Abstract

The Kingman pegmatite, located in the Cerbat Range in northwestern Arizona, is hosted by orogenic 1.7 Ga Paleoproterozoic granitic rocks. This post-orogenic pegmatite was intruded into these granites during the middle Proterozoic (ca. 1.5 Ga) subsequent to the Yavapai and Mazatzal orogenies. Extreme LREE-enrichment, as well as the HREE-depletion, is exemplified by the presence of abundant large crystals of LREE-enriched allanite and a near-absence of HREE-enriched minerals. Most of the allanite is Nd-rich allanite-(Ce). However, several of the samples have domains that are Nd-dominant, and thus are allanite-(Nd). This represents the fourth world location and the first reported U.S. location for allanite-(Nd). The Nd-dominant domains, which typically occur near the rim of grains and along fractures, may be the result of alteration by highly oxidizing late-stage fluids that replaced allanite with bastnäsite-(Ce). Late-stage carbonate fluids, enriched in F from the dissolution of biotite, could have preferentially redistributed Ce into bastnäsite-(Ce), resulting in Nd-enriched recrystallized allanite near fractures and along the recrystallized rim. As the Kingman pegmatite is not associated with a parental granite and as it exhibits unusual depletions in F, Nb, Ta, and HREE and an extreme enrichment in the LREE, we favor an anatectic origin.

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