21 Advancements in the Proterozoic tectonics of western North America

This session highlights recent developments in understanding of the Proterozoic tectonic record of western Laurentia, particularly its relation to critical mineral resources. Insights derived from all disciplines and approaches are welcome.

Tuesday, 19 May 2026
9:00 AM - 12:00 PM
Alvarado F

Mesoproterozoic Deposition, Alteration, and Metamorphism of the Trampas Group Basin, Picuris Range, New Mexico

Paper number: 21-6
Program: Technical Sessions
Time: 10:35 AM – 10:55 AM

Authors

Abstract

          The Trampas Group depositional basin of the Picuris Range, New Mexico, contains the Mesoproterozoic Pilar and Piedra Lumbre Formations. The Pilar Formation is a graphite-rich phyllite with interlayered metatuffs, and the Piedra Lumbre Formation is a quartzofeldspathic schist with abundant quartzite lenses. Volcanic and detrital zircon U-Pb ages from the respective units constrain the minimum duration of Pilar Formation deposition to ~1490–1470 Ma and the maximum depositional age of the Piedra Lumbre Formation to ~1475 Ma, indicating that these units are at least partially syndepositional. Preserved lithologies and sedimentary structures suggest the Pilar Formation was deposited in a relatively quiet, distal marine setting while the Piedra Lumbre Formation was deposited in a shallower, more active shelf environment. Although the appearance and occurrence of the Pilar Formation metatuffs are consistent with silicic volcanic deposits, bulk compositions (~50–65 wt. % SiO2, ~20–30 wt. % Al2O3, ~6–10 wt. % alkalis) are not. Mass balance modeling suggests low-temperature diagenetic or metasomatic alteration of glassy, distally sourced rhyolitic tuff shortly after deposition. We show that metatuffs experienced major volume and mass loss (~15–35% each), likely facilitated by alteration of glass to clays and up to ~33 wt. % silica loss.

 

          Presumed metatuff layers in the Piedra Lumbre Formation contain Paleoproterozoic and Mesoproterozoic zircon populations, suggesting tuffs were mechanically mixed with detrital material following deposition. Piedra Lumbre Formation shists contain equilibrium assemblages of white mica + quartz + plagioclase + biotite + garnet + ilmenite ± staurolite. Peak metamorphic conditions for the Picuris Mountains come from the Paleoproterozoic, aluminosilicate-bearing Hondo Group and were previously estimated to be very near the aluminosilicate triple point (~510–525 °C and ~4.0–4.2 Kbar). We construct new phase equilibria models for Piedra Lumbre schists in the hinge of the Hondo syncline that constrain peak temperatures between 525 and 600°C and peak pressures up to 2 kbar higher than previously estimated. The stability field of staurolite-bearing assemblages is sensitive to bulk Fe3+; however, over a reasonable range of Fe3+ contents, new models only overlap with previous P-T estimates at the extreme lower pressure end of their stability fields. The Trampas Group records higher peak metamorphic conditions for the Picuris Mountains and extends the P-T path for this "triple-point" terrane.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410494

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