Xenocrystic Zircons from Mafic Volcanic Rocks in the Bikou Terrane: A Window to Trace the Paleoarchean to Mesoproterozoic Crustal Evolution of the Northwestern Yangtze Block, South China

The early crustal properties of the northwestern Yangtze domain are a crucial puzzle piece in elucidating the Archean to Mesoproterozoic crustal evolution of the Yangtze Block. However, related comprehension remains constrained by the scarcity of surface exposures. The accidental discovery of xenocrystic zircons in the Bikou Terrane offers valuable insights into the information of the deep crust in the region. This study presents systematic zircon U-Pb geochronology, zircon Hf isotopes and whole-rock geochemistry on this specific set of mafic meta-volcanic rocks identified from the Bikou Terrane. The zircon properties, alongside the dating results, indicate a xenocrystic origin for all the extracted zircons, with ages scattered at ca. 3.50, 2.88 and 2.72-2.62 Ga in the Archean, ca. 2.52-2.37, 2.23-2.16, 2.07-1.95 and 1.90-1.77 Ga in the Paleoproterozoic, ca. 1.16-1.05 Ga in the Mesoproterozoic and ca. 820 Ma in the Neoproterozoic. The youngest zircon age clusters, plus the regional geological fact that the Bikou Group was covered by the Cryogenian, indicate their formation in the late Tonian. The geochemical fingerprints demonstrate the OIB-like nature of the mafic magma but with apparent Nb-Ta depletion, confirming their deep-seated crustal contamination. Combining Hf isotopic analysis reveals that the xenocrystic zircons not only encompass the information reflected in the Yudongzi Complex but also preserve additional new imprints pointing to the presence of a unified deep crust within the Bikou Terrane and a more intricate early evolutionary history. An intergraded examination of the early geological records in the northwestern Yangtze Block reveals multiple early crustal evolution events, including pulsed crustal growth/ reworking at ca. 3.50, 2.88-2.78 and 2.72-2.62 Ga in the Paleoarchean-Neoarchean, ancient crust reworking at ca. 2.52-2.37 and 2.23-2.16 Ga plus reworking alongside high-grade metamorphism at ca. 2.07-1.95 Ga in the early Paleoproterozoic, followed by recycling events involving new component at ca. 1.90-1.76 and 1.16-1.05 Ga in the latest Paleo- and Mesoproterozoic, respectively. Comparative considerations indicate a discrete history in various Yangtze domains during the Archean, gradually converging by the early Paleoproterozoic. The latest Paleo- and Mesoproterozoic brought allied crustal remelting and vertical accretion, implying the potential unity of the Yangtze Block by the late Paleoproterozoic.