DETRITAL ZIRCON GEOCHRONOLOGY OF THE PERMIAN ABO FORMATION AND PERMIAN LABORCITA FORMATION, SACRAMENTO MOUNTAINS, NEW MEXICO, USA

The exposures of Paleozoic strata, which are more than 2500 m thick and range in age from Cambrian to Permian, in the Sacramento Mountains of New Mexico reveal important information for understanding of the paleogeography and timing of the late Paleozoic Ancestral Rocky Mountain orogeny in general, and the Pedernal Uplift in particular. The lower Permian (Wolcampian) strata consists of two formations: the Laborcita and the Abo Formations, resting above the Ancestral Rocky Mountain Unconformity. The late synorogenic Laborcita is an ~300 m thick succession of gray and red limestone pebble conglomerate, sandstone, siltstone, shale and limestone deposited in fan delta or deltaic systems. The flat-lying, post-tectonic Abo Formation ranges from 120-450 m in thickness and consists of dark-red mudstones, arkoses, and basal quartzite cobble conglomerate. Otte (1959) recognized that paleocurrent data indicated that Abo sediment was transported south and west along the axis of the Pedernal uplift. U-Pb geochronology on detrital zircons were conducted on both the Laborcita (n=103) and Abo Formation (n=103) sandstones to determine the sedimentary provenance and sediment dispersal patterns. Zircons in the Laborcita ranged in age 1125-1865 Ma, and have a unimodal peak age of 1220 Ma, and about 80% being Grenville in age. Zircons in the Abo were exclusively Grenville in age, ranging from 1193-1308 Ma with a peak age of 1235 Ma. The most likely source for these zircons are recycling Grenville synorogenic strata that were exposed to the south during the late Paleozoic. These include the Lanoria Quartzite, which occurs in the Franklin Mountains, and the Hazel Formation, which occurs near Van Horn, Texas. The protolith sandstone of the Lanoria Quartzites was derived from the central Texas Llano region before being transported west to the Rodinian continental margin ~1110 Ma where it was buried and metamorphosed into quartzite. The similarity of the age spectra for these two units indicates that the same source area supplied sediment during and after the local compressional tectonism.