Zircon U-Pb ages and Hf isotopes of the Huai'an gneisses from the Tianzhen-Xinpingbu area: Implications for the tectonic evolution of the Trans-North China Orogen

One of the major advancements in understanding the formation and evolution of the North China Craton was the recognition of a Himalayan-type collisional belt in the central part of the craton, called the Trans-North China Orogen (TNCO), along which two micro-continental blocks, named the Eastern Block and Western Block, joined together to form a coherent craton. However, controversy still remains about the pre-collisional tectonic setting and architectures of the TNCO. In this contribution, field-based zircon U-Pb and Lu-Hf isotopic investigations were carried out on the Huai'an gneisses from the Tianzhen-Xinpingbu area in the northern part of the TNCO. The Huai'an gneisses consist predominantly of tonalitic-trondhjemitic-granodioritic (TTG) gneisses with minor gabbroic, dioritic and granitic gneisses. Our zircon U-Pb results show that the plutonic protoliths of the Huai'an gneisses were emplaced at 2.51-2.40 Ga and experienced a granulite-facies metamorphic event at 1.86-1.80 Ga. The magmatic zircons of these samples yield epsilon(Hf)(t) values from +1.9 to +7.0 and T-DM(C) ages of 2862-2619 Ma. These positive epsilon(Hf)(t) values, associated with the lack of old inherited zircons, suggest the derivation of their precursor magmas from a juvenile crust without contamination of old continental crust. The T-DM(C) model ages indicate that 2.8-2.6 Ga is the best estimation of the mantle extraction time for the source rocks in the Huai'an gneisses, revealing a major period of juvenile crustal growth. Thus it can be inferred that the partial melting of subducted oceanic crust slab produced the magmas of the Huai'an gneisses, which subsequently interacted with overlying mantle wedge during the ascent, suggesting an infra-oceanic arc environment for the Huai'an gneiss complex. Taken together, the development of the Huai'an gneiss complex was involved in a subduction-related magmatic event in an intra-oceanic arc at similar to 2.5 Ga, followed by collision between the Eastern and Western blocks at similar to 1.85 Ga.