216-11 Geochronology of Proterozoic pegmatites in the Transition Zone of Arizona: A composite, diachronous origin for the Arizona Pegmatite Belt

Session: Geochronology of Critical Mineral Deposits with Special Reference to U-Th-Pb Dating of Common-Pb-Rich Minerals

Presenting Author:

Authors:

Abstract:

Pegmatites are exposed throughout Arizona’s Transition Zone, a physiographic province between the Colorado Plateau and the Basin and Range dominated by exposures of Proterozoic plutonic and supracrustal rocks. Pegmatites range from simple to complex with widely varying geochemical affinities, including well-known Li- and REE-bearing localities. This area of abundant pegmatites has been termed the Arizona Pegmatite Belt (APB), which generally overlaps with exposures of higher-grade Proterozoic rocks in the Transition Zone. Despite its exploration potential for critical elements, the age and tectonic setting of the APB is understudied and few pegmatites have been dated using modern analytical techniques. We overcome many of the inherent challenges with U-Pb geochronology of pegmatites using a more campaign-style approach and dating multiple U-Pb-bearing phases.

We present new U-Pb geochronology and geochemical data from pegmatites within and adjacent to the APB. Zircon, monazite and xenotime ages of pegmatites range from c. 1715 to c. 1330 Ma, including both NYF and LCT-type pegmatites. The Wagon Bow pegmatites are late-stage segregations within the Aquarius pluton, yielding monazite dates of c. 1715 Ma. Accessory minerals within these pegmatites include xenotime, fluorite, and Nb-bearing phases, indicating an NYF affinity. Deformed pegmatites veins, representative of a widespread swarm, in the Thorn Peak area are c. 1715 Ma.

Ages from well-known districts, including the Rare Metals, White Picacho, Amblya, and Shannon pegmatites, are typically 1400-1330 Ma. These pegmatites are generally isolated, spodumene, lepidolite, columbite-bearing bodies, indicating LCT affinities. The Proterozoic evolution of North America involved the voluminous intrusion of A-type granites at c. 1.4 Ga. The Mesoproterozoic pegmatites dated during this study are clearly broadly related to this event. However, known ages of Mesoproterozoic granites in Arizona are 1440-1400 Ma, distinctly older than pegmatites in the same area. LCT Proterozoic pegmatites in Arizona likely represent an evolving tectonic regime, post-dating regional plutonism.

The Arizona Pegmatite belt is thus a composite feature that does not meaningfully convey a sense of genesis, classification, nor potential for hosting critical minerals.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9068

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