[1] Subduction zone magmatism produces calc-alkaline andesite melts that combine the high SiO 2 , Na 2 O, and K 2 O abundances of the differentiated continental crust with low FeO, FeO/MgO, and TiO 2 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, TiO 2 , 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 + Fe 2+ )) $60-70 of these melts still requires additional differentiation to arrive at the lower Mg # $55 of average continental crust.

[1] The magmatic record of the easternmost part of the Trans-Mexican Volcanic Belt elucidates how temporal changes in subduction parameters influence convergent margin volcanism. In the Palma Sola massif, three phases of magmatic rocks with distinct chemical characteristics were emplaced in a relatively short time span (17 Ma): Miocene calc-alkaline plutons, latest Miocene-Pleistocene alkaline plateau basalts, and Quaternary calc-alkaline cinder cones. Plutons have arc-like trace element patterns (Ba/Nb = 16-101), and their Sr, Nd, and Pb isotopic compositions become more ''depleted'' with increasing SiO 2 contents. Their Pb isotopes are bracketed by the subducted sediments and Pacific mid-ocean ridge basalts (MORB), requiring the participation of an unradiogenic component that mixes with a sediment contribution. High Sr/Y and Gd/Yb ratios in the least radiogenic pluton might indicate a melt coming from the subducted MORB. Trace element patterns of the plateau basalts show moderate or negligible subduction contributions (Ba/Nb = 6-31). Rocks without subduction signatures are similar to ocean island basalts, indicating melting of an enriched mantle wedge. The plateau basalts form an array in 206 Pb/ by an essentially flat subduction angle that favored melting of the subducted oceanic crust. Slab rollback in the Pliocene allowed melting of deeper portions of the wedge by the injection of dehydrated sediment melts. In the Quaternary, an even steeper subduction angle provided negligible slab contributions to the Palma Sola region, and upper crustal contamination largely controls the petrogenesis.

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