HR: 1400h
AN: V53B-03    [Abstracts]
TI: Rhyolite genesis by slab melting and mantle interaction: an example from the Trans-Mexican Volcanic Belt
AU: * Rincon-Herrera, N
EM: nelly@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: Mori, L
EM: lmori@geociencias.unam.mx
AF: Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, 76230, Mexico
AU: Ortega-Gutierrez, F
EM: fortega@servidor.unam.mx
AF: Instituto de Geologia, UNAM, Cd. Universitaria, Mexico City, 04510, Mexico
AB: A remarkable set of rhyolitic rocks emplaced in the Trans-Mexican Volcanic Belt indicate that slab-derived siliceous melts can ascend through the mantle and crust while suffering only minor compositional modifications. These rocks were discovered in the isolated rhyolitic domes of Chalcatzingo, located at 40 km SW from Popocatepetl Volcano, in Central Mexico. New 40Ar/39Ar data on plagioclase and amphibole yielded an age of 20.7 Ma, thus revealing some of the oldest tracks of magmatic activity in the Mexican arc. The rhyolitic domes are covered by small andesitic lava flows of unknown age, that are chemically similar to the Quaternary Chichinautzin volcanic field. The rhyolitic rocks are petrographically homogeneous with a porphyritic texture made of plagioclase and amphibole set in a groundmass of plagioclase and quartz. Interestingly, the rhyolitic rocks host a large variety of crustal xenoliths: amphibolites, granulites, gabbros and gneisses. Younger andesites are also porphyritic with phenocrysts of plagioclase, two pyroxenes and minor olivine set in a microcrystalline groundmass of the same mineralogy, but they did not carry crustal xenoliths. The Chalcatzingo rhyolites (SiO2=71%) have relatively high MgO (1.2%), Mg# (60), CaO (3%), Na2O (6%) and Ni (24 ppm), but unusually low K2O (0.7%) contents. Incompatible trace element patterns are typical of arc magmas, but concentrations in the rhyolites are lower than in the andesites, and much lower than those of more typical Mexican rhyolites. In addition, Sr, Nd and Pb isotopes of the rhyolites are the most depleted compositions ever measured in the Mexican arc, completely overlapping with those of the Pacific MORB, with values that are different from the crustal xenoliths. These characteristics are inconsistent with a crystal fractionation process of a more primitive magma, and cannot be explained by direct anatexis of the continental crust. Instead, the overall geochemical features are what would be expected in melts from subducted oceanic basalt that acquired its high Mg and Ni contents through interactions with mantle peridotites, in a process that did not modify the incompatible trace element budget.
DE: 1020 Composition of the continental crust
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly