- The basement of the Tonga intraoceanic forearc comprises Eocene arc volcanic crust formed during the earliest phases of subduction. Volcanic rocks recovered from the forearc include boninites and arc tholeiites, apparently erupted into and upon older mid-oceanic ridge tholeiites. Rock assemblages suggest that the forearc basement is a likely analog for large supra-subduction zone (SSZ) ophiolites not only in structure and Ethology, but also in the style of hydrothermal alteration.
Dredged volcanic samples from the central Tonga forearc (20-24° S) exhibit the effects of seafloor weathering, low (<200°C, principally <100°C) alteration, and high temperature (>200°C) alteration. Tholeiites and arc tholeiites are significantly more altered than boninites. Seafloor weathering is due to extensive interaction with cold oxidizing seawater, and is characterized by red-brown staining and the presence of Fe-oxyhydroxides. Low temperature alteration is due to circulation of evolving seawater-derived fluids through the volcanic section until fluid pathways were closed by secondary mineral precipitation. Low temperature alteration is characterized by smectites, celadonite, phillipsite, mixed-layer smectite/chlorite, carbonates, and silica. All phases fill veins and cavities; clay minerals and silica also replace the mesostasis and groundmass phases. Low temperature alteration enriches the bulk rock in K, Ba, and Na, and mobilizes other elements to varying extents. The few high temperature samples are characterized by mobilizes other elements to varying extents. The few high temperature samples are characterized by epidote, chlorite, quartz, oxides, and fibrous amphibole, which replace groundmass and phenocrysts, and fill cavities, and are presumed to have originated in zones of concentrated hydrothermal upflow.These three alteration types are similar to those seen in many ophiolites such as Troodos, where low temperatures prevailed in the volcanic section except in localized upflow zones. Alteration mineral chemistries are also broadly similar to those observed for the Troodos Ophiolite. Tonga forearc alteration differs from mid-oceanic ridge alteration in the presence of Al-rich dioctahedral smectites (not common in mid-oceanic ridge crust), the high Al content of saponite, and the predominance of K as an interlayer cation in clays. Hydrothermal alteration of the Tonga forearc is likely the product of extensive interaction with compositionally evolving seawater-derived fluids beginning at the time of emplacement. The distribution and intensity of alteration in these crustal sections depend principally on the porosity and permeability of the crust during alteration, which are influenced by the primary porosity, igneous morphology, and the presence of faults and fractures which could affect fluid flow.