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Contributions to the geology and petrology of the Trans-Mexican Volcanic belt Nixon, Graham Tom


The composition and spatial distribution of Quaternary volcanism in the Trans-Mexican Volcanic Belt (TMVB) exhibit some remarkable correlations with the seismicity, age, and structure of ocean lithosphere being consumed at the Middle America Trench. In the west, the TMVB is related to aseismic subduction of the Rivera plate (2 cm/yr) and in the east to a moderately dipping (20-30°) rapidly subducting Cocos plate (6-9 cm/yr). These contrasting arc segments are bounded by the Colima Graben, a zone of high-angle faulting and contemporaneous alkaline/calc-alkaline volcanism, situated above a sinistral transform fault (4 cm/yr) developed in the downgoing slab at the Cocos/Rivera juncture. Geologic mapping and K-Ar dating of the lavas of Iztaccíhuatl, a major calc-alkaline volcano in the TMVB, have established two main phases of eruptive activity that began prior to 0.9 Ma. The substructure of Iztaccíhuatl (>0.6 Ma) is composed principally of two-pyroxene andesites and dacites (300 km³) erupted from Llano Grande and Ancestral Pies volcanoes. The second stage of cone construction (<0.6 Ma) involved horn- blende dacites and andesites (150 km³) extruded from NNW-SSE oriented vents to form the modern summit region. The earliest glacial deposits date prior to 0.27 Ma but moraine complexes on the flanks are Wisconsin to Neoglacial. The phenocryst mineralogy and chemistry of the Younger Andesites and Dacites indicate that magma mixing has played an important role in the evolution of Iztaccíhuatl magma chambers. Mixed lavas characterized by disequilibrium phenocryst assemblages involving magnesian olivine and in the crust and olivine-phyric basaltic magma ascending from depth. Mixing and homogenization are effected by liquid blending and dynamic fractional crystallization in turbulently convecting hybrid liquids. The whole-rock geochemistry of mixed lavas and hornblende dacites is used to derive the composition of each batch of basaltic magma periodically replenishing crustal magma chambers. Basaltic end-members exhibit significant variations in large-ion lithophile elements and Sr isotopic composition which are attributed to heterogeneities in mantle source regions. The primitive compositions of these magmas are compatible with an origin involving partial melting of fertile peridotite under hydrous high-pressure conditions.

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