Paleoproterozoic meta-carbonates from the central segment of the Trans-North China Orogen: Zircon U-Pb geochronology, geochemistry, and carbon and oxygen isotopes

The Trans-North China Orogen (TNCO) represents one of the major Paleoproterozoic collisional orogens across the central part of the North China Craton. Here we investigate the meta-carbonates (marbles and calc-silicate rocks) from the Fuping and Zanhuang Complexes in the TNCO through a combination of detrital zircon U-Pb geochronology, geochemistry and stable isotopes (carbon and oxygen). The U-Pb ages obtained from detrital zircons in both sequences show age populations of 2.50-2.57 Ga, 2.2-2.4 Ga, 2.0-2.1 Ga and 1.85-1.90 Ga. The depositional age for the Wanzi Group in the Fuping Complex is further constrained as 1.95-1.93 Ga and the Central Zanhuang Domain (Zanhuang Complex) as 2.03-1.90 Ga. All samples show nearly flat REE patterns below the concentration level of 1 in the shale-normalized REE distribution diagram. The dolomite-calcite marbles from the Fuping Complex have delta C-13 values ranging from -0.31 parts per thousand to 3.65 parts per thousand and delta O-18 values vary from -9.12 parts per thousand to -2.96 parts per thousand. The calcsilicate rocks from the Fuping Complex show wide variation of delta C-13 (-1.56 parts per thousand to 3.30 parts per thousand) and delta O-18 (-12.89 parts per thousand to -5.84 parts per thousand). Dolomite marbles from the Zanhuang Complex show low delta C-13 and delta O-18 values ranging from -3.31 parts per thousand to -3.69 parts per thousand and -12.98 parts per thousand to -13.59 parts per thousand. The dolomite-calcite marbles from the Zanhuang Complex display delta C-13 and delta O-18 compositions of -2.68 parts per thousand to 1.44 parts per thousand and -11.77 parts per thousand to -7.76 parts per thousand, respectively. The correlations between the isotopic results (delta C-13 and delta O-18) and geochemical compositions (Mn, Sr, Mn/Sr, Mg/Ca, Ca/Sr and Fe/Sr) suggest that the dolomite marbles from the Zanhuang Complex are strongly disturbed and the calc-silicate rocks are moderately affected by the post-depositional processes. The salient lithological, geochemical and isotopic features imply that the protoliths of the meta-carbonates were formed in a proximal and shallow marine environment. The Wanzi Group was deposited in a forearc basin and the Central Zanhuang Domain was formed in a back-arc basin. The geochemical imprints and biospheres of the oceanic basin in the Fuping area were possibly affected by the Lomagundi or Jatulian Event, resulting in the positive excursion of delta C-13 values in the dolomite-calcite marbles. However, the back-arc basin in the Zanhuang area was formed after the 2.1-2.0 Ga rifting event, thus it was generated after the Jatulian Event, thus preserving normal delta C-13 values of marine carbonates. (C) 2016 Elsevier B.V. All rights reserved.