Zircon trace elemental and Hf-O isotopic constraints on the petrogenesis of the REE-fertile and barren alkaline rocks in the South Qinling Belt, China

Abstract

Alkaline silicate rocks and/or associated carbonatites are important hosts of rare earth elements (REE) resources. Previous studies have suggested that REE-fertile alkaline rocks and/or carbonatites were derived from a mantle source that may have once been metasomatized by recycled crustal materials. However, it is still not clear if the source metasomatism is a prerequisite of REE mineralization. In this study, we investigated and compared both REE-fertile and barren alkaline plutons in South Qinling, Central China with emphasis on zircon elemental and Hf-O isotopic compositions. The REE-fertile alkaline pluton, i.e. the Miaoya (MY) complex, is dominated by syenite with minor coeval carbonatite dykes/stocks, whereas the REE-barren alkaline plutons, i.e. the Huangyangshan (HYS) and Guanzishan (GZS) plutons, are composed mainly of quartz-bearing syenite without carbonatites. Our new U-Pb dating of magmatic zircon demonstrates that the fertile and barren alkaline complexes have identical ages of ca. 440 Ma. The syenites in both fertile and barren alkaline plutons have similar affinities of A-type granitoids with comparably high Ti-in-zircon temperatures of 632 to 878 °C. In particular, the fertile syenites have zircon Hf and O isotopic compositions (ε_Hf(t): −3.4 to + 8.2, δ¹⁸O: +3.3 to + 6.9 ‰) also broadly similar to the barren syenites (ε_Hf(t): −1.6 to + 10.1, δ¹⁸O: +3.5 to + 8.0 ‰), indicating that they may be sourced from a common, enriched mantle source that likely included components from oceanic crustal material, such as sediments and/or altered oceanic crust. However, compared to the fertile syenites, the barren syenites exhibit much higher SiO₂ (up to 75 %) but lower MgO contents, and thus were suggested to be likely formed through partial melting of a juvenile mafic lower crust. It is reasonable to speculate that the mafic lower crust was likely formed from mafic magmas derived from the enriched mantle due to their undistinguishable zircon Hf and O isotopic ratios. Zircon trace element analyses reveal that the barren syenites exhibit ΔFMQ values (+0.95 to + 6.74) obviously higher than those of the fertile syenites (−1.14 to + 0.88), indicating a more oxidized source characteristic. Since an oxidized source is conducive to carbon stability, promoting the formation of carbonatite-bearing syenite complexes (which are favorable for REE mineralization), the absence of carbonatites in the relatively oxidized Huangyangshan and Guanzishan syenites might be attributed to a lack of carbon in their source. Our findings emphasize the critical role of oceanic crust recycling in producing REE-fertile, carbonaceous silicate melts and underscore that source variations exert a primary control on REE fertility of alkaline plutons within specific alkaline magmatic belts.