ZIRCON RECYCLING FROM THE GONDWANAN MARGIN OF THE RHEIC OCEAN: THE ROLE OF MEGACRYSTIC GRANITOIDS OF THE ACATLÁN COMPLEX, SOUTHERN MEXICO

The Acatlán Complex of southern Mexico exposes Paleozoic rocks that preserve a record of the evolution of the Rheic Ocean. The complex is faulted against, and likely lies structurally above, the Mesoproterozoic Oaxaca Complex. Lower Permian sedimentary rocks overstep the fault zone, but an earlier relationship between the two complexes is widely inferred.

In the Acatlán Complex, a suite of Ordovician to early-Silurian, variably deformed, commonly megacrystic, calcalkaline granitoids with mixed arc/within-plate affinities form part of a major bimodal suite. The suite is inferred to be related to rifting along the southern Gondwanan margin of the Rheic Ocean. The granitoids are distinct lithologically and also in the abundance and character of their inherited zircons. Our new U-Pb data, along with those previously published, indicate that the majority of inherited zircons in the granitoids fall between 900-1300 Ma; few zircons older than 1300 Ma have been analyzed and younger analyses commonly form discordant trends generally consistent with mixtures between Ordovician and Mesoproterozoic end-members. The 900-1300 Ma ages match closely the bedrock crystallization ages and detrital zircon spectra of Paleozoic sedimentary cover in the Oaxaca Complex. Some Paleozoic sedimentary units in the Acatlán Complex show a similar pattern of Mesoproterozoic detrital zircon ages, but with added Ordovician or younger components. Neoproterozoic and Cambrian apparent ages commonly form discordant trends on concordia diagrams in a manner consistent with Ordovician-Mesoproterozoic mixed ages.

Although not unique, the simplest explanation for these data is that the granites are either sourced in or contaminated by the Oaxaca Complex or its recycled sedimentary derivatives. Paleozoic sediments again recycled these zircons, adding zircons from other Acatlán units. This interpretation would tie the two complexes together since at least the Early Ordovician. Two megacrystic granitoids in the Acatlán Complex have been interpreted to yield Mesoproterozoic crystallization ages; that interpretation is equivocal, but if substantiated would imply a much longer history of interaction between the two complexes.