A proposed model for subduction zone magmatism with

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Unformatted text preview: with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford. q q q The parent magma for the calc-alkaline series is a high alumina The basalt, a type of basalt that is largely restricted to the subduction basalt type zone environment, and the origin of which is controversial Some high-Mg (>8wt% MgO) high alumina basalts may be primary, Some as may some andesites, but most surface lavas have compositions too evolved to be primary too Perhaps the more common low-Mg (< 6 wt. % MgO), high-Al Perhaps (>17wt% Al2O3) types are the result of somewhat deeper (>17wt% types fractionation of the primary tholeiitic magma which ponds at a density equilibrium position at the base of the arc crust in more mature arcs q Fractional crystallization thus takes place at a number of levels Figure 16-11b. A proposed model for subduction zone magmatism with particular reference to island arcs. Dehydration of slab crust causes hydration of the mantle (violet), which undergoes partial melting as amphibole (A) and phlogopite (B) dehydrate. From Tatsumi (1989), J. Geophys. Res., 94, 4697-4707 and Tatsumi and Eggins (1995). Subduction Zone Magmatism. Blackwell. Oxford. Figures not used Figures Figure 16-9. Major phenocryst mineralogy of the low-K tholeiitic, medium-K calc-alkaline, and high-K calc-alkaline magma series. B = basalt, BA = basaltic andesite, A = andesite, D = dacite, R = rhyolite. Solid lines indicate a dominant phase, whereas dashes indicate only sporadic development. From Wilson (1989) Igneous Petrogenesis, AllenUnwin/Kluwer. Figures not used Figures Figure 16-11b. MORB-normalized spider diagrams for selected island arc basalts. Using the normalization and ordering scheme of Sun and McDonough (1989) with increasing compatibility to the right. Data from BVTP. OIB data from Sun and McDonough (1989) In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. 313-345....
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