Magmatism in the South Sandwich arc
P. T. Leat, J. L. Smellie, I. L. Millar, R. D. Larter, 2003. "Magmatism in the South Sandwich arc", Intra-Oceanic Subduction Systems: Tectonic and Magmatic Processes, R. D. Larter, P. T. Leat
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The South Sandwich Islands are one of the world’s classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas–Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.
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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.