Ziron U-Pb ages and geochemistry of the Qinglong volcano-sedimentary rock series in Eastern Hebei: Implication for ∼2500 Ma intra-continental rifting in the North China Craton

Voluminous Archean rocks (including orthogneisses and supracrustal rocks) are exposed in Eastern Hebei, northern part of the North China Craton (NCC), and nearly all of these Archean rocks have experienced high-grade metamorphism of granulite-facies or upper amphibolite facies from 2.54 Ga to 2.52 Ga. The Qinglong area of the eastern part of Hebei Province has a set of low-grade (greenschist- to lower amphibolite-facies) supracrustal rocks, e.g., the Qinglong volcano-sedimentary rock series (QVRS), unconformably overlying on the high-grade metamorphosed Qianxi Complex. Zircons yield a weighted mean Pb-207/Pb-206 age of 2511 +/- 12 Ma meta-felsic volcanic rocks and 2503 +/- 13 Ma meta-basic volcanic rocks. These ages indicate that the QVRS formed during the period from 2511 Ma to 2503 Ma. The meta-felsic volcanic rocks display low MgO (0.96-2.67 wt.%), Cr (50.90-87.10 ppm), and Ni (2.78-6.18 ppm) contents, and low Nb/Ta ratios (13.56-14.54), and high light rare earth elements (LREEs) ((La/Yb)(n) = 14.75-22.04), which are interpreted to have been produced by the partial melting of the lower crust. The meta-basic volcanic rocks (low-grade amphibolites) have abundance levels from 3.86 wt.% to 7.74 wt.% of MgO, and from 46.11 wt.% to 51.99 wt.% of SiO2. They are enriched in LREEs and depleted in high field strength elements (HFSEs) with positive epsilon(Nd)(t) values (+2.6 to +2.9). The volcanic rocks in the QVRS show bimodal compositions. The epsilon(Hf)(t) (t=2511 Ma) values of amphibolite range from -4.7 to +5.7, and their single-stage Hf model ages vary from 2622 Ma to 3030 Ma (peak at 2835 Ma). For the meta-felsic volcanic rocks, the epsilon(Hf)(t) values range from -3.2 to +5 with two-stage model ages changing from 2683 Ma to 3180 Ma (peak at 2872 Ma). The QVRS is proposed to have formed during an intra-continent rift at 2500 Ma, after the regional high-grade metamorphism from 2.52 Ga to 2.54 Ga. The QVRS probably formed during an intra-continental rift, as a part of the late Neoarchean cover of the NCC. (C) 2012 Elsevier B.V. All rights reserved.