From arc accretion to continental collision in the eastern Jiangnan Orogen: Evidence from two phases of S-type granites

The NE Jiangxi ophiolitic m lange separates the Huaiyu and Jiuling terranes in the eastern Jiangnan Orogen of the South China Block. Two phases of S-type granites, which are geochemically and mineralogically similar to the Himalayan leucogranites, are newly recognized in mantle peridotites of the NE Jiangxi ophiolite. The first phase, a tourmaline-bearing granite porphyry, has a SIMS zircon U-Pb age of 904.4 +/- 4.8 Ma and is strongly enriched in K2O (4.00-4.26 wt%) and strongly peraluminous (A/CNK = 2.94-3.24). It further shows high Rb/Sr ratios and highly fractionated REE patterns, indicating an origin due to fluid-absent melting of metapelites at relatively high temperatures. The granite porphyry exhibits highly evolved zircon Hf ( -6.0 to -1.0) and whole rock Nd ( -4.2 to -5.1) isotopes, which is in contrast to juvenile-derived magmatic rocks in the Huaiyu Terrane. We thus propose that the granite porphyry was formed by melting of the metapelites sourced from the Yangtze Block subsequent to the accretion of the Huaiyu arc to the Jiuling Terrane (the continental margin of the Yangtze Block). The second phase, a muscovite-bearing quartz porphyry, is strongly peraluminous (A/CNK = 1.29-1.34) with low CaO/Na2O ratios (0.11-0.14) and low FeOt + MgO + TiO2 contents (2.39-2.75 wt%), similar to melts derived from fluid-fluxed melting of metapelites. Zircon grains from the quartz porphyry yielded U-Pb ages of ca. 803-801 Ma with mainly positive epsilon(tif)(t) values ( + 1.9 to + 8.7), which are younger than the depositional ages of the basement sequences and nearly identical to the accumulation of the overlying unmetamorphosed cover in the Jiangnan Orogen. Therefore, it is suggested that the quartz porphyry was formed in a post-collisional extensional setting subsequent to the final assembly of the Yangtze and Cathaysia blocks. In conjunction with published data, we conclude that multiple subduction-accretion events may have been involved in the formation of the South China Block during Neoproterozoic times.