GEOCHRONOLOGIC INSIGHT INTO RARE EARTH ELEMENT MINERALIZATION AND REMOBILIZATION IN HIGH-GRADE METAMORPHIC ROCKS OF THE SOUTHERN MOJAVE DESERT, CA

Domains of rare earth element (REE) mineralization have previously been identified within Proterozoic gneiss in the Music Valley region (MVR) of southern California. Despite their potential economic significance, little is known about the mechanisms by which mineralization occurred, or the subsequent geologic history of the deposits. This study employs the relatively new technique of in-situ Laser Ablation Split Stream petrochronology to develop a petrogenetic model for mineralization and document the nature and extent of subsequent metasomatic events affecting the ore bodies. Data indicate primary mineralization occurred at ~1.7 Ga with REE hosted in monazite and xenotime that are largely restricted to biotite folia within quartzofeldspathic gneiss. Based on trace element geochemistry, the presence of oscillatory zonation, and ages that are consistent with ‘igneous’ zircon from the same rocks, initial REE-mineralization is inferred to have occurred during late stage crystallization of felsic plutonic bodies. At least three texturally distinct phases of metasomatic alteration of ore-forming monazite and xenotime are recognized. The earliest phase of ore-mineral resportion, Pb-loss and/or recrystallization occurred at ~1.4 Ga. Formation of secondary xenotime and uranothorite via breakdown of monazite and vice-versa (secondary monazite and uranothorite forming from original xenotime) overprinting relict oscillatory zonation provide evidence for widespread fluid-assisted dissolution/re-precipitation event(s) occurred at ~165 Ma. In addition, the breakdown reaction: monazite + anorthite + biotite + fluid à apatite + allanite is spectacularly preserved in various stages of evolution, yielding evidence of ore-body metasomatism and REE redistribution at ~160 Ma. Detailed petrochronology of the alteration textures as well as other rock units in the region enable temporal and genetic correlations between REE re-mobilization and specific regional-scale thermal, magmatic fluid flow events that have affected the southern Mojave tectonic block. Taken together, these data yield new insight into the timing and mechanisms of REE mineralization as well as identification of subsequent, and distinct, fluid-induced alteration events.