The Slab Component and Island Arc Igneous Rocks

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Explain how the slab component is generated in island arc igneous rocks. Consider both the fore-arc and back-arc basin environments

Island arcs form as oceanic plate subducts under oceanic plate. Volcanism is concentrated in an arc of volcanoes, generally approximately located above the leading edge of the subducting plate. A trench often forms where the slabs meet and subduction begins. On the non subducting slab a series of basins form, with a fore-arc basin nearest the subduction/trench, then the main arc, and a back-arc basin on the far side (Mitchell and Reading, 1971; Frisch, Meschede and Blakey, 2010).

An island arc subduction zone. Modified from Frisch et al. (2010).

Magmas in island arc settings consist primarily of components from two different origins, the slab component, and the mantle wedge. The mantle wedge may melt due the descent of the slab, giving the main portion of the non-slab component. The slab component is derived from the subducting slab as it descends. This may consist of melting of the crustal portion of the slab, but also melting of the mantle wedge due to addition of water driven off the slab. Since the descending slab is composed of old, cold oceanic crust, there is a slight paradox in the idea of melts forming from it. However, frictional heat, as well as the heat of the underlying mantle, can drive hydrous fluids off the slab. (Machado, Chemale Jr., Conceição, Kawashita, Morata, and Van Schmus, 2003; Kimura and Yoshida, 2006).The addition of water to the mantle wedge results in the lowering of melting temperatures, allowing melts to be formed at much lower temperatures than might otherwise be expected. Also, the circulation of hot fluids allows materials dissolved in them to migrate upwards ...

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...eology. 16 (12), p1108-1111.

Paterson, S., and Tobisch, O. (1992). Rates of processes in magmatic arcs: implications for the timing and nature of pluton emplacement and wall rock deformation. Journal of Structural Geology. 14 (3), p291-300.

Schaltegger, U., Zeilinger, G., Frank, M., and Burg, J-P. (2002). Multiple mantle sources during island arc magmatism: U–Pb and Hf isotopic evidence from the Kohistan arc complex, Pakistan. Terra Nova. 14 (6), p461-468.

Smith, I. E. M., Price, R. C., Stewart, R. B., and Worthington, T. J. (2009). An assessment of the mantle and slab components in the magmas of an oceanic arc volcano: Raoul Volcano, Kermadec arc. Available: http://researchcommons.waikato.ac.nz/bitstream/10289/2685/1/Price assessment.pdf. Last accessed 10th Jul 2011.

van der Pluijm, B., and Marshak, S. (2004). Earth Structure. New York: WW Norton & Company.

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