Petrologia Ignea en Ingles
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Transcript of Petrologia Ignea en Ingles
http://www.mhhe.com/earthsci/geology/plummer/student/olc/chap03boxsumm.mhtml
http://www.geologyclass.org/Igneous%20Concepts.htm
http://geologicalintroduction.baffl.co.uk/?p=3
http://saddleback.edu/faculty/jrepka/notes/GEOigneousLAB.pdf
http://csmres.jmu.edu/geollab/Fichter/IgnRx/IgHome.html
IGNEOUS ROCKS AND IGNEOUS PROCESSES
Introduction
Magma- Molten rock material composed mostly of _____________________. Magmas may also include dissolved gases and minor amounts of solid minerals.
Magma at Earth's Surface. United States Geological Survey image.
Magmas can occur deep within the Earth, or at Earth's surface.
Igneous rock- A silicate-rich rock that forms when magma solidifies.
There are two types of igneous rocks:
Intrusive/Plutonic- Igneous rock formed when magma solidifies deep underground (includes granites, the main rock of the continents).
Extrusive/Volcanic- Igneous rock formed when magma solidifies at the Earth’s surface as lava (includes basalts, the main rock of ocean floors).
Unlike extrusive volcanism, intrusive igneous activity has never directly witnessed! However, we can infer much about igneous processing based on indirect evidence.
The Textures of Igneous Rocks
Texture refers to the size, shape and arrangement of crystal grains within a rock.
Under the microscope, the mineral grains of igneous rocks tend to display an interlocking texture that represents the growth of minerals from a melt.
A thin section of gabbro showing plagioclase, clinopyroxene and olivine (GNU Image by Siim Sepp, 2006).
In igneous rocks, crystal size is primarily controlled by
Extrusive/volcanic rocks cooled quickly at or near Earth’s surface, giving crystals little time to grow. These rocks tend to be fine-grained or aphanitic (most crystals <1 mm).
In contrast, intrusive/plutonic rocks cooled slowly deep within the Earth and are coarse-grained or phaneritic (most crystals >1 mm)
Basalt, an extrusive igneous rock. Individual minerals are not easily seen
in hand-specimen.
Granite, an intrusive igneous rock.
Individual minerals can be seen in hand-specimen.
These photos are from R. Welleror of Cochise College, © 2008.
To classify igneous rocks, we will need to identify minerals in hand specimen.
Let's review the rock forming minerals, and divide each mineral into "light" and "dark" categories based on color.
The Rock-Forming Minerals
FeldsparAlkali Feldspar: (K,Na)Al Si 3O8
FeldsparPlagioclase Feldspar: NaAlSi3O8 - Ca Al 2Si2O8
Quartz: SiO2
AmphiboleHornblende:
Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2
MicaMuscovite: K Al 2(AlSi3O10)
(F,OH)2
MicaBiotite:
K(Fe,Mg)3AlSi3O10(F, OH)2
OlivineFayalite: Fe2SiO4
Forsterite: Mg2Si2O4
PyroxeneEnstatite: Mg2Si2O6
Ferrosilite: Fe2Si2O6
These photos are from R. Welleror of Cochise College, © 2008.
The Classification of Igneous Rocks by Color
Igneous rocks can be readily classified into three categories based on color: felsic, mafic, intermediate, and ultramafic. This color-based classification scheme may seem simplistic, but it turns out that a rock's color tells us much about its mineralogical makeup and overall composition.
Felsic rocks are rich in light-colored minerals (quartz, alkali feldspar, and some plagioclase feldspar). They are compositionally rich in Si, Na, Al, and K and poor in Fe and Mg (the dark-colored minerals biotite and amphibole are present, but only in minor amounts).
A felsic rock known as granite (R. Welleror of Cochise College, © 2008).
Mafic rocks contain abundant dark-colored minerals (olivine, pyroxene, and plagioclase). They are compositionally rich in Fe, Mg, and Ca.
A mafic rock known as basalt (R. Welleror of Cochise College, © 2008).
Intermediate rocks contain roughly equal amounts of dark- and light-colored minerals.
An intermediate rock known as andesite (R. Welleror of Cochise College, © 2008).
Ultramafic rocks consist almost exclusively of Fe and Mg-rich minerals from the mantle (olivine and pyroxene, but no plagioclase). They are compositionally rich in Fe, Mg, and Ca, but poor in Si.
An ultramafic rock known as peridotite consisting chiefly of olivine (R. Welleror of Cochise College, © 2008).
The Classification of Igneous Rocks Based on Color, Texture, and Mineralogy
The most useful system for classifying igneous rocks utilizes color, texture, and mineralogy.
Felsic Intermediate Mafic Ultramafic
Coarse-grained/
Phaneritic
(Intrusive)
Granite
Diorite
Gabbro
Peridotite
Fine-grained/
Aphanitic
(Extrusive)
Rhyolite
Andesite
Basalt
Komatiite
(Not Pictured)
QuartzContent
High Intermediate None
Alkali Feldspar
Content
High Low None
(Na, K)
Plagioclase
Content
(Al, Ca)
Low Intermediate High None
As we go from left to right (from felsic to ultramafic): Color darkens.
Mg and Fe increases. K and Na decreases.
The above photos are by R. Welleror of Cochise College, © 2008.
Special Textures in Igneous Rocks Xenolith: A fragment of rock within an igneous rock that differs compositionally from the host rock. The host rock and zenolith inclusions formed from different magmas. Vesicule: A bubble or hole formed by escaping gas (common in basalts).
Olivine xenolith in vesicular basalt, © Dr. Richard Busch
Pegmatite: A very coarse grained igneous rock (crystal sizes > 5 cm) in which crystal growth was enhanced by the presence of fluids.
Pegmatite, © Marli Miller
Porphyritic: Igneous rock with large crystals (called phenocrysts) in a fine-grained matrix. Porphyritic rocks may represent a two-state cooling history:
1) slow cooling at depth followed by...
2) rapid uplift and fast cooling near Earth's surface.
Porphyritic rock, © Dr. Richard Busch
How Does Magma Form?
1) The temperature of the Earth increases from crust to core at approximately 30 C/km (this is called the geothermal gradient). The core temperature is > 5000 C, and heat moves upward from the very hot core (where temperatures exceed 5000°C) and melts the upper mantle and crust. 2) Melting can also result from a decrease in pressure. Since pressure favors solids, mineral melting points decrease with decreasing pressure. This decompression melting occurs when hot mantle rock moves upward. 3) The presence of water vapor reduces the melting point of rock. Wet magma (magma with water vapor) melts at a lower temperature than dry magma (magma with no water vapor). For example, wet granite melts at 700°C whereas dry granite melts at 900°C. 4) Mixtures of minerals always have lower melting points than the pure minerals would. For example, Quartz melts at ~1650°C and K-Feldspar melts at ~1300°C. However, a 50/50 mixture of these two minerals will melt at ~1150°C.
Magma Crystallization and Melting Sequence
Minerals crystallize in a predictable order over a large temperature range (and melt in the reverse order). This sequence of mineral crystallization is described by Bowen’s Reaction Series, named after N.L. Bowen who used laboratory experiments to determine the sequence of mineral crystallization.
Lessons from Bowen’s Reaction Series: 1) The chemistry of a magma will determine the type of rock that can form from it.
2) For a given magma composition, the first magmas to solidify will be mafic (rich in Fe, Mg, Ca) such as a basalt or gabbro.
3) Later, more evolved felsic magmas (rich in K, Na, Si, and quartz) will produce rhyolites and basalts.
4) During heating, the order of mineral melting will be reversed from the order of crystallization.
Magma Evolution
Magmas that solidify close to their source rock will be the most like the source rock, whereas magmas that solidify far from the source rock will be changed or evolved. Magma evolution (called magma differentiation) can occur by 4 different processes:
1) Partial melting produces magmas less mafic than their source rocks, because the first minerals to melt will be felsic in composition.
2) Fractional crystallization involves the changing of magma composition by the removal of denser early-formed ferromagnesian minerals by crystal settling. The remaining magma becomes more felsic.
3) Assimilation occurs when a hot magma melts and incorporates surrounding country rock. If mafic magma assimilates more felsic continental crust an intermediate rock will result.
4) Magma mixing involves the mixing of more and less mafic magmas to produce a magma of intermediate composition.
Intrusive Rock Bodies
Intrusive rocks exist in intrusions that penetrate or cut through pre-existing country rock.
Intrusive bodies are given names based on their size, shape and geometric relationship to the country rock.
Two basic types of intrusions are:
A. Shallow intrusions (formed < 2 km beneath Earth’s surface). These cool and solidify fairly quickly resulting in fine-grained rocks.
Dike: Tabular structure that cuts across the layering in the country rock.
Igneous intrusions at Alaska's Glacier Bay National Park © Bruce Molnia, Terra Photographics.
Sill: Tabular structure that parallels layering in the country rock.
Basaltic sill near Logan Pass in Montana's Glacier National Park. © Larry Fellows.
Volcanic Neck: Shallow intrusion formed when magma solidifies in the throat of a volcano (i.e., Ship Rock, New Mexico).
The volcanic neck, Shiprock. Copyright © Louis Maher
B. Deep intrusions (formed > 2 km beneath Earth's surface). These cool and solidify slowly resulting in coarse-grained rocks.
Plutons are large, blob-shaped intrusive bodies formed when rising blobs of magma (diapirs) get trapped within the crust (commonly granite
Summit of Harney Peak in the Black Hills of South Dakota. © Bruce Molnia, Terra Photographics.
Small plutons (exposed over <100 km2) are called stocks, whereas large plutons (exposed over >100 km2) are called batholiths.
The interface between instrusions and country rock are called contacts.
Rapid cooling of igneous rock near the contact (called a chill zone) often results in a smaller crystal size near the contact.
The Link Between Igneous Activity and Plate Tectonics
Igneous activity occurs mainly at or near tectonic plate boundaries.
Mafic igneous rocks commonly form at divergent boundaries. Here, the low overburden pressure (decompression) contributes to the formation of
mafic magmas formed by partial melting of the asthenosphere (upper mantle).
Intermediate igneous rocks commonly form at convergent boundaries. Partial melting of subducted asthenosphere produces basaltic magma which evolves into intermediate magma by differentiation, assimilation, and magma mixing.
Felsic igneous rocks are also common adjacent to convergent boundaries. Hot mafic magmas produced near the subducting slab may induce partial melting and assimilation of continental (granitic) crust.
Some ingneous rocks form within plates (not at a plate boundary). Rising mantle plumes (of controversial origin) can produce localized hotspots and volcanoes as they rise through continental or oceanic crust.
2. Igneous Rocks
Posted on 31/12/2008 by Vitor Pacheco
2.1 Origin and Composition
Convection cells must have developed in the Earth’s
Mantle at a very early stage, consequently initiating
the differentiation of the elements composing the
original magma. The less dense elements, silicon rich,
accumulated at the top of the up flow side of the
convection cells, just as foam in a boiling pot. Thus,
this lighter material concentrated at the surface and
consolidated forming the continents which are
therefore silicon rich rocks containing an abundance of
quartz and are classified as oversaturated (acid). They
encompass the granite family, of which the volcanic
equivalent is rhyolite. Of the remaining magma, the
most common member and the one which forms the
oceanic floors, does not have enough silicon for quartz
to form, is classified as saturated, and its most common
rock family is the gabbro, with basalt as its volcanic
equivalent. The rocks with least silicon content are
classified as undersaturated (alkaline), and one of its
rock types is peridotite.
It is easy to understand that along crustal plate
diverging boundaries numerous cracks will form
through which the fluid magma from the mantle can
flow. Thus, igneous rocks associated with diverging
boundaries, if within an ocean and forming its ridge,
like the one along the centre of the Atlantic Ocean, will
have a basaltic composition since its source is also
basaltic. If the divergence is within a continent
breaking up like the Rift Valley in Africa, the igneous
rocks will be basaltic, but only if the magma being
tapped is from the mantle.
Along converging boundaries, where the rock masses
are under compression, it is not so straight forward,
especially since either the two plates are compressing
against each other, or the heavier density plate is
being subducted under the other one. So, I think that
in the majority of cases the igneous rocks originate
from the melting of the local rocks due to the
incredibly high temperatures and pressures caused by
the friction developed during compression. Thus, their
composition will differ in accordance to their relative
location, with basic rocks for the sector close to the
subduction trench because they will be fed by oceanic
floor rocks. Within continental masses, acidic rocks will
predominate.
2.2 Type of Occurrence
2.2.1 Volcanic Rocks
Molten magma is continuously being spewed from the
mantle through all sorts of existing fractures. If ejected
into the atmosphere, it is known as lava, and the ducts
through which the lava pours are the volcanos.
Further, because the surrounding
atmospheric temperature is markedly lower, the lava
will cool very rapidly and the resulting rock will tend to
be fine grained. Nowadays volcanos typically have pipe
like structures through which the magma flows and as
it cools, it creates the well known conic shapes (fig. 1).
Figure 1 – The top of the Teide volcanic cone (Tenerife, Canarias Archipelago).
Also, they frequently develop lateral vents (fig. 2).
However, magma may also outpour along fissures as
presently in Iceland and in the past, for example during
the Karroo volcanicity (Jurassic), in South Africa.
Figure 2 – lateral volcanic vent of the Teide (Tenerife, Canarias Archipelago).
Lava flows will enlarge the volcanic cone and spread in
a fan shape at the base. In the example shown on
figure 3 in Tenerife, the fan actually entered into the
sea, and that is where the town of Garuchio was built.
Figure 3 – Town built on a lava flow fan into the sea (Garuchio, Tenerife).
Volcanic exhalations may be gentle and fairly
continuous, in which case it takes the form of a very
plastic fluid termed lava flow, as for example the upper
dark layer of figure 4. Or, like the lower layer of the
same figure, the out pour may take the form of ash,
termed pyroclastic, with small fragments
predominating, but larger clasts may also be common
and in the present case they are easily identified
because of their much darker colour.
Figure 4 – Layer of volcanic ash (pyroclasts) overlain by basalt (view approximately 6 m high) (Tenerife, Canarias Archipelago).
These pyroclastic explosive bursts are due to the
magma high gas content, as well as the stage of
consolidation of the lava being spewed out. In extreme
cases we will have volcanic breccias (fig. 4B)
Figure 4B – Volcanic breccia (Barberton Mountain Land, S. Africa)
The appearance of the consolidated lava will also be
affected by:
• its degree of plasticity, which, when very high gives a
very contorted appearance (fig. 5);
Figure 5 – Contorted appearance of a very plastic lava flow (view approximately 1 m high) (Tenerife, Canarias Archipelago).
• the rate of cooling which, when very rapid, yields
volcanic glass, obsidian (fig. 6);
Figure 6 – Lava field with abundant obsidian (black), (Tenerife, Canarias Archipelago).
• high fluidity as well as gaseous content, will cause
the lava to be very porous, pumice stone, and the
porosity will make these rocks very light (fig. 7).
Figure 7 — Demonstration on how light the pumice stone is (Tenerife, Canarias Archipelago).
Further, this porosity will allow water to flow through
the hollows and, with time, the diluted substances will
precipitate and fill the holes, giving rise to what is
known as amygdaloidal lava (fig. 8).
Figure 8 – Amygdaloidal lava (Ventersdorp lavas, Carletonville, S. Africa).
When the size of those hollows is sufficiently large we
have the formation of the famous agates and
geodes (fig. 9), which will tend to broadly have a
spherical shape but may reach quite a considerable
size and present a huge variety of internal shapes. The
term agate is used when the precipitate is not
crystalline, and geode when it is.
Figure 9 — Agates/geodes from the Karroo lavas (Lebombo Mountains, Mozambique).
• Lava that flows into the sea freezes as it tumbles in
and forms very characteristic spherical units, termed
pillows. As these pillows fall on top of of those already
settled and if the lava is still sufficiently plastic, its
lower portion will become sort of squeezed between
the ones more solid below (fig. 10).
Figure 10 – Outcrop of pillow lavas (Barberton, S. Africa).
If, on the other hand the pillows fall on soft ground,
their spherical shapes are preserved and they squeeze
the paleosol below (fig. 11).
Figure 11 – Pillow lavas overlying VCR (East Driefontein Mine, Carletonville, S. Africa).
• Lava cooling on land often develop a very
characteristic hexagonal jointing, columnar. This
occurs both with basalt (fig. 12).
Figure 12B – Volcanic plug basalt showing columnar jointing (view approximately 6 m high) (Mafra region, Portugal)
as well as rhyolite (figs. 13).
Figure 13 – Close up of columnar rhyolite (view approximately 2 m high) (Castro Verde, Portugal).
2.2.2. Hypabyssal Rocks
A significant proportion of the magma flowing through
the tension cracks will actually consolidate along them.
The resulting rocks are termed hypabyssal, that is,
intermediate between plutonic and volcanic. The
majority of the ducts through which magma flows are
narrow and very long (fig. 14). As such, the magmas
filling these fissures will cool quite fast and the
resulting rocks will predominantly be fine to medium
grained. If these intrusives are parallel to the
surrounding strata they are termed sills and when
cutting across, they are called dykes.
Figure 14 – Aerial photo of a dyke outcrop on a peneplane (Central Angolan Plateau).
Also, these fractures are a consequence of the
breaking away of continental plates, and fracturing of
non homogenous brittle materials usually have
associated splitting, termed conjugate faulting. Thus
dykes tend to occur in conjugate sets (fig. 15).
Figure 15 – Set of conjugate dykes (Estoril beach, Potugal).
Hypabysal rocks occasionally also have pipe like forms
which may have considerably large diameters, hence
taking longer to cool and becoming therefore more
coarse grained. They are predominantly associated
with rifting and if I’m not mistaken, their magma
source is very deep, as with carbonatites (fig. 16),
Figure 16 – Aerial view of a large carbonatite plug outcrop on a peneplane (Central Angolan Plateau).
kimberlites (fig. 17), and some others.
Figure 17 — Kimberly diamond mine (South Africa).
Volcanic breccias are moderately frequent (fig. 4B),
but I think the Boula Igneous Complex in India is a
rather unique example (fig. 18)
Figure 18 – Ultramafic Igneous breccia (Boula, Orissa, India).
In fact I put it here rather than with the volcanic rocks,
because, according to Augé and Thierry, this breccia
was caused by a violent explosion within the magma
ducts with the clasts belonging to the intruded, rather
than the intruding rock and it must have happened at a
considerable depth since the intruding basalt is very
coarse grained, often pegmatitic. However the
brecciated wall-rock shows very little movement. For
example, the position of the very large chromite clast
shown in figure 19, is very close to its initial position
relative to sector of the chromite lens unaffected by the
explosive burst.
Figure 19 – Igneous breccia containing chromite clasts (view approximately 16 m high) (Boula, Orissa, India).
Other than the in situ shattering, what we had was the
rotation of the clasts within a very hot chamber which
partially melted the wall-rock (fig. 20).
Figure 20 – Metasomatised igneous breccia clast showing roundness and concentric reaction rim due to partial melting
(Boula, Orissa, India).
2.2.3 Plutonic Rocks
Plutonic rocks are formed by magmatic intrusions at
great depths. Since we are dealing with a fluid
intrusion, the contacts with the surrounding rocks tend
to be irregular (fig. 21).
Figure Figure 21 – Granite/limestone intrusive contact (Sintra Mountain, Portugal).
Also, with the exception of the marginal areas of
contact and the fact that they generally have very large
volumes, this magma has a very long time to cool,
allowing the development of coarse grained rocks.
When the magma is rich in volatiles it often has
associated hydrothermal pegmatitic (ultra coarse
grained) veins, giving rise to magnificently well
developed crystals (fig. 22).
Figure 22 – Pegmatitic minerals: book of muscovite (back) (Perth, Canada); black tourmaline, red and green tourmaline
and blue beryl (front) (Ligonha, Mozambique); Wolframite (Panasqueira, Portugal)
2.3 Magmatic Differentiation
Magmatic differentiation was already mentioned (item
2.1) but here I’m just referring two rather unique
examples, the Boula Igneous Complex in India and the
Bushveld Igneous Complex (B.I.C.) in South Africa.
Both these igneous lopoliths have a basic to ultrabasic
composition, meaning that the intruding magma has
already had a significant amount of chemical
differentiation from the initial mantle magma.
2.3.1 Differential Crystal Settling
While cooling within the intruded chamber, further
differentiation took place due to the rate of settling of
the various minerals as they crystallised at the top, the
coolest area, and slowly dropped to the bottom. The
reason why these two cases are so spectacular is
because both assemblages consist of a light coloured
member, peridotite in India and anorthosite in South
Africa, inter-layered with a black member, chromite.
Also, the SG of the latter is far higher than either of the
other two, thus allowing for a much more clear
separation of the respective minerals (figs. 23 and 24).
Figure 23 – Magmatic differentiation by crystal settling (view approximately 30×20 cm) (Boula, Orissa, India).
Figure 24 – Magmatic differentiation by crystal settling (Dwars River, South Africa).
The similarity between a normal sedimentation process
and the crystal settling in these two cases is
remarkable. So much so, that initially a school of
geology in South Africa believed the B. I. C. to be an
assemblage of metamorphosed sediments. Take also
the example shown in figure 25. I have never seen such
perfect graded bedding in real sediments. In the
present case we have granular magnetite forming the
base of the sequence with feldspar crystals
progressively increasing in quantity upwards, just like
in sediments where the heavier clasts are the ones that
reach the bottom first.
Figure 25 – Graded bedding by crystal settling (view approximately 1 m high) (Dwars River, South Africa).
Another example, still with close similarities with
sedimentation, but now with igneous crystal settling
characteristics more apparent, is the occurrence
observed at the sector of this rock sequence where the
locally termed pyroxenite boulder horizon occurs. This
member of the succession is approximately 50cm
above a very well defined and continuous pyroxenite
band and consists of a layer of spotted anorthosite,
containing scattered coarse grained pyroxenite nodules
with an average diameter of 15 cm (fig. 25B).
Figure 25B – Normal pyroxenite “boulder” horizon, about 50 cm above the distinct pyroxenite band (Bafokeng Mine,
Rustemberg, South Africa).
However, as shown in figure 26, one of the “boulders”,
considerably larger than normal, appears to have fallen
through the semi fluid mush of the already settled
pyroxenite band. Note that the “boulder” was not
entirely solid, since it looks as if it is rather frayed at
the edges. Both these photos were taken along one of
the mine adits, within 2 m of each other, and I think
this example is rather useful in helping to understand
the notion of a crystal settling environment.
Figure 26 – Pyroxenite “boulder” falling through pyroxenite beds (Bafokeng Mine, Rustemberg, South Africa).
2.3.2 “Pot Holes” Within the Marensky Reef
The Marensky Reef (MR) is a platinum bearing,
generaly conformable horizon of the B. I. C.. It is
accepted that this band is the first layer after a new
magma influx was injected into the settling chamber,
bringing the platinum and also raising the
environmental temperature. That is the reason why the
MR has a pegmatitic texture with a much coarser grain
size than that of the lower layers. This temperature
rise also caused the development of convection
currents within the settling chamber causing what are
locally called “potholes” and for which a tentative
explanation follows:
Figure 27 was taken underground at the face of a MR
stope. The right hand portion of the picture is a
pegmatitic pyroxenite, with practically a vertical
contact, representing the edge of a MR “pothole”. On
the left side of the ruler, we have a mottled
anorthosite, filling in the centre of the “pothole”, with
vague suggestions of normal horizontal layering, due to
a latter period of crystal settling.
Figure 27 – Marensky reef “pothole” edge (Bafokeng Mine, Rustemberg, South Africa).
Figure 28 is an interpretative cross section along a
diamond drill hole which intersected a different
“pothole”, but I think helps to understand the situation.
M3 and M2 are anorthosites that cover a normal MR,
shown in pink at the upper section of the diagramme.
Below that, the bore hole intersected another mottled
anorthosite interpreted as the inner fill of the pothole.
Next comes the MR horizon again, this time consisting
of a very thin chromite seam. Following is a norite
footwall below which we have the final segment of MR
at the base of the “pothole”, and consisting of a rather
thick chromite horizon very rich in platinum. Thus we
have a situation indeed similar to an ordinary river pot
hole with irregularities close to the bottom, where the
heavier materials concentrate.
Very important as well is that, as logically expected,
the footwall below the base of the “pothole” is not the
same as the horizon under an ordinary MR, but rather
a unit which is stratigraphically considerably lower.
Some of these “potholes” actually cut down more than
5m through the presumably semi solid mush within the
magma chamber.
Figure 28 – Diagrammatic interpretation of a “pothole” edge intersected by a surface diamond drill prospecting hole
(Maricana, South Africa).
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