U-PB GEOCHRONOLOGY OF NEWLY RECOGNIZED MESOPROTEROZOIC (CA. 1.50 GA) METATUFFS IN THE UPPER SALT RIVER CANYON, ARIZONA AND THEIR POTENTIAL CORRELATION WITH METATUFFS IN THE PICURIS MOUNTAINS, NEW MEXICO: IMPLICATIONS FOR THE PROTEROZOIC TECTONIC EVOLUTION OF LAURENTIA

Newly recognized metatuffs within the Yankee Joe Fm., upper Salt River Canyon area, Arizona and the Pilar Fm., Picuris Mountains, northern New Mexico are separated by about 540 km, yet show striking, nearly identical depositional ages within the Mesoproterozoic metasedimentary successions in the two study areas. The metatuffs in both locations are interlayered with siliciclastic metasedimentary rocks previously thought to be Paleoproterozoic (ca. 1.70-1.65 Ga). Euhedral to subhedral zircon, 30 to 150 µm long, show distinct igneous growth zoning in cathodoluminescence (CL) imaging. Four metatuff layers within the lower to middle Yankee Joe Formation vary in thickness from 5 cm to 30 cm and yield preliminary, near-concordant ages of ca. 1502-1479 Ma. Three of the samples show significant reworking with older detrital grains making up 39% to 71% of the analyzed zircon population. The minimum depositional age of the Yankee Joe and Blackjack formations is constrained by the crosscutting ca. 1.43 Ga Ruin granite. Metatuff layers in the Pilar Fm. vary in thickness from 10 cm to about 3 m and yield preliminary, near-concordant ages ranging from ca. 1504-1479 Ma. One sample shows about 8% older grains consistent with minor sedimentary reworking. The volcanic source of the metatuffs is not presently known as there are few plutonic complexes in this age range exposed in southwestern Laurentia. We speculate that the protoliths could be air-fall tuffs derived from a volcanic arc on an exotic, formerly adjacent craton such as Australia ca. 1.50-1.48 Ga.

Following deposition, both areas experienced greenschist to amphibolite facies metamorphism and ductile deformation at depths of 6-15 km. Proterozoic deformation and metamorphism in these two areas was previously attributed to the ca. 1.65 Ga Mazatzal Orogeny. However, this model is no longer viable given recent detrital zircon work and the new ages from the metatuffs. We attribute Mesoproterozoic deformation and metamorphism of both metasedimentary successions to the ca. 1.45-1.40 Ga Picuris Orogeny.