DETRITAL ZIRCON CONSTRAINTS ON THE PALEOZOIC PROVENANCE OF PERI-GONDWANAN TERRANES IN SOUTHERN MEXICO

Several terranes of peri-Gondwanan affinity are exposed in central and southern Mexico. They include: (1) the Oaxaquia terrane, a ~1 Ga crustal block that underlies much of central Mexico and is overlain by a veneer of unmetamorphosed latest Cambrian-Ordovician and Silurian strata containing Gondwanan fauna, and (2) the Mixteca and Sierra Madre terranes, which mainly comprise metamorphosed Paleozoic siliciclastic and oceanic rocks juxtaposed against the Oaxaquia terrane along major, N-S dextral faults of Permian age. Detrital zircon age populations from: (1) the latest Cambrian-Pennsylvanian cover of the Oaxacan Complex (Oaxaquia terrane), (2) the Paleozoic Acatlán Complex (Mixteca terrane), and (3) the ?Silurian Granjeno Schist (Sierra Madre terrane), are dominated by Mesoproterozoic (~950-1300 Ma), late Neoproterozoic-Cambrian (~500-700 Ma), Ordovician (~440-480 Ma), and Permo-Carboniferous (~290 Ma) ages, with additional early Neoproterozoic (~800-950 Ma) and mid-Proterozoic and older (~1300-2200 Ma) signatures. These ages suggest Precambrian provenances in: (1) the Oaxaquia terrane or other ~1 Ga basement complexes of the northern Andes, (2) the ~500-600 Ma Brasiliano orogens and ~600-950 Ma Goias magmatic arc of South America, and the Pan-African Maya terrane of the Yucatan Peninsula, and (3) ~1.4-3.0 Ga cratonic provinces that most closely match those of Amazonia. Exhumation of ~440-480 Ma and ~290 Ma granitoids within the Acatlán Complex likely provided additional sources in the Paleozoic.

These data collectively suggest palinspastic linkages to the northwest margin of Amazonia during the late Proterozoic-Paleozoic and support continental reconstructions that place the Oaxaquia terrane adjacent to Amazonia throughout the Paleozoic rather than those that either accrete Oaxaquia to Laurentia in the late Ordovician-early Silurian or advocate more complex Paleozoic Oaxaquia-Laurentia-Gondwana relationships. They also support a broad correlation between the Paleozoic strata in the Sierra Madre terrane (Granjeno Schist) and similar Paleozoic rocks (e.g. Cosoltepec Farmation) in the Mixteca terrane, and suggest that both were deposited along the southern, Gondwanan (Oaxaquia) margin of the Rheic Ocean in the ?Siluro-Devonian and were accreted to Laurentia with the closure of this ocean during the late Paleozoic amalgamation of Pangea