U-PB GEOCHRONOLOGY OF LARAMIDE MAGMATISM IN NORTH-CENTRAL SONORA, MEXICO

The Laramide magmatic arc is well represented in the Arizpe-Mazocahui quadrangle of north-central Sonora where dioritic to granitic plutons intrude a rhyolitic-dacitic, andesitic and volcaniclastic succession assigned to the Tarahumara Formation. Measured stratigraphic columns indicate that this succession is at least 2-km-thick, and regionally extends to northern Sonora where it grades laterally into the Campanian-Maastrichtian Cabullona Group. Consistent U-Pb ages range from 76 to 70 Ma for this volcanic succession, whereas ⁴⁰Ar/³⁹Ar ages north of the quadrangle are as young as ~60 Ma. The lowermost member of the Tarahumara Formation is a conglomerate and rhyolitic succession that unconformably overlies thrusted and folded rocks of Proterozoic to early Late Cretaceous age. The upper members are composed of rhyolitic to dacitic ash-flow tuffs, ash-fall tuffs, andesitic flows and agglomerates, volcaniclastic sandstone and conglomerate. Several large plutons younger than the Tarahumara range in age from 71 to 50 Ma (U-Pb), although four apparent magmatic pulses around ~70, ~62, 56-53, and ~50 Ma are present. This magmatism was high-K and calc-alkaline, and the plutonic rocks are metaluminous to strongly peraluminous, exemplified by the muscovite-bearing El Babizo (70 Ma) and El Jaralito (55 Ma) granites.

The youngest tectonically deformed unit which the Tarahumara Formation unconformably overlies is the Cocóspera Formation, a syntectonic conglomerate from where an interbedded andesitic flow yielded a ~93 Ma age (⁴⁰Ar/³⁹Ar hornblende; Lawton et al., 2009). These data constrain the age of shortening between Cenomanian-Turonian to middle Campanian time, as the Tarahumara postdates it. Nevertheless, the age-equivalent Cabullona Group, the basin-fill of the southernmost basin of the belt of Laramide uplifts extending northward into the USA, is deformed by open folds, suggesting northward migration of the deformational front with time. Other ~90 Ma ages assigned to Tarahumara volcanism in southern Sonora may in fact represent a volcanic and sedimentary succession correlative with the Cocóspera Formation, considering that most of the reported older ages for the Tarahumara in that region cluster near 74 Ma.