Carbonate metasomatism in sub-continental lithospheric mantle: Insights from decoupled Nd-Hf isotopes of Mesozoic gabbros in Jinan, eastern North China Craton

The ancient sub-continental lithospheric mantle (SCLM) of the eastern North China Craton (NCC) underwent metasomatism in the Triassic due to fluids derived from subducted Yangtze continental crust, followed by further metasomatism during the Jurassic-Cretaceous caused by fluids originating from subducted Paleo-Pacific oceanic slabs. However, the occurrence of carbonate metasomatism during these two geological events is a subject of debate, and the origin of carbonate-bearing metasomatic agents remains ambiguous. Here, we present an integrated study of petrography, whole-rock trace element data and Sr-Nd-Hf-Pb isotope data for the Jinan gabbros in the Luxi block of the eastern NCC. The Jinan gabbros show arc-like trace element patterns and enriched Sr-Nd-Hf-Pb isotopic compositions, suggesting that they originated from partial melting of the enriched ancient SCLM. In addition, there are remarkable similarities in Sr-Nd-Hf-Pb isotopes between the Jinan gabbros and HP-UHP metamorphic rocks in the Dabie-Sulu orogenic belt, indicating the involvement of continental crustal components from the subducted Yangtze block in the mantle source of the Jinan gabbros. The gabbro samples show decoupled Nd-Hf isotopes with positive Delta eHf values (Delta eHf = eHf - 1.55 x eNd - 1.21; Vervoort et al., 2011), which were mainly caused by a metasomatic agent with high Nd/Hf ratios. The increase in eNd values with increasing Hf/Hf* (Hf/Hf* = HfPM/[(NdPM x SmPM)0.5]) and Nb/La further indicates that the metasomatic agent in the mantle source was depleted in high field strength elements (HFSEs) but enriched in rare earth elements (REEs), resembling the mantle-derived carbonatites in the Luxi area. The Jinan gabbros show significant variations in eNd and eHf, which cannot be explained by the involvement of C-O-H fluids and pure carbonatitic melts in their mantle source. Instead, carbonate-bearing felsic melts from the deeply subducted Yangtze continental crust are proposed to have acted as metasomatic agents. Carbonate metasomatism can significantly weaken the strength and decrease the melting temperature of the overlying ancient SCLM. Therefore, carbonate metasomatism that occurred in the lithospheric mantle of the eastern NCC during the Triassic, likely associated with deep subduction of the Yangtze block, might have played a significant role in promoting lithospheric thinning and mafic magmatism in the late Mesozoic.