The magma source of the rare earth element-mineralized carbonatite–syenite complexes in the Oligocene Mianning–Dechang belt, western Sichuan, Southwest China

Abstract

Carbonatite–syenite complexes contain over half of the rare earth element (REE) resources worldwide, particularly the light REEs. These deposits are thought to be mainly sourced from REE-enriched subcontinental lithospheric mantle (SCLM). However, the precise nature of the mantle source is debated due to ambiguous interpretations of Sr–Nd–Pb isotope and trace element data. Four Oligocene REE deposits in the Mianning–Dechang (MD) REE belt in Sichuan Province, China (Maoniuping, Lizhuang, Muluozhai, and Dalucao) are ideal for investigating the magma sources in such deposits. These deposits have well-documented geological features and previously published Li–B–Sr–Nd–Pb isotope data, and contain fresh zircons in syenite (nordmarkites) formed by immiscibility from the carbonatites. Despite the abundant SrNd isotope data, distinguishing between metasomatic contributions from the upper continental crust and marine sediments is challenging. To address this, we present zircon HfO isotope data for the MD REE belt, obtained by laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry, respectively. We use these data to trace the origins of the REE-enriched magmas. Given the complex geological history of this region, including ca. 260 Myr of subduction, we tested a multi-end-member magma source model that includes depleted mid-ocean ridge basalt mantle (DMM), high- and low-temperature altered oceanic crust (AOC), and marine sediments. Using a Monte Carlo simulation for this multi-end-member mixing, coupled with the longest increasing subsequence algorithm, we modeled the time-dependent contributions of these components during Proto-Tethyan oceanic subduction beneath the Yangtze Craton. Our results suggest that, by the end of subduction, marine sediments contributed approximately 13 %, 24 %, 25 %, and 22 % among these three endmembers to the magma sources of the Maoniuping, Dalucao, Lizhuang, and Muluozhai deposits, respectively. The DMM and AOC were the main components in the source. Although marine sediments that are enriched in REEs relative to DMM and AOC contributed significantly to the REE contents of the lithospheric mantle, our findings indicate that the scale of the REE deposits was not solely controlled by the incorporation of marine sediments. The results indicate the need for a reassessment of the role of marine sediments in the petrogenesis of carbonatite magma and formation of REE deposits.