10 Advancements in the Proterozoic tectonics of western North America (Posters)

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.

Monday, 18 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

1.45 Ga Sub-volcanic Intrusion, Crustal Assimilation, and Quaternary Lead Loss Recorded in the Zuni Mountains of Central New Mexico

Paper number: 10-0
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

Updated age data from igneous rocks in the Zuni Mountains and adjacent portions of north-central New Mexico: 1) further document the range of Hf isotope composition of the Mazatzal crust and reveal the first instance of ca. 1450 Ma sub-volcanic rhyolitic magmatism identified along the southwestern Laurentian margin, and 2) reveal the extent to which relatively recent volcanism and hydrothermal activity has induced lead loss of varying degrees upon the Proterozoic basement.

Plutons from the Zuni and surrounding region yield new crystallization ages between 1.68 and 1.65 Ga with variable degrees of discordance. εHf values of concordant zircon (n = 63) demonstrate magmas were derived from mantle sources followed by assimilation of dominantly juvenile Mazatzal-age crustal materials (e.g. 10.1 to 2.9 ε-units; εHf(1650)= ~10).

A new zircon crystallization age of ca. 1445 Ma from a weakly-foliated hypabyssal metarhyolite in the Zuni mountains indicates the occurrence of the first documented shallow magmatism associated with the Picirus orogeny. Together with 1.5 to 1.45 Ma detrital zircon ages of the near-by Yankee Joe-Defiance Basin, this sample suggests the Zuni mountains represent a shallower crustal position than the Picuris range crustal section. Zircon εHf values (n=24) in the metarhyolite reveal relatively low values (εHf =2.8 to 7.36, with a single datum of 10.7) suggesting primarily partial melting of Mazatzal crust by small volumes of mantle-derived melts.

A dike cross-cutting the hypabyssal metarhyolite contains abundant discordant xenocrystic zircon with upper-intercept ages near ~1.7 Ga (upon projection to Concordia) and a lower-intercept of ~1 Ma, indicating extreme recent Pb loss. Populations of minimally discordant xenocrysts cluster at ~1.65 Ga (n=16) and have εHf values of 1.6 to 9.2, and five grains define an ~1.4 Ga population consistent with the host intrusion age and εHf ratios (2.1 to 5.1). Degrees of discordance correlates with high U concentrations and U/Th ratios, suggesting progressive metamictization increased susceptibility to Pb mobility during lead loss events. Zircon εHf measured from concordant xenocrystic zircon in the cross-cutting dike correspond to the ages and εHf composition of those reported for the Mazatzal province.

Lead loss may result from ~25 Ma years of regional Neogene volcanic and hydrothermal activity, thus caution is required while interpreting regional detrital and Proterozoic crystalline rock zircon datasets.

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

doi: 10.1130/abs/2026RM-410710

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