Zircon U-Pb and Lu-Hf isotopic and whole-rock geochemical constraints on the provenance and age of the Shuangshanzi and Qinglonghe Groups in Eastern Hebei: Implications for the tectonic evolution of the Eastern Block

Abstract

Eastern Hebei is located in the northern part of the Eastern Block, which is considered to have collided with the Western Block along the Trans-North China Orogen to form the basement of the North China Craton. It consists mainly of Eoarchean to Mesoarchean rocks, Neoarchean granitoid plutons and supracrustal rocks. The minor amounts of supracrustal rocks can be further divided into the high-grade Zunhua, Qianxi, Luanxian and Dantazi Groups and the low-grade Shuangshanzi and Qinglonghe Groups. The Shuangshanzi Group is composed of amphibolites, leptynites, schists and quartzites, whereas the overlying Qinglonghe Group consists predominantly of meta-conglomerates and leptynites. Geochemistry of the metasedimentary samples from the two low-grade groups indicates weak to moderate source weathering, dominantly chemically immature and major felsic sources with few mafic source rocks. On the tectonic discrimination plots, most of the metasedimentary samples display continental island arc feature. U-Pb ages of detrital zircons from the Shuangshanzi and Qinglonghe Groups yielded three age populations of 2600–2400Ma, 2400–2161Ma and 3662–2609Ma. The primary 2600–2400Ma detrital zircons were most probably sourced from the late Neoarchean to early Paleoproterozoic granitoids and amphibolites in Eastern Hebei. The subordinate 2400–2161Ma detrital zircons may have mostly derived from the meta-volcanic rocks in the Shuangshanzi Group, whereas minor amounts of 3662–2609Ma detrital zircons were most likely directly derived from the Paleoarchean to Neoarchean granitics or recycled from the Caozhuang quartzites in Eastern Hebei. The crystallization age of 2352±4Ma of a meta-porphyritic dacite and the youngest detrital zircon age peaks of 2337–2329Ma from the Shuangshanzi Group suggest that the sedimentation of the group began at ∼2.35Ga. The nearly contemporary youngest age peaks of 2331–2329Ma have also been identified from the overlying Qinglonghe Group. In combination with the ∼1.85Ga metamorphic event in the Trans-North China Orogen, the formation ages of the two groups can be constrained in the period between ∼2.35Ga and ∼1.85Ga. Taking into account the lithostratigraphic features, provenances and formation ages, the Shuangshanzi and Qinglonghe Groups are interpreted as having formed possibly in a continental intra-arc basin.