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

摘要

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 2445-2533 Ma, and register three later tectonomagmatic events with ages of similar to 2360, similar to 2150 and similar to 1850 Ma. The similar to 2360 Ma magmatism is recorded by a dioritic gneiss (2358 +/- 34 Ma) from the Dongyanghe area and a granodioritic gneiss (2370 +/- 24 Ma) from the Hunyuanyao area. The similar to 2150 Ma magmatism is recognized in a dioritic gneiss (2172 +/- 7 Ma), granodioritic gneiss (2159 +/- 63 Ma) and the Mesozoic mafic dyke (2144 +/- 57 Ma). The similar to 1850 Ma magmatism is represented by zircon populations with ages of 1877 +/- 48 Ma in the dioritic gneiss, 1844 +/- 69 Ma in the granodioritic gneiss and similar to 1800 Ma 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 +/- 32 Ma, 1952 +/- 25 Ma and 1901 +/- 25 Ma. These three age groups suggest that the Kouzicun area experienced a tectonothermal evolution diagnostic of the Khondalite Belt, including protolith ages (2100-2000 Ma), peak metamorphism (similar to 1950 Ma) and post-orogenic exhumation (similar to 1900 Ma). 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 similar to 2500 Ma mantle-derived mafic magmas not only provided enormous heat for partial melting of the 2.7-2.8 Ga 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 2.7-2.8 Ga and at 2.5 Ga. (C) 2014 Elsevier B.V. All rights reserved.