Water-fluxed melting and back-arc extension in the continental arc: Evidence from I-type granites, adakitic rocks and high-Nb mafic rocks at the western margin of the Yangtze Block, South China

The Neoproterozoic western margin of the Yangtze Block in South China records significant continental crust-forming and modification processes through two distinct magmatic episodes. Using integrated geochemical and petrological approaches, we demonstrate that the 811-802 Ma Yuanmou Complex comprises alkaline high-Nb mafic rocks characterized by high Nb (15.7-41.9 ppm), TiO₂ (2.13-3.39 wt%) contents and positive ε_Nd(t) (+4.8 to +6.9), coupled with adakitic granodiorites showing high Sr/Y (17.4-49.0), (La/Yb)_N (16.3-52.6) and consistent bulk rock ε_Nd(t) (-0.5 to -1.5) and zircon ε_Hf(t) (0.0 to +2.3). The younger 750 Ma Jinping I-type granites exhibit high SiO₂ (71.2-73.5 wt%) and alkalis contents, enriched LREE patterns and depleted isotopic signatures (ε_Nd(t): -0.4 to +1.3; zircon ε_Hf(t): +4.83 to +8.37). Thermodynamic modeling reveals how crustal water content-controlled magma generation at different depths - low water-fluxed melting (2.0-3.5 wt% H₂O) produced I-type granites at medium pressure (6-9 kbar), while deeper settings with higher water content generated adakitic melts. The high-Nb mafic rocks in the Yuanmou Complex, derived from metasomatized mantle wedge, provide evidence for crustal-mantle interaction during back-arc extension. These coupled magmatic processes demonstrate how water content variations with depth influenced continental crust formation and evolution in arc settings.