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The NE Jiangxi ophiolite in the eastern Jiangnan Orogen is a tectonic melange that mainly consists of individual tectonic blocks comprising pyroxenite, gabbro, basalt, diorite, granite and chert in a matrix of serpentinite or tuffaceous greywacke. A combined geochemical and geochronological study of the NE Jiangxi ophiolite was undertaken to constrain the timing and tectonic setting of its formation. The basalts were geochemically subdivided into three groups with different FeOt and TiO2 contents. Group 1 basalts have the lowest FeOt (12.17–13.07 wt.%) and TiO2 (1.48–1.62 wt.%) contents and the lowest Nb/Yb (0.80–0.88) and Th/Nb (0.02–0.03) ratios. Furthermore, they have normal mid-ocean ridge basalt (N-MORB)-like trace element patterns, suggesting derivation from an N-MORB-type mantle source without subduction input. Group 2 Fe–Ti basalts have the highest FeOt (15.52–16.30 wt.%) and TiO2 (3.06–3.23 wt.%) contents, Nb/Yb and Th/Nb ratios from 1.75 to 1.89 and from 0.11 to 0.15, respectively, and trace element patterns similar to those of back-arc basin basalts. The geochemical characteristics suggest that Group 2 basalts were derived from a slightly enriched MORB-type mantle source with a minor subduction contribution. In contrast, Group 3 Fe–Ti basalts have moderate FeOt (12.98–13.40 wt.%) and TiO2 (2.37–2.71 wt.%) contents, and Nb/Yb and Th/Nb ratios from 1.28 to 1.45 and from 0.27 to 0.30, respectively. These basalts further display markedly negative Nb–Ta anomalies and show a geochemical affinity to island-arc basalts (IAB), indicating a slightly enriched MORB-type mantle source that was significantly influenced by subduction-derived fluids and/or melts. SIMS zircon U–Pb dating on gabbros gave ages of 995 ± 22 Ma and 993 ± 12 Ma, which are interpreted as the formation age of the NE Jiangxi ophiolite. Positive zircon eHf(t) (+ 8.8 to + 13.8) values for the gabbros and whole-rock eNd(t) (+ 5.5 to + 6.6) values for the basalts indicate that the NE Jiangxi ophiolite originated from an isotopically homogeneous depleted mantle source. The diversity of MORB- to IAB-like basalts and the presence of Fe–Ti basalts favor a formation of the NE Jiangxi ophiolite during the initial rifting phase of an intra-oceanic back-arc basin between an oceanic arc (Huaiyu Terrane) and the continental margin of the Yangtze Block (Jiuling Terrane) at ca. 990 Ma. Both the present and previous studies imply that multiple arc–back-arc systems formed during long-lasting subduction–accretion processes between the Yangtze and Cathaysia blocks during the early Neoproterozoic. |
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