Hydrothermal alteration in a modern suprasubduction zone: The Tonga forearc crust

Abstract

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. Seawater‐derived hydrothermal fluids first penetrated the lower crust along grain boundaries and microscopic fracture networks at temperatures >800°C. As the plutonic sequence cooled, amphibole progressively replaced the mafic phases, followed by chlorite and epidote below ∼550°C. Basalts record peak alteration temperatures up to 773°C; however, most were altered at lower temperatures typical of mid‐ocean ridge (MOR) volcanic sequences [Kelman, 1998]. Epidosites formed by pervasive alteration of basalt and plagiogranite at greenschist facies conditions and at high water‐rock ratios. Pervasively altered gabbros and basalts display evidence of late cataclastic deformation and/or contain veins that is likely related to a later tectonic event such as trench rollback. The range of alteration temperatures and mineral assemblages in basalts and gabbros are similar to those described from both SSZ ophiolites and 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. The initiation of high‐temperature brittle deformation in the absence of ductile deformation suggests that the Tonga forearc crust was constructed in a magma‐rich environment, similar to fast spreading MORs.