EVIDENCE AND CONTEXT FOR THE FIRST OCCURRENCE OF MESOPROTEROZOIC CA. 1450 MA RHYOLITE MAGMATISM IN THE SOUTHWEST, MARQUEÑAS FORMATION, PICURIS MOUNTAINS, NEW MEXICO, USA

The lowermost member of the Marqueñas Formation is a metamorphosed and ductily deformed boulder to cobble conglomerate with abundant quartzite and rhyolite clasts. Preliminary U-Pb zircon ages from two boulder- to cobble-size rhyolite clasts preserved within the lowermost conglomeratic layer of the Marqueñas Formation yield concordant ages of 1447 ± 8 Ma (N=7) and 1456 ± 4 Ma (N=7). These clasts represent the first evidence of Mesoproterozoic, ca. 1450 Ma, rhyolite in the southwest United States. Two additional cobble-size rhyolite clasts yield preliminary U-Pb zircon ages of ca. 1689 Ma, consistent with derivation from older Paleoproterozoic rhyolites in the region. The clast ages match well with previously reported detrital zircon peak ages of ca. 1470 Ma and 1710 Ma from the matrix grains surrounding the clasts and from overlying quartzite layers of middle Marqueñas Formation. There is no known source for the ca. 1450 Ma clasts preserved in northern New Mexico; however, the large clast size and poor sorting is consistent with deposition proximal to the source area.

The lower contact of the Marqueñas Formation is interpreted to represent an unconformity that separates the Mesoproterozoic Marqueñas Formation from the Paleoproterozoic ca. 1700 Ma Vadito Group. Map relationships within the study area point to a corrugated contact representing 300 meters of paleorelief that was infilled by lower Marqueñas Formation conglomerates. In some areas, strike changes in the contact do not coincide with strike changes in compositional layering in the underlying Vadito Formation. The Vadito compositional layering exhibits a 14 degree angular discordance with the contact and overlying Marqueñas bedding along a 1.5 kilometer segment of the contact. In the same area, distinctive lithologic layers within the Vadito terminate against this contact. Locally, slickenlines can be found on the contact between the Marqueñas and Vadito; however, there is no evidence of faulting on this surface at other localities, suggesting this surface is not a regional fault contact. Our discovery of ca. 1450 Ma rhyolite clasts immediately above this unconformity further supports evidence for the 1480 to 1400 Ma Picuris orogeny.