PROVENANCE OF DETRITAL ZIRCONS IN ORDOVICIAN IAPETUS OCEAN-BASIN QUARTZITES IN SONORA, MEXICO

Stratigraphic, paleontological, and detrital zircon studies of Ordovician ocean-basin rocks in the Sonora allochthon help document their depositional setting and sediment source. These deep-marine sandstone, mudstone, chert, and minor limestone beds probably were deposited in an Iapetus ocean-basin setting along the margin of Laurentia and later were thrust northward onto the Laurentian carbonate shelf during late Paleozoic closure of the Rheic Ocean. Although Sonora and Sinaloa quartzites are now located close geographically, their different compositions and zircon populations indicate widely separated depositional settings within the Iapetus Ocean.

Paleontological data indicate that the entire Ordovician is represented in Sonora and that thick quartzite beds are characteristic of the upper part. The quartzites are dated as late Middle and Late Ordovician by graptolite faunas in interbedded argillites. These mature quartzites consist mostly of rounded, fine to medium quartz grains. Associated detrital-zircon grains also are well rounded, indicating local recycling and/or abrasion during distant transport. U-Pb (SHRIMP-RG) dating of zircons in five beds yielded concordant to near-concordant latest Paleoarchean to latest Mesoproterozoic ages (~3250-1025 Ma), in contrast to zircon age spectra of coeval ocean-basin quartzites in northern Sinaloa that include dominant Cambrian through Neoproterozoic (Gondwanan?) populations. Predominant populations (N=170) in Sonoran quartzites include Trans-Hudsonian (~1800-1880 Ma), Penokean (~1900-1950), and Neoarchean (~2675-2780 Ma) ages. All five beds contain specific igneous sources at ~1840, 1920, and 2725 Ma. All these ages have been found in older Laurentian rocks, although other paleo-continental sources are possible. The near absence of 1.6-1.8, 1.3-1.5 and 1.0-1.2 Ga-age zircons, typical of the southwestern and southern Laurentian basement, is consistent with paleogeography of Laurentia in the Middle and Late Ordovician when most exposed crust was Canadian shield rocks, the zircons in which match most of the Sonora detrital-zircon populations. The presence of Laurentian zircons and absence of Gondwanan zircons in Ordovician quartzites of the Sonora allochthon indicate a predominant Laurentian continental provenance.