Subducted sediment contributions to REE deposits recorded by alkaline mafic dikes in the Lizhuang REE deposit, Panxi area, southwest China

Abstract

Carbonatite-associated rare earth element (REE) deposits are the most important source of REEs worldwide. Recycled subducted sediments are critical for REE mineralization, but it remains unclear how the subducted sediments contribute to the REE mineralization. This study presents new whole-rock and apatite geochemical data for alkaline mafic dikes from the Lizhuang REE deposit, southwest China. These alkaline mafic dikes were emplaced at ca. 28.4 Ma (monazite U-Pb age), broadly coeval with the carbonatite–syenite complex and REE mineralization in the Lizhuang REE deposit. The alkaline mafic dikes have enriched Sr-Nd-Pb isotopic compositions, and variable Ba/Th and La/Sm ratios, demonstrate they were derived from an enriched mantle source, which had been modified by both slab-derived fluids and sediment melts. The apatite grains in the alkaline mafic dikes have distinctive geochemical features in their irregular cores (apatite A) and rim overgrowths (apatite B). Apatite A has low REE contents (18.6–161 ppm) and apatite B has high REE contents (1869–4288 ppm). The different REE contents in these apatites could have been caused by the addition of REE-rich carbonatitic melts at lower crustal depths. We propose that the REE-rich carbonatitic melts could be formed by melting of a sediment-rich mantle source, and that collision between India–Asia triggered the melting.