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Journal of the Geological Society

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Geochemistry of volcanic rocks from the Naga Hills Ophiolites, northeast India and their inferred tectonic setting

¹ Geological Survey of India, Calcutta-700 016, India
² International Institute for Aerospace Survey and Earth Science, Delft, Netherlands

Highly disrupted and deformed slices of ophiolitic rocks occurring along a linear belt in Nagaland and Manipur states, NE India, are known as the Naga Hills Ophiolites (NHO). The principal rock types include dunite, harzburgite, lherzolite, wehrlite, pyroxenite and mafic volcanics.

The volcanic rocks of NHO have been classified into low-Ti and high-Ti groups with 2 wt.% TiO₂ as the boundary, which is corroborated by the differences in their immobile trace element and Rare Earth element characteristics. The two groups are probably not cogenetic; their chemical differences reflect variable source composition. Within group variation of incompatible elements such as La/Ta in the high-Ti group may be related to varying degrees of partial melting.

The Ti/V ratios and REE distribution pattern of low-Ti group show overlapping Mid-Ocean Ridge Basalt and island arc-like characteristics, and possibly suggest back-arc basin setting. The high-Ti group, on the other hand, has similarities with within-plate basalts erupted at off-axis sea-mounts and ocean islands. The sea-mounts and ocean islands brought to the subduction zone acted as barriers. Within the imbricate fault slices are components from the oceanic crust which were accreted to the continental margin and which represent clipped off portions from these barriers.