SUBDUCTION-RELATED VOLCANISM
Volcanism at convergent margins is a major physical and chemical process on the Earth. It is related to subduction of ocean crust below other ocean crust (island arc volcanism) or below the continental crust (andesitic volcanism). They are generated along active margins of tectonic plates.
Physical Characteristics of Subduction-zone Volcanism
- Andesitic volcanoes occur over Benioff zones, the seismically active regions at the interface between the subducting lithosphere and overlying crust or mantle.
- Subduction related volcanoes are typically stratavolcanoes, in contrast to shield volcanoes formed hot-spot volcanism.
- In island arcs, there may be a development of a back-arc basin, where tholeiitic magmas may erupt. This is observed, for example, in the Japanese arc. Back-arc structures are lacking andesitic-type subductions.
Principal Chemical and Mineralogic Characteristics:
- The majority and average of erupted lavas are andesites (~55 – 60 wt.% SiO2) but they range from tholeiitic basalt, andesites, dacites, to rhyolites.
- Equivalent plutonic rocks that crystallize in subvolcanic magma chambers range from equivalent gabbro, diorite, granodiorite, granite. Plutonic rocks are, however, minor.
- Because most of the magmas have relatively high calcium and sodium concentrations, they are called calc-alkaline. However, tholeiites also occur.
- A major distinction of volcanism at subduction zones compared to ocean floor volcanism is the usual explosive nature of subduction-related volcanism. This is due to the fairly high content of water in andesitic magmas.
- Aside from plagioclase and clinopyroxene as important minerals, hornblende and biotite also occur. Their presence implies the presence of H2O in the magmas.
Detailed Chemical Characteristics of Subduction-zone Lavas
Wide range of compositions
As noted above, magmas erupted out of a single volcano range in composition from basalt to rhyolite. Harker diagrams show smooth trends indicative of a continuous genetic relationship, probably fractional crystallization.
Calc-alkaline and Tholeiitic Trends
Subduction-related rocks show two trends – calc-alkaline and tholeiitic. The trends are illustrated well on an AFM diagram:
A) Tholeiitic trend is marked by iron enrichment caused by crystallization of olivine and pyroxene, which have high Mg/Fe ratios relative to melt. The Fe enrichment is then followed by late swing toward alkalis. Tholeiitic trends occur mostly in island arcs.
B) Calc-alkaline trend exhibits only small amount of iron enrichment during differentiation. This is because of the crystallization of amphibole, which has approximately the same Mg/Fe ratio as the coexisting melt. The calc-alkaline series generally has more H2O than the tholeiitic series. Calc-alkaline series are dominant in andesitic-type subduction zones. Rocks in both series can range from basalt to rhyolite.
K2O concentration (classification of Gill, 1981)
The concentration of K2O in subduction zone rocks range from ~0.5 to ~2.5 wt.%. There are low-K, medium-K and high-K suites. The tholeiitic series is mostly low-K, whereas the calc-alkaline series is mostly medium to high-K. The high concentration of K2O in calc-alkaline melts may be related to its enrichment in the source region during fluxing by water (see below).
Thus, compared to the tholeiitic series magmas, the calc-alkaline series magmas have
- higher water contents, indicated by the presence of abundant hydrous minerals;
- higher K2O concentrations;
- on average, somewhat higher SiO2 concentrations;
- is the dominant series in andesitic-type subduction zones, where the tholeiitic series is not very common.
Petrogenesis – The Importance of Water
Water plays a very significant role in the generation of magmas in subduction-zone environments – from magma generation to eruption.
By the time the ocean crust begins to subduct, mafic minerals in it have become hydrated, where olivine and pyroxene may have turned to serpentine and pyroxene also to amphibole, depending on the amount of seawater that has interacted with the crust. In addition, overlying sediments may have substantial amount of water. As the ocean crust, together with the underlying mantle lithosphere, begins to subduct and heat-up, metamorphic dehydration reactions result in expulsion of water from the slab into the overlying mantle wedge (see attached).
The slab eventually becomes eclogitic, meaning that it has lot of garnet, and becomes very dense. Garnet is produced by reaction of pyroxenes with plagioclase. Note that subducting slabs usually do not become hot enough near Benioff zones to melt.
In the mantle wedge, the influx of water has several effects:
1) water reduces the solidus temperature of the peridotite mantle wedge.
Water has a profound effect on the melting temperature of rocks. As the attached figure shows, if there is free water around, the solidi of rocks can be lowered several hundreds of degrees. Therefore, the influx of water into the overlying mantle wedge induces its melting. Typically, calc-alkaline magmas are more water-rich than tholeiitic magmas.
2) water carries soluble elements from the subducting slab into the source region.
Many elements, including K, Ba, U, B, are soluble in aqueous fluids. Therefore, the escaping water may carry these elements from the subducting slab (where they may also be incorporated in subducted sediments), into the overlying source region of the magma in the mantle wedge. This process may explain the generally higher concentration of the elements in the calc-alkaline series than tholeiitic series.
3) water affects the major element composition of melts generated in the mantle.
Melting of the mantle peridotite under conditions of high P(H2O) results in topology of the Mg2SiO4-CaMgSi2O6-SiO2 phase diagram that looks like its 1 atm analogue – melting of enstatite is incongruent and the peritectic melt composition has high SiO2/(MgO+FeO) ratio.
All the contrasting characteristics of the calc-alkaline vs. tholeiitic series that are noted above suggest that water plays greater role in petrogenesis of the dominant calc-alkaline series.
4) water promotes explosive volcanism.
The solubility of water in silicate melts decreases with decrease in pressure (see the diagram below). Furthermore, as crystallization proceeds, most water that is dissolved in the melt has to exsolve from it. Because the molar volume of the supercritical vapor is very large, exsolution of water during ascent of melt through the crust and crystallization of the melt in a subvolcanic magma chamber both promote explosive eruptions, as the exsolved water creates over-pressure in the magma chamber under a volcano.
Water solubility in melts of various compositions as a function of pressure at 1100°C (after Burnham, 1979).