Coupled alkaline high-Nb mafic rocks and adakitic granodiorites: Products of Neoproterozoic back-arc extension at the western margin of the Yangtze Block, South China

The newly identified Neoproterozoic Yuanmou high-Nb mafic and adakitic rocks at the western margin of the Yangtze Block in South China are investigated for petrogenesis and geodynamic implications. Zircon U-Pb ages show the adakitic granodiorite host and mafic microgranular enclaves (MMEs) were synchronously emplaced at 811−806 Ma, and were cut by an 802 ± 11.5 Ma mafic dike. The medium-grained granodiorites consist mainly of plagioclase, quartz, K-felspar, biotite, and amphibole. High SiO₂ (62.8−67.5 wt%), Al₂O₃ (13.1−16.8 wt%) contents, high Sr/Y (17.4−49.0), and (La/Yb)_N (16.3−52.6) but low Y (6.83−16.1 ppm) and Yb (0.74−1.82 ppm) concentrations of the granodiorites point toward their calc-alkaline and adakitic nature. Further, their high Dy/Yb (1.52−2.31) and Gd/Y (2.35−4.43) and low K₂O/Na₂O (0.22−0.48) and Th/La (0.14−0.20) ratios, constant bulk rock ε_Nd(t) (−0.5 to −1.5) and zircon ε_Hf(t) (0.0 to +2.3) values, suggest that the granodiorites were derived from partial melting of a mafic lower crustal source. The MMEs in the granodiorites and mafic dike intrusions are composed of amphibole, plagioclase, clinopyroxene, and olivine, and have similar chemical compositions, such as high Nb (15.7−41.9 ppm) and TiO₂ (2.13−3.39 wt%) contents and high Nb/U (18.7−30.9) values, akin to alkaline high-Nb basalts (HNB). Their variable Ba/Nb (7.68−79.2), Nb/Zr (0.09−0.16), high Nb/Th (5.47−6.68), and low Th/Zr (0.01−0.03) and La/Nb (1.03−1.37) ratios indicate a mantle source modified by melts derived from subducted oceanic crust and sediments. The Yuanmou HNB with high ε_Nd(t) (+4.8 to +6.9) were probably generated during back-arc extension by decompression melting of a mantle source metasomatized by adakitic slab melts formed during earlier stages of the subduction. This implies that the location of the back-arc was far from the subducted oceanic slab, possibly due to a slab rollback process. In this scenario, HNB intrusions provided the heat for partial melting and adakitic melt formation in the mafic lower crust. The model suggests that the Yuanmou and other similar associations of HNBs and adakitic rocks emplaced along the western margin of the Yangtze Block are products of a widespread Neoproterozoic back-arc extension.