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, 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 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 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:
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.
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.
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.
Copyright © 1998-2004 Dr B.M.Gunn