Subduction zone magmatism produces calc-alkaline andesite melts that combine the high SiO2, Na2O, and K2O abundances of the differentiated continental crust with low FeO, FeO/MgO, and TiO2 typical of melts from depleted mantle. Ni-rich olivines in basaltic andesites and andesites of the central Mexican Volcanic Belt suggest that this dichotomy reflects a particular mechanism of mantle processing in the subduction environment. Hydrous slab components rich in Si, Na, and fluid mobile large-ion lithophile elements (LILE) transform mantle olivine to "reaction orthopyroxene.'' Along the ascent paths, and embedded into surrounding peridotite, secondary pyroxenite lithologies are created that are composed of "reaction orthopyroxene'' next to mantle clinopyroxene and orthopyroxene. Partial melts from peridotite and pyroxenite then mix to produce primary calc-alkaline basaltic andesites and andesites that are rich in Na and LILE. The steady slab flux maintains high levels of Na and LILE in the mantle source but also induces repetitive melting that steadily depletes the subarc mantle in FeO, TiO2, and other high field strength elements. If mantle processing thus creates primary basaltic andesite and andesite melts with the fractionated major element signature of the continental crust, the high magnesium number (Mg # (= Mg/ Mg + Fe2+)) similar to 60-70 of these melts still requires additional differentiation to arrive at the lower Mg # similar to 55 of average continental crust.
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