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

Abstract

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 2.50–2.57Ga, 2.2–2.4Ga, 2.0–2.1Ga and 1.85–1.90Ga. The depositional age for the Wanzi Group in the Fuping Complex is further constrained as 1.95–1.93Ga and the Central Zanhuang Domain (Zanhuang Complex) as 2.03–1.90Ga. 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 δ13C values ranging from −0.31‰ to 3.65‰ and δ18O values vary from −9.12‰ to −2.96‰. The calc-silicate rocks from the Fuping Complex show wide variation of δ13C (−1.56‰ to 3.30‰) and δ18O (−12.89‰ to −5.84‰). Dolomite marbles from the Zanhuang Complex show low δ13C and δ18O values ranging from −3.31‰ to −3.69‰ and −12.98‰ to −13.59‰. The dolomite–calcite marbles from the Zanhuang Complex display δ13C and δ18O compositions of −2.68‰ to 1.44‰ and −11.77‰ to −7.76‰, respectively. The correlations between the isotopic results (δ13C and δ18O) 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 δ13C values in the dolomite–calcite marbles. However, the back-arc basin in the Zanhuang area was formed after the 2.1–2.0Ga rifting event, thus it was generated after the Jatulian Event, thus preserving normal δ13C values of marine carbonates.