Pb isotope evidence for Colombia-southern México connections in the Proterozoic

Identical present-day lead isotope compositions of whole rocks from the Mesoproterozoic Guichicovi and Oaxaca Complexes in southern México and Grenville rocks from the Mesoproterozoic Santa Marta and Garzón Massifs in the Colombian Andes indicate these now widely separated Grenville-age rocks shared a U/Pb history that was distinct from Grenville-age rocks from northern and east-central México, and Texas and parts of the Appalachians in the United States. The southern México-Colombian rocks are characterized by a large range of ²⁰⁶Pb/²⁰⁴Pb values (17.5-24.6) and a corresponding range of ²⁰⁷Pb/²⁰⁴Pb values (15.53-16.31) that scatter along two 1,250 ± 50 Ma reference isochrons that lie above and below the average crustal growth curve. Variable Th/U values, indicated by a wide range of ²⁰⁸Pb/²⁰⁴Pb values (36.8-45.1), characterize the complexes, with rocks of the Guichicovi Complex and the Garzón Massif sharing low time-integrated Th/U values whereas the Santa Marta Massif has a tendency toward elevated Th/U values. These Pb isotopic similarities provide additional constraints to reconstructions of the pre-Pangea collisions between Gondwana and Laurentia. The Pb isotopic data, coupled with paleontologic data from unconformably overlying Early Paleozoic rocks, suggest that these Grenville-age basement blocks shared a common history in the late Proterozoic, and that they were part of Gondwana during the early Paleozoic. The southern México rocks, the Oaxaca and Guichicovi Complexes, are considered to have been transferred from that part of northwestern Gondwana now occupied by Colombia to the southern tip of Laurentia during Paleozoic orogenic events related to the opening and closing of the proto-Atlantic Ocean. This model requires a major suture separating México Grenville-age rock, which are considered to be offset pieces of the Grenville Province of Laurentia, from Gondwanan crust in southern México; the suture must lie north of the Guichicovi-Oaxaca Complexes. The Pb isotopic data also require that the Mesoproterozoic megaterrane Oaxaquia consists of several discrete blocks assembled in the late Proterozoic(?) or Paleozoic.