Felsic volcanism in the Kermadec arc, SW Pacific: crustal recycling in an oceanic setting
Ian E. M. Smith, Timothy J. Worthington, Robert B. Stewart, Richard C. Price, John A. Gamble, 2003. "Felsic volcanism in the Kermadec arc, SW Pacific: crustal recycling in an oceanic setting", Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes, R. D. Larter, P. T. Leat
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Large-scale felsic volcanic systems are a common, but not ubiquitous, feature of volcanic arc systems in continental settings. However, in oceanic volcanic arcs the erupted materials are dominated by basalts and basaltic andesites, whereas intermediate compositions are rare and dacites and rhyolites relatively uncommon. The Kermadec arc is an intraoceanic convergent system in the SW Pacific. Volcanoes occur as a continuous arc that is mainly submarine. Despite its simple tectonic setting, felsic magmatism is widespread. In the Kermadec Islands, Macauley Volcano is a basaltic volcano that produced a large felsic eruption about 6000 years ago. A comparable pattern of magmatic evolution is seen on adjacent Raoul Volcano, where basaltic activity built the main edifice of the volcano and where activity during the last 3000 years has been characterized by felsic eruptions of varying size. Elsewhere in the Kermadec arc and in its northward extension, the Tonga arc, felsic eruptions are recorded from 11 of the 30 volcanoes for which petrographic information is available, and in many cases these are the most recent eruptions. Felsic europtions are a widespread recent feature of the arc, and the scale and extent of this magmatism appears to be unusual for a tectonically simple oceanic subduction system. One explanation of the origin of the felsic magmatism is prolonged fractional crystallization from a parental basalt composition, but modelling of the chemical compositions of the felsic rocks does not support this. A second explanation, albeit apparently at odds with the oceanic setting, is crustal anatexis. An important feature of the felsic eruptives from the Kermadec arc is that each tephra sequence or occurrence has a unique chemical composition, although all show the same generalized characteristics. We suggest that this feature supports a model of crustal anatexis rather than fractionation of a range of parental magmas. We also suggest that in the thermal evolution of an oceanic arc system the processes of underplating, together with the continuous magmatic (and thermal) flux, can generate a crustal thickness in which dehydration melting of underplated arc material generates felsic magmas. Further, this condition can represent a unique ‘adolescent’ stage in a developing oceanic arc, as once the felsic melts are extracted the lower crust becomes an infertile residue.
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Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes
Recycling of oceanic plate back into the Earth’s interior at subduction zones is one of the key processes in Earth evolution. Volcanic arcs, which form above subduction zones, are the most visible manifestations of plate tectonics, the convection mechanism by which the Earth loses excess heat They are probably also the main location where new continental crust is formed, the so-called ‘subduction factory’. About 40% modern subduction zones on Earth are intra-oceanic. These subduction systems are generally simpler than those at continental margins as they commonly have a shorter history of subduction and their magmas are not contaminated by ancient sialic crust. They are therefore the optimum locations for studies of mantle processes and magmatic addition to the crust in subduction zones.
This volume contains a collection of papers that exploit the relative simplicity of intra-oceanic subduction systems to provide insights into the tectonic, magmatic and hydrothermal processes associated with subduction.