Monazite ([LREE]PO4, where LREE stands for light rare earth element) and xenotime (Y[HREE]PO4, where HREE stands for heavy rare earth element) occur in ore-grade concentrations within the Pinto Gneiss in the Music Valley region of southern California. However, both the age and petrogenesis of this potentially economically significant rare earth element (REE) deposit remain uncertain. New petrologic and geochronologic data enable assessment of the textural and temporal relationships between REE-bearing minerals and the host rock and development of a petrogenetic model for REE mineralization. Ore-forming monazite and xenotime are typically restricted to biotite folia within the host Pinto Gneiss, with greatest modal abundances occurring within a few meters of contacts between the host gneiss and a metadiorite intrusive unit, the latter of which is crosscut by pegmatite veins generated by partial melting of the Pinto Gneiss. Ore-forming monazite and xenotime preserve complex internal elemental zonation defining two distinct textures: (1) oscillatory zoning interpreted to represent primary crystallization, overprinted by (2) irregular embayed textures inferred to be the result of fluid-mediated dissolution re-precipitation reactions. The altered domains in monazite consist of primary monazite replaced by secondary monazite along with xenotime and uranothorite [(U,Th)SiO4] inclusions. Similarly, primary xenotime is replaced by secondary xenotime with monazite and uranothorite inclusions. Localized breakdown of monazite, anorthite, and biotite to apatite and allanite provides further evidence for postmineralization metasomatism of the ore bodies. In situ monazite and xenotime U-Pb geochronology constrains the timing of primary REE mineralization to ca. 1.71 Ga, consistent with zircon dates obtained from the Pinto Gneiss. Based on the similarity in ages of monazite, xenotime, and zircon in the Pinto Gneiss, along with relict “igneous” zoning in the ore-bearing phosphate minerals, REE mineralization is inferred to have occurred during crystallization of the igneous protolith to the Pinto Gneiss. Metadiorite emplacement at ca. 1.4 Ga and pegmatite genesis at ca. 165 Ma both postdate the main phase of REE mineralization but likely played a role in fluid-assisted alteration, breakdown, and partial resetting of monazite and xenotime U-Pb systematics in the Pinto Gneiss.

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