MESOPROTEROZOIC CA. 1.5–1.45 GA DEPOSITION AND VOLCANISM IN THE SOUTHWESTERN UNITED STATE: ONSET OF THE PICURIS OROGENY

Metamorphosed siliciclastic rocks interlayered with thin metatuff layers in Arizona and north–central New Mexico record the onset of Mesoproterozoic deposition and volcanism that marks the likely formation of a convergent margin following the ca. 1.60–1.50 Ga tectonic/magmatic gap. Four, cm–thick metatuff layers in the lower to middle Yankee Joe Fm, near Clay Springs, upper Salt River Canyon, AZ yield euhedral, zoned igneous zircon with crystallization ages between about 1.50 and 1.48 Ga. The Yankee Joe and the overlying Blackjack formations together have an estimated minimum thickness of about 1500 m and contain abundant ca. 1.6-1.47 detrital zircon. The minimum depositional age is constrained by the crosscutting, ca. 1.43 Ga Ruin granite.

In the Picuris Mountains, New Mexico, four, cm– to m–thick metatuff layers in the lower to upper Pilar Fm yield small, euhedral, zoned igneous zircon that yield crystallization ages between ca. 1.5 and 1.47 Ga, and constrain the maximum depositional age of the formation. The Pilar Fm is overlain by the Piedra Lumbre Fm and together they have an estimated thickness of 900-1000 m. The Piedra Lumbre Fm also contains aboundant ca. 1.6–1.47 Ga detrital zircon. The Marquenas Fm structurally overlies the Piedra Lumbre Fm and yields metarhyolite clasts as young as ca. 1.45 Ga and is interpreted as a syntectonic, cobble-to-boulder metaconglomerate. The minimum and maximum depositional age constraints of the Marquenas Formation are identical within error, with the minimum depositional age constrained by a ca. 1.46 ± 0.02 Ga Lu/Hf garnet isochron growth age.

Preliminary geochemical analyses of the metatuff layers show that they are calc-alkaline trachyandesites and dacites. Normalized spider diagrams show enrichments in fluid mobile elements Rb, Ba, Th and U, depletions in fluid immobile elements including Nb and Ta, with distinct enrichments in K, consistent with a subduction zone origin. We interpret the overlapping depositional ages and the eruption/deposition of metatuff layers to reflect the onset of basin formation in Arizona and New Mexico and the initiation of a convergent margin associated with the beginning of the Picuris Orogeny.