2.9 GA Magmatism in Eastern Hebei, North China Craton

We report the results of a comprehensive isotopic investigation of zircons from metamorphosed volcanic rocks from Caochang village in Zunhua complex, Eastern Hebei. A detailed investigation of internal structures of zircons, combined with SIMS U-Pb and oxygen isotope analyses and ICP-MS Hf zircon analyses as well as whole-rock geochemical study revealed the previously unknown 2.9Ga magmatic event and 2.87Ga metamorphic event in Eastern Hebei. The Mesoarchean rocks include high silica rhyolites, high silica sodic rhyolites, dacites and basalts. They together constitute an ancient bimodal volcanic rock series, which may be the largest outcrop of Mesoarchean supracrustal rocks in North China craton. Low δ18O compositions and inherited cores of zircons, low Cr, Ni composition and negative Eu anomaly in whole rock geochemical characteristic, as well as the relevant study of experimental petrology indicate a genesis of shallow crustal recycling with deep hydrothermal circulation. Their lower ɛNd(t) and ɛHf(t) (close to CHUR) indicate rapid reworking of altered juvenile crust. Thus these rhyolites may be formed by remelting of young hydrothermally altered basaltic crust or altered felsic wallrocks. All the geochemical evidence and the existence of Archean potassic rhyolites as well as the bimodal volcanic rock series imply a continent rift zone setting. The existence of Mesoarchean rocks and widespread ancient inherited and detrital zircons (>2.6 Ga) imply a possible ancient continent in Eastern Hebei, not only in Caozhuang nearby but also in Zunhua-Qinglong area. They provide valuable information about the Mesoarchean magmatic evolution. Meanwhile, they may imply the mesoarchean era (especially the 2.9 Ga) represent another major magmatic event of equal importance with 2.6-2.5 Ga period, at least in the eastern block. The magmatic and metamorphic history in Eastern Hebei are far more complex than previously thought.