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Abstract

This study reports, for the first time, Sr and Nd isotope ratios from the mafic rocks in the Manipur Ophiolite Complex (MOC), along with new elemental abundances to show the subduction zone influence. The initial 87Sr/86Sr ratios (for t = 127 Ma) range from 0.705230 to 0.709734. The initial 143Nd/144Nd and ɛNdt (t = 127 Ma) range from 0.512611 to 0.512900 and +2.7 to +8.3, respectively. The high field strength element (HFSE) ratios vary widely, with Nb/Ta ranging from c. 3 to 18 and Zr/Hf ranging from 20 to 41, indicating fluid–rock interaction in the presence of rutile. The correlated variation in the Nd and Sr isotope ratios and the HFSEs, including TiO2, reflects the variation in the slab-derived fluids. The light rare earth element (LREE) enriched and flat patterns yielded by the mafic rocks are modelled by varying the degree of melting of the fluid-metasomatized mantle. The subsequent influx of the slab-derived fluid at a greater depth caused the re-melting of the previously depleted wedge to produce the LREE-depleted patterns.

We propose that the geochemical variation recorded in the MOC rocks indicates the changing nature of fluid metasomatism of the mantle wedge across the subduction zone with time.

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