DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF JURASSIC-EOCENE SANDSTONES IN NORTHERN MEXICO: AN ARCHIVE OF POST-PENNSYLVANIAN MAGMATIC ARC HISTORY

Detrital zircon U-Pb age populations from Jurassic through Eocene sandstones of northern Mexico contain modal age groups that help constrain the Permian to Paleogene arc history of Mexico and the southwestern US. Analyses (n=2669 individual zircon grains) of foreland-basin sandstones of the Cretaceous-Eocene Difunta Group of northeastern Mexico and intra-arc and backarc Lower Jurassic-Lower Cretaceous sandstones of northern Sonora have broadly comparable age population modes despite their geographic separation. Post-Pennsylvanian zircons in both data sets record three prominent grain-age populations: (1) Permian-Triassic grains ranging ~290-235 Ma; (2) Jurassic grains ranging 180-150 Ma; (3) Early Cretaceous grains ranging 140-115 Ma. A younger grain population with ages ranging 110-54 Ma is present in the NE Mexico analyses. Marked grain-age minima are present in both sample sets from approximately 240-180 Ma, separating the Permian-Triassic and Jurassic populations, and from approximately 150-140 Ma, separating the Jurassic and Early Cretaceous age populations. A third grain-age minimum is present near 111 Ma in the NE Mexico data set.

The zircon grain-age populations correlate with documented magmatic arc systems of Mexico and the southwestern US. Population 1 records the Permo-Triassic East Mexican arc, Population 2 equates to the Nazas arc of Mexico and time-equivalent Cordilleran arc of California and Arizona, and Population 3 records the Alisitos and time-equivalent arcs of southern California and the Baja Peninsula. The younger age group in the NE Mexico samples was derived from the arc recorded by the Peninsular Ranges batholith and batholiths of the western Mexican mainland. The striking similarity of grain-age populations across Mexico likely resulted from temporal variation in magnitude of arc magmatism or changes in the zircon fertility of individual arc systems rather than variable exposure to erosion of the igneous rock suites through time.