UNRAVELING THE MAZATZAL OROGENY: GEOCHRONOLOGICAL CONSTRAINTS AND SPECULATION ON THE NATURE AND TIMING OF PALEOPROTEROZOIC AND MESOPROTEROZOIC OROGENESIS IN THE PICURIS MOUNTAINS, NORTHERN NEW MEXICO, USA

Detrital zircon ages from the Picuris Mountains help bracket the timing of regional sedimentation, deformation and metamorphism along a proposed lithospheric suture zone between the Yavapai (>1.7 Ga) and Mazatzal (1.7–1.6 Ga) crustal provinces. Quartzite from the Paleoproterozoic Vadito Group show narrow unimodal age distributions with maxima near 1.71 Ga. The overlying Ortega and Rinconada Formations show somewhat broader unimodal age distributions with an increase in peak age from 1.72 Ga to 1.77 Ga, respectively. Cross-cutting plutons constrain the minimum age of the Vadito Formation to be ca. 1.68 Ga. The minimum age of the Ortega and Rinconada Formations is based upon ca. 1.44 Ga metamorphic monazite. The Piedre Lumbre Formation shows a distinct change in zircon age distribution with multiple age peaks and Mesoproterozoic ages between ca. 1.5–1.6 Ga. The youngest population indicates a maximum depositional age of 1.49 Ga for the upper Hondo Group. The diversification of sediment sources likely reflects an influx of exotic detritus from Australia and/or Antarctica. Basin formation may indicate the onset of the ca. 1.45 Picuris orogeny. The Marquenas Formation is dominated by a 1.71 Ga peak with a significant age peak of ca. 1.46 Ga. The absence of 1.5–1.6 Ga detrital zircon in Marquenas Formation indicates a more local provenance perhaps due to basin closure during regional shortening.

The interpreted CD-ROM seismic reflection profile across the Yavapai-Mazatzal crustal province boundary shows a bivergent structure that formed as the Mazatzal crustal province overrode a south-dipping subduction zone and was accreted to the Yavapai margin culminating in the Mazatzal Orogeny (ca. 1.7 to 1.6 Ga). Following orogenesis, metamorphosed and deformed rocks were interpreted to remain in the mid-crust from ca. 1.65 Ga to ca. 1. 45 Ga followed by isobaric heating and cooling at 1.45–1.40 Ga. Such a model is inconsistent with our detrital zircon data. We do not refute evidence for possible Mazatzal age deformation in Paleoproterozoic rocks. However the occurrence of regionally metamorphosed and deformed Mesoproterozoic sediments within presumed Paleoproterozoic successions requires a re-examination of longstanding tectonic models for the Proterozoic tectonic evolution of the Laurentia.