Provenance of Mexican Terranes: Isotopic Constraints

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Abstract
Pb isotopes in Mesozoic and Cenozoic lavas and mineral deposits of eastern Mexico (Oaxaquia and Maya terranes) are highly radiogenic, in contrast to signatures from areas underlain by Laurentian Grenville basement. The sharp boundary between these Pbisotope domains and its NE trend nearly perpendicular to the Laramide suture and the Cenozoic trench suggest that the main source of radiogenic Pb lies in the continental basement. Similar Pbisotope signatures in the lavas and mineral deposits in the western Mexican (Pacific) oceanic-arc terranes suggest a source in sediments derived from eastern Mexico. The ca. 0.55 to 3.1 Ga Nd model ages of lower-crustal granulite-facies xenoliths in Cenozoic lavas is consistent with the hypothesis that much of eastern Mexico may be underlain by rocks of the ca. 1 Ga Oaxacan Complex of southern Mexico or Late Proterozoic-Cambrian rocks in the basement of the Yucatan Peninsula. Comparison of the Mexican Pb- and Nd-isotopic signatures with Laurentian and Gondwanan data indicates that they are generally distinct from Laurentian Grenvillian basement, but are similar to those in the Arequipa-Antofalla terrane (in Peru, western Bolivia, and northern Chile), the southwestern Amazonian craton in Brazil, and the Appalachian-Ouachita terranes derived from Gondwana. This suggests that a possible provenance of the Mexican Proterozoic basement lies in northwestern South America, which is consistent with the Late Cambrian-Early Ordovician faunal similarities between Oaxaca (Mexico) and Bolivia. Interaction between eastern Laurentia and western South America during the Ordovician appears to be absent in Oaxaquia, the earliest deformation being Devonian. A provenance for Oaxaquia off Colombia, outside the sphere of such Ordovician interactions, is supported by lithological comparisons.