Evolution of alkaline magmas and enrichment of rare earth elements: Insights from the geochemistry of apatite in the Saima alkaline igneous complex, Liaodong Peninsula, China

Abstract

Rare earth element (REE) mineralization related to alkaline magmas is an important source of REEs, and some deposits are enriched in heavy REEs (HREEs). However, the mechanisms of HREE enrichment in alkaline igneous rocks are unclear. In this study, we conducted petrographic, U–Pb geochronological, and in situ elemental and isotopic analyses of apatite in the Saima alkaline igneous complex, Liaodong Peninsula, China, the aim was to constrain the HREE geochemical behavior during alkaline magma evolution. The Saima complex consists of hornblende–pyroxene syenite, biotite syenite, syenite, nepheline syenite, and lujavrite (in order of magmatic evolution). Apatite U–Pb geochronology has yielded Late Triassic (hornblende–pyroxene syenite: 223 ± 5 Ma; biotite syenite: 220 ± 3 Ma; syenite: 219 ± 6 Ma; nepheline syenite: 219 ± 10 Ma) ages. Apatite in the hornblende–pyroxene syenite, biotite syenite, and syenite has similar geochemical compositions and textures, contains melt inclusions, and is classified as type Ⅰ apatite that formed in a purely magmatic system. Two types of apatite occur in the nepheline syenite. The type Ⅱ apatite has high Sr/Y and non-chondritic Y/Ho ratios, contains melt inclusions and scarce fluid inclusions, and is formed in a H₂O-saturated magmatic system. The type Ⅲ apatite is characterized by abundant fluid inclusions and has higher Sr contents, Th/U ratios, and ¹⁴⁷Sm/¹⁴⁴Nd ratios than the other apatite types. It has lower light REE (LREE) contents and higher HREE contents as compared with the type Ⅱ apatite and is formed by the reaction of type Ⅱ apatite with Cl-rich fluids. The calculated REE patterns of the equilibrium melt, based on $D_{\mathrm{REE}}^{\mathrm{apatite}-melt}$ values, are different from the corresponding whole-rock geochemical data. This finding, combined with the results of a Rayleigh fractionation model, indicates that a crystal mush accumulation model can explain the generation of the Saima complex. The enrichment of volatile components (e.g., H₂O) and crystal accumulation during the evolution of the magma mush were key controls on the anomalous HREE enrichment in the evolved rocks of the complex.