DETRITAL ZIRCON RECORD OF LARAMIDE SOURCE AREAS, UPPER CRETACEOUS (CAMPANIAN) RINGBONE FORMATION, LITTLE HATCHET MOUNTAINS, SW NEW MEXICO

U-Pb ages of detrital zircons in the Ringbone Formation (n = 282; N = 3), Hidalgo County, NM, indicate their derivation from contemporary arc sources west and south of the basin and from sedimentary strata that occupied nearby Laramide uplifts. The Ringbone consists of 1600 m of conglomerate, sandstone and shale deposited in a Laramide intermontane basin. Clasts of Lower Cretaceous sandstone (Mojado Formation) and limestone are prominent throughout the formation; clasts from recognizable Paleozoic formations are subordinate and were probably derived from Lower Cretaceous conglomerate. Fluvial facies in the middle part of the formation contain feldspathic-lithic sandstones rich in volcanic grains. A maximum weighted-mean depositional age of 72.8 ± 0.6 Ma for a sandstone sample from the middle member of the Ringbone overlaps with U-Pb zircon tuff ages of 73.4 ± 1.0 Ma (middle member) and 73.3 ± 0.7 Ma (upper member).

A composite of three Ringbone samples contains nine recognizable detrital-zircon age groups. Arc-derived groups are (a) 77-66 Ma (27%), (b) 111-93 Ma (5%), (c) 137-122 Ma (2%), and (d) 246-147 Ma (12%). These groups were derived from the contemporary Laramide (“Tarahumara”) arc in northern Sonora and southern Arizona, the Peninsular Ranges or Sierran arcs to the west, the Alisitos arc, and the Triassic-Jurassic Nazas arc of northern Mexico and Arizona, respectively. Pre-arc age groups are (e) 632-330 Ma (9%), (f) 1.3-0.95 Ma (15%), (g) 1.5-1.4 Ga (9%), (h) 1.8-1.6 Ga (14%), and (i) 2.8-1.9 Ga (7%). These groups are attributed to peri-Gondwanan, Grenville, granite-rhyolite, Yavapai-Mazatzal, and unspecified Archean initial sources, respectively. The age groups are present in similar percentages in a single sample of the lower part of the Mojado Formation (n = 92), except that Grenville grains are more abundant (40%) in the Mojado sample, and grains younger than 156 Ma are absent. A basal Ringbone sample contains abundant grains in age group b, but lacks grains from syndepositional group a, suggesting that pre-depositional Cretaceous arc grains (group b) were reworked from subjacent Cretaceous strata. The Nazas age group (group d) is of similar abundance (13%) in the Mojado sample as in our composite Ringbone data, indicating that the older arc grains were likely also recycled through Lower Cretaceous strata.