UNUSUAL MINERALOGY OF CHEMISTRY OF THE KINGMAN FELDSPAR PEGMATITE, MOJAVE PEGMATITE DISTRICT, NORTHWESTERN AZ

The Mojave Pegmatite District, located near Kingman, AZ is host to numerous post-orogenic pegmatites that range from simple, meter wide dikes to complexly zoned sill-like bodies. All of the pegmatites, with the exception of the Kingman (KM) pegmatite, are ~1.44 Ma and occur as dike-like segregations that are intrusive into anorogenic granitic host rocks in the Aquarius Range, approximately 65 km SW of Kingman. The KM pegmatite, located in the Cerbat Range ~1 ½ km northeast of Kingman, is a unique sill-like body that is geographically isolated, slightly older (ca. 1.56 Ma), and is the only pegmatite that is not associated with a parent granite. Instead, it is intrusive into the Paleoproterozoic (1.740 – 1.720) orogenic Diana Granite.

The KM pegmatite is zoned with a composite quartz-microcline core, is extremely LREE enriched, and depleted in HFSE, HREE and F. Extreme LREE enrichment in the KM pegmatite is evidenced by the unusually large and abundant allanite that occurs as large, extensively fractured pods or crudely formed crystals up to 0.5 m in size. The individual crystals are dark brown to black, euhedral to subhedral, reach 25 cm in length, and in some cases contain inclusions of thorogummite. Although allanite is predominantly Nd-rich allanite-(Ce), some of the crystals exhibit domains that are Nd dominant, thus are allanite-(Nd). Exposed allanite surfaces are coated with a reddish iron oxide crust and Nd-enriched bastnäsite-(Ce). HREE-bearing minerals are conspicuously absent.

The LREE enrichment and HREE depletion exhibited by the KM pegmatite is atypical for NYF pegmatites that form during the final stages of fractionation in a granitic magma. However, an anatectic melt generated from only a small percentage of partial melting could account for the absence of a parental granite, as well as the extreme enrichment in LREE and depletion in F, Nb, Ta, and HREE, if the protolith were a garnet-biotite±hornblende metamorphic rock. These differences may be, in part, due to the origin of the melts. Unlike many typical NYF pegmatites, the Mojave pegmatites were not formed during large-scale extension. It may be that only small amounts of partial melt were generated during the localized, small-scale extension, likely due to back-arc rifting that occurred during the time between the Mazatzal and Grenville orogenies.