Zircon U-Pb ages and geochemistry of the Huai'an TTG gneisses terrane: Petrogenesis and implications for ～2.5Ga crustal growth in the North China Craton

摘要

The Huai'an gneiss terrane is a typical Archean terrane within the Trans-North China Orogen of the North China Craton (NCC) and is composed of Tonalite-Trondhjemite-Granodiorite (TTG) suite and dioritic granulite facies gneisses. Precise SHRIMPII and Cameca IMS1280 zircon U-Pb ages demonstrate that the Huai'an gneisses formed around 2.55-2.50Ga and underwent granulite facies metamorphism at 1.85-1.80Ga. The TTG gneisses are compositionally equivalent to high-SiO _2 adakites, with high SiO _2 and Al _2O _3 concentrations, strongly fractionated rare earth element (REE) patterns ((La/Yb) _N=10-54) with positive or negligible Eu anomalies (Eu/Eu *=0.9-3.4), low heavy REE (HREE) and Y concentrations, high Sr (435-938ppm) concentrations, and high Sr/Y ratios (48-261). High Mg numbers (33-60) and high compatible element concentrations (e.g., Cr=11-183ppm and Ni=11-82ppm) indicate that the magmas that formed these gneisses were derived from interaction between adakitic melts and mantle material. The dioritic gneisses have compositions similar to high-Mg diorites or sanukitoids, with high Al _2O _3 (14.46-17.61wt%), MgO (3.0-6.4wt%), Cr (100-304ppm), and Ni (57-109ppm) concentrations, and fractionated REE patterns with weak negative or no Eu anomalies (Eu/Eu * =0.8-1.0). All the TTG and dioritic gneiss samples analysed during this study have negative Nb, Ta, and Ti anomalies, compared to neighbouring elements in spider-diagrams, with the majority having high positive e{open} _(Nd)(t) and e{open} _(Hf)(t) values, close to values of the depleted mantle. These geochemical characteristics strongly suggest that protoliths of the Huai'an TTG and dioritic gneisses originated in a juvenile crust-forming event at 2.55-2.50Ga through partial melting of subducted oceanic crustal material; these magmas also assimilated mantle peridotite material during their ascent. This indicates that ～2.5Ga mantle-derived magmatism led to significant crustal growth in the North China Craton, and was probably associated with subduction.