However the position of the batholith and age of the oceanic crust at the trench during the Miocene preclude slab melting. Instead partial melting of newly underplated Miocene crust is proposed. In this dynamic model newly underplated basaltic material is melted to produce high-Na, low HREE, high Al- "trondhjemitic" type melts with residues of garnet, clinopyroxene and amphibole. Such Na-rich magmas are characteristic of thick Andean crust; they are significantly different from typical calc-alkaline, tonalite-granodiorite magmas and their presence along the spine of the Andes provokes questions about models of trondhjemite genesis by melting of subducted oceanic crust, as well as any generalised, circum-Pacific model involving consistent isotopic/chemical changes inboard from the trench.
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