Rare-earth Enrichment Mechanism of Magmatic Evolution in the Zhuopan Alkaline Complex, Yunnan Province:Evidence from Apatite Geochemistry

The Zhuopan alkaline complex in Yunnan is characterized by a relatively high content of rare earth elements (REE) (approximately 1 319×10^–6), showing high potential for REE exploration.However, the magmatic evolution process and the enrichment mechanism of REEs within the Zhuopan alkaline rocks warrant further investigation.This study selected the exposed lithology of the Zhuopan intrusion (syenite-pyroxenolite and gabbro-syenite), and conducted major and trace element tests on the main rare earth mineral apatite, delving into the geochemical characteristics of apatite, aiming to explore the magmatic evolution process and the mechanism of REE enrichment.The results from Electron Probe Microanalysis (EPMA) and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) analyses indicated that the apatite found in both syenite-pyroxenolite and gabbro-syenite was fluorapatite, with fluorine contents of 2.92% and 3.31%, respectively.There were differences in the REE contents of apatite between the two rock types, with average values of 12 288×10^–6 and 16 503×10^–6, respectively.The REE distribution patterns for both types of apatite show an enrichment in light rare earth elements (LREEs) and a strong depletion in heavy rare earth elements (HREEs).The estimation results of the apatite Mn thermometer, combined with the relative levels of SO₃ content, suggest that the oxygen fugacity in the syenite-pyroxenolite is relatively lower than that in the gabbro-syenite.The high Ce/Pb and low Th/U characteristics in apatite indicate a strong fluid activity during the magmatic evolution process, with the former exhibiting higher fluid activity than the latter.The content and ratio characteristics of volatiles (F, Cl) in apatite indicate that the magma source region was influenced by metasomatism associated with oceanic subduction.Based on the aforementioned research results, the enrichment process of rare earth elements in the Zhuopan alkaline complex can be delineated as follows: (1) During the magma chamber stage at depths of 18 to 10 kilometers, due to extensive crystallization of silicate minerals, the phosphate content in the magma chamber gradually saturates, leading to the crystallization of abundant apatite.At this point, the magma system exhibits higher fluid activity and lower volatile content, resulting in relatively lower enrichment of rare earth elements in the apatite crystallized within the syenite-pyroxenolite.(2) During the magma chamber stage at around 10 kilometers, due to the extensive crystallization of early apatite, the phosphate content decreases in the magma chamber, leading to the formation of a small amount of apatite.At this stage, the magma system exhibits lower fluid activity and higher volatile content, resulting in a higher enrichment of rare earth elements in the apatite crystallized within the gabbro-syenite.Therefore, both the syenite-pyroxenolite and gabbro-syenite in the Zhuopan alkaline complex show promising prospects for rare earth exploration to a certain extent.