Subduction-RelatedMagmatic Arcs, with an Emphasison Primitive Andesiteand Lower Crust

3.18.3 CHARACTERISTICS OF ARC MAGMAS 7 3.18.3.1 Comparison with MORBs 7 3.18.3.1.1 Major elements 7 3.18.3.1.2 We are cautious about fractionation correction of major elements 9 3.18.3.1.3 Distinctive, primitive andesites 11 3.18.3.1.4 Major elements in calc-alkaline batholiths 11 3.18.3.2 Major and Trace-Element Characteristics of Primitive Arc Magmas 18 3.18.3.2.1 Primitive basalts predominate 18 3.18.3.2.2 Are some low Mgx basalts primary melts? Perhaps not 22 3.18.3.2.3 Boninites, briefly 22 3.18.3.2.4 Primitive andesites: a select group 23 3.18.3.2.5 Three recipes for primitive andesite 23 3.18.3.3 Trace Elements, Isotopes, and Source Components in Primitive Magmas 29 3.18.3.3.1 Incompatible trace-element enrichment 29 3.18.3.3.2 Tantalum and niobium depletion 36 3.18.3.3.3 U-series isotopes 37 3.18.3.3.4 Geodynamic considerations 40