Late Neoarchean reworking of the Mesoarchean crustal remnant in northern Liaoning, North China Craton: A U-Pb-Hf-O-Nd perspective

摘要

How the earth's crust formed and evolved during the Precambrian times is one of the key questions to decipher the evolution of the early Earth. As one of the few cratons containing well-preserved Eoarchean to Neoarchean basement on Earth, the North China Craton is an ideal natural laboratory to unravel the early crustal evolution. It is controversial whether the Archean tectonothermal events in this area represents reworking or growth of the continental crust. To solve this issue, we have compelled field-based mapping, zircon U-Pb dating by SHRIMP RG and LA-ICP-MS U-Pb, zircon SHRIMP SI oxygen and LA-MC-ICP-MS Hf isotope, and whole-rock Nd-O isotope analyses from the Archean granitoids in northern Liaoning, North China Craton. On the basis of zircon U-Pb isotopic dating andmeasured geological section investigation, two distinct magmatic suites as enclaves in the Jurassic granites are recognized, viz. a newly discovered 3.0 Ga crustal remnant and a 2.5 Ga granitoid. The Mesoarchean zircons from the 3.0 Ga granodioritic gneisses exhibit heterogeneous Hf isotopic compositions, with the most radiogenic analysis (epsilon(Hf)(t) = +3.8) following the depleted mantle evolution array and the most unradiogenic epsilon(Hf)(t) extending down to -3.4. This implies that both ancient continental crust at least as old as 3.4 Ga and depleted mantle contributed to the magma source of the protoliths of the Mesoarchean gneisses. The epsilon(Hf)(t) values of the Neoarchean zircons from these gneisses overlap the 3.4-3.0 Ga zircon evolution trend, indicating that the ancient crustal materials have been reworked during the late Neoarchean. The Neoarchean zircons from the 2.5 Ga granitoids have a relatively small variation in the Hf isotope and are mainly plotted in the 3.0-2.8 Ga zircon evolution field. However, taking all the epsilon(Hf)(t) values of the Neoarchean zircons into the consideration, we find that the Hf model age of the Neoarchean zircon does not represent the time of crustal growth or reworking but are artifacts of magma mixing. The interaction between the magmas derived from the ancient crustalmaterials and the depleted mantle is also supported by zircon O isotopic data and Hf-O isotopic-modeling of the Neoarchean granitoids. Both Mesoarchean and late Neoarchean tectonothermal events involved synchronous crustal growth and reworking, which may be applicable to other parts of the world. (c) 2019 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.