Carbonate recycling during intracontinental lithospheric extension recorded by Zn-Sr-Nd isotopes in an alkaline complex

The subcontinental lithospheric mantle (SCLM) represents a significant carbon reservoir, releasing CO2 through continental rift systems and impacting climate. However, the processes of carbonate recycling in the SCLM remain uncertain. Here we present Zn-Sr-Nd isotopes and bulk elemental compositions for the Late Cretaceous Gejiu alkaline complex, Southwest China. The monzogabbros exhibit low SiO2 and high CaO, MgO, Ba, and Sr contents, together with enriched Sr-Nd isotopes [initial 87Sr/86Sr = 0.7100−0.7110; εNd(t) = −6.8 to −6.5], indicating derivation from a metasomatized SCLM. Although their mid-oceanic-ridge basalt−like δ66Zn (0.25 ± 0.02‰) values are consistent with an unmodified mantle source, the high Zr and Hf contents, elevated Th/Nb (0.48−0.83) and Ba/Th (87.3−250) ratios, and low Ti/Eu ratios collectively suggest mantle metasomatism by “ghost-like,” carbonate-bearing sediment-derived fluids. The monzonites display geochemical and isotopic affinities similar to those of the monzogabbros (δ66Zn = 0.25‰ ± 0.03‰), supporting a cogenetic origin. The nepheline syenite exhibits extremely high K2O, Na2O, and light rare earth element contents, elevated La/Yb and Sm/Yb ratios, and enriched Sr-Nd isotopes [initial 87Sr/86Sr = 0.7097−0.7101; εNd(t) = −7.5 to −6.7], consistent with derivation from a metasomatized SCLM source. However, their heavy and variable δ66Zn values (0.25‰−0.39‰), together with high Th/Ta (29.0−69.4) ratios and positive Gd anomalies, are attributed to the injection of ∼5%−10% carbonated silicate melt. These results record a considerable carbonate signature in the petrogenesis of the Gejiu alkaline complex. Combined with the regional geology, these carbonate-bearing sediments could have been introduced into the SCLM during Paleo-Tethys subduction, after which the heterogeneous carbonated SCLM was remobilized during Late Cretaceous intracontinental extension. This study demonstrates that intraplate alkaline magmatism can effectively remobilize ancient, subduction-introduced carbonate in the SCLM, highlighting its critical role in carbon recycling over long-term geodynamic processes.