The Fuchuan ophiolite in Jiangnan Orogen: Geochemistry, zircon U–Pb geochronology, Hf isotope and implications for the Neoproterozoic assembly of South China

Abstract

The Fuchuan ophiolite occurs along the easternmost domain of the Jiangnan Orogen, considered as the suture between Yangtze and Cathaysia Blocks in South China. Here we report results from our new field observations, zircon U–Pb ages and Hf isotope of the cumulate units, chemistry and tectonic discrimination of Cr-spinel from the harzburgite, and whole rock geochemistry of the pillow lava from the ophiolite in an attempt to evaluate the Neoproterozoic tectonic evolution of South China. The zircon data indicate that the ophiolite suite formed at 840–820Ma, and the high εHf(t) (8–13) and εNd(t) (3.3–5.7) values suggest that the cumulates were derived from significantly depleted mantle sources. The pillow lavas and the keratophyres, show typical calc-alkalic continental arc-signatures including the enrichment in LREE and LILE, and depletion in HFSE (Nb/La=0.3 to 0.5) and a wide range of εNd(t) values (−1.2 to 2.5). The chromium-spinels from the serpentinized harzburgite show Cr# ranging from 40 to 67 and Mg# from 33 to 60 comparable with the chemistry of spinels from suprasubduction zone (SSZ)-type ophiolites. The geochemical features as well as the association of coeval I–S type granites and several thousand meters of flysch-type sedimentary sequences correlate the Fuchuan ophiolite to SSZ-type. We suggest that the emplacement of the Fuchuan ophiolite suite marks the time of final welding between the Yangtze and Cathaysia blocks in South China at ca.820Ma