HR: 1400h
AN: V53B-02    [Abstracts]
TI: Variable Slab Contributions Characterize the Miocene Magmatic Record of the Central Trans- Mexican Volcanic Belt
AU: * Mori, L
EM: lmori@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, 76230, Mexico
AU: Gomez-Tuena, A
EM: tuena@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, 76230, Mexico
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964, United States
AB: Two distinct magmatic events, related to different petrogenetic processes, characterize the early activity of the central Trans-Mexican Volcanic Belt. These episodes are represented by the stratovolcanoes of the Palo Huerfano-La Joya-Zamorano Volcanic Complex (PH-LJ-Z; 12-10 Ma), located at 500 km from the Mesoamerican Trench, and by the Queretaro Volcanic Succession (QVS; 9-6 Ma), a mafic plateau that stratigraphically overlies the stratovolcanoes. PH-LJ-Z products are calc-alkaline andesites and dacites, while QVS rocks range from basalts to basaltic andesites. Despite their different major element contents, the two rock groups exhibit the same Mg# (40-70). Both sequences display arc-like trace element patterns; however, the stratovolcanoes present a stronger subduction signature than the QVS, and they also show compositional analogies with slab melts, such as high Sr/Y ratios, HREE depletions, and a tendency to MORB-like isotopic compositions. The two suites cannot be genetically related by simple fractional crystallization; moreover, the geochemical features of the PH-LJ-Z sequence are not compatible with lower crustal melting. Therefore, even though both rock groups have experienced contamination at different crustal levels during ascent, we attribute their primary compositional differences to distinct mechanisms of element recycling in the subduction zone: the geochemical features of PH- LJ-Z high-Mg# andesites and dacites are consistent with the interaction of silicate slab melts with mantle peridotites, while the chemical features of QVS products suggest partial melting of a fluid-fluxed mantle wedge. The modification of the subduction component from slab melts to slab fluids might be related to a variation in the thermal structure of the subduction zone during a period of prolonged sub-horizontal subduction. Indeed, the slab-melt features of PH-LJ-Z rocks, and their emplacement at a large distance from the trench, are consistent with a flat slab scenario that promotes slab melting at relatively low pressures. A protracted flat subduction would produce a gradually cooler thermal state that will hinder slab melting and instead favor slab dehydration.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly