Petrological and mineralogical constraints on the formation of Neoproterozoic calc-alkaline continental crust: The example of the Luding Intrusive complex, western Yangtze Block, South China

Decoding the trans -crustal magmatic evolution processes from exposed paleo-depth that are otherwise inaccessible above continental arc zone is instructive to understand the geochemical fractionation of arc magmatism and geochemical stratification of continental crust. To unravel trans -crustal magmatic processes leading to the formation of Neoproterozoic continental arc, we combined a comprehensive dataset of petrography, zircon U-Pb geochronology, whole -rock major and trace elements, and Sr-Nd isotopes as well as zircon Lu-Hf isotopes for the Neoproterozoic calc-alkaline Luding Intrusive Complex (LIC) in the western Yangtze Block, South China. The LIC is composed of monzodiorites, gabbro-diorites, diorites, and granodiorites that formed at ca. 783-779 Ma. The LIC lithologies are calc-alkaline with highly variable Mg# values (40.8-70.6) and subductionrelated trace elemental patterns. In combination with predominantly depleted and comparable whole -rock Sr-Nd (initial 87Sr/86Sr = 0.703244-0.704248 and epsilon Nd(t) = +2.35 - +5.10) and zircon Hf isotopes (epsilon Hf(t) = +4.47 - +9.99), the LIC originated from a common subduction fluids-metasomatized mantle source. The hornblende geothermobarometry and whole -rock normative mineral trajectories as well as previously experimental results collectively suggest that the crystallization of LIC occurred within the middle crustal level (11.4-20.9 km) at various temperatures (776-875 degrees C) and pressures (2.50-5.50 kbar), and underwent different degrees of fractional crystallization of predominant hornblende and plagioclase phases during the assemblage and solidification of cogenetic and multiple batches magmatic pluming. The diorites-granodiorites result from fractionation of early olivine + clinopyroxene phases followed by subsequent amphibole + plagioclase phases, and the monzodiorites mainly represent accumulation residues of dominant hornblende + plagioclase phases from more fractionated melts, while the gabbro-diorites underwent the fractionation from an isotopically homogeneous mantle source, but differ in their elemental compositions. The LIC from Neoproterozoic continental arc suggests that the cogenetic fractional crystallization of mantle -derived melts in intra-crustal magmatic system can account for the petrochemical diversities of arc magmatic rocks during the construction of the Neoproterozoic continental arc crust in the western Yangzte Block, South China.