Allanite-(Nd) from the Kingman Feldspar Mine, Mojave Pegmatite District, Northwestern Arizona, USA

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 bastnasite-(Ce). Late-stage carbonate fluids, enriched in F from the dissolution of biotite, could have preferentially redistributed Ce into bastnasite-(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.