Altered Oceanic Crust Source for the Alkaline Rocks with Superchondritic Nb/Ta Ratios Due to the Steep Subduction

Abstract

The cause resulting in alkaline rocks with superchondritic Nb/Ta ratios remains a geochemical enigma. This paper presents zircon and apatite geochronology, mineralogy, geochemistry, and Nd-Hf-Zn isotope investigation on the Ejinao nepheline syenite from South China to constrain the petrogenesis of alkaline rocks. The occurrence of alkaline minerals, including alkaline feldspar, arfvedsonite, and nepheline, indicate an alkaline feature of the Ejinao nepheline syenite. Zircon and apatite from the rock show U-Pb concordia age of 135.4 ± 0.9 and 137.5 ± 5.8 Ma, respectively, suggesting generation in the Early Cretaceous. The rock is characterized by low MgO (0.17 wt.%–0.26 wt.%) contents, but high Fe₂O₃^T (4.66 wt.%–6.16 wt.%) and total alkalis (Na₂O + K₂O = 12.1 wt.%–13.1 wt.%) contents, consistent with the melts from subducting oceanic crust rather than mantle peridotite. The rock has low TiO₂ (0.13 wt.%–0.21 wt.%) contents and superchondritic Nb/Ta and Zr/Hf ratios, resulting from rutile residue in the source area. The ε_Nd(t) (−1.35 to −1.09), ε_Hf(t) (−3.03 to −1.52), and δ⁶⁶Zn (0.32‰ to 0.34‰) values of the nepheline syenite manifest the involvement of 15%–20% carbonate-bearing sediments in the altered oceanic crust. Geochemical modeling further reveals that the nepheline syenite was sourced from 10%–20% melting of such a source. This superchondritic Nb/Ta nepheline syenite was generated during the steep subduction of the Paleo-Pacific Ocean, which is needed to bring oceanic slab to a hot mantle and resulted in the altered oceanic crust melting. Our study highlights that the altered oceanic crust represents a novel source for superchondritic Nb/Ta alkaline rocks.

Keywords

nepheline syenite / superchondritic Nb/Ta ratios / altered oceanic crust / Zn isotope / South China / geochemistry / structural geology

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