
which the name is derived.
Episyenite
Episyenite (or epi-syenite) is a term used in petrology to describe to the result of alteration of a
SiO2 rich rock to a more SiO2 depleted rock.
The process which results in SiO2 depletion can be termed episyenitization. This process is only
referring to the macroscopic result of relative SiO2 depletion in a rock. The actual physical
process leading to this SiO2 depletion may vary in a given metamorphic environment. Diffusion
of chemical components in a stagnant fluid, related to differences in chemical potential or
pressure as well as advection of a SiO2- undersaturated fluid may lead to the dissolution of quartz
from the un-altered rock, thus depleting it of this component.
Nepheline syenite
Nepheline syenite from Sweden
Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali
feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not
unlike granites, but dark green varieties are also known. Phonolite is the fine-grained extrusive
equivalent.
Petrology
Nepheline syenites are silica-undersaturated and some are peralkaline (terms discussed in
igneous rock). Nepheline is a feldspathoid, a solid-solution mineral, that does not coexist with
quartz; rather, nepheline would react with quartz to produce alkali feldspar.

34. They are distinguished from ordinary syenites not only by the presence of nepheline but also by
the occurrence of many other minerals rich in alkalis and in rare earths and other incompatible
elements. Alkali feldspar dominates, commonly represented by orthoclase and the exsolved
lamellar albite, form perthite. In some rocks the potash feldspar, in others the soda feldspar
predominates. Fresh clear microcline is very characteristic of some types of nepheline syenite.
Sodalite, colorless and transparent in thin section, but frequently pale blue in the hand
specimens, is the principal feldspathoid mineral in addition to nepheline. Reddish-brown to black
triclinic aenigmatite occurs also in these rocks. Extremely iron-rich olivine is rare, but is present
in some nepheline syenite. Other minerals common in minor amounts include sodium-rich
pyroxene, biotite, titanite, zircon, iron oxides, apatite, fluorite, melanite garnet, and zircon.
Cancrinite occurs in several nepheline-syenites. A great number of interesting and rare minerals
have been recorded from nepheline syenites and the pegmatite veins which intersect them.
Genesis
Silica-undersaturated igneous rocks typically are formed by low degrees of partial melting in the
Earth's mantle. Carbon dioxide may dominate over water in source regions. Magmas of such
rocks are formed in a variety of environments, including continental rifts, ocean islands, and
supra-subduction positions in subduction zones. Nepheline syenite and phonolite may be derived
by crystal fractionation from more mafic silica-undersaturated mantle-derived melts, or as partial
melts of such rocks. Igneous rocks with nepheline in their normative mineralogy commonly are
associated with other unusual igneous rocks such as carbonatite.
Distribution
Nepheline syenites and phonolites occur in Canada, Norway, Greenland, Sweden, the Ural
Mountains, the Pyrenees, Italy, Brazil, China, the Transvaal region, and Magnet Cove igneous
complex of Arkansas, as well as on oceanic islands.
Phonolite lavas formed in the East African rift in particularly large quantity, and the volume
there may exceed the volume of all other phonolite occurrences combined, as discussed by
Barker (1983).
Nepheline-normative rocks occur in close association with the Bushveld Igneous Complex,
possibly formed from partial melting of the wall rocks to that large ultramafic layered intrusion.
Nepheline syenites are rare; there is only one occurrence in Great Britain and one in France and
Portugal. They are known also in Bohemia and in several places in Norway, Sweden and
Finland. In the Americas these rocks have been found in Texas, Arkansas and Massachusetts,
also in Ontario, British Columbia and Brazil. South Africa, Madagascar, India, Tasmania, Timor
and Turkestan are other localities for the rocks of this series.
Rocks of this class also occur in Brazil (Serra de Tingua) containing sodalite and often much
augite, in the western Sahara and Cape Verde Islands; also at Zwarte Koppies in the Transvaal,

35. Madagascar, São Paulo in Brazil, Paisano Pass in West Texas and Montreal, Canada. The rock of
Salem, Massachusetts, United States, is a mica-foyaite rich in albite and aegirine: it accompanies
granite and essexite. Litchfieldite is another well-marked type of nepheline-syenite, in which
albite is the dominant feldspar. It is named after Litchfield, Maine, United States, where it occurs
in scattered blocks. Biotite, cancrinite and sodalite are characteristic of this rock. A similar
nepheline-syenite is known from Hastings County, Ontario, and contains hardly any orthoclase,
but only albite feldspar. Nepheline is very abundant and there is also cancrinite, sodalite,
scapolite, calcite, biotite and hornblende. The lujaurites are distinguished from the rocks above
described by their dark color, which is due to the abundance of minerals such as augite, aegirine,
arfvedsonite and other kinds of amphibole. Typical examples are known near Lujaur on the
White Sea, where they occur with umptekites and other very peculiar rocks. Other localities for
this group are at Julianehaab in Greenland with sodalite-syenite; at their margins they contain
pseudomorphs after leucite. The lujaurites frequently have a parallel-banding or gneissose
structure. Sodalite-syenites in which sodalite very largely or completely takes the place of
nepheline occur in Greenland, where they contain also microcline-perthite, aegirine, arfvedsonite
and eudialyte.
Cancrinite syenite, with a large percentage of cancrinite, has been described from Dalekarlia,
Sweden and from Finland. We may also mention urtite from Lujaur Urt on the White Sea, which
consists very largely of nepheline, with aegirine and apatite, but no feldspar. Jacupirangite (from
Jacupiranga in Brazil) is a blackish rock composed of titaniferous augite, magnetite, ilmenite,
perofskite and nepheline, with secondary biotite.
Nomenclature
There is a wide variety of silica-undersaturated and peralkaline igneous rocks, including many
informal place-name varieties named after the locations in which they were first discovered. In
many cases these are plain nepheline syenites containing one or more rare minerals or
mineraloids, which do not warrant a new formal classification. These include;
Foyaite: foyaites are named after Foya in the Serra de Monchique, in southern Portugal. These
are K-feldspar-nepheline syenites containing <10% ferromagnesian minerals, usually pyroxene-,
hornblende- and biotite.
Laurdalite: The laurdalites, from Laurdal in Norway, are grey or pinkish, and in many ways
closely resemble the laurvikites of southern Norway, with which they occur. They contain
anorthoclase feldspars, biotite or greenish augite, much apatite and in some cases, olivine.
Ditroite: Ditroite derives is name from Ditrau, Transylvania, Romania. It is essentially a
microcline, sodalite and cancrinite variety of nepheline syenite. It contains also orthoclase,
nepheline, biotite, aegirine, acmite.
Chemical composition
The chemical peculiarities of the nepheline-syenites are well marked. They are exceedingly rich
in alkalis and in alumina (hence the abundance of felspathoids and alkali feldspars) with silica

36. varying from 50 to 56%, while lime, magnesia[disambiguation needed] and iron are never present in
great quantity, though somewhat more variable than the other components. A worldwide average
of the major elements in nepheline syenite tabulated by Barker (1983) is listed below, expressed
as weight percent oxides.
 SiO2 — 54.99%
 TiO2 — 0.60%
 Al2O3 — 20.96%
 Fe2O3 — 2.25%
 FeO — 2.05%
 MnO — 0.15%
 MgO — 0.77%
 CaO — 2.31%
 Na2O — 8.23%
 K2O — 5.58%
 H2O — 1.47%
 P2O5 — 0.13%
The normative mineralogy of this average composition contains about 22 percent nepheline and
66 percent feldspar.
Monzonite
Photomicrograph of thin section of monzonite (in cross polarised light)

37. The QAPF diagram, by which a monzonite is defined
Photomicrograph of thin section of monzonite (in plane polarised light)
An intrusion (Notch Peak monzonite) inter-fingers (partly as a dike) with highly-metamorphosed
host rock (Cambrian carbonate rocks). From near Notch Peak, House Range, Utah.
Monzonite is an intermediate igneous intrusive rock composed of approximately equal amounts
of sodic to intermediate plagioclase and orthoclase feldspars with minor amounts of hornblende,
biotite and other minerals. Quartz a minor constituent or is absent; with greater than 10% quartz
the rock is termed a quartz monzonite.
If the rock has more orthoclase or potassium feldspar it grades into a syenite. With an increase of
calcic plagioclase and mafic minerals the rock type becomes a diorite. The volcanic equivalent is
the latite.
Tonalite

38. A piece of tonalite on red granite gneiss from Tjörn, Sweden
Tonalite is an igneous, plutonic (intrusive) rock, of felsic composition, with phaneritic texture.
Feldspar is present as plagioclase (typically oligoclase or andesine) with 10% or less alkali
feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are
common accessory minerals.
In older references tonalite is sometimes used as a synonym for quartz diorite. However the
current IUGS classification defines tonalite as having greater than 20% quartz and quartz diorite
with from 5 to 20% quartz.
The name is derived from the type locality of tonalites, adjacent to the Tonale Line, a major
structural lineament and mountain pass, Tonale Pass, in the Italian and Austrian Alps.
Trondhjemite is an orthoclase-deficient variety of tonalite with minor biotite as the only mafic
mineral, named after Norway's third largest city, Trondheim.
Igneous rocks by composition
Intermediate-
Ultramafic Mafic Intermediate Felsic
Type Felsic
< 45% SiO2 < 52% SiO2 52–63% SiO2 >69 % SiO2
63–69% SiO2
Volcanic Komatiite Basalt Andesite Dacite Rhyolite
rocks: Kimberlite, Diabase Aplite—
Subvolcanic Lamproite (Dolerite) Diorite Granodiorite Pegmatite
rocks: Peridotite Gabbro Granite
Plutonic rocks:
Diorite

39. Diorite
Diorite classification on QAPF diagram
Diorite (pronounced /ˈdaɪəraɪt/) is a grey to dark grey intermediate intrusive igneous rock
composed principally of plagioclase feldspar (typically andesine), biotite, hornblende, and/or
pyroxene. It may contain small amounts of quartz, microcline and olivine. Zircon, apatite,
sphene, magnetite, ilmenite and sulfides occur as accessory minerals.[1] It can also be black or
bluish-grey, and frequently has a greenish cast. Varieties deficient in hornblende and other dark
minerals are called leucodiorite. When olivine and more iron-rich augite are present, the rock
grades into ferrodiorite, which is transitional to gabbro. The presence of significant quartz makes
the rock type quartz-diorite (>5% quartz) or tonalite (>20% quartz), and if orthoclase (potassium
feldspar) is present at greater than ten percent the rock type grades into monzodiorite or
granodiorite. Diorite has a medium grain size texture, occasionally with porphyry.
Diorites may be associated with either granite or gabbro intrusions, into which they may subtly
merge. Diorite results from partial melting of a mafic rock above a subduction zone. It is
commonly produced in volcanic arcs, and in cordilleran mountain building such as in the Andes
Mountains as large batholiths. The extrusive volcanic equivalent rock type is andesite.
Occurrence

40. Diorite
Diorite is a relatively rare rock; source localities include Leicestershire; UK [2] (one name for
microdiorite - Markfieldite - exists due to the rock being found in the village of Markfield),
Sondrio, Italy; Thuringia and Saxony in Germany; Finland; Romania; Northeastern Turkey;
central Sweden; Scotland; the Darrans range of New Zealand; the Andes Mountains; the Isle of
Guernsey; Basin and Range province and Minnesota in the USA; Idahet in Egypt
An orbicular variety found in Corsica is called corsite.
Gabbro
Gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.
Close-up of gabbro specimen; Rock Creek Canyon, eastern Sierra Nevada, California.

41. Photomicrograph of a thin section of gabbro.
Gabbro (pronounced /ˈɡæbroʊ/) refers to a large group of dark, coarse-grained, intrusive mafic
igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten
magma is trapped beneath the Earth's surface and cools into a crystalline mass.
The vast majority of the Earth's surface is underlain by gabbro within the oceanic crust, produced
by basalt magmatism at mid-ocean ridges.

Petrology
A gabbro landscape on the main ridge of the Cuillin, Isle of Skye, Scotland.
Gabbro as a xenolith in a granite, eastern Sierra Nevada, Rock Creek Canyon, California.
Gabbro is dense, greenish or dark-colored and contains pyroxene, plagioclase, amphibole, and
olivine (olivine gabbro when olivine is present in a large amount).

42. The pyroxene is mostly clinopyroxene; small amounts of orthopyroxene may be present. If the
amount of orthopyroxene is substantially greater than the amount of clinopyroxene, the rock is
then a norite. Quartz gabbros are also known to occur and are probably derived from magma that
was over-saturated with silica. Essexites represent gabbros whose parent magma was under-
saturated with silica, resulting in the formation of the feldspathoid mineral nepheline. (Silica
saturation of a rock can be evaluated by normative mineralogy). Gabbros contain minor amounts,
typically a few percent, of iron-titanium oxides such as magnetite, ilmenite, and ulvospinel.
Gabbro is generally coarse grained, with crystals in the size range of 1 mm or greater. Finer
grained equivalents of gabbro are called diabase, although the vernacular term microgabbro is
often used when extra descriptiveness is desired. Gabbro may be extremely coarse grained to
pegmatitic, and some pyroxene-plagioclase cumulates are essentially coarse grained gabbro,
although these may exhibit acicular crystal habits.
Gabbro is usually equigranular in texture, although it may be porphyritic at times, especially
when plagioclase oikocrysts have grown earlier than the groundmass minerals.
Distribution
Gabbro can be formed as a massive, uniform intrusion via in-situ crystallisation of pyroxene and
plagioclase, or as part of a layered intrusion as a cumulate formed by settling of pyroxene and
plagioclase. Cumulate gabbros are more properly termed pyroxene-plagioclase orthocumulate.
Gabbro is an essential part of the oceanic crust, and can be found in many ophiolite complexes as
parts of zones III and IV (sheeted dyke zone to massive gabbro zone). Long belts of gabbroic
intrusions are typically formed at proto-rift zones and around ancient rift zone margins, intruding
into the rift flanks. Mantle plume hypotheses may rely on identifying mafic and ultramafic
intrusions and coeval basalt volcanism.
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase
labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro
without thin section study under the petrographic microscope. It occurs with gabbro and other
mafic to ultramafic rocks in layered intrusions which are often associated with platinum
orebodies such as in the Bushveld Igneous Complex in South Africa, the Skaergaard igneous
complex of Greenland, and the Stillwater igneous complex in Montana, USA. Norite is also the
basal igneous rock of the Sudbury Basin complex in Ontario which is the site of a meteorite
impact and the world's second largest nickel mining region. Norite is a common rock type of the
Apollo samples. On a smaller scale, norite can be found in small localized intrusions such as the
Gombak Norite in Bukit Gombak, Singapore.
The name Norite is derived from the Norwegian name for Norway: Norge.

43. Anorthosite
Anorthosite from Poland
Lunar anorthosite from Apollo 15 landing site
Anorthosite (pronounced /ænˈɔrθəsaɪt/) is a phaneritic, intrusive igneous rock characterized by a
predominance of plagioclase feldspar (90–100%), and a minimal mafic component (0–10%).
Pyroxene, ilmenite, magnetite, and olivine are the mafic minerals most commonly present.
Anorthosite on Earth can be divided into two types: Proterozoic anorthosite (also known as
massif or massif-type anorthosite) and Archean anorthosite. These two types of anorthosite have
different modes of occurrence, appear to be restricted to different periods in Earth's history, and
are thought to have had different origins.
Lunar anorthosites constitute the light-coloured areas of the Moon's surface and have been the
subject of much research.[1]
Proterozoic anorthosite

44. Age
Although a few Proterozoic anorthosite bodies were emplaced either late in the Archean Eon, or
early in the Phanerozoic Eon, the vast majority of Proterozoic anorthosites were emplaced, as
their name suggests, during the Proterozoic Eon (ca. 2,500-542 Ma).
Mode of occurrence
Anorthosite from southern Finland
Anorthosite plutons occur in a wide range of sizes. Some smaller plutons, exemplified by many
anorthosite bodies in the U.S. and Harris in Scotland, cover only a few dozen square kilometres.
Larger plutons, like the Mt. Lister Anorthosite, in northern Labrador, Canada, cover several
thousands of square kilometres.
Many Proterozoic anorthosites occur in spatial association with other highly distinctive,
contemporaneous rock types (the so-called 'anorthosite suite' or 'anorthosite-mangerite-
charnockite complex'). These rock types include iron-rich diorite, gabbro, and norite; leucocratic
mafic rocks such as leucotroctolite and leuconorite; and iron-rich felsic rocks, including
monzonite and rapakivi granite. Importantly, large volumes of ultramafic rocks are not found in
association with Proterozoic anorthosites.
Occurrences of Proterozoic anorthosites are commonly referred to as 'massifs'. However, there is
some question as to what name would best describe any occurrence of anorthosite together with
the rock types mentioned above. Early works used the term 'complex' The term 'plutonic suite'
has been applied to some large occurrences in northern Labrador, Canada; however, it has been
suggested (in 2004-2005) that 'batholith' would be a better term. 'Batholith' is used to describe
such occurrences for the remainder of this article.
The areal extent of anorthosite batholiths ranges from relatively small (dozens or hundreds of
square kilometres) to nearly 20,000 km2 (7,700 sq mi), in the instance of the Nain Plutonic Suite
in northern Labrador, Canada.
Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the Appalachian
Mountains, eastern Canada, across southern Scandinavia and eastern Europe. Mapped onto the
Pangaean continental configuration of that eon, these occurrences are all contained in a single

45. straight belt, and must all have been emplaced intracratonally. The conditions and constraints of
this pattern of origin and distribution are not clear. However, see the Origins section below.
Anorthosites are also common in layered intrusions. Anorthosite in these layered intrusions can
form as cumulate layers in the upper parts of the intrusive complex[2] or as later-stage intrusions
into the layered intrusion complex. [3]
Physical characteristics
Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites
appear, in outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue,
or grey, and may exhibit an iridescence known as labradorescence on fresh surfaces. The
feldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is a
compositional term for any calcium-rich plagioclase feldspar containing between 50–70
molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence. The
mafic mineral in Proterozoic anorthosite may be clinopyroxene, orthopyroxene, olivine, or, more
rarely, amphibole. Oxides, such as magnetite or ilmenite, are also common.
Most anorthosite plutons are very coarse grained; that is, the individual plagioclase crystals and
the accompanying mafic mineral are more than a few centimetres long. Less commonly,
plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoic
anorthosites are deformed, and such large plagioclase crystals have recrystallized to form smaller
crystals, leaving only the outline of the larger crystals behind.
While many Proterozoic anorthosite plutons appear to have no large-scale relict igneous
structures (having instead post-emplacement deformational structures), some do have igneous
layering, which may be defined by crystal size, mafic content, or chemical characteristics. Such
layering clearly has origins with a rheologically liquid-state magma.
Chemical and isotopic characteristics
The composition of plagioclase feldspar in Proterozoic anorthosites is most commonly between
An40 and An60 (40-60% anorthite). This compositional range is intermediate, and is one of the
characteristics which distinguish Proterozoic anorthosites from Archean anorthosites. Mafic
minerals in Proterozoic anorthosites have a wide range of composition, but are not generally
highly magnesian.
The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has been
examined in some detail by researchers with the aim of arriving at a plausible genetic theory.
However, there is still little agreement on just what the results mean for anorthosite genesis; see
the 'Origins' section below. A very short list of results, including results for rocks thought to be
related to Proterozoic anorthosites. [4]
Some research has focused on neodymium (Nd) and strontium (Sr) isotopic determinations for
anorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic

46. determinations are of use in gauging the viability of prospective sources for magmas that gave
rise to anorthosites. Some results are detailed below in the 'Origins' section.
Origins of Proterozoic anorthosites
The origins of Proterozoic anorthosites have been a subject of theoretical debate for many
decades. A brief synopsis of this problem is as follows. The problem begins with the generation
of magma, the necessary precursor of any igneous rock.
Magma generated by small amounts of partial melting of the mantle is generally of basaltic
composition. Under normal conditions, the composition of basaltic magma requires it to
crystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magma
crystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content
(90–100% plagioclase), and are not found in association with contemporaneous ultramafic rocks.
This is now known as 'the anorthosite problem'. Proposed solutions to the anorthosite problem
have been diverse, with many of the proposals drawing on different geological subdisciplines.
It was suggested early in the history of anorthosite debate that a special type of magma,
anorthositic magma, had been generated at depth, and emplaced into the crust. However, the
solidus of an anorthositic magma is too high for it to exist as a liquid for very long at normal
ambient crustal temperatures, so this appears to be unlikely. The presence of water vapour has
been shown to lower the solidus temperature of anorthositic magma to more reasonable values,
but most anorthosites are relatively dry. It may be postulated, then, that water vapour be driven
off by subsequent metamorphism of the anorthosite, but some anorthosites are undeformed,
thereby invalidating the suggestion.
The discovery, in the late 1970s, of anorthositic dykes in the Nain Plutonic Suite, suggested that
the possibility of anorthositic magmas existing at crustal temperatures needed to be reexamined.
However, the dykes were later shown to be more complex than was originally thought. In
summary, though liquid-state processes clearly operate in some anorthosite plutons, the plutons
are probably not derived from anorthositic magmas.
Many researchers have argued that anorthosites are the products of basaltic magma, and that
mechanical removal of mafic minerals has occurred. Since the mafic minerals are not found with
the anorthosites, these minerals must have been left at either a deeper level or the base of the
crust. A typical theory is as follows: partial melting of the mantle generates a basaltic magma,
which does not immediately ascend into the crust. Instead, the basaltic magma forms a large
magma chamber at the base of the crust and fractionates large amounts of mafic minerals, which
sink to the bottom of the chamber. The cocrystallizing plagioclase crystals float, and eventually
are emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals form
ultramafic cumulates which stay at the base of the crust.
This theory has many appealing features, of which one is the capacity to explain the chemical
composition of high-alimuna orthopyroxene megacrysts (HAOM). This is detailed below in the
section devoted to the HAOM. However, on its own, this hypothesis cannot coherently explain
the origins of anorthosites, because it does not fit with, among other things, some important

47. isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Sr
isotopic data shows the magma which produced the anorthosites cannot have been derived only
from the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites must
have had a significant crustal component. This discovery led to a slightly more complicated
version of the previous hypothesis: Large amounts of basaltic magma form a magma chamber at
the base of the crust, and, while crystallizing, assimilating large amounts of crust.[5]
This small addendum explains both the isotopic characteristics and certain other chemical
niceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on the
basis of geochemical data, that the mantle's role in production of anorthosites must actually be
very limited: the mantle provides only the impetus (heat) for crustal melting, and a small amount
of partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almost
entirely from lower crustal melts. [6]
High-alumina orthopyroxene megacrysts
The high-alumina orthopyroxene megacrysts (HAOM) have, like Proterozoic anorthosites, been
the subject of great debate, although a tentative consensus about their origin appears to have
emerged. The peculiar characteristic worthy of such debate is reflected in their name. Normal
orthopyroxene has chemical composition (Fe,Mg)2 Si2O6, whereas the HAOM have anomalously
large amounts of aluminium (up to about 9%) in their atomic structure.
Because the solubility of aluminium in orthopyroxene increases with increasing pressure, many
researchers,[7] have suggested that the HAOM crystallized at depth, near the base of the Earth's
crust. The maximum amounts of aluminium correspond to a 30–35 km (19–22 mi) depth.
Other researchers consider the chemical compositions of the HAOM to be the product of rapid
crystallization at moderate or low pressures. [8]
Archaean anorthosite
Smaller amounts of anorthosite were emplaced during the Archaean eon (ca 3,800-2,400 Ma),
although most have been dated between 3,200 and 2,800 Ma. They are distinct texturally and
mineralogically from Proterozoic anorthosite bodies. Their most characteristic feature is the
presence of equant megacrysts of plagioclase surrounded by a fine-grained mafic groundmass.
Diabase

48. Diabase
Diabase (pronounced /ˈdaɪ.əbeɪs/) or Dolerite is a mafic, holocrystalline, subvolcanic rock
equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase
refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase
refers to altered material. [1][2] Diabase dikes and sills are typically shallow intrusive bodies and
often exhibit fine grained to aphanitic chilled margins which may contain tachylite (dark mafic
glass).
Petrology
Diabase normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals
(62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3%
up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%).[3] Accessory and alteration
minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and
calcite. The texture is termed diabasic and is typical of diabases. This diabasic texture is also
termed interstitial[4]. The feldspar is high in anorthite (as opposed to albite), the calcium
endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.
Diabase/dolerite

49. The Candlestick, Tasman Peninsula, Tasmania, is composed of Jurassic Dolerite. Tasmania has
the world's largest areas of dolerite.
In non-North American usage dolerite is preferred due to the various conflicting uses of diabase.
Dolerite (Greek: doleros, meaning "deceptive") was the name given by Haüy in his 1822 Traité
de minéralogie. In continental Europe diabase was reserved by Brongniart for pre-Tertiary (pre-
Cenozoic) material[5], with dolerite used for more recent rock. The use of diabase in this sense
was abandoned in Britain in favor of dolerite for rocks of all ages by Allport (1874)[6], though
some British geologists continued to use diabase to describe slightly altered dolerite, in which
pyroxene has been altered to amphibole.[7]
Locations
A diabase dike crosscutting horizontal limestone beds in Arizona

50. Diabase is usually found in smaller relatively shallow intrusive bodies such as dikes and sills.
Diabase dikes occur in regions of crustal extension and often occur in dike swarms of hundreds
of individual dikes or sills radiating from a single volcanic center.
The Palisades Sill which makes up the New Jersey Palisades on the Hudson River, near New
York City, is an example of a diabase sill. The dike complexes of the British Tertiary Volcanic
Province which includes Skye, Rum, Mull, and Arran of western Scotland, the Slieve Gullion
region of Ireland, and extends across northern England contains many examples of diabase dike
swarms. Parts of the Deccan Traps of India, formed at the end of the Cretaceous also includes
dolerite[8]. It is also abundant in large parts of Curaçao, an island off the coast of Venezuela.
In Western Australia a 200 km long dolerite dike, the Norseman–Wiluna Belt[9] is associated
with the non-alluvial gold mining area between Norseman and Kalgoolie, which includes the
largest gold mine in Australia[10], the Super Pit gold mine. West of the Norseman–Wiluna Belt is
the Yalgoo–Singleton Belt, where complex dolerite dike swarms obscure the volcaniclastic
sediments.[11]
The vast areas of mafic volcanism/plutonism associated with the Jurassic breakup of
Gondwanaland in the Southern Hemisphere include many large diabase/dolerite sills and dike
swarms. These include the Karoo dolerites of South Africa, the Ferrar Dolerites of Antarctica,
and the largest of these, indeed the most extensive of all dolerite formations worldwide, are
found in Tasmania. Here, the volume of magma which intruded into a thin veneer of Permian
and Triassic rocks from multiple feeder sites, over a period of perhaps a million years, may have
exceeded 40,000 cubic kilometres.[12] In Tasmania alone dolerite dominates the landscape.
Ring dikes are large, near vertical dikes showing above ground as circular outcrops up to 30 km
in diameter, with a depth from hundreds of metres to several kilometres. Thicker dikes are made
up of plutonic rocks, rather than hypabyssal and are centred around deep intrusions. The central
part may be a block sunken into underlying magma, the ring dikes forming in the fracture zone
around the sunken block.
Peridotite
Peridotite
— Igneous Rock —
Peridotite xenolith from San Carlos, southwestern United

51. States. The rock is typical olivine-rich peridotite, cut by a
centimeter-thick layer of greenish-black pyroxenite.
Composition
olivine, pyroxene
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine
and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in
magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron.
Peridotite is derived from the Earth's mantle, either as solid blocks and fragments, or as crystals
accumulated from magmas that formed in the mantle. The compositions of peridotites from these
layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes,
chromite, plagioclase, and amphibole.
Peridotite is the dominant rock of the upper part of the Earth's mantle. The compositions of
peridotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest,
because they provide samples of the Earth's Mantle roots of continents brought up from depths
from about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserve
isotope ratios of osmium and other elements that record processes over three billion years ago,
and so they are of special interest to paleogeologists because they provide clues to the
composition of the Earth's early mantle and the complexities of the processes that were involved.
The word peridotite comes from the gemstone peridot, which consists of pale green olivine. [1]
Types of peridotite
 Dunite: more than 90% olivine, typically with Mg/Fe ratio of about 9:1.
 Wehrlite: mostly composed of olivine plus clinopyroxene.
 Harzburgite: mostly composed of olivine plus orthopyroxene, and relatively low
proportions of basaltic ingredients (because garnet and clinopyroxene are minor).
 Lherzolite: mostly composed of olivine, orthopyroxene (commonly enstatite), and
clinopyroxene (diopside), and have relatively high proportions of basaltic ingredients
(garnet and clinopyroxene). Partial fusion of lherzolite and extraction of the melt fraction
can leave a solid residue of harzburgite.

52. Classification diagram for peridotite and pyroxenite, based on proportions of olivine and
pyroxene. The pale green area encompasses the most common compositions of peridotite in the
upper part of the Earth's mantle (partly adapted from Bodinier and Godard (2004)).
Composition
Peridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine.
The compositions of peridotites from layered igneous complexes vary widely, reflecting the
relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Minor minerals and
mineral groups in peridotite include plagioclase, spinel (commonly the mineral chromite), garnet
(especially the mineral pyrope), amphibole, and phlogopite. In peridotite, plagioclase is stable at
relatively low pressures (crustal depths), aluminous spinel at higher pressures (to depths of 60
km or so), and garnet at yet higher pressures.
Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/or
clinopyroxene; minerals that may be present in lesser abundance include olivine, garnet,
plagioclase, amphibole, and spinel.
Distribution and location
Olivine in a peridotite weathering to iddingsite within a mantle xenolith
Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that
depth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist of
up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick,
consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal
peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually
subducted back into the mantle in subduction zones. However, pieces can be emplaced into or
overthrust on continental crust by a process called obduction, rather than carried down into the
mantle; the emplacement may occur during orogenies, as during collisions of one continent with
another or with an island arc. The pieces of oceanic plates emplaced within continental crust are
referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks such
as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses of
peridotite have been emplaced into mountain belts as solid masses but do not appear to be related
to ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites."