Abstract

The Paleozoic Acatlan complex and Grenville-age Oaxaca terrane of southern Mexico have been suggested to be the southern continuation of North American orogenic systems. The Oaxaca terrane yields interpreted ages of ∼1.0 Ga and has many of the characteristics typical of Grenville belt rocks. The Paleozoic Acatlan complex consists of multiply deformed metasedimentary rocks, schists, granitoids, and eclogites that have been compared to rocks of the Appalachian belt. The northward extension of both the Oaxaca terrane and Acatlan complex, how-ever, are obscured by younger rock cover as they enter central Mexico. Furthermore, the configuration of these orogenic systems in southern Mexico is the reverse of that of the rest of North America, with the Grenville Oaxaca terrane to the east of the Paleozoic Acatlan complex.

Isotopic studies show that the Acatlan complex records three tectonothermal events. The Sm-Nd whole-rock/mineral isochrons from schists as well as eclogites yield metamorphic ages of 410-380 Ma. This age of metamorphism is supported by U-Pb zircon data from a granitoid which yields an age of 370 ± 34 Ma. A later intrusion of a large stock in the Late Pennsylvanian (287 ± 2 Ma) was probably closely followed by a less significant deformational event. Small granitic intrusions and migmatites were later emplaced at 205-170 Ma (Rb-Sr and Sm-Nd mineral/ whole rock). The metasedimentary rocks and Paleozoic granitoids of the Acatlan complex have present-day εNd of -9 to -11, with crustal residence ages (TDM) of 1.3-1.6 Ga. These rocks must have been derived at least in part from a Proterozoic source area, and it is significant that crustal residence ages are identical to those of the Oaxaca terrane. The Oaxaca terrane, along with some South American Precambrian complexes, of which the Oaxaca terrane was probably once a part, may be considered the most likely source areas for the Acatlan complex. The less extensive eclogite, trondjhemite, and amphibolite bodies in the Acatlan complex yield model ages that show them to be juvenile additions to the crust.

Neodymium model ages of the Acatlan complex are unlike those of some accreted crustal blocks of the Pacific margin, such as the Alexander, Stikine, and Wrangellia terranes, which have little signs of crustal recycling. Similarities between the Acatlan complex and the Acadian belt, as well as current Paleozoic paleogeographic and paleomagnetic reconstructions, suggest that the Devonian metamorphic event that affected these two areas was the result of a Laurentia-Gondwana collision. A later collision in late Carboniferous time caused deformation in the Acatlan complex, plutonic activity in southern Mexico, and deformation in the Ouachita, Marathon, and Appalachian belts. Both the Acatlan and Oaxaca terranes would have been continuous with South America until the break-up of Pangea in the Mesozoic era.

First Page Preview

First page PDF preview
You do not currently have access to this article.