Zircon U–Pb ages and Hf isotope of gneissic rocks from the Huai’an Complex: Implications for crustal accretion and tectonic evolution in the northern margin of the North China Craton

Abstract

The Precambrian tectonic evolution of the North China Craton (NCC) has long been a matter of debate. The Huai’an Complex, situated at the conjunction of the Khondalite Belt and Trans-North China Orogen (TNCO), undoubtedly can provide some key constraints on the geological evolution of the Khondalite Belt and the TNCO. We report zircon U–Pb ages and Hf-isotope results for four gneissic rocks and one Mesozoic mafic dyke in the Huai’an Complex to constrain the Precambrian tectonothermal evolution and crustal accretion in the northern margin of the NCC. The zircons from three dioritic to granitic gneisses and inherited zircons from the mafic dyke all yield Neoarchean to early Paleoproterozoic protolith ages of 2445–2533Ma, and register three later tectonomagmatic events with ages of ∼2360, ∼2150 and ∼1850Ma. The ∼2360Ma magmatism is recorded by a dioritic gneiss (2358±34Ma) from the Dongyanghe area and a granodioritic gneiss (2370±24Ma) from the Hunyuanyao area. The ∼2150Ma magmatism is recognized in a dioritic gneiss (2172±7Ma), granodioritic gneiss (2159±63Ma) and the Mesozoic mafic dyke (2144±57Ma). The ∼1850Ma magmatism is represented by zircon populations with ages of 1877±48Ma in the dioritic gneiss, 1844±69Ma in the granodioritic gneiss and ∼1800Ma in the mafic dyke. These Neoarchean to Paleoproterozoic ages reveal that the Huai’an Complex underwent a successive, complicated tectonomagmatic evolution, similar to the Hengshan–Wutai–Fuping Complexes, which represent a long-lived Neoarchean to Paleoproterozoic magmatic arc. In contrast, a sillimanite–garnet gneiss from the Kouzicun area yields completely different zircon U–Pb ages with peaks at 2056±32Ma, 1952±25Ma and 1901±25Ma. These three age groups suggest that the Kouzicun area experienced a tectonothermal evolution diagnostic of the Khondalite Belt, including protolith ages (2100–2000Ma), peak metamorphism (∼1950Ma) and post-orogenic exhumation (∼1900Ma). Thus, the geological boundary between the Khondalite Belt and the TNCO can be constrained to a position between the Kouzicun and Hunyuanyao areas. Coupled with zircon U–Pb ages, zircon Hf isotopic results reveal that ∼2500Ma mantle-derived mafic magmas not only provided enormous heat for partial melting of the 2.7–2.8Ga juvenile crust to form widespread TTG rocks, but also contributed directly to the generation of dioritic rocks. The Huai’an Complex in the northern margin of the NCC witnessed two Neoarchean episodes of continental-crust growth at 2.7–2.8Ga and at 2.5Ga.