PALEOGEOGRAPHIC IMPLICATIONS OF DETRITAL ZIRCON AGES FROM THE OAXACA TERRANE OF SOUTHERN MEXICO

The striking density of concordant ages at ~990 Ma of detrital zircon grains derived from Paleozoic platformal rocks of the Oaxaca Terrane in southern Mexico give the Oaxaca Terrane a highly distinctive age signature. This signature is absent in both detrital zircon age distributions of the miogeoclinal rocks of the southwestern United States and the Ouachita orogen. Furthermore, the major peaks in the age distributions of detrital zircons from the southwestern US (~1110 and 1430 Ma.) and Ouachita (~1025 Ma.) appear to be absent in the Oaxacan detrital zircon age distribution.

Samples were collected near Santiago-Ixtaltepec in Oaxaca, Mexico from the Cambrian-Ordovician Tinu Formation, the lower Mississippian Santiago Formation, and the lower-middle Pennsylvanian Ixtaltepec Formation. Sixty-one detrital zircon grains derived from these rocks were individually analyzed using a conventional ID-TI mass spectrometer and were compared to detrital zircon ages from previous studies of Paleozoic miogeoclinal rocks from the southwestern United States and the Ouachita orogen. Detrital zircons from the Tinu Formation yielded U-Pb ages ranging from 980 Ma to 1230 Ma, with a pronounced peak of concordant ages at ~995 Ma. The Santiago Formation yielded U-Pb ages between 455 Ma and 1020 Ma, with a pronounced peak of concordant ages at ~990 Ma. The Ixtaltepec Formation yielded U-Pb ages ranging from ~350 Ma to 1180 Ma, with a major peak of concordant ages at ~995 Ma.

Our results suggest that the ~1.0 Ga Grenvillian basement rocks of the Oaxaca Terrane did not originate along the region of the Grenville Belt associated with the southern and southwestern United States. Furthermore, our results suggest that the Oaxaca Terrane was isolated from the southern margin of Laurentia throughout most of the Paleozoic.