Provenance mixing in an intraoceanic subduction zone: Tonga Trench-Louisville Ridge collision zone, southwest Pacific

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Dredging on the lower slope of the Tonga forearc, at the intersection between the Louisville Ridge hotspot chain and the trench, yielded both Late Tertiary volcaniclastic sediments and Late Cretaceous slightly tuffaceous pelagic sediments. Petrography and phase chemistry of volcanic debris of Tertiary age samples indicates derivation from a low-K tholeiitic arc: volcanic glass has a low K2O content and shows an Fe enrichment trend; plagioclase grains are high in An and low in Or components; pyroxene grains (calcic clinopyroxene, orthopyroxene and pigeonite) have low TiO2 contents and show an Fe enrichment trend; alkali feldspar and biotite are absent. The composition of the Tertiary samples is similar to other Tonga forearc sediments and is consistent with their derivation from the adjacent magmatic arc. The character and composition of volcanogenic debris in Cretaceous age samples indicates derivation from an intraplate alkali igneous source: plagioclase compositions show an increasing Or component with increasing Ab; alkali feldspar is a rare additional component of these samples; calcic clinopyroxene has high TiO2 contents and is titanaugite; amphibole is also rich in TiO2 and is kaersutite; minor volcanic glass is rich in alkalis. Samples were dredged from the seaward slope of a small terrace on the lower trench slope. A planar reflector, located 2.6 km below the terrace is marked by an abrupt jump in seismic velocity and is interpreted as the top of an essentially undamaged Late Cretaceous guyot of the Louisville chain that was subducted at the Tonga trench about 0.5 Ma. This guyot is the likely source of the Late Cretaceous, intraplate igneous detritus collected at the dredge site. Some dredge samples yielded a mixed volcanic arc/intraplate provenance and/or Late Tertiary/Late Cretaceous ages. This probably represents the mixing of the Cretaceous seamount and Tertiary magmatic arc sources during post-collision slumping of the wedge of lower slope material as it re-establishes its critical taper.

Original languageEnglish
Pages (from-to)35-53
Number of pages19
JournalSedimentary Geology
Issue number1-2
Publication statusPublished - 1 Jan 1990
Externally publishedYes

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