Cordilleran Section - 108th Annual Meeting (29–31 March 2012)

Paper No. 8
Presentation Time: 11:30


GALAZ-ESCANILLA, Gonzalo, Departamento Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, México D.F, 04510, Mexico, KEPPIE, J. Duncan, Departamento de Geología Regional, Instituto de Geología, Universidad Nacional Autónoma de México, México, DF 04510, Mexico, MURPHY, J. Brendan, Department of Earth Sciences, St. Francis Xavier University, Antigonish, NS B2G 2W5, Canada, LEE, James K.W., Department of Geological Sciences and Geological Engineering, Queen's University, Bruce Wing 325, Miller Hall, Kingston, K7L 3N6, Canada and ORTEGA-RIVERA, Amabel, Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, L.D. Colosio y Madrid S/N, Campus UNISON, Hermosillo, 83000, Mexico,

High-pressure (HP) rocks at Tehuitzingo, are composed of metabasites, serpentinite, granite (482 Ma) and mica schist (433 Ma: youngest concordant detrital zircon), which forming klippe that was thrust over low-grade clastic rocks on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México). Low-grade rocks yielded U-Pb youngest cluster of 481 ± 16 Ma and a youngest nearly concordant detrital zircon of 334 ± 3 Ma, therefore, its deposition and thrusting is bracketed between either 481-329 Ma (Ordovician-Mississippian) or 334-329 Ma (Mississippian: Visean), and was followed by F3 deformation producing a synform, and later a HP klippe after erosion.

The granite and mica schist are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. The associated metabasites represent a juvenile cumulate magma (εNdt=450 Ma= 5.3-8.9) generated in an island arc environment of unknown age (pre-Mississippian), and the serpentinite body is an ophiolotic harzbugite generated in a suprasubduction environment. This set of HP rocks was subducted, undergoing HP metamorphism and polyphase deformation at depths of ~50 km (~16 kbar and 750 °C: eclogite facies), and subsequent retrogression passed through amphibolite-epidote to greenschist facies, which was synchronous with W-vergent Mississippian thrusting over the low-grade clastic rocks. Cooling through ca. 385ºC is indicated by ~329 Ma (metabasite), ~316 Ma (granitoid), and ~317 Ma (mica schist) 40Ar-39Ar muscovite plateau ages, which are 5-17 my younger that those of the adjacent Piaxtla eclogites suggesting younger thrusting. These data are consistent with a subduction-extrusion process on the active western margin of Pangæa during initial stages of its amalgamation, instead of collision as was previously suggested. Igneous detrital zircon populations in the low-grade psammite and the HP schist indicate derivation from the Ordovician Acatlán granitoids (490-440), Neoproterozoic Brasiliano orogens (750-500), 900-750 Ma Goiás arc (Amazonia), 1.3-1.0 Ma Oaxaquia, and more ancient sources suggest derivation from Oaxaquia/Amazonia.

  • GSA 2012-Galaz presentation.ppt (13.0 MB)