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45 Cards in this Set
- Front
- Back
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Igneous textures
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A. Igneous textures commonly arise from a balance between nucleation rate and crystal growth rate.
B. But also from changes in mineral stability with pressure and volatile content. |
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Homogeneous nucleation
Heterogeneous nucleation |
-is such that a crystal forms without aid of foreign material, when a cluster of atoms fortuitously reaches a critical size
-is such that the formation of nuclei of critical size is catalyzed by a solid surface in contact with the liquid (impurities |
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Crystal growth
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-increase in dimensions of a crystal. It may be very directional, depending upon the free energy of individual planes of the crystal.
-requires that chemical components be transported to the crystal surface, which is usually achieved by diffusion, and oriented into the crystal lattice. As crystallization proceeds, diffusion probably becomes limiting, determining crystal growth rate. |
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slow cooling
rapid very rapid |
-only minor undercooling (Ta), so that rapid growth and slow nucleation produce coarse-grained crystals.
- more undercooling (Tb), slower growth and rapid nucleation produce many fine-grained crystals -little if any nucleation or growth (Tc) producing a glass |
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phenocrysts
ground mass |
-larger grains in porphyritic texture
-material surrounding phenocrysts |
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Tectonic-Igneous Associations
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-An attempt to address global patterns of igneous activity by grouping provinces based upon similarities in occurrence and genesis
1.) Mid-Ocean Ridge Volcanism 2.) Subduction-related volcanism and plutonism -Island Arcs -Continental Arcs 3.) Intra-plate -Ocean Intra-plate (Island) volcanism -Continental Plateau Basalts |
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crystalization sequence
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olivine, plag, pry
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MORB Petrogenesis
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Separation of plates
Upward motion of mantle material into extended zone Decompression partial melting of lherzolite Olivine tholeiite melt separates from lherzolite at about 25-35 km Melt rises to shallow magma chamber (4-8 km depth) and fractionates to produce quartz tholeiite and cumulate layered gabbro. |
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seamounts
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contains several linear belts of extinct submarine volcanoes
-pacific plate |
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hotspots
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intraplate regions of voluminous volcanism
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Tholeiitic (dominant type)
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Ocean island/continental tholeiitic basalt * and fractionation products.
*Similar to MORB, |
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Alkalic series (subordinate)
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Two principal alkalic sub-series
Silica undersaturated NaAlSiO4 (nepheline) or KAlSi2O6 (leucite-rare) in the norm Slightly silica-oversaturated (hypersthene in the norm) |
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stage 1 of continent breakup
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Individual shield volcanoes
Found in failed rift in East Africa Highly volatile-rich alkalic magmas often potassic and sodic rocks Silica-unsaturated basalts with nepheline and leucite in the norm. carbonatites -produced by CO2 and H2O-rich peridotite |
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Kimberlites:
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a complex group of hybrid volatile-rich (dominantly CO2), potassic, ultramafic rocks with a fine-grained matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite, chromite. Xenocrysts and xenoliths are also common.
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Carbonatites
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Carbonatite (most commonly calcitic) intrudes the silicate plutons, and is itself cut by later dikes or cone sheets of carbonatite and ferrocarbonatite.
The last events in many complexes are late pods of Fe and REE-rich carbonatites. A fenite aureole surrounds the carbonatite phases and perhaps also the alkaline silicate magmas. |
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Stage 2 of continental breakup
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Individual shield volcanoes
Found as ocean island and shield volcanoes on continents, also at lowest levels below flood basalts on continents. Alkalic magmas and tholeiites (oceanic crust only) and their differentiates. Hydrous magmas (2a) and later drier ones (2b). |
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Sodic silica-saturated alkalic suites
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Can be produced by fractionation at the base of 30 km crust.
Can be produced from ordinary continental tholeiite. |
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Ocean island tholeiitic suite
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Ol
Ol+plag Ol+plag+augite |
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Sodic silica under-saturated alkalic suites
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augite
augite+Ti- amph Ti-amphibole + ternary feldspar |
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2b
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-Potassic silica-saturated alkalic suites
-The series is characterized by the clearly tholeiitic nature of its least evolved mafic rocks and the mildly alkalic nature of its intermediate and silicic rocks. volcanics of hotspot-related large igneous provinces, (i.e., Yellowstone-Snake River Plain volcanic province) and the fine-grained rocks associated with massif anorthosite complexes |
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2a
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hydrous, sodic silica saturated
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Benioff zone
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The inclined zone where the earthquakes focus
It represents upper boundary of cool and brittle subducting slab. After 700 km, it loses its brittle behavior and softens. |
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Forearc
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the area between volcanic front and trench and it is composed of lava, pyroclastic materials from volcanic arc, sediments from subducting plate and eroded sediments from growing arc and also called accretionary prism/wedge
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Volcanism at Convergent Plate Boundaries
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Convergent plate boundaries rank next to mid ocean ridges in their rate of production of igneous rocks.
Unlike MORB, instead of following the fate of subduction and disappearance, igneous rocks created by these margins survive by being accreted to the continents – continent building ! They are distinctly different from mainly basaltic provinces. |
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Continental Arcs
Island Arcs |
-oceanic lithosphere subducted beneath mature continental lithosphere
-oceanic lithosphere subducted beneath oceanic lithosphere or oceanic lithosphere subducted beneath thinner, commonly immature continental fragments/ peninsulas |
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Composition is more diverse and silicic than MORB
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of 2 different magma types where K2O is key to discriminate:
Tholeiitic series (aka low K series): mostly island arcs Calc-alkaline series (aka medium- to high-K series): mostly continental arcs |
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Volcanic Rocks of Island Arcs
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Complex tectonic situation and broad spectrum
High proportion of basaltic andesite and andesite |
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More explosive and forms stratovolcanoes
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Andesitic, dacitic, and rhyolitic rocks of the spectrum have water-bearing minerals such as HORNBLENDE and BIOTITE implying high water content of magma.
WATER is the key component controlling explosivity. |
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Magma series
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Tholeiitic
Calc-Alkaline (~ restricted to continental arcs of SZ) why? Minor alkalic |
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subduction
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Oceanic crust is partially melted as it is subducted and heated at a certain depth
Melts rise through the overriding plate to form volcanoes just behind the leading plate edge Partial melts (mafic basalts) are less mafic than their parent material (ultramafic mantle). These mafic basalts differentiate and processed into andesites. [Mantle peridotites basaltic oceanic crust arc andesites] Unlimited supply of oceanic crust to melt |
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anorthosite
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- plagioclase feldspar a. Pyroxene, ilmenite, magnetite, and olivine
-continental hotspots 2a -not at mid ocean ridge b/c pressure not high enough |
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basalt
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-pyroxene, plag, olivine
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monzodiorite
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-plag, pyrox, iron oxide
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apatites
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small hexagonal grains in plehochroic minerals that have low birefringence
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rhyolite
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-mainly quartz
- alkali feldspar |
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gabbro
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pyrox and plag, ol biotite
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trachytic
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magnetite pyrox, hornblende
2 hotspot |
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carbonatite
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-calcite
stage 1 |
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syenite
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alkali feldspar, quartz, biotie
stage 1 |
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pillow basalt
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quartz tholeites
-plag pyrox |
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ophiolites
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- no hornblende as primary mineral
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subduction zones
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hornblende+ biotite
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tholeitic series
calc-alkaline series |
-low K island arcs
-med high K continental arcs (subduction zone) |
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ultramafics
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ol and pyrox. nooo plag
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nepheline
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Na0.75K0.25Al(SiO4)
1 stage |
