Petrology and Geochemistry of Igneous Rocks from the Tonga Trench: A Non-Accreting Plate Boundary

Abstract

Petrologic and geochemical examination of a varied suite of intermediate, mafic, and ultramafic rocks dredged from the deep flanks of the Tonga Trench between 20°S and 21°S show that the landward slope has not developed by accretion of material from the subducted Pacific plate. The lowermost trench slopes (> 9000 m) are part of a graben in the Mesozoic Pacific plate; the west and east walls of this graben expose normal- and enriched-type ocean-ridge basalts. The distribution of recovered rock types suggests that the shoaler (< 9000 m) nearshore flank is crudely layered from peridotite (up to 8500 m), to gabbro (to 7000 m), to volcanic rocks (7000-5000 m). Peridotites are fresh to moderately serpentinized harzburgites, with some dunite and minor lherzolite. They are distinctly more depleted in clinopyroxene and have more magnesian mineral compositions than are characteristic of tectonite periedotites from ocean-ridges, fracture zones, and many ophiolites. Volcanics from the upper nearshore slope are basalts, andesites, and dacites. Unlike the N- and E-type basalts being subducted, they are most similar to low-Ti, low-Ba arc volcanics from the Lau Ridge and Mariana forearc. Overall, the coherent 4 + km crustal section of the nearshore flank exposed above the structural plate boundary at 9000+ m is geochemically unlike crust generated in mid-ocean. It may be primitive island-arc crust, older oceanic crust modified by island-arc volcanism, or atypical ocean-ridge crust upon which the Tonga arc was constructed. There is no evidence of accretion of material like that being subducted-non-accretion or tectonic erosion are inferred to be the principal processes which have shaped this margin.