FINGERPRINTING CARBONATITE SOURCES WITH ZIRCON PETROCHRONOLOGY

Carbonatites are relatively rare igneous rocks that are of considerable economic interest due to their common compositional enrichment in critical minerals, including rare earth elements. Despite their economic significance, the petrogenesis and source(s) of carbonatites and associated alkaline silicate rocks are poorly understood. Many petrogenetic models propose either carbon-rich melts derived from a mantle plume or a mantle source that was enriched by subduction-related metasomatism.

Here, we use the trace element compositions of zircon to fingerprint carbonatite-alkaline complex sources, focusing on the economically significant 1.4 Ga Mountain Pass intrusive suite. Autocrystic zircon from alkaline silicate rocks in the Mountain Pass intrusive suite yield compositions enriched in Th and U and depleted in Nb and suggest an oxidized source, consistent with a source region that was affected by subduction. Inherited zircons (1.8–1.5 Ga) and regional geologic constraints support protracted Paleoproterozoic convergence and subduction associated with mantle metasomatism and oxidation. Lower Sc/Yb, higher Ti concentrations, and the absence of Eu anomalies in autocrystic Mountain Pass intrusive suite zircons suggest derivation from a less hydrous, hotter, and deeper mantle source relative to inherited zircons derived from arc-related sources.

We propose that these data are best explained by the Mesoproterozoic reactivation of a mantle source which had previously undergone Paleoproterozoic subduction-metasomatism. Mesoproterozoic magmatism likely occurred in a syn- to post-collisional environment associated with the Picuris orogeny. This contrasts with other Mesoproterozoic carbonatites, Gifford Creek (Australia) and Bayan Obo (China), which exhibit plume-like zircon compositions and are spatially associated with rifts. This suggests different sources and tectonic settings for these economically significant Mesoproterozoic carbonatite deposits These results, along with a global compilation of trace element compositions from carbonatite complexes, demonstrate that zircon petrochronology is a robust method for distinguishing carbonatite source types and can inform more targeted exploration strategies for critical mineral resources.