EENS 211

Earth Materials

Tulane University

Prof. Stephen A. Nelson

Igneous Rocks and Plate Tectonics

This lecture is designed to give you a general overview of how igneous rocks are related to tectonic setting.  In this course, we will not consider in detail the origin of the various rock types and volcanic rock types will be emphasized over plutonic rock types, mainly because the tectonic setting is usually more clear in recent volcanic environments.  The main characteristics of rocks that occur in the various environments will be discussed.  The meaning of these characteristics and the clues that they give us to to origin of the original magmas will be left to the petrology course next semester.

Because basaltic magmas of one type or another occur in nearly all tectonic environments, we will start  with a general discussion of the types of basalts.


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 into three compositional volumes, separated by planes. 
  1. The plane Cpx-Plag-Opx is the critical plane of silica saturation. Compositions that contain Qtz in their norms plot in the volume Cpx-Plag- Opx-Qtz, and would be considered silica oversaturated. Basalts that plot in this volume are called Quartz Tholeiites
  1. The plane Ol - Plag - Cpx  is the critical plane of silica undersaturation. Normative compositions in the volume between the critical planes of silica undersaturation and silica saturation are silica saturated compositions (the volume Ol - Plag - Cpx - Opx).  Silica saturated basalts are called Olivine Tholeiites.
  2. Normative compositions that contain no Qtz or Opx, but contain Ne are silica undersaturated (the volume Ne-Plag-Cpx-Ol). Alkali Basalts, Basanites, Nephelinites, and other silica undersaturated compositions lie in the silica undersaturated volume.

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. 

Igneous Rocks of the Ocean Basins

The ocean basins cover the largest area of the Earth's surface. Because of plate tectonics, however, most oceanic lithosphere eventually is subducted.  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 section 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 about 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. ophiolite.gif (8443 bytes)
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".

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).
divergingpltbndy.gif (12926 bytes)


At Iceland, the rate of magma production is so high that volcanism has built the oceanic ridge above sea level. Most of the active volcanism occurs within two central rift zones that cut across the island. Again, the predominant type of basalts erupted are tholeiitic basalts, however, these are somewhat different from NMORBs, showing higher concentrations of incompatible trace elements. Thus, they are often referred to as Enriched MORBs (EMORBs). Unlike normal oceanic ridges, a significant volume of rhyolite is also erupted in Iceland.  This was once thought to suggest that continental crust underlies Iceland, but it is more likely that the rhyolites are produced by either crystal fractionation of the basalts or partial melting of the oceanic crust beneath Iceland. iceland.gif (13682 bytes)


Small volumes of alkali basalt have also erupted in Iceland.  This alkaline volcanism occurs on top of the tholeiitic basalts and erupts in areas to the east and west of the main rift zones that cut across the island.



  • 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. As shown, MORBs show a restricted range of compositions that fall along a linear trend extending away from the compositions of Mg-rich pyroxenes and olivines.
afm_morb.gif (5752 bytes)

This is the trend that would be 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.

  • The rock suite erupted at Iceland shows a much broader range of chemical compositions. While EMORBs predominate, intermediate rocks like icelandites and siliceous rocks like rhyolites also occur.  Plotted on an AFM diagram, we see that the EMORBs show a range of compositions that likely result from crystal fractionation of early crystallizing Mg-rich olivines and pyroxenes.  With continued fractionation, the liquids follow an Fe enrichment trend to produce the Icelandites. 
afm_ice.gif (8488 bytes)

During this sequence the olivines and pyroxenes are expected to become more Fe enriched which would tend to cause the trend to bend somewhat.  But at the peak of Fe enrichment it appears that the liquids have become so rich in Fe that an Fe-rich phase, like magnetite, joins the early crystallizing mineral assemblage. Fractionation of this Fe-rich mineral assemblage would then cause Fe to become depleted in successive liquids, driving the liquid compositions toward rhyolite.  Thus, crystal fractionation appears to be responsible for the main variety of rocks found at Iceland.

Ocean Island Basalts (OIBs)

The 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.

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. hotspot.gif (13585 bytes)
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. linearislands.gif (19512 bytes)
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.


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
Réunion 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
Major Elements.  For those islands that produce tholeiitic basalts, the range of compositions is similar to those that occur in Iceland.  The more alkaline suites show a somewhat different range of rock types.

    Shown here is the more common alkaline suite produced at oceanic islands. The most basic rocks are alkaline basalts that show a range of compositions and an Fe-enrichment trend that likely results from fractionation of Mg-rich olivines and pyroxenes. As these magmas become more enriched in iron, hawaiites are produced.  The fractionating assemblage then becomes more Fe-rich, likely caused by the addition of magnetite to the crystallizing assemblage. This causes an Fe-depletion trend causing magmas to evolve to mugearites, benmoreites or trachytes, and eventually rhyolites and/or peralkaline rhyolites.

afm_alkbas.gif (10584 bytes)

    Perhaps not typical of all oceanic islands, but certainly the most well studied oceanic islands are the Hawaiian Islands.  The main portion of each of the islands exposes tholeiitic basalts which appear to make up the most volume.

    At the northwestern end of the island chain is the island of Kauai.  Tholeiitic volcanism built Kauai about 5.5 million years ago.  As one moves to southeast along the chain the age of tholeiitic volcanism becomes younger, occurring at 3.8 and 2.8 million years ago on Oahu, 2 and 1.7 million years ago on Molokai, and 1.3 and 0.9 million years ago on Maui.

hawaiianislands.gif (13628 bytes)

    The big island, Hawaii, is composed of 5 major volcanoes.  The oldest is Kohala whose tholeiitic shield was built about 800,000 years ago. Hualalai and Mauna Kea are somewhat younger, and Mauna Loa and Kilauea are still actively erupting tholeiitic lavas.  

    Although the lower parts of Mauna Kea, on the big island, and Haleakala on Maui are composed of tholeiitic basalts, they are capped by steeper sided composite cones that consist of alkali basalts, hawaiites, mugearites, and trachytes.  These alkaline caps are thought to have existed on top of the tholeiitic shields that make up the older islands, but the alkaline rocks have been eroded.

    The island of Oahu is deeply eroded, but recent volcanism has occurred on the eroded shield.  This post-erosional volcanism, as it is called, consists of highly alkaline basanites and nephelinites.

    In terms of total alkalies and silica the three suites of rocks each show trends of increasing alkalies with increasing SiO2.

    In basic magmas the alkaline elements K and Na behave as incompatible elements, so crystallization of Mg & Fe- rich phases tends to cause both SiO2 and alkalies to increase. Thus the general trends are consistent with crystal fractionation as a mechanism to explain chemical variation in each suite.

alksilicahawbas.gif (9663 bytes)

    The least siliceous rocks of each suite show a trend of decreasing alkalies with increasing SiO2.  Again, because the alkali elements are incompatible in mantle mineral assemblages this could be explained by increasing degrees of partial melting.  But, in the Hawaiian Islands, the tholeiitic magmas are produced first.  During the tholeiitic stage the volcanoes are located directly over the rising mantle plume the forms the hotspot.  In this area we might expect higher mantle temperatures and thus higher degrees of melting to produce tholeiitic magmas. 

    As the island moves away from a position directly over the hotspot the temperatures in the underlying  mantle would be lower, but the rising plume at the hot spot could still drive the somewhat cooler mantle upward.  Because the temperatures are lower, lower degrees of partial melting would be expected, and the slightly more alkaline magmas would be produced and erupt to form the alkalic caps such as seen at Mauna Kea and Haleakala.

    Finally, after the passage of several million years the volcano has moved farther away from a position above the hotspot, and thus temperatures are even lower.  Melting of the mantle at this lower temperature would result in lower degrees of melting to produce the very alkali-rich basanite magmas.

    If this is the case we might expect a symmetrical distribution of magma types being produced on both sides of the hotspot.  The difference, of course, would be that volcanism on the plate before it has reached the hotspot would only occur on the seafloor, because the volcanic island will not yet have been built.  The only evidence presently available to support this idea is that just to the southeast of the big island there is currently an active submarine volcano called Loihi seamount.  Chemical compositions of lavas erupted from Loihi are alkali basalt.


Convergent Plate Margins

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 (and probably is resulting at present) in significant additions to the crust in the form of plutonic igneous rocks.  Here, we look at this magmatism in terms of the volcanic rocks that appear to be related to subduction.


The "Pacific Ring of Fire" is often discussed in relation to both earthquakes and volcanoes. It 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. ringfire.gif (23123 bytes)
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.
oceanocean.gif (12446 bytes)
  1. 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. 
oceancont.gif (13396 bytes)
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. stratovolpet.gif (39308 bytes)
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.  Basalts commonly contain phenocrysts of olivine, augite, and plagioclase.  Andesites and dacites commonly have phenocrysts of plagioclase, augite, and hypersthene, and some contain hornblende.  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:

  • Kinetic factors during crystal growth.  As one zone of the crystal is precipitated the liquid immediately surrounding the crystal becomes depleted in the components necessary for further growth of the same composition.  So, a new composition is precipitated until diffusion has had time to renourish the surrounding liquid in the components necessary for the equilibrium composition to form.
  • Cycling through a chemically zoned magma chamber during convection.  As crystals grow, they are carried in convection cells to warmer and cooler parts of the magma chamber.  Some zones are partially dissolved and new compositions are precipitated that are more in equilibrium with the chemical compositions, pressures, and temperatures present in the part of the magma chamber into which the crystal is transported.

  • Magma mixing.  As magmas mix the chemical compositions of liquids and temperatures change during the mixing process.  This could result in dissolution of some zones, and precipitation of zones with varying chemical composition.

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.


Before discussing the major element chemistry of subduction related volcanic rocks we first need to clarify some terminology concerning rock suites.  In the early 1900s a petrologist by the name of Peacock examined suites of rocks throughout the world. On a plot of CaO and total alkalies versus SiO2, Peacock noted that the two curves intersected at different values of SiO2 for different suites.  He used the value of SiO2 where the two curves intersect (now known as the Peacock Index or Alkali-Lime Index) to divide rock suites into the following:  peacockindex.gif (6718 bytes)
Peacock Index Name of Suite
<51 Alkalic
51-56 Alkali-Calcic
56-61 Calc-Alkalic
>61 Calcic
Although Peacock's classification of rock suites is rarely used today, some of the terminology has survived in slightly different forms.  For example the general term "alkaline suite" is used to describe rock suites in which the basic rocks have relatively high values of total alkalies, like the alkali basalt -hawaiite - mugearite - trachyte suite  or the basanite - nephelinite suites discussed previously.  Most subduction related volcanic and plutonic rocks fall into the calc-alkalic suite of Peacock, and thus the term calc-alkaline is often given to the suite of rocks found associated with subduction.  But, it is notable that all four suites of rocks defined by Peacock are found in subduction-related areas.
The modern definition of the calc-alkaline suite is based on the AFM diagram.  On this diagram subduction-related volcanic rocks show a trend along which the ratio of MgO to total iron (MgO/FeO*) remains nearly constant.  This trend is often referred to as the Calc-Alkaline trend.  Note that the calc-alkaline trend is distinct from the Fe-enrichment trends shown by the alkaline and tholeiitic suites discussed previously.  Also, calc-alkaline basalts, like tholeiitic basalts are subalkaline, but they differ from tholeiitic basalts in their higher concentrations of Al2O3, with values of 17 to 20 wt %.   Thus, these calc-alkaline basalts are often referred to as high alumina basalts. afm_calcalk.gif (9677 bytes)
Recall that the Fe-enrichment trend exhibited by the tholeiitic and alkaline rock series can be explained by crystal fractionation involving removal of early crystallizing Mg-rich olivines and pyroxenes from the parental basaltic magmas.  The calc-alkaline trend, however, would require early removal of mineral assemblages with a higher Fe/Mg ratio, or some other process.  Over the years several explanations for the calc-alkaline trend have been discussed.  Among these are:
  • Crystal fractionation by early removal of an Fe-rich mineral assemblage.  Because the basaltic compositions are similar to tholeiitic basalts, they would crystallize the same Mg-rich olivines and pyroxenes as tholeiitic basalts.  So, this process would require early crystallization of additional Fe-rich phases to raise the Fe/Mg ratio of the early crystallizing assemblage. Likely candidates for the Fe-rich phase or phases would be magnetite or an Fe-rich amphibole. Experiments conducted in the 60s through 80s failed to show that magnetite or Fe-rich amphibole would be early crystallizing phases in basalts or andesites under geologically reasonable conditions.  So, at least initially, this mechanism appeared to be unacceptable (but see below).

  • Assimilation of crustal material by basaltic magmas.  Since rhyolites and granites are chemically similar, and since the continental crust contains a higher proportion of granitic rocks, it could be possible that the calc-alkaline trend is due to assimilation of crustal granites by basaltic magmas.  We have previously discussed the difficulty of such a process operating on a large scale because of the energy requirements involved.   A larger hindrance to this mechanism, however, is that the calc-alkaline suite occurs both in island arcs, where there is little or no continental crust, as well as in continental margin arcs where there is such crust.

  • Magma Mixing.  The calc-alkaline trend could be explained by mixing of basaltic magmas with rhyolitic magmas to produce the intermediate andesites and dacites.  This involves the problem of first, how are the rhyolites generated, and second that such rhyolitic and basaltic magmas would have to be present beneath all arcs. While mixing does seem to play a role, it is unlikely that it always occurs and is always able to generate the large volumes of magma required to build a mostly andesitic stratovolcano.

  • Andesites as primary magmas.  In the early years, when it was not recognized that basalts do occur in the arcs or at least that andesites were the predominant type of magma erupted, it was suggested that andesites were primary magmas.  Since it was known that the mantle would not likely be able to produce silica oversaturated andesitic magma by partial melting, except at very low pressure, it was suggested that the subducted oceanic crust partially melted to produce andesitic magmas. This seemed like a good hypothesis in light of the new theory of Plate Tectonics that was coming out at the time. But, as we will see later, there are serious obstacles to this theory in the trace element composition of the magmas.  Nevertheless, this early theory became popular and was put into introductory physical geology textbooks, many of which still advocate that andesitic magmas are generated by partial melting of the subducted oceanic crust. 

In recent years more light has been shed on the possible origin of the calc-alkaline suite.  Perhaps the best evidence comes from experimental petrology and recent advances in experimental techniques. 

First, however, consider experiments conducted at low pressure on tholeiitic basalt magmas.  These experiments show that at low pressure Plagioclase and Olivine crystallize first, with proportion of plagioclase crystallizing being higher than that of olivine.  On the projected phase diagram, removal of Olivine and Plagioclase drives the liquid composition away from the Olivine corner until it intersects the the Ol + Plag  + Cpx  + Liq. cotectic.  Further crystallization of these phases will then drive the liquid composition along the cotectic to eventually crystallize pigeonite (low Ca-pyroxene) as shown by the light colored path on the diagram. 2kbh2o.gif (11750 bytes)
Analyses of the liquids produced in these experiments showed that, as expected the liquids would follow a trend of Fe-enrichment and thus the calc-alkaline trend could not be produced by fractional crystallization at low pressures. 

Next, experiments were conducted at a pressure of 2 kb with enough H2O in the capsules to assure that the liquid would be H2O saturated at this pressure (i.e. a free vapor phase would coexist with the liquid).  These experiments were conducted because it was known that H2O would lower the temperature of appearance of the silicate minerals, but would lower the temperature of appearance of oxide minerals, like magnetite to a lesser extent, and could stabilize a hydrous phases like hornblende at a higher temperature.

The experiments show that
  • The position of the Ol + Plag  + Cpx  + Liq. cotectic shifts toward the Olivine corner of the projected phase diagram.

  • The proportion of Olivine relative to plagioclase becomes much higher than in the dry low pressure experiments.

  • Magnetite becomes an early crystallizing phase and hornblende also crystallizes early if the liquids have a high enough concentration of Na2O. 

  • Most importantly, analyses of the liquids produced in the experiments plot along the calc-alkaline trend in the AFM diagram.

Furthermore, if subduction related arc rocks are plotted on the projection there are seen to lie in a field surrounding the 2 kb H2O saturated cotectic.  This indicates that the calc-alkaline suite could be produced by fractional crystallization under moderate pressure water saturated conditions.  This would suggest that the main difference between tholeiitic rocks and calc-alkaline rocks might be the presence (in calc-alkaline basalts) or absence (in tholeiitic basalts) of H2O in the parental magmas and/or the source rocks that melt.  From our previous discussion, we know that it is possible to introduce water into the subduction related environment by dehydration of the subducting lithosphere, whereas it is more difficult to envision a mechanism to add water to the source where tholeiitic magmas are generated.


Continental Igneous Rocks

A wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere. In addition, because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study.  We start with granitic rocks and their associated pegmatites, next consider large volume continental rhyolites and basalts, and finish with continental rift valleys.


Granitic Rocks

Here we discuss a group of plutonic igneous rocks usually referred to as "granitic rocks", "granitoids", or loosely as "granites". Included are true granites, but our discussion will include all medium to coarse-grained rocks that are mostly felsic with a few mafic minerals.

A variety of classification schemes have been proposed for granitic rocks.  The easiest to employ uses the modal mineralogy of the rocks, while others attempt classification on the basis of the tectonic setting, or type of source rock which melted to produce the granitic magma.

  • Mineralogical Classification. The IUGS mineralogical classification scheme shown here is based modal mineralogy. 

    Note that 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. The base of the composition triangle is a thermal divide, that separates quartz-bearing rocks from feldspathoid-bearing rocks.

    The feldspathoid bearing rocks include the feldspathoidal syenites, which will not be considered to any large extent here.

  • Tectonic/Chemical Classification.  Tectonic classification is more appropriately called a chemical classification, because, as we will see, the various chemical types are not necessarily restricted to certain tectonic environments. 
    • 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. during the Meszoic.  I-type granites are also found in the Himalayas, which are 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 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.

      An example of the a convergent margin plutonic suite is found in the Sierra Nevada Batholith and associated plutons in eastern California and western Nevada that 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.

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  • 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 sometimes occurs.

    They 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 & 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, 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 (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 mesozone 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.

Before considering the origin of granitic magmas we will first discuss the related rocks, the pegmatites, then consider continental rhyolites, which are likely closely related to granitic plutons.



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 tourmaline, [(Na,Ca)(Mg,Fe,Mn,Li,Al)3(Al,Fe+3)6Si6O18(BO3)2(OH)4], beryl [Be3Al2Si6O18], lepidolite [K2(Li,Al)5-6Si6-7Al2-1(OH,F)4, and spodumene [LiAl2Si2O6], 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.  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.

Three Late Tertiary examples are notable. All have formed large calderas associated with the emplacement of pyroclastic flow deposits as well as fall deposits.  These include Yellowstone Caldera in Wyoming, Long Valley Caldera in eastern California, and Valles Caldera in New Mexico.  Note that all three are located on the margins of the Basin and Range Province.  Pyroclastic fall deposits are widespread, although not well-preserved, as would be expected.  Ash from the youngest eruptions from Yellowstone occur in sediment cores from the Gulf of Mexico.  Ash from Long Valley caldera is found as far east as Nebraska. Eruptions of volumes of ash similar to these would certainly be devastating to any country in which the occurred. yellowstonessh.gif (21891 bytes)
As stated above, most of the preserved rocks from these large silicic systems are preserved as pyroclastic flow deposits that spread out close to the eruptive vents.   Most display various degrees of welding that results from compaction and annealing of the glassy fine-grained ash particles contained in the deposits.  It is the welded portions of the pyroclastic flow deposits that have the greatest potential for preservation in the geologic record, as they are more resistant to erosion.
Non-welded pyroclastic flow deposits are usually poorly sorted and poorly indurated.  They contain blocks and lapilli-sized fragments of pumice and lithic fragments in a matrix of fine-grained glassy ash particles. welding.gif (19669 bytes)
Deeper in the deposit heat remains available for longer times and the weight of the overlying material tends to start the welding process.  Lumps of pumice become compressed into disc-like forms called fiamme.  The original vesicular texture is lost and the flattened pumice loses its vesicular character.   The glassy ash particles in the matrix start to weld together, while the brittle lithic fragments are usually not compressed.  If temperature remains high and the lower parts of the deposit are under a high load, welding may progress to turn the rock into a vitrophyre, wherein the ash matrix becomes a coherent glass, the fiamme are further flattened and the vesicles and pore space are completely pressed out.

Two examples of continental rhyolite complexes will be discussed.

  • Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 million years.  The first formed 2.0 million years ago, producing 2,500 km3 of ash, the second 1.3 million years ago producing 250km3, and the latest  600,000 years ago producing 1,000 km3 of ash.  Thus, the repose time is on the average about 650,000 years.  That magma is still present beneath Yellowstone is evidenced both by the intense hydrothermal activity that takes place within the most recent caldera, and by seismic profiling which indicates magma at a depth of about 3 km.
  • Long Valley Caldera.  Long Valley caldera is located on the eastern side of the Sierra Nevada Mountains in California, along the western edge of the Basin and Range extensional zone. Between 2 and 3 my ago basaltic and andesitic volcanism produced lava flows that filled the down-dropped graben between the Sierra Nevada and the White-Inyo Mountains.  Rhyolitic volcanism began about 1.9 m.y. ago and produced lava flows and domes of Glass Mountain.  This activity continued until about 0.9 my ago.
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    720,000 years ago an eruption produced about 600 km3 of pyroclastic material, both as fall deposits and pyroclastic flows. The pyroclastic flows, known as the Bishop Tuff, are still preserved in Owens Valley to south, and in the Mono Basin to the north.  Some flows crested the Sierra Nevada and reached the San Joaquin valley to the west.  This eruption resulted in the collapse of the area above the magma chamber to produce Long Valley Caldera.  The Bishop Tuff shows chemical zonation, having SiO2 concentrations of 77.4% in lower, first erupted units and 75.5% SiO2 in the upper, later erupted units.  This likely reflects chemical zonation in the magma chamber.  Rhyolite domes and lava flows were then emplaced on the floor of the caldera between 0.73 and 0.61 my ago.  During this time the central part of the caldera floor was uplifted to form a structural dome, called a resurgent dome, likely due to re-intrusion of magma below.  The resurgent dome shows a central graben on the map above.  Rhyolite and rhyodacite domes and flows were also emplaced in the moat around the resurgent dome and along the ring fractures of the caldera between 0.5 my and 0.1 my ago.  One of these, Mammoth Mountain, located on the southwestern margin of the caldera, is now a popular ski resort. Between 0.2 and 0.06 my ago basaltic lavas were erupted on the floor of the caldera and in areas to west of the caldera. 

    The most recent activity in the Long Valley area has occurred just to the north at Mono Craters.  Here rhyolite domes have been erupted along an arcuate zone that may be a developing ring fracture for another caldera in the near future.

Origin of Large Volumes of Silicic Magma

In the early part of the century a debate among igneous petrologists ensued concerning the origin of granitic rocks (known as the "Granite Controversy").  One group referred to themselves as the granitizationists and argued that granitic rocks were produced by ultrametamorphism at high temperatures and pressures in the Earth's crust.  The other group, referred to as the magmatists, argued that granites were produced by melting and intruded as liquids into higher levels of the crust.  The granitizationists used evidence mainly based on what are now recognized as catazonal plutons to make their case.  They further argued that making room for such large bodies of magma in the brittle crust would be near impossible and that production of granites in place by granitization of the pre-existing rock would do away with this "room problem".  The magmatists argued their case using evidence from mostly mesozonal and epizonal plutons, which clearly show evidence of the intrusive origin of these bodies and evidence that they were liquid when emplaced.

Clearly there are several ways that granitic rocks could be produced, but it is highly unlikely that all granitic rocks were formed by granitization, although some catazonal bodies could have been.  The fact that contact relations clearly show that many granites were liquid upon intrusion and the fact that large volumes of silicic magma actually erupt in continental rhyolite centers is plentiful evidence for the existence of liquids with granitic composition.  The "room problem" argued by the granitizationists is largely solved when we recognize that most intrusive events occur during stages of deformation wherein the stress regime changes from one of compression to extension.  Extension of the brittle crust can make the space into which magmas intrude.

Nevertheless, it is highly unlikely that large volumes of granitic magma can be produced by crystal fractionation of basaltic magmas.  Such crystal fractionation would require initial volumes of basalt 10 to 100 times greater than the siliceous liquids produced.  There is no evidence for the existence of such large bodies of crystallized basalt magma in the crust. 

Among the mechanisms by which large volumes of granitic magma could be produced are:

  1. Anatexis of metasedimentary/sedimentary rocks to form S-type granitic magmas.

  2. Anatexis of young crustal basic meta-igneous rocks to form I-type granitic magmas.

  3. Melting/Assimilation of lower crustal rocks by mantle-derived basic magmas.

  4. Crystal fractionation/Assimilation of basaltic and andesitic magmas.

  5. Granitization, wherein high grade metamorphism bordering on melting converts rocks into those that appear texturally and mineralogically similar to granitic rocks.

Continental Flood Basalts

Like the large submarine plateaus discussed in our lecture on the ocean basins, large volumes of basaltic magma have erupted on the continents at various times in Earth history.  The most recent of these outpourings, but by no means the largest, is the Columbia River basalts erupted in Oregon and Washington states in the mid-Miocene.  Other important flood basalt provinces are listed in the Table below.



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
Paraná, Brazil Cretaceous 2,000,000 72 28 0
Deccan, India Eocene 500,000 55 35 10
Columbia River Mid-Miocene 163,000 30 70 -

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.  
The Columbia River Basalts represent a series of lava flows all erupted between 17 and 8 million years ago, with the bulk of the volume being erupted between 16.5 and 14 million years ago.  One of the basalt flows, the Roza flow, was erupted over a period of a few weeks traveled about 300 km and has a volume of about 1500 km3.  Vents for the eruptions are thought to be a series of dikes located in southwestern Washington.  Despite the fact that these lavas appear to be fractionated, they are poor in phenocrysts, and individual flows are very homogeneous in composition.  Furthermore, it is difficult to reconcile chemical variation among the various flows to be the result of crystal fractionation processes.  Thus, the origin of the Columbia River basalts, as well as most other flood basalts, remains one of petrology's great unsolved problems.
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Continental Rift Valleys

Continental Rift valleys are linear zones of extension within continental crust.  Some of these extensional zones may 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.  Among the ancient and modern examples are:

  • A series of Triassic to Jurassic grabens that occur along eastern North America and extend from Canada to Georgia that appear to have formed in a failed attempt to rift North America away from Eurasia/Africa.  About 50 m.y. later, these continents successfully rifted apart along a zone further to the East.  These grabens are filled with mostly tholeiitic basalts.

  • Another failed rift is the Oslo Graben of southern Norway, of late Paleozoic age.  Here, both volcanic and intrusive igneous rocks are exposed.  The volcanic rocks consist of an early group of alkalic basalt lavas and a later group of siliceous ignimbrites. The plutonic rocks are also alkaline.  One group consists of alkaline gabbros containing alkali feldspar, called essexites,  that form dikes, sills, and stocks that apparently fed the basaltic volcanic rocks, and the group consist of syenites and peralkaline granites, all of which contain an abundance of alkali feldspar.  Some of these are nepheline bearing while others are quartz bearing.

  • The Rhine graben, between Germany and France is an active rift in which have erupted both silica undersaturated phonolitic ignimbrites along with alkaline trachytes and rhyolites.

  • 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.
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Evolution of Alkaline Rock Suites
The alkaline rock series found in many rifting areas are very unusual, and far less common than the basalt - andesite - dacite suites found in other environments.  We here discuss how some of these unusual magmas might have come into existence. We have already discussed how low degrees of partial melting of the mantle at higher pressure can produce silica-undersaturated basaltic magmas, while increasing degrees of melting, or melting at lower pressure tends to produce silica-saturated to oversaturated basaltic magmas.  Once such magmas have evolved to the point where they begin to crystallize alkali feldspar, we can see what would happen by looking at the simple three component system Ne-SiO2-Ks (kalsilite).  Note that in this system, the join Ab -Or forms a partial thermal divide at low pressure, and separates Silica oversaturated compositions from silica undersaturated compositions. 

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  • Imagine that a silica-saturated basaltic liquid has evolved to a silica-oversaturated trachyte.  Continued fractionation of alkali feldspar solid solution from such a trachyte would cause the liquid to change composition toward the minimum in the sub-system Ab - Or - SiO2, eventually crystallizing quartz and producing a rhyolitic liquid.

  • On the other hand, a silica undersaturated basaltic liquid that has evolved to a silica undersaturated trachyte will change in the opposite direction with alkali feldspar crystallization, and eventually reach a phonolite composition crystallizing nepheline, or leucite + nepheline at the minimum in the silica undersaturated part of the system Ab- Or - Ne - Ks. 

This is further illustrated by looking at a total alkalies versus SiO2 diagram, showing the approximate compositions of various alkaline rock types.

Also shown is the approximate position of the critical plane of silica-undersaturation. Note how a slightly silica-saturated basalt will evolve through hawaiites, mugearites, benmoreites and trachytes that will eventually continue to produce rhyolites. While a slightly silica-undersaturated composition will follow a similar path, but eventually produce phonolites with decreasing SiO2. alksilicadifftrends.gif (11373 bytes)
Note how small amounts of crustal contamination of silica-undersaturated basalts could also cause these silica-undersaturated magmas to become silica-saturated, and result in the bifurcation of the trends. 

Thus, upwelling of the mantle beneath the continental rift zones likely results in various degrees of melting of the mantle by decompression melting.  The presence of continental crust, favors small amounts of contamination of these already alkali rich magmas resulting in the production of basalt-trachyte-rhyolite suites.  Basaltic magmas that reach low pressure and are still silica-undersaturated results in basalt-trachyte-phonolite suites.  Only small changes in composition of the original mantle-derived magmas are necessary to produce these diverging magma types.

Peralkaline Rhyolites
Peralkaline rhyolites are common in continental rift settings, although they also occur in oceanic island settings, and our discussion here includes such settings.  In nearly all cases, peralkaline rhyolites are associated with mildly alkaline silica-saturated basalts, hawaiites, mugearites, and trachytes.  The question becomes - why are peralkaline rhyolites produced instead of normal metaluminous rhyolites?

One answer could come from fractional crystallization of plagioclase.  Ca-rich plagioclase contains twice as much Alumina as the alkali feldspars, and very little alkalies.

Thus removal of Ca-rich plagioclase will result in depletion of Al and enrichment of Na and K (as well as Si).  This is termed the "plagioclase effect".  (Recall that peralkaline rocks are those that have a molecular amount of Na2O + K2O > Al2O3).  On the other hand, if a ferromagnesium phase with high Ca, but little Al, such as augite, also fractionates, then Al depletion will be minimized with increasing Si, thus offsetting the plagioclase effect to produce normal metaluminous rhyolites.

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