U-Pb geochronology of the type Nazas Formation and superjacent strata, northeastern Durango, Mexico: Implications of a Jurassic age for continental-arc magmatism in north-central Mexico
U-Pb ages of ignimbrites and detrital-zircon analyses from Middle Jurassic through lowermost Cretaceous strata in northeastern Durango, Mexico, indicate the age of local volcanism and the age range, respectively, of the Jurassic continental-margin arc in north-central Mexico, where it is termed the Nazas arc. The type Nazas Formation along the Rio Nazas consists of red continental sandstone and siltstone, ignimbrites, and intermediate flows that provide an important record of the extensional Jurassic Nazas magmatic arc. Overlying shallow-marine sandstone and shale of Late Jurassic to earliest Cretaceous age represent deposits of younger extensional basins that are part of the Border rift system, which extended from southern Arizona to the Gulf of Mexico. The younger strata constitute volcanic litharenite and lithic arkose derived from the Jurassic volcanic rocks and from basement rocks of the Coahuila block to the north. Two ignimbrites in the upper part of the Nazas Formation have statistically indistinguishable ages of 170 ± 2 and 169 ± 2 Ma. The ages are consistent with their stratigraphic order and position above volcanic-lithic sandstone with a maximum depositional age of ca. 180–178 Ma calculated from detrital zircons. Six detrital-zircon age populations were defined from four sandstone samples ( n = 377 zircon grains) of the Nazas Formation and overlying La Casita and Carbonera Formations. The most prominent age clusters (≥10% of the entire grain population) have ranges ca. 1277–890 Ma (Grenville, 22% of total), ca. 486–310 Ma (early Paleozoic, 11%), and ca. 202–159 Ma (Jurassic, two populations constituting 54% of total grains). The Jurassic age cluster indicates that Nazas arc magmatism in the region took place during the Early and Middle Jurassic and ended by early Late Jurassic time. The Nazas Formation correlates with arc-related Middle Jurassic strata in Sonora, southern Arizona, and southeastern California that contain interbedded eolian sandstone derived from the Jurassic ergs of the Colorado Plateau; absence of interbedded eolianite in the Nazas Formation thus indicates that the outcrops of north-central Mexico were likely not translated to their present locations along a regional, Late Jurassic transcurrent fault system, as postulated by some workers. Detrital-zircon data from the Upper Jurassic and lowermost Cretaceous strata indicate that Jurassic volcanic rocks covered the Coahuila block, directly north of the study area, at the end of Middle Jurassic time. Sandstone composition and detrital-zircon ages record erosional unroofing of the Coahuila block, whereby removal of Lower and Middle Jurassic volcanic and volcaniclastic strata exposed Grenville, Pan-African, and peri-Gondwanan basement of the block during Late Jurassic time.