Orogenic Andesites


The Orogenic Andesite Series ( or Calc-Alkaline Series) includes lavas with composition ranging from basalt through basaltic andesite to dacite and rhyolite. Virtually all andesites are associated with a subduction zone where oceanic crust underthrusts a continent or has done so in the past, the andesites generally becoming more potassic as well as richer in Cs, Rb, Ba, U & Th inland from the subduction zone in regions of thicker crust. A marked negative Nb, Ta anomaly is always present along with rather low Ti and Ni and high K, U, Th, Ba, Rb, Cs and Pb, these being features not seen in rocks derived from sub-oceanic mantle. Sr and Ba are usually high (1000 - 2000 ppm) and erratic.The underthrusting takes place along the line of an associated off shore oceanic trench, usually of great depth, up to 32,000ft (10,000m) in the Marianas and Tonga-Kermadec trenches..

The subduction plane is assumed to coincide with the Benioff zone, a plane of high earthquake intensity, which dips inward at an angle of up to 40 degrees under Peru or as low as 20 under southern Chile. The volcanoes tend to lie 50-100 miles on the continental side of the trench. There are many pages on the NET explaining plate tectonics and subduction at continental margins, so we will not discuss it in detail here. Just type "Plate Tectonics" or "Subduction" in the "Google" search engine. The USGS maintains a good page with diagrams.

However, some orogenic lavas (mainly dacites-rhyolites) form where two continental masses are compressing each other, as in Tibet, Turkey etc. These tend to be very potassic and may have been formed by partial re-melting of the lower crust. The most potassic of all are those of the Roman Province in Italy (with up to 8% K2O) where no active subduction activity seems to be taking place. Harvey Belkin (USGS) and I have been looking at Vesuvius, (2002) and though the region is claimed to be anorogenic, the fingerprint is classically Orogenic Andesite. Partial melting of deep-seated metamorphosed sediments may be seen in the deep valleys of the Himalayas, and diapirs are seen as well as rhyolitic dikes extending up towards the surface. Partial melting of deep-seated crust may be taken as established fact.

Andesite series tend to have a highly variable composition, so much so it is difficult to define precisely what we mean by an andesite. The plagioclase may not always be andesine, silica is generally in the range 58-63% but very potassic rock of andesite type may have less. Large ion elements such as Rb, Ba may vary from 0.5 times the average

Ocean Ridge Basalt, (in juvenile island arcs) up to as much as 100 times in continental sectors such as the North or Central Andean Zones while Cs can vary even more.

Ternary Na-Mg-K2O diagram showing the range in Na/K ratio in the Andesite suite:
* = The Quill (St Eustatius, Lesser Antilles)
o = Sangay (Ecuadorean Andes)
x = Vulcano (Liparian Arc, Sicily)
+ = Mt Vesuvius (Roman Province)
    The range of K(60), ie the average weight percent K2O at 60% silica is:
    0.6 for The Quill (Gunn, Roobol, & Smith unpub.)
    1.05 for Deception Is (Gunn, Schultz unpub.)
    1.8-3.0 for N.Chile (Roobol et al 1974)
    2.4 for Sanguay, Ecuador (Monzier et al, 1999)
    5.0 for Vulcano, Liparian Arc, Sicily (Esperanza, 1992)
    8.0 for Vesuvius (approx. Gunn & Belkin, unpub.)

The K(60) for Vesuvius cannot be measured directly as the K trend does not intersect the silica at 60%, but for andesites is approximately 8%.

Island arcs are curved chains of andesite-type volcanoes usually occurring a few tens to a few hundred miles to seaward of a continental margin. The Lesser Antilles or West Indian Island arc is typical being about 360 miles long and including five active cones on a dozen major and many minor volcanic islands, but it is unusual, in that it does not lie parallel to the immediate continental coast though parallel to the general trend of N and S America.. The East Indian arc is about 2,500 miles long with >100 active cones but is more mature with overlapping centres linked together to form Sumatra, Java, Bali, Lombok, Sumbawa, Flores and other islands. The Tongan Arc south-east of Fiji is formed of a chain of volcanoes, many not yet emergent from the sea and none more than a few miles across.

Primitive Island arcs are very depleted in incompatible elements, resembling N-type MORBS except for the characteristic low Ti, Nb-Ta and relatively elevated K, U, Th, Ba, Rb, Cs, the stress being on the word Relatively. Many have lower concentrations of LILE than are seen in enriched MORB and only 2-3 times mantle levels. Other primitive arcs include the South Sandwich group, the northern part of the Lesser Antilles, the Bonin-Marianas Arc, the Solomons-New Hebrides Arc, the Tonga-Kermadec Arc etc. They are always very sodic, especially Deception Island in the South Shetlands, south-west of Cape Horn where soda dacites may have 7% Na2O. Rocks with 1% or less of K2O at 60% silica are the norm.

Even more depleted rocks, the Boninites which are high Mg andesites, may occur in some arcs but these are so depleted in the lighter elements as to be close to the detection limit for most analytical methods so that that good data is very hard to find. Basaltic extrusions may also be found behind the arc (the so-called "Back Arc spreading centres", or to seaward of it, but these do not seem to be distinguishable chemically from other primitive arc rocks. Crustal warping consequent on crustal compression may produce a ridge or chain of islands to seaward of the main arc. These may consist of albitised old MORB rocks overlain by reef limestone, as seen in Anguilla, Antigua. East Guadaloupe-Desirade etc in the Lesser Antilles but keratophyres and albitised andesites may be present suggesting early andesitic volcanism also affected the outer arc.

Mixture of primitive and more mature arc series from Sth Sandwich arc.
Old version of above diagram.
This diagram dates from GEOKEM in 1998, the best we could do in those days!

Continental Marginal Andesites are rocks with usually about 2-3% K2O at 60% silica with Rb, Ba levels at 5-10 times E-type MORB. These are the most common and found in much of the Andes, Central America, NW USA, Alaska, Kamchatka, Japan, Philippines, New Zealand but similar rocks are seen in the more massive mature arcs such as the East Indies, Solomons-Vanuatu, Aleutians, Kuriles etc. Usually on the continental sides of the andesite chain there occur the more potassic rocks with 3-4% K2O at 60% SiO2. If two parallel chains of volcanoes are found lying parallel to the coast, the chain on the continental side will be the most potassic and most enriched in the light REE, Rb, Ba, U, Th etc.
However some continental areas of thin crust such as Central America and Southern Chile produce andesites which are more depleted than some from the more mature oceanic arcs.

A histogram for all Andesite series world wide., a special compilation by GEOROC, to whom we tender thanks. Note the tendency towards trimodality with peaks at 52-4% (Proto and Early Arc IAB,'s), at 60-62% (continental andesites) and at 74-6%, (rhyolites-ignimbrites).
All Orogenic Andesites and associated rocks variation diagram. This basically reflects the composition of the continental crust. Note that human beings cannot live on soils even peripheral to the concentrations shown, we have adapted to a crust of this composition only.
Exaggerated TiO2, P2O5 vs SiO2 for 12,000 andesites series. No crystal fraction effects are seen, instead both decline steadily all the way from about 43% Silica. Alkaline rocks have excluded, but minettes and shoshonites remain. Once again the maximum seen is about the same as the maximum in ORBs.
Metals vs MgO. Unlike ORBs etc, Zn declines towards higher silica and K (on left). Some high Cr but there is an obvious cut-off point for Ni, Co.
Some mineralisation of Cu. V alone shows much the same dispersion as seen in ORBs and quite unlike Fe, Ti in this case. Why?
Y and Nb both decline with increasing SiO2, again suggesting no fractionation. Zr, quite unlike ORBs, OIBs, CFBs, remains roughly constant.
REE for all (>5000) andesites. Alkaline rocks excluded but some high K andesites and minettes are present. Average La/Sm = ~ 6. Average Ce/La ~ 2. The occasional high Nd>Ce is not likely to be real.
The wide dispersion of Na2O, K2O for ~ 12,000 andesites.
This data from Rick Conrey's (of W.S.U.) massive study of Mt Jefferson in the Cascades, Or. is typical of a mature continental calc-alkaline basalt to rhyolite series. Notice the major differences when compared to the juvenile arc series below.
FMA diagram of the Izu-Bonin Arc of the Western Pacific. The Marianas Arc is closely similar. This diagram shows resemblences to Iceland in the build up of iron in intermedate rocks! Most of the rocks are IAB's (Island Arc basalts) Though they include Proto-arc, Fore-arc and Back Arc, all for which we have data show the classic andesitic Nb-Ta anomaly. Though the ferrobasalts are very high in iron, (on Myake Jima Island, to 26 -28%, the TiO2 while variable, remains very low and less than 1.5%. V is unusually high, to 600ppm. While Na/K ratios are highly variable, they cover the same range as seen in continental andesites, though not as plentiful. Some sectors of primitive arcs such as the Tafahi-Ata sector of Tonga may be almost entirely depleted basalt, yet still has the low Nb-Ta signature. (see section on Tonga-Kermadec)
All Orogenic Andesites - FMA diagram. In general the trends in orogenic series are iron-enriched in the juvenile arcs, (sometimes called a "tholeiitic" trend) becoming richer in alumina and poorer in Fe-Ca-Mg in the mature sectors of the older continents. Much work remains to be done in defining the effects on minor and trace elements (June 2003)
All Orogenic Andesites - Na-Ca-K diagram, (Error on plot will be fixed, we swear!). The spread of Na-K is wide. Note this diagram does not include the Roman province which are even more potassic. Nevertheless a wide spread in NA-K can been seen in quite juvenile arcs such as the IBM, but not in Tonga-Kermadec or Deception Id. in the Scotia Arc.
The ultra-high K rocks near the base are leucitic andesites from Tambora, Muriah and Batu Tara in the Sunda Arc, and from Stromboli, Vulcano in the Aeolian Arc.

How can such variable rocks have the same origin and belong to the same series? Continental andesites give every indication of having been partially melted from Oceanic crustal basalts, yet IABs may be more depleted than the Oceanic crust. Therefore they must be derived from the mantle where possibly melting tempartures have been lowered by the introduction of water by the down going crustal slab..

Let us adopt an uproven hypothesis and assume that in a primitive arc the downgoing crustal slab is cold and has not yet begun to melt. It does however releases large amounts of entrapped and combined water and volatiles into the overlying mantle which lowers it's melting point and IABs are formed. The general small but distinct increase seen in Cs, Rb, Ba, Th, U, K suggests that these LILE are also carried up in small amount into the overlying mantle.
With time the down going slab is heated to the point where partial melting occurs and variable mixtures of mantle enriched by the melt fraction from the crustal slab below. Finally, and we have no time scale to put on this, lavas are produced which are almost entirely produced by partial melting of the oceanic crustal slab, and are predominantly andesite. Crystal fractionation seems to be minor in andesites, there are no kinks as different phases form, andesite series form straight line variation diagrams, only MgO may show a slight curve. Are some rhyolites formed by fractionation as in Iceland? It is not likely, the majority seem to be partial remelts of the crust. However some rare sodic rhyodacites found in primitive arcs such as the slab of airfall tuff on Macauley (TK) or the similar tuffs of The Quill on Statia in the Lesser Antilles, which could have a fractionation origin and are all highly sodic..
Much more work remains to be done on IABs especially, as they appear to be yet another form of basalt distinct from NMORB-EMORB; CFB; OIB and IPB (Intra-plate basalts) We will them divide into sectors and get averages and average NORMs etc and try to define their limits when time permits (This is July, 2003). Are those rocks with more than 20% Alks on the FMA diagram, also enriched in other LILE and if so to what degree? At this point we do not know.

Copyright © 1998-2004 Dr B.M.Gunn