LU-HF GARNET GEOCHRONOLOGY CONSTRAINS THE BOUNDARIES OF THE YAVAPAI, MAZATZAL, AND PICURIS OROGENIES
Garnet schist samples from central and northern New Mexico exhibit garnet growth restricted to the time period between ~1460 and 1400 Ma. A schist from the Manzano Mountains in central New Mexico yields an age of 1461 ± 14 Ma (MSWD = 2.6); two schist samples from the Picuris Mountains in northern New Mexico yield ages of 1399.9 ± 3.4 Ma (MSWD = 0.18) and 1407 ± 17 Ma (MSWD = 2.9), respectively. Previous ages determined from garnet-bearing schists of the Tusas and the Picuris Mountains are consistent with these ages and suggest metamorphism migrated northward over time.
Garnet bearing samples from the Needle and Wet Mountains in southern Colorado display a range of garnet ages between ~1750 and 1470 Ma. A garnet gneiss from the Needle Mountains in southwestern Colorado yields an age of 1747.7 ± 4.6 Ma (MSWD = 1.2); an orthogneiss from the northern Wet Mountains in south-central Colorado yields an age of 1601.4 ± 5.7 Ma (MSWD = 3.6); and gneisses from the southern and northern Wet Mountains yield ages of 1496.8 ± 5.7 Ma (MSWD = 1.7) and 1476.3 ± 3.8 Ma (MSWD = 0.4), respectively.
The age distribution of these samples shows that the region that is now southern Colorado underwent amphibolite facies metamorphism during both the Paleo- and Mesoproterozoic, whereas rocks in New Mexico only underwent garnet growth in the Mesoproterozoic. This contrast implies that the boundary between rocks deformed and metamorphosed during the ~1800-1600 Ma Yavapai and Mazatzal orogenies and those only deformed and metamorphosed during the ~1460-1400 Ma Picuris orogeny lies in northern New Mexico, along what has previously been considered the Mazatzal front.