EVIDENCE FOR CA. 1520 MA METATUFF IN THE PILAR FORMATION, PICURIS MOUNTAINS, NORTHERN NEW MEXICO, USA

Two metatuff layers within the Pilar Formation yield preliminary zircon U-Pb crystallization ages of 1518 ± 6 Ma and 1523 ± 13 Ma, some 30 m.y. older than a previously reported metatuff age of 1488 ± 6 Ma from the upper Pilar Formation. These ages represent the earliest depositional age constrains for the lower-to-middle Pilar formation and confirm a Mesoproterozoic protolith age for the Pilar and overlying Piedra Lumbre formations in northern New Mexico. A few zircon grains yielded ages between ca. 1680 Ma and ca. 2650 Ma and may reflect xenocrysts or detrital grains.

Metatuff sample MR13-01 was collected from a folded, 6-cm thick layer in the southern Picuris west of the Copper Hill anticline. Sample MR13-05 was collected from a 0.5 m thick metatuff/phyllite layer in the lower Pilar Formation on the northern limb of the Hondo synclinorium. Approximately 40 zircon grains were analyzed in each metatuff sample. Zircon grains in both samples are generally euhedral to subhedral, prismatic with sharp terminations and square cross-sections and range in length from 40 to 90 µm, with an average width of 40 µm. The general absence of rounding suggests little sedimentary reworking. Cathodoluminescence imaging of zircon reveals concentric growth zoning with generally darker cores and brighter overgrowths consistent with an igneous origin.

The eruptive source for the metatuff is not known. There are no known intrusive rocks of this age in the southwestern United States. Distal sources could be buried in the subsurface of the midcontinent or possibly associated with equivalent age rocks in a long-since rifted continental fragment such as Australia. Subsequent regional metamorphism and deformation is attributed to the 1480–1400 Ma Picuris Orogeny.