Textural and chemical variation of apatite from Ziyang-Zhuxi alkaline rocks (Central China): Implications for REE migration during magmatic and hydrothermal processes

Apatite is a ubiquitous rare earth elements (REE)-rich accessory mineral of great significance in revealing the behavior of REE in alkaline rocks. This study investigates the textural, elemental, and RbSr isotopic compositions of apatite from a series of alkaline volcanic-intrusive rocks in the Ziyang-Zhuxi region of the South Qinling Belt (China), including gabbroic rocks (pyroxenite, basalt, gabbro, and diabase), dioritic rocks (pyroxene diorite and amphibole monzonite), and syenitic rocks (biotite syenite and trachyte). Two main types of apatite, magmatic and metasomatic, have been identified in the studied rocks. The magmatic apatite (Ap-1) is represented by euhedral hexagonal crystals and commonly occurs as inclusions in pyroxene, amphibole and feldspar. The metasomatic apatite (Ap-2) generally occurs in syenitic rocks, exhibiting obvious metasomatic textures and is commonly associated with secondary monazite. In Ap-1, the LILE (e.g., Sr, Ba) and compatible elements (e.g., V, Sc) decrease from alkaline mafic rocks to felsic rocks, indicating that fractionation of feldspars and mafic minerals such as pyroxene and amphibole dominated the magma evolution process. The REE contents (ΣREE from 4060 to 37,100 ppm), however, increase accordingly, suggesting that magma evolution promotes REE enrichment in the residual melt. On the other hand, the metasomatic apatite (Ap-2) displays obvious lower ΣREE contents (mean of 10,480 ppm) than their magmatic apatite (Ap-1) counterparts (mean of 30,890 ppm), indicating a leaching process of REE from primary apatite during hydrothermal fluid metasomatism. The leached REE from apatite (∼30 %) either precipitated into monazite as an in-situ replacement or were remobilized and migrated locally. The higher Sr contents and ⁸⁷Sr/⁸⁶Sr ratios of Ap-2 than those of Ap-1 (0.7090–0.7107 vs 0.7033–0.7046) indicate an external contribution of Sr from the late hydrothermal fluids. Therefore, we propose that the REE enrichment mechanism of the studied alkaline rocks can be attributed to two major processes: magma fractional crystallization followed by hydrothermal metasomatism.