DETRITAL ZIRCON AND MUSCOVITE GEOCHRONOLOGY FROM SOUTHWEST USA MESOPROTEROZOIC ROCKS

U/Pb detrital zircon and 40Ar/39Ar detrital muscovite geochronology from several Mesoproterozoic sequences from the southwest USA document a long and punctuated tectonic history of Laurentia. The units sampled include the Unkar Group, Grand Canyon; Apache Group, Arizona; lower Crystal Springs Formation, Death Valley; Debaca sequence, New Mexico; and Lanoria Formation, west Texas. Most samples display strong peaks in age at 1.8 to 1.6 Ga and 1.4 Ga that represent the nearby basement ages of juvenile crust additions to Laurentia and later orogenic activity, respectively. Overall, there is only a minor population of grains older than ca. 2 Ga indicating a relatively small contribution of detritus from Archean crust. Interestingly, many grains are younger than about 1300 Ma, indicating non-local sources. A robust age population at about 1.25 Ga indicates significant volcanic/plutonic zircon input that is probably derived from the west Texas areas associated with the Grenville orogen. In addition, several Dox Formation samples from the Grand Canyon yield detrital zircon ages between 1150 and 1200 Ma, corroborating muscovite ages that also support a Grenville source terrane. Ca. 1.8 to 1.9 Ga zircon ages present within the Lanoria Formation and the Crystal Springs Formation suggest Mojave basement sources or possibly a Trans-Hudson-aged source region for detritus in southern Laurentia during the late Mesoproterozoic. Combining the zircon and muscovite data is a powerful tool for constraining maximum depositional ages and identification of potential sources. It appears that many of the units sampled are less than 1250 Ma and the presently scattered outcrops could have been much more continuous representing a structurally complex foreland basin of broad extent developing in-board of the Grenville orogen. This depositional history may help explain the apparent lack of basement exhumation between 1.4 and 1.2 Ga that is suggested by basement thermochronological studies.