ANDESITIC VOLCANISM IN WESTERN MEXICO BY MELTING OF AMPHIBOLITIC MANTLE WEDGE PLUMES

The western Mexican subduction zone is characterized by the steep subduction of the young Rivera plate, and by the presence of a continental rift at ~230 km from the trench under which the slab is either extremely deep or even absent (>300 km). Magmatism is distributed in a dual volcanic chain with the unusual character to become less potassic and more volumetric towards the rear. With the exception of Colima volcano, volcanism at the frontal chain is mostly monogenetic and mafic, with abundant potassic lamprophyres that display strong subduction (Rb/Ta= 40-150), rutile (Nb/Ta= 17-27) and garnet (Gd/Yb= 2-8) signatures. In contrast, mafic rocks at the rear chain are intraplate-like basalts that appear to derive from low extents of melting of a dryer (Rb/Ta= 4-20) and shallower (Gd/Yb= 2-2.5) mantle source. Although a transition from a volcanic arc-front to an extensional back-arc is apparent if only the more mafic rocks are considered, at least five andesitic stratovolcanoes have been emplaced at the rear volcanic chain over the last million years (San Juan, Sangangüey, Tepetiltic, Ceboruco and Tequila). These volcanoes erupt monotonous two-pyroxene, high-Mg# (~60) calc-alkaline andesites with occasional hornblende, have strong subduction signatures (Rb/Ta= 25-105), but extend to low Gd/Yb (1.2) and Nb/Ta (~12) ratios that suggest residual amphibole. Calc-alkaline basalts are surprisingly rare and unlikely to be parental to andesites unless enormous quantities (>70%) of fractionating amphibole are considered. Potassium-rich rocks at the volcanic front appear to be influenced by deep and hot slab-derived melts or supercritical fluids, whereas calc-alkaline andesites erupting from stratovolcanoes at the rear chain appear to be sampling a much shallower source in which amphibole plays an important role. Direct melting of a crustal amphibolites to form andesites is unlikely due their high-Mg#, but delivering shallow slab melts to the rear chain is problematic due to the steep subduction angle. This either indicates that inherited amphibolitic heterogeneities may be present in the local upper mantle, or that mixtures of hydrous and partially molten basalts and sediments can rise buoyantly away from the down-dragging slab as discrete "wet plumes" that can be trapped into the upwelling regime of the continental rift.