NEW U-PB ZIRCON DATES FROM THE ACATLAN COMPLEX, MEXICO: IMPLICATIONS FOR THE AGES OF TECTONOSTRATIGRAPHIC UNITS AND OROGENIC EVENTS

The Acatlan Complex of southern Mexico is inferred to comprise: (i) low-grade, Lower Paleozoic trench/forearc metasedimentary rocks (Petlalcingo Group) thrust beneath (ii) eclogitic oceanic lithospheric rocks (Piaxtla Group) during the Late Ordovician-Early Silurian, Acatecan Orogeny, unconformably overlain by (iii) Siluro-Devonian, arc-related volcanic and sedimentary rocks (Tecomate Formation), all deformed during (iv) the low-grade, Middle Devonian Mixtecan Orogeny and intruded syntectonically by a Late Devonian granitoid pluton, and unconformably overlain by (v) Late Devonian-Early Permian sediments (San Salvador Patlanoaya and Matzitzi formations). Respective correlation of the Acatecan and Mixtecan orogenies with the Taconian and Acadian orogenies of the Appalachians implied that the Acatlan Complex was a vestige of the Iapetus Ocean. But this interpretation was based upon limited geochronology and poorly preserved fossils. New data indicates a revised sequence of events: (1) intrusion of Ordovician granitoids (~460-480 Ma); (2) polyphase deformation and high-grade metamorphism at 442 ± 1 Ma of rift-related mafic rocks, and eclogite facies metamorphism at 346 ± 3 Ma followed by migmatization at ~350-330 Ma, the latter associated with tectonic interleaving of the low- and high-grade units (the youngest detrital zircons in the uppermost unit of the former are ~375-355 Ma: J. Ruiz, writt. comm.); (3) Late Devonian-Middle Permian arc magmatism and deposition of the Tecomate Formation (which contains granite pebbles with 280-230 Ma ages) and San Salvador Patlanoaya and Matzitzi formations (new fossil data); (4) Early-Middle Permian polyphase deformation accompanied by low-grade metamorphism and continued arc magmatism; and (5) Middle Jurassic (174-170 Ma), plume-related, polyphase deformation, metamorphism and bimodal magmatism. These new data indicate that the main Paleozoic tectonothermal events are Carboniferous and Permian (rather than Ordovician and Devonian) suggesting a correlation with the Variscan-Alleghanian-Ouachita orogen. Such a correlation is more compatible with an origin in the Rheic Ocean (rather than Iapetus), and is consistent with paleogeographic reconstructions that place the Acatlan Complex along the margin of the Precambrian Amazonia-Oaxaquia throughout most of the Paleozoic.