CORRELATION OF PALEOZOIC AND PROTEROZOIC TERRANES OF SOUTHERN MEXICO WITH THE NORTHERN ANDES BASED ON U-PB GEOCHRONOLOGY OF DETRITAL ZIRCONS AND ND ISOTOPIC DATA

Much has been written about the relationship of the Proterozoic Oaxaca complex and Paleozoic Acatlan Comples of southern Mexico with the northern Andes. Here we show new U-Pb geochronology of detrital zircons show that show that the provenance and the depositional history of the Acatlan complex and the age of source rocks and their orogenic affinities is from either Gondwana, Laurentia or Oaxaquia, even though the sedimentary packages are presently juxtaposed. These new data clearly show that our understanding of the history of the Acatlan Complex is poor, at best.

The U-Pb zircon ages from the Xayacatlan Formation range from 447 to 3115 Ma. Most zircons are magmatic. The cumulative age pattern shows the most important zircon clusters at 447-550 (peak at 477 Ma) and 590-795 (peaks at 603 and 708 Ma). Smaller but distinctive populations occur at 800-1400 (peaks at 946 and 1128 Ma) and 1651-1964 (peak at 1821 Ma). Zircons from the Cosoltepec Formation range from 374 to 3451 Ma. The age-probability curve shows important populations at 374-665 (peaks at 394 and 543 Ma) and 718-1460 (peak at 936 Ma). A smaller cluster occurs at 1780-1997 (peak at 1960 Ma) and a few zircons between 2780-3470 Ma. Zircons from the two Chazumba samples (which likely correlates with the Silgara Fm of Colombia) yield nearly identical ages from 249 to 1772 Ma with a single older grain (2637). The zircons from Chazumba show low U/Th ratios typical of magmatic zircons.

The Xayacatlan Formation represents a Laurentian fragment, whereas that the Cosoltepec Formation is a South American (Gondwanan) suite. The Chazumba and the underlying Magdalena Formation seem to be derived from Oaxaca-like crustal source with additional contributions of a local source of middle to late Paleozoic age. Our data further indicate that the Xayacatlan and Cosoltepec Formations evolved independently at least until the Middle Devonian and that amalgamation occurred probably during the Carboniferous. The Chazumba and Magdalena Formations were deposited during or after Early Permian time and accordingly, their amalgamation with Cosoltepec and Xayacatlan Formations could not have occurred until the final assembly of Pangea during Triassic or Early Jurassic times. The U-Pb data agree with Nd model ages and common Pb isotopic data, which show slight differences between Gondwana and Laurentia data.