MESOPROTEROZOIC TO PERMIAN ZIRCON RECYCLING IN THE ACATLÁN COMPLEX OF SOUTHERN MEXICO

The Acatlán Complex of southern Mexico preserves a record of the opening and closing 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 based on detrital and inherited zircon studies.

In the Acatlán Complex, a suite of Ordovician to early-Silurian variably deformed, commonly megacrystic, calcalkaline granitoids with mixed arc/within-plate affinities is inferred to be related to rifting along the Gondwanan margin of the Rheic Ocean. The granitoids are distinct lithologically and also in the abundance and character of their inherited zircons. 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. 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 could be interpreted as Ordovician-Mesoproterozoic mixed ages.

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. No, or only a very minor, contribution from foreign cratonic source regions is needed to explain the detrital zircon spectra of the Paleozoic sedimentary successions in the Acatlán Complex.