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 Mylonites, REE Mineralization, and Orogeny at Mountain Pass

Paper number: 21-7
Program: Technical Sessions
Time: 10:55 AM – 11:15 AM

Authors

Abstract

The Mountain Pass rare earth element (REE) deposit, one of the world’s largest REE resources, is hosted in carbonatitic rocks of the 1450–1370 Ma Mountain Pass Intrusive Suite (MPIS) in the Mojave Desert of southeastern California. Despite its economic significance, the tectonic setting during emplacement of the MPIS remains poorly constrained, limiting genetic models for carbonatite-hosted REE mineralization. We integrate structural observations, mineral imaging, and multi-phase petrochronology (zircon and titanite U-Pb, apatite Lu-Hf/U-Pb, biotite Rb-Sr) for the MPIS and basement rocks to constrain the timing of deformation in the eastern Mojave Desert. Our results show top-to-the-east upper greenschist to lower amphibolite facies deformation at ca. 1400 Ma in the Lucy Gray Mountains (Beer Bottle Pass shear zone; 30 km northeast of MPIS) and at 1630–1600 Ma and 1450–1400 Ma in the New York Mountains (Nipton area; 30 km east of MPIS). The Mesoproterozoic mylonitization event(s) overlap peak MPIS magmatism (1420–1400 Ma) and regional deformation and shear zone reactivation across southern Laurentia during the Picuris orogeny (1470–1350 Ma). A mylonite sample from the MPIS records a deformation event at or before ca. 1360 Ma, which overlaps within error of the youngest deformation ages determined for mylonite samples from the Lucy Gray and New York Mountains. We suggest that the MPIS formed in a transpressional regime inboard of a convergent margin in Laurentia with post-collisional extension and lithospheric relaxation facilitating low percentage melting of the MPIS source region and magma ascent. Our work documents a spatially expanded Picuris orogeny across the southwestern USA. Our results challenge the simple rift-based models traditionally invoked for carbonatite-hosted REE deposits and implies that post-collisional domains may represent underexplored, fertile terranes for world-class REE mineralization.

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

doi: 10.1130/abs/2026RM-410621

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