Hydrothermal alteration in a modern suprasubduction zone : the Tongo forearc crust




Banerjee, Neil Raoul

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An extensive suite of hydrothermally altered basalts, gabbros, and plagiogranites, was recovered from the trench-facing slope of the Tonga forearc. The tectonic setting, lithology, and geochemistry of these samples make them a unique collection for comparison with suprasubduction zone (SSZ) ophiolites. Petrography, mineral chemistry, and geothermometry are used to constrain the metamorphic evolution of ocean crust formed in a modern SSZ setting. We report the discovery of the first suite of oceanic epidosites. Tongan epidosites metasomatically replaced basaltic and plagiogranite protoliths and formed under similar conditions to epidosites hosted in many SSZ ophiolites. The range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both SSZ ophiolites and mid-ocean ridges (MORs). However, the degree of alteration in basalts and the presence of epidosites in the Tonga collection are most similar to alteration characteristics in SSZ ophiolites. We show that the trace element chemistry of epidote may be linked to the composition of fluids circulating deep in hydrothermal systems. This is possible due to the subordinate role crystal chemistry may play in controlling the trace element chemistry of hydrothermal minerals. Whole rock oxygen isotope ratios of the Tonga samples are generally similar to values determined from MOR and SSZ ophiolite samples; however, enriched values in plagiogranites and gabbros may indicate a late, low temperature metamorphic overprint associated with tectonic unroofing during trench rollback. Basalts show an interesting northward decrease in oxygen isotope ratios that remains unresolved.



Hydrothermal alteration, Ophiolites