Monazite [(LREE)PO4] and xenotime [(HREE,Y)PO4] occur in ore-grade concentrations within the Pinto Gneiss in the Music Valley region (MVR) of southern California. However, both the age and petrogenesis of this potentially economically significant rare earth element (REE) deposit remain uncertain. A combined petrologic and geochronologic study yields insight into the textural and temporal relationships between REE mineralization and the host rock. Ore-forming monazite and xenotime are typically restricted to biotite folia within the host Pinto Gneiss with highest modal abundances occurring within a few meters of contacts between the host gneiss and a diorite/amphibolite intrusive unit, the latter of which is cross-cut by pegmatite veins that appear to have been generated by partial melting of the Pinto Gneiss. Ore-forming monazite and xenotime preserve complex internal elemental zoning.

Textures inferred to be the result of dissolution re-precipitation reactions overprint relict oscillatory zoning with xenotime and uranothorite [(U,Th)SiO4 ] forming from the breakdown of monazite and monazite and uranothorite forming from the breakdown of xenotime, suggesting late-stage fluid mediated alteration of the ore minerals. Localized breakdown of monazite, anorthite and biotite to apatite and allanite provides further evidence for post-mineralization metasomatism of the ore bodies. In-situ monazite and xenotime U-Pb geochronology constrains REE mineralization to ∼1.71 Ga, consistent with zircon dates obtained from the Pinto Gneiss. Zircon from the diorite/amphibolite are ∼1.4 Ga, inferred to represent the emplacement age of this unit, whereas zircon in the pegmatite veins yield ages of ∼1.65Ma.

Zircon from a porphyritic hornblende diorite pluton exposed in the northern part of the MVR also yield an age of 165 Ma, suggesting a genetic link between emplacement of this intrusive suite and melting of the Pinto Gneiss to form the pegmatite veins. Based on the similarity in ages between monazite, xenotime and zircon in the Pinto Gneiss, along with relict 'igneous' zoning in the ore-bearing phosphate minerals, REE mineralization is interpreted to have occurred during crystallization of the igneous protolith of the Pinto Gneiss. Diorite/amphibolite emplacement at ∼1.4 Ga and pegmatite genesis at ∼165 Ma post-date the main phase of REE mineralization, but likely played a role in fluid-assisted alteration of the ore bodies and partial resetting of monazite and xenotime ages in the Pinto Gneiss.