Igneous Petrology

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Igneous Petrology  / Origins and Differentiation of Magma

source http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=65EA2ABEB61A2F4CDFF3FE48119B4014?doi=
            lecture notes of Prof. Stephen A. Nelson Tulane University
            IUGS Subcommission on the Systematics of Igneous Rocks

 Igneous Petrology
    Continental Igneous Rocks
    Coastal Igneous Rocks
    Oceanic Igneous Rocks
    Igneous Rock Classification

Origins and Differentiation of Magma
    Origins of Magma
    Magmatic Differentiation

molten basalt lava

Continental Igneous Rocks

A wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere.
Because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study.

Granitic Rocks

1. Classification of Granitic rocks based on visible pewrcentages  of "Q" quartz / "A" alkalai feldspar  / "P" plagioclase feldspar
granitic rocks classification based on rarios of "A"  alkalai feldspar,"P"  plagioclase feldspar and "Q"quartz
Note: true granites have between 10% and 65% of their feldspars as plagioclase, and between 20% and 60% quartz. 
All rocks will likely contain mafic minerals such as biotite, hornblende, and perhaps pyroxenes, along with opaque oxide minerals.

"S", "I" and "A" Type Granitic Rocks
Mechanisms by which large volumes of granitic magma could be produced are:

    S-type Granites.
S-type granites are thought to originate by melting (or perhaps by ultrametamorphism) of a pre-exiting metasedimentary or sedimentary source rock.
These are peraluminous granites [i.e. they have molecular
Al2O3 (Na2O + K2O)].
Mineralogically this chemical condition is expressed by the presence of a peraluminous mineral, commonly muscovite, although other minerals such as the Al2SiO5 minerals and corundum may also occur. Since many sedimentary rocks are enriched in Al2O3  as a result of their constituents having been exposed to chemical weathering near the Earth's surface (particularly rocks such as shales that contain clay minerals), melting of these rocks is a simple way of achieving the peraluminous condition.

Many S-type granitoids are found in the deeply eroded cores of fold-thrust mountain belts formed as a result of continent-continent collisions, such as the Himalayas and the Appalachians, and would thus be considered orogenic granites.

    I-type Granites.
I-type granites are granites considered to have formed by melting of an original igneous type source.
These are generally metaluminous granites, expressed mineralogically by the absence of peraluminous minerals, and the absence of peralkaline minerals, as discussed below.Instead these rocks contain biotite and hornblende as the major mafic minerals.
Mesozoic or younger examples of I-type granites are found along continental margins such as the Sierra Nevada batholith of California and Nevada, and the Idaho batholith of Montana.  In these regions the plutonism may have been related to active subduction beneath the western U.S. in Mesozoic times.I-type granites are also found in the Himalayas, related to continent-continent collisions.
Plutonic suites that were emplaced in convergent continental margin settings, show many of the same characteristics as the calc-alkaline volcanic suite that likely erupted on the surface above.  The suites include gabbros, diorites, quartz monzonites, granodiorites, and granites.  They show only mild to no Fe-enrichment, similar to calc-alkaline volcanic rocks, and a range of isotopic compositions similar to the associated volcanic rocks. Nearly all are I-type granitoids.
The rocks were emplaced during the Mesozoic Era.  Exposed rocks are generally older toward the east and southeast.  Kistler and Peterman showed that the Sr isotopic ratios vary across the batholith in a systematic way.  The younger rocks in the western portion of the batholith are mostly quartz diorites with Sr isotopic ratios less than 0.704, ratios expected from melting of the mantle or young crustal rocks. Plutons farther east are mostly quartz monzonites and granodiorites with ratios increasing along with age of the plutons toward the east and southeast.  One interpretation of the data is that the older rocks contain a higher proportion of older crustal material than the younger plutonic bodies.

    A-type Granites.
A-type granites are generally peralkaline in composition [molecular (Na2O + K2O) > Al2O3].
Minerals like the sodic amphiboles riebeckite and arfvedsonite and the sodic pyroxene aegerine are commonly found in these rocks.  In addition, they tend to be relatively Fe-rich and thus fayalitic olivine may sometimes occur.

These are considered anorogenic granites because they are generally found in areas that have not undergone mountain building events.Instead, they appear to be related to continental rifting events wherein continental lithosphere is thinned as a result of upwelling asthenosphere.The upwelling raises the geothermal gradient resulting in melting.Young peralkaline granites are found in the Basin and Range Province of the western U.S., and older examples are found throughout southeastern Australia

Depth of Emplacement.
Because the conditions under which a magma cools can play an important role in the texture and contact relationships observed in the final rock, plutons can be characterized by the depth at which they were emplaced.  This is because depth, to a large extent controls the contrast in temperature between the magma and its surroundings.
    Catazonal Plutons
The catazone is the deepest level of emplacement, usually considered to be at depths greater than about 11 km.  In such an environment there is a low contrast in temperature between the magma and the surrounding country rock.   The country rock itself is generally high grade metamorphic rock.  Contacts between the plutons and the country rock are concordant (meaning the contacts run parallel to structures such as foliation in the surrounding country rock) and often gradational. The plutons themselves often show a foliation that is concordant with that in the surrounding metamorphic rocks.  Migmatites (small pods of what appears to have been melted rock surrounded by and grading into metamorphic rocks) are common.  Some catazonal plutons appear to have formed by either melting in place or by ultrametamorphism that grades into actually melting.  Others appear to have intruded into ductile crustal rocks.  Most, but not all, Catazonal plutons are S-type granitoids.
    Mesozonal Plutons
The metazone occurs at intermediate crustal depths, likely between 8 and 12 km.  The plutonic rocks are more easily distinguished from the surrounding metamorphic rocks.  Contacts are both sharp and discordant (cutting across structures in the country rock), and gradational and concordant like in the catazone.  Angular blocks of the surrounding country rock commonly occur within the plutons near their contacts with the country rock.  The plutons generally lack foliation and are often chemically and mineralogically zoned.
    Epizonal Plutons
The epizone is the shallowest zone of emplacement, probably within a few kilometers of the surface. In such an environment there is a large contrast between the temperatures of the magma and the country rock. The country rock is commonly metamorphosed, but the metamorphism is contact metamorphism produced by the heat of the intrusion.  Contacts between the plutons and surrounding country rock are sharp and discordant, indicating intrusion into brittle and cooler crust. The margins of the plutons often contain abundant xenoliths of the country rock.
Pegmatites are very coarse grained felsic rocks that occur as dikes or pod-like segregations both within granitic plutons and intruded into the surrounding country rock.  They appear to form during the late stages of crystallization which leaves H2O-rich fluids that readily dissolve high concentrations of alkalies and silica.  Thus, most pegmatites are similar to granites and contain the minerals alkali feldspar and quartz.  But other chemical constituents that become concentrated in the residual liquid, like B, Be, and Li are sometimes enriched  pegmatites.  This leads to crystallization of minerals that are somewhat more rare, such as:
which are sometimes found.

Continental Rhyolites
Rhyolites are much more common and voluminous on the continents than in the ocean basins.  They range from small domes and lava flows to much larger centers that have erupted volumes measured in 100s of km3 and emplaced as pyroclastic material.  Most of the preserved volume is represented as pyroclastic flow deposits, often termed "ash flow tuffs" or "ignimbrites.  Large quantities of these deposits were erupted during the middle Tertiary in the western United States, northern Mexico, throughout Central America, and on the western slopes of the Andes mountains.  The composition of these deposits is usually metaluminous although peralkaline varieties are known.  None are peraluminous in composition.  Although the recent examples occur near continental margins. Most seem to be associated with episodes of continental extension, such as in Basin and Range Province of the Western U.S. and Mexico.

classification of volcanic rocks based on percent of quartz, alkalai feldspar ahd plagioclase feldspar

Continental Flood Basalts
Like the large submarine plateaus discussed in our large volumes of basaltic magma have erupted on the continents at various times in Earth history
Province Age Original Area Covered (km2) Types of Basalts %
Qtz -Thol. Oliv. Thol. Alk. Bas.
Lake Superior Precambrian 125,000 42 51 7
Siberia Permo-Triassic 2,500,000 28 69 3
Karoo, S. Africa Jurassic 2,000,000 57 37 6
ParanĂ¡, Brazil Cretaceous 2,000,000 72 28 0
Deccan, India Eocene 500,000 55 35 10
Columbia River Mid-Miocene 163,000 30 70 -

continental flood basalts

    Chemical Composition
Although each flood basalt province differs somewhat in the composition of magmas erupted, most provinces have erupted tholeiitic basalts. With the exception of a few early erupted picrites in some provinces, the tholeiitic basalts tend to have lower concentrations of MgO (5 - 8%) than would be expected from melts that have come directly from the mantle without having suffered crystal fractionation. Thus, despite their large volume, they are differentiated magmas that are similar in many respects to MORBs. Still, they show incompatible trace element concentrations more similar to EMORBs, and have 86Sr/87Sr and 143Nd/144Nd ratios that extend from the OIB field toward and overlapping with continental crust.  This latter feature indicates that they have likely suffered some crustal contamination.

Continental Rift Valleys
Continental Rift valleys are linear zones of extension within continental crust.Some of these extensional zones may be eventually become zones along which the continents break apart to form a new ocean basin, however, there are many examples where such break-ups have failed.
The East African Rift which extends from Syria in the north to Mozambique in the south has been active throughout the Cenozoic.  During the initial stages of rifting fissure eruptions produced large volumes of basalt and siliceous ignimbrites.  During the late Miocene and Pliocene these eruptions became more focused, and produced shield volcanoes consisting of basanites, rhyolites and phonolites.  In Plio-Pleistocene times rhyolites were erupted along the main axis of the rift, while basalts continued to be erupted on the plateaus adjacent to the rift.  Quaternary volcanoes along the axis of the central rift zones, in Kenya and Tanzania consist of phonolite, trachyte, or peralkaline rhyolite.  This province illustrates the wide variety of unusual rock types found in continental rifting settings.  Note, however, that parts of the rift along the Red Sea and Gulf of Aden have evolved to oceanic ridges and produce MORBs to form new seafloor.

Coastal Igneous Rocks

The convergent plate margins are the most intense areas of active magmatism above sea level at the present time. Most of world's violent volcanic activity occurs along these zones. In addition, much magmatism also has resulted in (and probably is resulting at present) significant additions to the crust in the form of plutonic igneous rocks.

Pacific "Ring of Fire"

The "Pacific Ring of Fire" surrounds the Pacific Ocean basin and extends into the Indian Ocean and Caribbean Sea. Active subduction is taking place, along these convergent plate boundaries, as evidenced by the zone of earthquakes, called a Benioff Zone, that begins near the oceanic trenches and extends to deeper levels in the direction of plate motion. Earthquake focal depths reach a maximum of about 700 km in some areas. Volcanism occurs on the upper plate about 100 to 200 km above the Benioff Zone. For this reason, volcanism in these areas is often referred to as subduction-related volcanism.

Two situations occur.
  1. In areas where oceanic lithosphere is subducted beneath oceanic lithosphere the volcanism is expressed on the surface as chains of islands referred to as island arcs.  These include the Caribbean Arc, the Aleutian Arc, the Kurile Kamachatka Arc, Japan, the Philippines, the South Sandwich Arc, The Indonesian Arc, the Marianas, Fiji, and Solomon Islands.
  2. In areas where oceanic lithosphere is subducted beneath continental lithosphere volcanism occurs as chains of volcanoes near the continental margin, referred to as a continental margin arc. These include the Andes Mountains, Central American Volcanic Belt, Mexican Volcanic Belt, the Cascades, the part of the Aleutian arc on Continental crust, and the North Island of New Zealand.

Within these volcanic arcs the most imposing, and therefore most recognized by early workers, features of the landscape are large stratovolcanoes. These  usually consist of predominantly  andesitic lava flows and interbedded pyroclastic material.  But, in the late stages of volcanism more silicic lavas and pyroclastics like dacites and rhyolites are common.

Many of these stratovolcanoes pass through a stage where their upper portions collapse downward to form a caldera.
These caldera forming events are usually associated with explosive eruptions that emit silicic pyroclastic material in large-volume eruptions.
It is the sudden evacuation of underlying magma chambers that appears to result in the collapse of the volcanoes to form the calderas.
The imposing presence of these large mostly andesitic stratovolcanoes led to an early widespread perception among petrologists that basalts were rare or absent in these environments.In recent years, however, it has become more evident that basalts are widespread, but do not commonly erupt from the stratovolcanoes. 
Instead, they are found in areas surrounding the stratovolcanoes where they erupt to form cinder cones and associated lava flows.
One explanation for this distribution is that the magma chambers underlying the stratovolcanoes intercept the basaltic magmas before they reach the surface and allow the basalts to differentiate to more siliceous compositions before they are erupted. 
Basaltic magmas that are not intercepted by the magma chambers can make it to the surface to erupt in the surrounding areas.

Probably the most distinguishing feature of subduction-related volcanic rocks is their usually porphyritic nature, usually showing glomeroporphyritic clusters of phenocrysts. (A textural term used to describe igneous rocks that contain clusters of phenocrysts, which are large crystals in a finer-grained matrix or groundmass)
glomeroporphyritic phenocryst clusters

Basalts commonly contain phenocrysts of olivine, augite, and plagioclase.
Andesites and dacites commonly have phenocrysts of plagioclase, augite, and hypersthene, and some contain hornblende.

calcic plagioclase phenocrysts that show complex oscillatory zoning

The most characteristic feature of the andesites and dacites is the predominance of fairly calcic plagioclase phenocrysts that show complex oscillatory zoning. 
Such zoning has been ascribed to various factors, including:

Rhyolites occur as both obsidians and as porphyritic lavas and pyroclastics. Phenocrysts present in rhyolites include plagioclase, sanidine, quartz, orthopyroxene, hornblende, and biotite.
In addition to these features, petrographic evidence for magma mixing is sometimes present in the rocks, including disequilibrium mineral assemblages, reversed zoning etc. 
Xenoliths of crustal rocks are also sometimes found, particularly in continental margin arcs, suggesting that assimilation or partial assimilation of the crust could be an important process in this environment.

Possible explanation  Calc-alkaline Suite of rocks that are commonly associated with subduction...

Oceanic Igneous Rocks

The ocean basins cover the largest area of the Earth's surface.Because of plate tectonics, however, most oceanic lithosphere eventually is subducted and thus the only existing oceanic lithosphere is younger than about Jurassic in age and occurs at locations farthest from the oceanic spreading centers.Except in areas where magmatism is intense enough to build volcanic structures above sea level, most of the oceanic magmatism is difficult to access.Samples of rocks can be obtained from drilling, dredging, and expeditions of small submarines to the ocean floor.Numerous samples have been recovered and studied using these methods.Most of the magmatism is basaltic. Still, few drilling expeditions have penetrated through the sediment cover and into the oceanic lithosphere. Nevertheless, we have a fairly good understanding of the structure of the oceanic lithosphere from seismic studies and ophiolites.
Here we will first look at ophiolites, then discuss basaltic magmatism in general, and then discuss the various oceanic environments where magmatic activity has occurred.
An ophiolite is a sequence of rocks that appears to represent a setion through oceanic crust. Ophiolites occur in areas where obduction (the opposite of subduction) has pushed a section of oceanic lithosphere onto continental crust. During this process, most ophiolite sequences have been highly deformed and hydrothermally altered. Nevertheless, it is often possible to look through the deformation and alteration and learn something of the structure of oceanic lithosphere.
An idealized ophiolite sequence shows an upper layer consisting of deep sea sediments (limestones, cherts, and shales), overlying a layer of pillow basalts. Pillow basalts have a structure consisting of overlapping pillow-shaped pods of basalt.  Such pillow structure is typical of lavas erupted under water. The pillow basalts overly a layer consisting of numerous dikes, some of which were feeder dikes for the overlying basalts.  Beneath the sheeted dike complex are gabbros that likely represent the magma chambers for the basalts.  The upper gabbros are massive while the lower gabbros show layering that might have resulted from crystal settling.

At the base of the layered gabbros, there is a sharp increase in the density of the rocks, and the composition changes to ultramafic rocks. This sharp change in density is correlated with what would be expected at the base of the crust, and is thus referred to as the petrologic moho. At the top of the ultramafic sequence the rock type is harzburgite (Ol + Opx), a rock type expected to be the residual left from partially melting peridotite. The base of the ultramafic layer is composed of peridotite. Because most ophiolites have been hydrothermally altered, most of the mafic rocks have been altered to serpentinite. Note that ophiolite means "snake rock"
Volcanic Settings
Volcanism occurs at three different settings on the ocean floor.
  1. Oceanic Ridges - these are the oceanic spreading centers where a relatively small range of chemical compositions of basalts are erupted to form the basaltic layer of the oceanic crust. This chemical type of basalt is referred to as Mid Ocean Ridge Basalts (MORBs).In some areas, particularly Iceland, where there has been a large outpouring of basalts on the oceanic ridge, basalts called Enriched Mid Ocean Ridge Basalts (EMORBs)have been erupted.
  2. Oceanic Islands - these are islands in the ocean basins that generally occur away from plate boundaries, and are often associated with hot spots, as discussed previously.  A wide variety of rocks occur in these islands, not all are basaltic, but those that aren't appear to be related to the basaltic magmas.  In general these rocks are referred to as Oceanic Island Basalts (OIBs).
  3. Large Igneous Provinces (LIPs) - these are massive outpourings of mostly basaltic lavas that have built large submarine plateaus.  Most are mid-Cretaceous in age. They are not well studied, but most have compositions similar to OIBs, and some may have once had oceanic islands on top, but most of these have since been removed by erosion.
Thus, most oceanic magmatism is basaltic, so we will first discuss basaltic magmas in general.

On a chemical basis, basalts can be classified into three broad groups based on the degree of silica saturation. This is best seen by first casting the analyses into molecular CIPW norms (the same thing as CIPW norms except the results are converted to mole % rather than weight %). On this basis, most basalts consist predominantly of the normative minerals - Olivine, Clinopyroxene, Plagioclase, and Quartz or Nepheline. These minerals are in the 4 component normative system Ol-Ne-Cpx-Qtz, shown here as a tetrahedron.In the tetrahedron, plagioclase plots between Ne and Qtz, and Opx plots between Ol and Qtz. The basalt tetrahedron can be divided
basalt tetrahedron
Note that tholeiitic basalts are basalts that show a reaction relationship of olivine to liquid which produces a low-Ca pyroxene like pigeonite or Opx.Both olivine tholeiites and quartz tholeiites would show such a relationship and would eventually precipitate either Opx or pigeonite.
The critical plane of silica undersaturation appears to be a thermal divide at low pressure.This means that compositions on either side of the plane cannot produce liquids on the other side of the plane by crystal fractionation.To see this, look at the front two faces of the basalt tetrahedron.  These are in the three component systems Ol-Cpx-Qtz and Ol-Cpx-Ne.

Mid Ocean Ridge Basalts (MORBs)
The Oceanic Ridges are probably the largest producers of magma on Earth. Yet, much of this magmatism goes unnoticed because, with the exception of Iceland, it all takes place below the oceans. This magmatism is responsible for producing oceanic crust at divergent plate boundaries.
Magma is both erupted and intruded near the central depressions that form the oceanic ridges.  Thus, both basalts and gabbros are produced.  But, little is known of the gabbros since they are rarely exposed and most oceanic lithosphere eventually is subducted.The main melting mechanism is likely decompression melting as rising convection cells move upward through the mantle beneath the ridges. At most oceanic ridges the basalts that are erupted are tholeiitic basalts sometimes referred to as NMORBs (normal MORBs)

divergent mid-oean plates
At the oceanic ridges, the basalts erupted range in composition from Olivine tholeiites to Quartz tholeiites.The compositions are by and large restricted to basalt, i.e. less than about 52% SiO2. The diagram shown here is called an AFM diagram. It is a triangular variation diagram that plots total alkalies at the A corner, total iron at the F corner, and MgO at the M corner.MORBs show a restricted range of compositions that fall along a linear trend extending away from the compositions of Mg-rich pyroxenes and olivines.
his is the trend that would expected from fractional crystallization involving the removal of early crystallizing olivines and pyroxenes from a tholeiitic basaltic liquid.  Note that the trend is often referred to as an Fe-enrichment trend.

Some thoughts on the origin of MORBs. mantle plumes

Ocean Island Basalts (OIBs)
Oceanic islands are, in general, islands that do not occur along the divergent or convergent plate boundaries in the ocean basins. Nevertheless, EMORBs, such as those that occur in Iceland, as well as the Alkalic basalts of Iceland have much in common with magmas erupted in the oceanic islands. In the Atlantic Ocean, which is a slow-spreading oceanic basin, as well as in the Galapagos Islands of the eastern Pacific Ocean, some of the islands occur close to oceanic ridge spreading centers.
In all cases we must keep in mind that the parts of these islands that are accessible for sampling represent only a fraction of the mass of the volcanic structures which rise from the ocean floor at depths up to 10,000 m. Thus, as with the ocean ridge volcanic rocks, there is a potential sampling problem.
Here we discuss not only the magmatism that has occurred recently at Oceanic Islands, but also the magmatism that produced massive submarine plateaus on the sea floor during the Cretaceous. The latter are often referred to as Large Igneous Provinces (LIPs).
Oceanic Islands
Most oceanic islands appear to be related to ascending plumes of hot mantle. These plumes must be relative narrow features because they appear to operate independent of the main convection cells that ascend beneath the oceanic ridges and descend at subduction zones. Still, in places like Iceland on the ocean ridge, magma production rates are high, and compositions of rocks are similar to those found in oceanic islands. So Iceland could also be considered an oceanic island.
If these rising plumes of hot mantle remain stationary in their positions in the mantle, they produce hot spots, as discussed previously. Hot spots are most recognizable when they occur beneath plates that move with higher velocities. Beneath faster moving plates, like the Pacific Plate, this results in linear chains of islands.
At the position directly over the hotspot,  rising mantle melts to produce magma that erupts on the seafloor, eventually building a volcanic island directly over the hot spot.  As the lithospheric plate moves over the hot spot the volcano eventually is cut off from its source of magma, and becomes extinct, and a new volcano forms on the plate at the location directly above the hot spot.

oceanic hot spot volcanism
The volcanoes that have moved away from the hot spot eventually begin to erode until their elevations are reduced below sea level. At this point they are called seamounts.
Such linear chains of islands and sea mounts are most evident in the Pacific ocean. The largest of these is the Hawaiian - Emperor chain. The hot spot that produced this chain is currently located under the position of the big island of Hawaii, which has the only currently active volcanoes in the chain. The bend in the Hawaiian-Emperor chain must have resulted from a change in the direction of plate motion. Volcanic rocks dredged from the sea floor at the location of this bend are about 40 million years old. Thus, prior to 40 million years ago the Pacific Plate was moving in a more northerly direction. The most northerly seamount is dated at about 60 million years.Seamounts older than 60 million have apparently been subducted.

The reason such island/seamount chains are not as evident in the other oceans is because the plate velocity is lower and volcanoes tend to remain over the hot spots for longer periods of time, building elongated groups of islands rather than linear chains.
Large Igneous Provinces (LIP)s
Large igneous provinces are areas where large volumes of magma have been added to the Earth's crust over relatively short periods of time. Although here we discuss these in terms of the ocean basins, it should be noted that they also include the continental areas where large volumes of magma have been erupted as flood basalts. Eruption of large amounts of magma on the surface of the Earth can have drastic consequences.
For example: The mid Cretaceous Period was a time of higher than normal global temperatures and high stands of the oceans.Eruption of magma on the ocean floor at this time might have been the cause of these conditions.  Evidence is preserved on the sea floor in the form of large submarine plateaus that were emplaced during this time period.
In the Pacific Ocean, much of the oceanic lithosphere east of the position of the ridge 80 million years ago has been subducted.Thus, if the submarine plateaus formed at the ridge, then it would be expected that half of each plateau became separated at the ridge and have since been subducted.These probable plateaus are shown in the map, and if they were present would double the original size of the plateau.Thus, for example the largest of the plateaus is the Ontong-Java Plateau, now located in the southeastern Pacific, with a volume of about 50 million km3.But if the other half had been present, the total volume of magma erupted over a 25 million year period would have been over 100 million km3The few studies that have looked at the rocks in these submarine plateaus suggest that their compositions are similar to EMORBs and OIBs.
Unlike the ocean ridges, which have a rather limited range of rock compositions, the oceanic islands have produced a broader range. Basalts are still predominant, but other compositions are part of the series, and the types of rocks produced are variable from one island to the next. The table below shows that variety of rock types found at different oceanic islands. Some produce tholeiitic rocks similar to EMORBs and others produce alkalic basalts that are saturated to undersaturated with respect to silica.

Oceanic Island Rock Suites
Island or Group Rock Types
Ascension Oliv. Tholeiite (dominant) + Hawaiite + Mugearite + Trachyte + Peralk. Rhyolite
Azores Alk. basalt + Hawaiite + Trachyte
Fernando de Noronha Alk. Basalt + Nephelinite + Trachyte + Alkali Basalt + Trachyte + Phonolite
St. Helena Alk. Basalt + Mugearite + Hawaiite + Trachyte + Phonolite
Trinadade Nephelinite + Phonolite (dominant)
Tristan de Cunha Alk. Basalt + Trachybasalt (dominant) + Trachyte
Gough Alk. Basalt + Ol Tholeiite + Hawaiite + Trachyte
RĂ©union Ol Tholeiite (dominant) + Mugearite
Mauritius Alk. Basalt (dominant) + Mugearite + Phonolitic Trachyte
Hawaii Tholeiite (dominant) + Alkali Basalt + Hawaiite + Mugearite + Trachyte
Tahiti Alk. Basalt + Mugearite + Hawaiite + Trachyte
Galapagos Tholeiite + Alk. Basalt + Icelandite (minor) + Qtz Trachyte (minor)
Jan Mayen Alk. Basalt (dominant) + Trachyte

Some thoughts on the origin of OIBs.

Igneous Rock Classification

Based on the IUGS Subcommission on the Systematics of Igneous Rocks
  The main QAPF classification for plutonic and volcanic rocks which is based on the modal mineral proportions of quartz (Q), alkali feldspar (A) and plagioclase (P) or of alkali feldspar (A), plagioclase (P) and feldspathoids (F). Rocks with mafic content >90% have their own classification.
This is followed by the classification separates and individually classifies the pyroclastic, carbonatitic, melititic, lamprophyric and charnockitic rocks before entering If the mineral mode cannot be determined as is often the case for volcanic rocks, then a chemical classification of total alkalis versus silica (TAS) is used. The nomenclature for these classifiations necessitates only 297 rock names out of the about 1500 that exist

    Ten principles of  IUGS Igneous Rock Classification
  1. use descriptive attributes;
  2. use actual properties;
  3. ensure suitability for all geologists;
  4. use current terminology;
  5. define boundaries of rock species;
  6. keep it simple to apply;
  7. follow natural relations;
  8. use modal mineralogy;
  9. if mode not feasible, use chemistry;
  10. follow terminology of other IUGS advisory bodies
     Main QAPF classification

Pyroclastic Rocks

Carbonatitic Rocks

Melititic Rocks

Lamprophyric Rocks

Charnockitic Rocks

Origins and Differentiation of Magma

Magmas must require special circumstances in order to form and do not form just anywhere in the crust.
The Different Layers of the Earth Have Differing Chemical Compositions