CONFIRMATION OF MESOPROTEROZOIC (CA. 1.47 GA) DETRITAL ZIRCON FROM THE METAMORPHOSED MARQUENAS FORMATION, PICURIS MOUNTAINS, NEW MEXICO, USA

New detrital zircon 207Pb/206Pb ages were obtained with the LA-MC-ICPMS at the University of Arizona from the type locality of the Marquenas Formation exposed at Cerro de las Marquenas in the Picuris Mountains. The data confirm recent findings of Mesoproterozoic (ca. 1.48-1.46 Ga) detrital zircon reported by Jones et al. (2011) but also raise a question about the assignment of quartzite exposures in the eastern Picuris to the Marquenas Formation. Prior to the work of Jones et al. (2011) and this study, the Marquenas Formation was considered to be part of the Paleoproterozoic Vadito Group.

Detrital zircon in both analyzed samples show concentric zoning in backscatter electron imaging, little or no metamorphic overgrowths, and low U/Th ratios consistent with an igneous origin. Grains in LP11-3 are subhedral, fractured, clear in color, and uniform in size distribution compared to the mostly anhedral, rounded, highly fractured, slightly yellower grains in CD10-10. The ca. 1.4 grains in LP11-3 are more rounded than older grains. Sample LP11-3 was collected from the central part of the middle Marquenas quartzite. Isotopic analysis of 110 detrital zircon yields 98 concordant ages with 4 grains at 1.47 Ga, a broad peak at 1.69 Ga, and a minor peak at 1.9 Ga; the oldest grain is 2.7 Ga. This detrital age spectrum is nearly identical to the detrital zircon age spectra (PIC2 & PIC3) reported by Jones et al. (2011) for the same area. The second sample (CD10-10) was collected from a quartzite in the eastern Picuris near the Picuris-Pecos fault and tentatively correlated with the Marquenas formation. Ninety-five concordant detrital zircon ages show a unimodal distribution with a broad peak ranging from 1.69 Ga to 1.77 Ga with a peak age at 1.71 Ga. This detrital signature is unrepresentative of the type locality Marquenas Formation, lacking ca. 1.46 Ga age peak and Archean age grains. Sample CD10-10 quartzite may be correlative with the Paleoproterozoic Vadito Group.

The Marquenas is sheared and metamorphosed, and together with the detrital zircon age data indicates Mesoproterozoic (ca. 1.46 Ga) deposition, regional deformation and metamorphism associated with a poorly understood Mesoproterozoic intracratonic orogenic event in the southwestern US.

Jones III, J.V., Daniel, C.G., Frei, D., Thrane, K., Geosphere, 2011, doi:10.1130/GES00614.1