U–Pb–Hf–O–Nd isotopic and geochemical constraints on the origin of Archean TTG gneisses from the North China Craton: Implications for crustal growth

Abstract

Tonalite–trondhjemite–granodiorite (TTG), an Archean dominant lithological assemblage within the cratonic provinces, is believed to have been formed by partial melting of the hydrous basalts in the garnet stability field. The origin of TTG is a window to decipher the evolution of Archean continental crust, whereas the petrogenesis is not well constrained. To solve this issue, we have carried out field observations, zircon U–Pb–Hf–O isotope, as well as whole-rock Nd–O isotope and geochemical analyses on the TTG and associated dioritic gneisses from the North China Craton. U–Pb isotopic dating using the LA–ICP–MS on zircons reveals that the granitoid gneisses in northern Liaoning were contemporaneously generated during ca. 2.56–2.51 Ga and immediately experienced a regional metamorphic event at 2.48–2.45 Ga. The dioritic gneisses occur as quenched inclusions with rounded to ellipsoidal globules in shape rather than lithospheric relics. Such occurrence, together with isotopic and geochemical data, suggests that the mafic enclaves and host TTG represent two co-existing, compositionally distinct magmas. The TTG gneisses are featured by elevated SiO2 (64.72–86.46 wt%) contents, (La/Yb)N (68 on average) and Sr/Y (119 on average) ratios, and low MgO (0.35–2.58 wt%) contents and Mg# values, with radiogenic Hf and Nd isotopes (εHf(t) = +3.5 to +9.0 and εNd(t) = +0.8 to +5.4), indicating the derivation from partial melting of the juvenile basaltic sources at lower crustal levels. Compared with the TTG gneisses, the dioritic gneisses have lower SiO2 contents, higher MgO contents and Mg# values, with similar Hf–Nd isotopes (εHf(t) = +3.6 to +9.5 and εNd(t) = +3.3 to +5.1), implying the depleted mantle source. A model is proposed whereby partial melts from juvenile basaltic sources evolved via mixing with mafic enclaves (viz. dioritic magma) and fractional crystallization, and finally were contaminated by the sedimentary rocks, forming the TTG magma. The granitoid gneisses in northern Liaoning witnessed the major vertical growth of continental crust at the end of the Neoarchean, which is supported by Hf–Nd–O isotopic data and zircon Hf–O isotopic modeling. Our results show that the geochemistry of TTG (such as elevated La/Yb and Sr/Y) might be affected by complex petrogenetic processes, i.e., partial melting of individual sources, magma mixing, fractional crystallization, and crustal contamination, rather than solely controlled by the P–T conditions of crustal anataxis as previously thought, which might have been also overlooked in other Archean cratons.