Paper No. 4
Presentation Time: 8:45 AM
DETRITAL ZIRCON EVIDENCE FOR MESOPROTEROZOIC DEPOSITION, OROGENESIS AND NON-LAURENTIAN PROVENANCE IN THE PICURIS MOUNTAINS, NEW MEXICO, USA
The accretion of the juvenile Paleoproterozoic Yavapai and Mazatzal crustal provinces to southern Laurentia between ca. 1.8 and 1.6 Ga represents a rapid and widespread crustal growth event. New detrital zircon data from a deformed and metamorphosed sedimentary succession exposed in the Picuris Mountains of northern New Mexico challenge this view and suggest that final accretion of the Mazatzal province might not have occurred until ca. 1.45 Ga, 200 m.y. younger than previously thought. Quartzite assigned to the basal Pilar Formation yielded a nearly unimodal age distribution between ca. 1650 and 2730 Ma. The minimum age probability peak is 1679 Ma, and the dominant peak age is 1758 Ma. A metatuff within the Pilar Formation yielded small (50–100 µm), euhedral zircon with concentric igneous zoning. Concordant analyses give a weighted mean age of ca. 1488 Ma, providing a direct constraint on the depositional age of the Pilar Formation. The Piedra Lumbre Formation conformably overlies the Pilar Formation and shows a coarsening upwards sequence with increasing amounts of quartzite. One quartzite sample yielded a complex distribution of detrital zircon ages defining 11 age probability peaks between 1495 Ma and 1994 Ma with a dominant peak at 1628 Ma. About twenty percent of the grains are between ca. 1500 and 1600 Ma, an age range not widely represented in Laurentia.
The distribution of detrital zircon ages in the Piedra Lumbre Formation and depositional age constraints of the Pilar Formation are similar to other metasedimentary successions in Laurentia such as the Blackjack and Yankee Joe Formations of the Hess Canyon Group in eastern Arizona and the Belt Supergroup in Idaho and Montana. We speculate that ca. 1.48 Ga basins in Arizona and New Mexico were tectonically linked with multiple sediment sources that likely included Australia or Antarctica. Deposition of the metaconglomerate of the Marquenas Formation ca. 1.46 Ga is interpreted to represent basin closure in New Mexico and the onset of “Al2SiO5 triple-point” metamorphism, km-scale folding, and ductile thrusting. Our new age data indicate that these events cannot be related to the ca. 1.65 Ga Mazatzal Orogeny but must reflect a ca. 1.44–1.40 Ga orogenic event we refer to as the Picuris Orogeny.