Abstract
The Hekou Group, an outcrop at the west margin of the Yangtze Block, consists of volcanic and sedimentary rocks of sodic lava, sodic pyroclastic rocks, coarse to fine-grained siliciclastic rocks and carbonate rocks. These rocks have endured lower to upper greenschist-facies metamorphism. The Hekou Group represents the basement of the late Mesoproterozoic to early Neoproterozoic successions and hosts many Fe-Cu deposits. We conducted zircon U-Pb and Lu-Hf isotope analyses and whole-rock geochemistry analyses on igneous rocks from the Hekou Group to constrain the formation age of the Hekou Group and to understand the genesis of the Fe-Cu deposits. A zircon 207Pb/206Pb age of 1669 ± 6 Ma was obtained from the tuffaceous schist of the Hekou Group and is considered to represent the deposition age of the group. Most rocks show negative high field strength element (HFSE, e.g., Ta, Nb, P, Zr, Hf and Ti) anomalies, light rare earth element (LREE) enrichment ((La/Yb)n = 12.10-109.01) and positive zircon εHf(t) values (+1.5 to +4.5). The geochemical and isotopic compositions indicate that the Hekou Group was formed in a back-arc basin at 1.7 Ga. Our study also implies that Paleoproterozoic tectonic evolution of the western Yangtze Block is correlated with the assembly and fragmentation of the supercontinent Columbia.
