Tuesday, 27 April 2010

Orogenic volcanic and plutonic rocks in island arcs

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So, here it goes the first Small Geological Fact: Volcanic and plutonic activity in a island arc.

What is the point of this article? I am working right now on a project in western Java. I find the geology in there very complex and obscure, and I realise that understanding more about the type of volcanic and plutonic materials, would help me to understand more the evolution of the volcanic arc. This article won't deal with Java itself, but with a generic volcanic arc.

The main source of information is a nice, affordable and simple (but very useful!) book by Kearey and Vine: Global Tectonics. So, if you want to know more, just read it (full reference at the bottom)

An island arc is created by plutonic and volcanic activity at ca. 150-200 km from the trench axis when the subducting lithosphere reaches a depth of 80 km. The majority of aisland arcs are found in the Pacific Ocean, and some in the Atlantic.

Young arcs are relatively simple to understand, and they are underlain by a crust no thicker than 20 km (i.e. Tonga-Kermadec, New Hebrides, Aleutians and Lesser Antilles). Older and mature island arcs are far more complex, usually build upon previous generations of subducting plate margins. Generally, they are underlain by thicker crust, 20 to 35 km, and they occur in Japan and Indonesia.

Baker(1982) identified three series of volcanic rocks in island arcs:

  1. The low potassium tholeiitic series, dominated by basaltic lavasand lesser volumes or Fe-rich basaltic andesites and andesites.
  2. The calc-alkaline series, dominated by andesites, more enriched in potassium other incompatible elements and rare earth elements than the andesites mentioned above(in an Andean-type belt, dacites and rhyolites are more abundant).
  3. The alkaline series, including subgroups of alkaline basalts and shoshonitic lavas.

Now, how is that related with the evolution, structure and zonation of an island arc? How is that relevant for this blog?

The tholeiitic series are found in young arcs, as the are due to magmas formed at 80-120 km depth. The calc-alkali and alkaline series are formed in more mature arcs, because their originating magmas are formed at a deeper depth. In some mature arcs there seem to be a compsitional trend, from the trench, where a tholeiite - calc-alkaline - alkaline volcanis suite developes, representing magmas from progressive deeper levels.

In mature island arcs, plutonic rocks are exposed, which represent the residua of magma chambers which have crystallised at depth. They are generally granodiorites and related igneous rocks and exhibit similar variations to the volcanic rocks.



Sources:

- Philip Keary and Frederick Vine, Global Tectonics, Blackwell Science, 1996.pp 161-163
- Baker, P.E. (1982) Evolution and classification of orogenic volcanic rocks. in Thorpe, R.S. (ed.) Andesites, pp. 11-23, Wiley, New York.





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SGF: New tag for the blog

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In my daily activity as geologist I find many small facts that I have learnt in the past (i.e. during my degree or even before, in secondary school) but, after many years not using them, they simply vanish in the caves of memory.

So, why not a type of entries in the blog, "Small Geological Facts", about things I know are useful in my job at the moment or will be in the future?

Doing a blog requires time. I have many ideas, but not time enough. The result is that no matter how enthusiastic you are, if writing the entries you like takes longer that you expected, finally you will have big time gaps where no articles are created.

Small Geological Facts
, that is the answer!

So, soon (very soon!) the first one... Thanks for your understanding!