Igneous textures and structures
-
Upload
badal-mathur -
Category
Education
-
view
3.186 -
download
8
Transcript of Igneous textures and structures
Textures ofIgneous Rocks
Made By Anchit GuptaBadal Dutt MathurApurv Singh Deepak Rawat Dhruv Gaur
IGNEOUS ROCK TEXTURES - PRINCIPLE
The fundamental principle behind igneous rock textures is that grain size is controlled by cooling rate. Thus, rapid cooling at the Earth’s surface of extrusive molten material, or lava, results in the growth of smaller crystals, or prevents crystal growth altogether. Conversely, slow cooling within the Earth’s crust of intrusive molten material, called magma, results in the growth of fewer but larger crystals, because atoms are able to migrate through the liquid to attach themselves to crystals that have already begun to form. The many igneous rock textures are simply variations on or modifications of this principle.
Igneous Textures
Texture: Individual grains relate to grains immediately surrounding them. I)Textures are useful indicators of cooling and crystallization rates and of phase relations
between minerals and magma at the time of crystallization. ii)Texture deals with small-scale features seen in hand specimen or under the microscope,
such as • the degree of crystallinity.
• grain size. • grain shape,
• grain orientation, • grain boundary relations• crystal intergrowths.
1. Degree of crystallinity.
1. Holocrystalline: Consisting entirely of crystal.
2. HolohyalineConsisting both crystal and glass.
3. Hypocrystalline
Degree of crystallinity
Hornblendite
Consisting entirely of glass.
Phaneritic texture
Coarse crystals cooled slowly at great depth
Phaneritic – With Evident Crystals
Igneous intrusive rocks have evident crystals [the Greek word phaneros means visible or evident] that can be discerned without the aid of microscope.
Phaneritic – With smaller crystals
• Rock : Gabbro
• Crystals are small in size but easily distinguishable from each other
Phaneritic – Economic importance
Used as grave markers and facing stone for buildings owing to the coarse size of crystals.
Granite
A Spectacular Pegmatite Vein of Feldspar and Quartz
Phenocryst
• PegmatiteExtremely coarse-grained igneous intrusive rocks, usually of a felsic composition.
Crystal size > 5 cm. Usually formed by concentration of volatiles in magma lowering its viscosity in the late stages of cooling.Attractive and economically significant.
Porphyritic texture
Granite
Porphyritic
Phenocrysts – coarser grains Porphyry – contains numerous coarse grains
(phenocrysts) in an otherwise fine grained mass
Porphyritic
Large, evident crystals called phenocrysts are surrounded by an aphanitic matrix or groundmass.
Granite
Granodiorite
Granite
Porphyritic2 stage cooling process:I)Slow cooling of magma underground for growth of phenocrystsii)Eruption of magma as lava which solidifies quickly allowing growth of only small crystals
Cathedral Peak Granodiorite in which K-feldspar crystals are the phenocrysts
Granite Porphyry
2. Grain size
PHANERIC TEXTURE
Is characterized by LARGE SIZE MINERALS which can be easily seen by Naked eye (size at least 2mm or greater)
Coarse-grainedPhaneric- > 5mm
Medium-grainedPhaneric- 1 mm - 5mm
Fine-grainedPhaneric<1 mm
A. Equigranular: Rocks with equigranular texture have mineral grains that are generally the same size. Diameters of component minerals are comparable.
Equigranular granite
B. Inequigranular: Not of uniform size
Porphyritic texture: One or more mineral species or a generation of one or more mineral species that are conspicuously greater in size than those minerals constituting the rest of the rock. There are number of larger grains called
phenocrysts, surrounded by a population of grains of significantly
smaller size, the groundmass.
3. Grain shapeA.Anhedral-allotriomorphic
B. Subhedral-hypidiomorphic
C. Euhedral-idiomorphic
Allotriomorphic: All the component mineral grains are anhedral.
Hypidiomorphic: Some mineral species are anhedral, those of others subhedral, and those of some may even be euhedral. *Granitic rocks: Quartz and orthoclase- anhedral.*Plagioclase and biotite-subhedral to euhedral.
3. Idiomorphic TextureAll mineral grains euhedral
Figure 3.7. Euhedral early pyroxene with late interstitial plagioclase (horizontal twins). Stillwater complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.
4. Grain orientation
Trachytic texture - a texture wherein plagioclase grains show a preferred orientation due to flowage, and the interstices between plagioclase grains are occupied by glass or cryptocrystalline material.
Trachytic texture in which microphenocrysts of plagioclase are aligned due to flow. Note flow around phenocryst (P).
Photomicrograph showing strain bands in trachytic texture in Unit 3b (Sample 197-1205A-10R-2, 73-75 cm) (cross-polarized light; field of view = 5 mm; photomicrograph 1205A-202).
Photomicrographs illustrating mineral grains present within the sands and sandstones of Woodlark rift. 5. Hornblende and feldspar phyric colorless vitric volcanic lithic fragment displaying an internal trachytic texture (Sample 180-1115C-12R-4, 144-148 cm [394.34 mbsf]) (plane-polarized light).
Figure 3.12a. Trachytic texture in which microphenocrysts of plagioclase are aligned due to flow. Note flow around phenocryst (P). Trachyte, Germany. Width 1 mm. From MacKenzie et al. (1982). © John Winter and Prentice Hall.
Trachytoidal texture:The texture of a phaneritic extrusive igneous rock in which the microlites of a mineral, not necessarily feldspar, in the groundmass have a subparallel or randomly divergent alignment.
crystal intergrowths.
Sieve textured crystals Are those which contain abundant, small,
interconnected, box shaped glass inclusions, giving the crystals a spongy or
porous appearance.
Figure 3.11a. Sieve texture in a cumulophyric cluster of plagioclase phenocrysts. Note the later non-sieve rim on the cluster. Andesite, Mt. McLoughlin, OR. Width 1 mm. © John Winter and Prentice Hall.
Glomeroporphyritic texture Phenocrysts of the same or different minerals occur in cluster and grow together form a glomeroporphyritic texture. Large crystals that are surrounded by finer-grained matrix are referred to as phenocrysts
Poikilitic texture - Refers to small, typically euhedral crystals (chadacrysts), that are enclosed (included) within a much larger mineral of different composition. Unlike the porphyritic texture, the large crystals known as oikocrysts, are devoid of crystal faces. Chadacryst also refers to a grain that is foreign to the
rest of the rock a.k.a. xenocryst.
Poikilitic texture. Orthopyroxene oikocryst that encloses rounded chadacrysts of olivine
Ophitic Textures
An igneous texture in which plagioclase grains are completely surrounded by pyroxene grains.
Refers to a dense network of lath-shaped plagioclase microphenocryst included in larger pyroxene with little or no associated glass.
A single pyroxene envelops several well-developed plagioclase laths.
This refers to a common igneous texture found in gabbroic rocks, consisting of plagioclase laths which are partly surrounded by pyroxene grains, and that are partly in contact with other plagioclase grains.
Sub-Ophitic
A . Photomicrograph of subophitic texture with plagioclase partially enclosed by clinopyroxene
B. Photomicrograph of subophitic texture with plagioclase enclosed by olivine
Intergranular texture In this texture angular interstices between plagioclase grains are occupied by grains
of ferromagnesium minerals such as olivine, pyroxene, or iron titanium oxides.
Tiny, equant clinopyroxene grains interstitial to plagioclase laths.
Compositionally Zoned Plagioclase is abundant, almost
completely homogeneous in composition, and is virtually pure anorthite. No evidence of zoning . Large olivine grain (bottom center) shows compositional zoning from Mg-rich core to Fe and Ca-rich rims. Angrite in XPL.
Angrite in PPL.
Compositionally zoned
a.hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm.
b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm.
Graphic Texture
Exsolved or devitrified minerals form angular wedge like shapes which look like reminiscent of writing.
Graphic texture. The feldspar is white and roughly 10 x 10 centimeters. Quartz are the little gray ones
Graphic texture: a single crystal of cuneiform quartz (darker) intergrown with alkali feldspar (lighter).
Granophyric Texture
Intergrowth of quartz and alkali feldspar
the granophyric texture radiates out from large plagioclase grains (lower left-gray, lower right-gray/black). View is under crossed polarizers.
Granophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm Golden Horn granite, WA.
5. Grain boundary relations
Seriate texture Refers to a situation where there is a continuous range in grain size of one or more mineral species from that of phenocryst to groundmass size, and in which crystals of progressively smaller sizes are increasingly numerous. This texture is commonly shown by plagioclase in some andesite porphyrites.
Plagioclase and clinopyroxene phenocrysts in a groundmass of plagioclase, clinopyroxene, and Fe-Ti oxide minerals
Medium-grained diabase with interlocking grains of plagioclase, clinopyroxene, and Fe-Ti oxide minerals
Myrmekitic texture
An intergrowth of plagioclase feldspar (commonly oligoclase) and vermicular quartz, generally replacing potassium feldspar; formed during the later stages of consolidation in an igneous rock or during a subsequent period of plutonic activity. The quartz occurs as blobs. A related term is vermicular quartz..
Myrmekitic texture defined by wormy intergrowths of quartz and K-feldspar in plagioclase which is adjacent to K-feldspar.
Perthite is very common in igneous rocks and consists of quantitatively minor lamellae, shreds, patches and rims of an albite component within and around host orthoclase or microcline. Whatever the orientation in thin section, the albite component always has the higher birefringence and appears brighter under crossed nicols, a useful feature in identification, as the exsolution lamellae are generally far too small to show any diagnostic multiple twinning.
Perthitic texture
IGNEOUS STRUCTURES
55
IGNEOUS STRUCTURES
The structures of igneous rocks are large scale features, which are dependent on several factors like:
(a) Composition of magma. (b) Viscosity of magma. (c) Temperature and pressure at which
cooling and consolidation takes place. (d) Presence of gases and other
volatiles.
56
Structures developed in igneous rocks are of two types-
INTRUSIVE- which form by the crystallization of magma at a depth within the Earth.
Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual crystals interlocked together to form the rock mass.
The cooling of magma deep in the Earth is typically much slower than the cooling process at the surface, so larger crystals can grow.
57
EXTRUSIVE-which form by the crystallization of magma at the surface of the Earth.
They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow.
Rocks with this fine-grained texture are called aphaniticrocks. The most common extrusive rock is basalt.
58
INTRUSIVE AND EXTRUSIVE IGNEOUS ROCK STRUCTURES
59
Basalt dikes hosted in a granitoid pluton, with metasediment
roof pendant; Wallowa Mts,
Oregon
Igneous Structures
Intrusive (Plutonic) Magma cools slowly at
depth Characteristic rock texture Characteristic structures
60
• Extrusive (Volcanic)– Magma cools quickly at
surface– Characteristic rock textures– Characteristic structures
Igneous Structures Intrusive
Batholith Stock Lopolith Laccolith Volcanic
neck Sill Dike
Extrusive Lava flow or
plateau Volcano
(many types)
Crater Caldera Fissure
61
Intrusive Igneous Structures Contacts (boundary
between two rock bodies) can be:Concordant
○ Does not cross cut country rock (surrounding rock) structure, bedding, or metamorphic fabric
○ Ex: laccolith, sillDiscordant
○ Cross cuts country rock structure
○ Ex: dike, batholith, stock
62
Intrusive Igneous Structures Categorized by depth of emplacement
Epizonal Mesozonal Catazonal
Depth Shallow<6-10 km
Intermediate~8-14 km
Deep>~12 km
Contacts Discordant Variable Concordant
Size Small to moderate
Small to large Small to large
Contact metamorphism
Very common Uncommon Absent
Age Cenozoic Mesozoic-Paleozoic
Paleozoic or older
63
Intrusive Igneous Structures: Large Scale
Major scale intrusive bodies: PlutonsBatholith: >100 km2 in map area (usually discordant)Stock: <100 km2 in map area Lopolith: dish-shaped layered intrusive
rocks (concordant)
64
Intrusive Igneous Structures:Intermediate Scale
Concordant intrusives Sill: tabular shape Laccolith: mushroom-shaped Roof pendant (remaining country
rock) Discordant intrusives
Dike: tabular shape Volcanic neck: cylindrical
65
Intrusive Igneous Structures: Small Scale
Apophyses: Irregular dikes extending
from pluton Veins:
Tabular body filling a fracture (filled with 1-2 minerals)
Xenoliths: Unrelated material in an
igneous body Autoliths:
Genetically related inclusions (related igneous material)
66
Extrusive Igneous Structures Volcanism
Directly observable petrologic process Redistributes heat and matter (rocks) from the interior to the exterior
of the earth’s surface Occurs in oceanic & continental settings
Volcano: Anywhere material reaches earth’s surface
67
Extrusive Igneous Structures: Scale
Large scale structures Lava plateau (LIP; flood
basalt) Ignimbrite (ash flow tuff;
pyroclastic sheet) Intermediate scale
structures Shield volcano Composite volcano
(stratovolcano) Caldera, crater Lava flow or dome
Small scale structures Tephra (pyroclastic material) Lava flow features Cinder cone
68
Extrusive Igneous Structures: Eruption Styles
Effusive Eruptions Lava flows and domes Erupted from localized fissures or
vents Generally low silica content
(basalt, “primitive” magma) Explosive Eruptions
Tephra (fragmental material) Pyroclastic falls or flows Erupted from vents Generally high silica content
(felsic, “recycled” magma)
Photo glossary of volcano terms
69
Extrusive Igneous Structures: Eruption Controls Two main controls on eruption style:
VISCOSITY○ A fluid’s resistance to flow○ Determined largely by fluid composition
DISSOLVED GAS CONTENT○ Main magmatic gasses: H2O, CO2, SO2 (or H2S)○ At high pressure, gasses are dissolved in the magma○ At low pressure (near surface), gasses form a vapor, expand,
and rise = “boiling” Interaction controls eruption style:
Gas bubbles rise and escape from low viscosity magma = EFFUSIVE ERUPTION
Gas bubbles are trapped in high viscosity magma; increase of pressure = EXPLOSIVE ERUPTION
70
Extrusive Igneous Structures: Eruption Controls Two main controls on eruption style:
VISCOSITY and DISSOLVED GAS CONTENT
71
– In general, both viscosity and gas content are related to magma composition• High silica content –> higher viscosity, more dissolved gas• Low silica content –> lower viscosity, less dissolved gas
Types of Volcanic Products: Effusive Lava Flow
Dominantly basalt (low viscosity and gas)Thin and laterally extensive sheets
○ Pahoehoe flows: smooth, ropey flows○ Aa or block flows: rough and irregular flows○ Baked zones: oxidized zones due to contact with
high temperature lava flow
72
• Lava Dome– Dacite or rhyolite (high viscosity, low gas
content)– Thick,
steep-sidedflows
Types of Volcanic Products: Explosive Pyroclastic particles
Fragmental volcanic material (TEPHRA)○ Vitric (glass shards)○ Crystals○ Lithic (volcanic rock
fragments)Broken during
eruption of magmaTypically higher silica,
high gas contentCategorized by size:
○ Ash (< 2.0 mm)○ Lapilli (2-64 mm)○ Blocks and bombs (>64
mm)
73Ash
TephraBombs
Types of Volcanic Products: Explosive Pyroclastic fall (mainly Ash fall)
Material ejected directly from volcano (fallout, “air fall”)
Ash, lapilli (pumice, scoria), blocks, and bombs
Sorted (small particles carried further)Laterally extensive, mantles
topography Pyroclastic flow (nueé ardante or
ignimbrite) Fast moving, high density flow of hot
ash, crystals, blocks, and/or pumiceFollow topographic lows Can be hot enough after deposition to
weld, fuse vitric fragments74
Hydroclastic ProductsWater-magma interaction (phreatomagmatic) causes
explosive fragmentationTypically basaltic lavasAny water-magma interaction (sea floor, caldera lake,
groundwater)
75
Types of Volcanic Products: Explosive
– Great volumes of hydroclastics on the sea floor and in the edifice of submarine volcanoes
– Highly subject to alteration –> clay minerals, microcrystalline silica, and zeolite
Styles of Volcanic Eruption: Effusive Lava Plateaus and Floo
d Basalts (LIPs)Generally low viscosity,
low gas content effusive lava flows (basalt)
Hot spot and continental rift settings
Great areal extent and enormous individual flows
Erupted from fissuresExamples (no modern):
○ Columbia River Basalt Group
○ Deccan Traps
76
Styles of Volcanic Eruption: Effusive Shield volcanoes
Generally low viscosity, low gas content effusive lava flows (basalt)
Hot spot and continental rift settingsCentral vent and surrounding broad, gentle sloping
volcanic edifice
77Mauna Loa, Hawaii
– Repeated eruption of mainly thin, laterally extensive lava flows
– Modern examples:• Mauna Loa, Kiluaea
(Hawaii)• Krafla (Iceland)• Erta Ale (Ethiopia)
Styles of Volcanic Eruption: Effusive
Submarine eruptions and pillow lavaGenerally low viscosity, low gas
content effusive lava flows (basalt)Divergent margin (mid-ocean
ridge) settingsProduces rounded “pillows” of lava
with glassy outer rindCan produce
abundant hydroclastic material (shallow)
Modern examples:○ Loihi, Hawaii
78
Styles of Volcanic Eruption: Explosive
Cinder coneGenerally low viscosity, high gas content (basalt)Subduction zone settings (also continental rifts and
continental hot spots)
79
SP Crater, Arizona – Small, steep sided pile of loose tephra (mainly lapilli, blocks, and bombs)• Scoria or cinder
– Often form on larger volcanoes (shield or stratovolcano)
– Modern example:• Parícutin, Mexico
Styles of Volcanic Eruption: Explosive Composite cones and
StratovolcanoesGenerally higher viscosity,
high gas content (andesites)
Dominantly subduction zone settings
80
Mayon VolcanoPhilippines
– Composed of layers of loose pyroclastic material (fallout and flows) and minor lava flows, some shallow intrusions
– Form from multiple eruptions over hundreds to thousands of years
– Examples: • Mt. St. Helens, Mt. Rainier (USA)• Pinatubo (Indonesia)
Styles of Volcanic Eruption: Explosive Calderas and pyroclastic
sheet (ignimbrite) deposits Generally high viscosity,
high gas content (rhyolite)Subduction zone and
continental hot spots
81
Crater Lake,Oregon
– Form by collapse of volcano following evacuation of the magma chamber
– Often produce widespread ash, ignimbrite (pyroclastic flow)
– Examples:• Krakatoa, Indonesia (modern example)• Crater Lake, Yellowstone (USA)
References
http://publications.iodp.org/proceedings/304_305/102/102_2.htmhttp://www-odp.tamu.edu/publications/180_IR/chap_04/ch4_htm4.htm