PALEOZOIC PALEOGEOGRAPHY OF MEXICO: CONSTRAINTS FROM DETRITAL ZIRCON AGES

Several Mexican terranes that accreted to Laurentia during the late Paleozoic amalgamation of Pangea record Paleozoic histories pertinent to continental reconstructions for Pangea assembly. They include: (1) the Oaxaquia terrane, a ca. 1 Ga crustal block that underlies much of central Mexico and is overlain by a veneer of unmetamorphosed Paleozoic strata, and (2) the Mixteca terrane of southern Mexico and the Sierra Madre terrane of central Mexico, both of which are dominated by metamorphosed Paleozoic siliciclastic and oceanic rocks juxtaposed against the Oaxaquia terrane along major, north-south dextral faults of late Paleozoic age. Detrital zircon age populations from: (1) the latest Cambrian-Pennsylvanian cover of the Oaxacan Complex (Oaxaquia terrane), (2) the Ordovician-Triassic Acatlán Complex (Mixteca terrane), and (3) the ?Silurian Granjeno Schist (Sierra Madre terrane), collectively support palinspastic linkages to the northwestern margin of Gondwana (Amazonia) during the late Proterozoic-Paleozoic. The age spectra are dominated by Mesoproterozoic (ca. 950-1300 Ma), late Neoproterozoic-Cambrian (ca. 500-700 Ma), Ordovician (ca. 440-480 Ma) and Permo-Carboniferous (ca. 290 Ma) ages, with additional early Neoproterozoic (ca. 800-950 Ma) and mid-Proterozoic and older (ca. 1300-2200 Ma) signatures. These data suggest Precambrian provenances in: (1) the Oaxaquia terrane or other ca. 1 Ga basement complexes of the northern Andes, (2) the ca. 500-600 Ma Brasiliano orogens and ca. 600-950 Ma Goias magmatic arc of South America, and the Pan-African Maya terrane of the Yucatan Peninsula, and (3) ca. 1.4-3.0 Ga cratonic provinces that most closely match those of Amazonia. Exhumation of ca. 440-480 Ma and ca. 290 Ma granitoids within the Acatlán Complex likely provided additional sources in the Paleozoic.

These ages indicate sources dominated by local and Gondwanan provenances and support continental reconstructions that place the Oaxaquia terrane adjacent to northwestern South America throughout the Paleozoic. The data also support a broad correlation between Paleozoic strata in the Sierra Madre terrane and similar rocks 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 during the assembly of Pangea in the late Paleozoic.