DEPOSITIONAL SETTING AND SOURCE OF DETRITAL ZIRCONS IN THE PERMIAN MINA MÉXICO FOREDEEP IN SONORA, MEXICO

Sedimentological and geochronological studies indicate that the Early and Middle Permian Mina México foredeep contains detritus from multiple sources. The foredeep, which formed on the outer carbonate shelf of Laurentia in front of the northward advancing Sonora allochthon, is filled with synorogenic sediments of the Mina México Formation (MMF) derived from Laurentia to the north and from the evolving Sonora allochthon, and probably peri-Gondwanan terranes, to the south.

U-Pb (SHRIMP-RG) dating of detrital zircons in six turbiditic quartzite and one lime-grainstone beds in central and east-central Sonora yielded a wide range of ages, including Mesoarchean (~3135 Ma), Neoarchean to early Paleoproterozoic (~2325-2800 Ma), several Proterozoic (~1635-2230, ~1300-1440, ~565-1270 Ma), and Paleozoic (Cambrian through Pennsylvanian). The immature quartzites and lime grainstones within the MMF consist of angular to rounded, fine to very fine grains of quartz and some feldspar; detrital-zircon populations include both rounded and euhedral grains. Age spectra suggest that most of the rounded zircons were recycled from Laurentian continental crust to the north. Euhedral zircons of early and middle Paleozoic age imply direct derivation from igneous and metamorphic rocks exposed in Laurentia, and/or from exotic peri-Gondwana terranes believed to have docked along the southern margin of Laurentia during closures of the early Paleozoic Iapetus Ocean (Taconian Orogeny) and middle to late Paleozoic Rheic Ocean. Many of these detrital-zircon ages are compatible with sources in the Precambrian Amazon craton and associated orogenic belts in the South American part of Gondwana. It is likely that our inferred exotic terranes adjacent to southern Laurentia have comparable depositional, igneous, metamorphic, and tectonic histories to rocks in the Carolina superterrane and associated terranes in the southern Appalachian region. Ages of detrital zircons in the MMF support our speculation that, during late Paleozoic time, peri-Gondwana terranes docked along the southern edge of Laurentia and contributed Gondwana-age zircons to foredeeps in front of the Ouachita-Marathon-Sonora orogenic belt.