Recording features of Igneous Rocks
Why to study igneous rocks?
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The main reasons to study igneous rocks are to:
(1) Understand volcanoes and the hazards they pose to life on Earth;
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(2) Understand processes beneath the Earth ’ s surface that drive plate
tectonics and that contributed to
the evolution of Earth and other
planetary bodies;
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(3) Understand the distribution of ore minerals
(4) for chronology purpose
An igneous intrusion cut by
a pegmatite dike, which in turn is cut by a dolerite dike
https://en.wikipedia.org/wiki/Igneous_rock
#/media/File:Multiple_Igneous_Intrusion_P hases_Kosterhavet_Sweden.jpg
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Igneous rock are studied when exposed by
• natural erosion or
•quarrying.
https://www.britannica.com/technology/q uarry-mining
Relationships with surrounding rocks
The most fundamental attribute of a body of igneous rock is:
• whether it is intrusive.
• or extrusive.
https://rockandmineralplanet.com/what-are-igneous-rocks-intrusive-and-extrusive/ 6
If intrusive
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• First see the contact between igneous and the adjacent rocks.
• it can be discordant.
• it can be concordant.
Dyke (US spelling is dike) --- Discordant curtain of igneous rock, originally intruded into a steep or vertical fracture.
Typically tens of centimetres to several metres in width
Names for igneous intrusions, based on shape and relationship with country rocks.
BEWARE! A concordant contact must be parallel to the stratification of the country rock in three
dimensions, so be sure that you are not misled by
looking only at one surface in a specific orientation.
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A dyke of Cenozoic age cutting discordantly across complexly folded wackes (Augrim quarry, Co. Down, Northern Ireland) . The dyke is the 2 - m – wide reddish feature, passing nearly vertically up the rock face behind the person in the foreground.
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Dyke swarm ---A number of spatially associated dykes, typically radial, parallel or en échelon.
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Lopolith--- Large, generally concordant intrusion that is broadly saucer - shaped
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Laccolith---- Intrusion that is roughly circular in plan, generally concordant with the country rock and having a planar floor but a domed roof.
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Batholith ---Grouping of more - or - less contiguous plutons
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Diapir ----Dome - shaped body of igneous rock, inferred to have deformed and ruptured the
country rock during ascent
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Diatreme Breccia - filled volcanic pipe
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It can be particularly tricky to distinguish a sill from a lava flow, but you will be in less doubt if you can see (or deduce from mapping) a place where either the top or the bottom of the sill steps up or down the stratigraphy (Figure )
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(a) Sketch cross - section showing the typical
relationship between a sill (in this example fed by a dyke on the left) and horizontal strata intruded by it.
The sill is generally
concordant, but is locally
discordant where it steps
up or down between
strata.
The lava flow is
concordant. (b) As (a) but
in this case the strata into
which the sill and dyke
were intruded have been
tilted. At a later date lava
flowed over the area. In
this case the lava flow is
discordant.
Be careful:
A concordant contact must be
parallel to the stratification of the
country rock in three dimensions, so
be sure that you are not misled by
looking only at one surface in a
specifi c orientation.
Measure dip and strike of the intruded body
This helps in knowing the you are dealing with parallel dyke swarm or radial dykes coming from a common centre.
Figures:-Schematic plan view of the possible outcrop pattern of radial dykes around a volcanic centre: (a) in the absence of
regional stress and (b) with east – west extension.
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Sills are tabular bodies, typically intruded between beds of horizontal (or gently dipping) strata.
Although they may extend horizontally for tens of hundreds of kilometres, most sills are only a few tens of metres thick, so the evidence needed to confirm the concordant nature of both contacts may not be far away.
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A weathered (typically reddened, maybe even developing into soil) top to a
concordant igneous sheet is strong
evidence that it is a lava flow.
There are other ways to distinguish between lava flows and sills depend on medium - and large - scale textural features, which we here refer to as ‘ internal architecture ’.
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Internal architecture: Joints and veins
The pattern of jointing in a flow can be used to deduce its
• cooling and
• crystallization history (it may also tell you about the local palaeo
topography), and should be
documented by sketches or photos of
both vertical and horizontal faces.
Columnar Joint
(a) General view (b) Close - up
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• e.g., columnar joints----which are generally attributed to thermal contraction during cooling.
• columnar joints tend to be much better
developed in sills and in thick ( > 5 m) lava
flows that formed extensive, evenly
cooling, sheets.
• Joints in igneous rocks are not necessarily open fractures;
• they may have been completely filled by crystallization of an aqueous or late - stage magmatic fluid to form veins.
• Veins in igneous rocks may be much younger than their host.
http://earthsci.org/mineral/mindep/vein/v 27 ein.html
• Other features to seek out and record in lava flows include:
General morphology and texture.
• Are there any pillows (indicative of eruption into water or under ice)?
• Is there a rubbly top or base, suggesting emplacement on land.
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29 https://www.pitt.edu/~cejones/GeoImages
/2IgneousRocks/IgneousTextures/7Vesicula rAmygdaloidal.html
Vesicles:- Concentrations or layers of
vesicles within a flow (may record pauses in
inflation of the flow)
• Elongated vesicles form when lava flows and stretches the gas bubbles (sawn face). Photo credit: Randy Korotev.
30 https://sites.wustl.edu/meteoritesite/items/vesicles-in-meteorites/
•Non-spherical vesicles may indicate either
shearing during flow or later flattening.
Mineral infill of vesicles (turning them into amygdales) is a consequence of post -
emplacement fluid percolation.
Vesicular and amygdaloidal rocks
https://sites.wustl.edu/meteoritesite/items/vesicles-in-meteorites/ 31
• Vesicles in general, especially if concentrated towards the top, are a good indicator of a lava flow rather than a sill.
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Igneous rock emplaced at shallow depth in cold country rock
there will be decreasing grain size close to the contact forming a
“Chilled margin”.
Usually the effects of chilling extend inwards no more than a centimeter or two.
The very edge may be glassy, lacking groundmass crystals of any size but groundmass crystal are visible away from the contact.
Chilling margin
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Mineralogy and small - scale textures of igneous rocks
• Diagnostic mineralogical and textural evidence that can be seen in the field
• with the unaided eye for coarse - gained rocks,
• easily with a hand lens for medium -grained rocks,
• but with difficulty even with a hand lens for fine - grained rocks.
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Petrologic type
• In medium- or fine-grained rocks, pyroxene and amphibole can be particularly difficult to tell apart.
• Don’t worry – it doesn’t matter much.
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More than 40% of any mafic mineral means your rock is mafic; 20 – 40% makes it intermediate.
https://sciencestruck.com/mafic-vs-felsic- 36 rocks
Quartz crystals are usually clear unlike milky in quartz veins.
37 https://www.geocaching.com/geocache/G
C6KFB9_rhosydd-geology-trail-3-quartz- veins?guid=9b85d0ab-af8b-4a26-ba61-
5d20c8bae6bc
• Feldspar is often altered.
• Most igneous rocks contain some plagioclase
feldspar. If you can identify this, estimating
its percentage will help to determine the rock
type.
Mineral texture and fabric If the rock has an igneous texture--
Except glassy lava and matrix in pyroclastic rocks, the igneous rock have interlocking crystals.
Cumulate layering in a gabbro
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Look carefully for signs of mineral alignment or arrangement in planes
Flow banding ’ or ‘ flow foliation
This is a result of shear in pyroclastic deposits or viscous lavas such as andesite, dacite or rhyolite.
Cumulate layering
Result from crystals settling to the floor of a magma chamber.
Can also result from crystal growth against magma chamber walls.
Pegmatites
Pegmatites usually occur in veins or lenses.
Crystals is nothing to do with cooling rate, but was instead controlled by the availability of nucleation sites and/or the abundance of volatiles in the last residue of the melt.
Xenoliths? These would be fragments of country rock that were engulfed in the igneous magma
‘ Enclaves ’
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Autoliths,
Magma of different composition that has somehow avoided total assimilation into the main magma
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