A reworked ∼3.45 Ga continental microblock of the North China Craton: Constraints from zircon U-Pb-Lu-Hf isotopic systematics of the Archean Beitai-Waitoushan migmatite-syenogranite complex

Abstract

The Archean Beitai-Waitoushan migmatite-syenogranite complex (BWMC) is located in the northeast of the Anshan-Benxi continent block (ABCB) where has ∼3.8Ga rocks outcropped in the northeastern North China Craton (NCC). The BWMC consists chiefly of weakly migmatized tonalitic-trondhjemitic gneisses, migmatites with various volumes of potassium-rich granitoid veins from either syenogranite magma or anatexis of tonalitic- trondhjemitic gneisses, monzo- and syenogranitic intrusive rocks. LA-ICP-MS and SHRIMP zircon U-Th-Pb isotopic analyses reveal that the migmatized tonalitic-trondhjemitic gneisses were formed at 3454–3484Ma, and were the partial sources of the syenogranitic rocks and entire protolith of the migmatites. The dominant monzo- and syenogranitic magmas were emplaced at 2500–2540Ma and generally contain inherited Paleoarchean zircon grains with apparent 207Pb/206Pb ages of >3300Ma. An amphibolite sample in the form of boudin in the migmatized tonalitic-trondhjemitic gneisses formed at 3414Ma. These Paleoarchean rock samples and the inherited Paleoarchean zircons from the Neoarchean migmatites, monzo- and syenogranite samples indicate that a ∼3.45Ga continent microblock existed prior to the ∼2.50Ga migmatization and syenogranitic magmatism.When integrated with previous zircon Lu-Hf isotopic data from the study area, our new LA-ICP-MS zircon Lu-Hf isotope analyses for dated zircon spots from six dated samples reveal that the magmatic precursors of the ∼3.45Ga samples and the ancient inherited zircons from the Neoarchean migmatites and intrusive granitoid rocks were primarily derived from ∼3.45Ga partial melting of juvenile crustal materials from the depleted mantle, and either the mantle or more ancient continent crustal materials with CHUR-like Lu-Hf isotopic systematics. However, some of the Neoarchean monzo- and syenogranitoid rocks and the granitoid veins in the migmatites exhibit hybrid Lu-Hf isotopic features derived from both ancient ∼3.45Ga continental and Neoarchean juvenile crustal materials, including partial mixing between the Neoarchean granitoid magma and plastic Paleoarchean gneiss relicts.