HR: 0800h
AN: T41A-12    [Abstracts]
TI: Conflicting Stratigraphic and Geochronologic Data From the Acatlán Complex: "Ordovician" Granites Intrude Metamorphic and Sedimentary Rocks of Devonian-Permian age
AU: Elías-Herre, M
EM: elias@servidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F 04510, Mexico
AU: Macías-Romo, C
EM: mcmr@servidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F 04510, Mexico
AU: * Ortega-Gutiérrez, F
EM: fortega@servidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F 04510, Mexico
AU: Sánchez-Zavala, J L
EM: jlszqservidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México, D.F 04510, Mexico
AU: Iriondo, A
EM: iriondo@geociencias.unam.mx
AF: Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Querétaro, Qto 76230, Mexico
AU: Ortega-Rivera, A
EM: amabel@servidor.unam.mx
AF: Instituto de Geología, Universidad Nacional Autónoma de México, Estación Regional del Noroeste, Hermosillo, Son 83000, Mexico
AB: Stratigraphic relationships in the Patlanoaya area of the Acatlán Complex, southern México, reveal conflicting data between absolute dating of minerals and field observations of the host rocks. The issue has central implications for the origin and evolution of the Acatlán Complex in the margins of the Iapetus (traditional view) or Rheic (innovative view) oceans. Three undeformed granites (Palo Liso, Los Hornos and Cuajilote) that show intrusive relationships with adjacent metamorphic rocks and with late Paleozoic fossiliferous sedimentary units, all yield apparent SHRIMP U-Pb Middle Ordovician zircon ages. Palo Liso K-feldspar-biotite megacrystic granite is a pluton intruding basement units that include Piaxtla high pressure and El Rodeo low pressure metamorphic suites, as well as sedimentary rocks of Late Devonian (?) Otate and latest Devonian-Permian Patlanoaya formations, and yet this granite yielded an age of 467.8 ± 4.2 Ma. Moreover, the Piaxtla Suite, which is nonconformably covered by the Devonian-late Paleozoic formations, rendered an U-Pb zircon age of 353 ± 1 Ma for a retrogressed eclogite, and ⁴⁰Ar/³⁹Ar ages of 341.57 ± 3.68 Ma (glaucophane) and 341.50 ± 2.82 Ma (phengite) for interlayered blueschists. Los Hornos granite, exposed 20 km SE from the former, and intruding greenstones of El Rodeo Suite of inferred Ordovician age, also yields similar zircon ages at 464.9 ± 2.3 Ma. Finally, the Cuajilote granite, although showing sheared contacts with the Pennsylvanian-Permian Tecomate Formation, it bears xenoliths from this unit indicating a younger stratigraphic age; nevertheless, its apparent zircon age is also Middle Ordovician at 463.5 ± 3.7 Ma. Possible explanations for this paradox include: (a) misinterpretation of field data (contacts and unit correlations), or (b) possible erroneous assumptions regarding equivalence of absolute mineral dates with rock-forming events. If the geochronologic data indeed correspond to crystallization ages of the granites, the deformation and regional metamorphism of El Rodeo Suite and the deposition and deformation of Otate Formation would predate the Middle Ordovician plutons. Consequently, a pre-Middle Ordovician orogenic history in the Acatlán Complex is evidenced, and the interpreted sedimentary contact between the high pressure Piaxtla Suite and the Otate Formation would be a major subhorizontal normal fault related to extensional exhumation of the Early Mississippian high-pressure rocks. If our stratigraphic observations hold instead, zircon inheritance and the petrologic (crystallization and dissolution) and U-Pb systematics behavior of this phase in contaminated granitoids should be understood better before conflicting isotopic ages are accepted.
DE: 1100 GEOCHRONOLOGY
DE: 8100 TECTONOPHYSICS
DE: 8104 Continental margins: convergent
DE: 8149 Planetary tectonics (5475)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly