Very shallow melting of oceanic crust during spreading ridge subduction: Origin of near-trench Quaternary volcanism at the Chile Triple Junction

Abstract

Recent dacites and basaltic andesites carrying a subduction-related geochemical imprint were dredged within the active Chile Trench off the Taitao Peninsula, where the Chile Ridge is being subducted beneath the South America Plate. Their maximal Ar/Ar ages range from 70-127 ka to 2 Ma. The basaltic andesites, which have a predominantly mantle-derived geochemical signature are thought to result from the mixing of dacitic magmas with MORB-type liquids derived from the buried spreading ridge. Two groups are distinguished among the dacites: the low-Si group has the chemical characteristics of adakitic slab melts, with depleted heavy rare earth element (HREE) abundances suggesting the occurrence of residual garnet in their source. The high-Si group has less depleted HREE contents. The Sr, Nd, and O isotopic signatures of both groups are ∈Sr = -14.9 to +0.8, ∈Nd = +2.9 to +3.8, and δ 18 O values = +6.4 to +6.9‰ respectively, consistent with mixed magma sources that include a MORB-type component and sediments. We propose that the high-Si dacites are derived from the hydrous melting of a mixture of MORB and sediments at high temperatures (800°-900°C) under low pressures ( < 0.8 GPa). The low-Si dacites originate from the melting of a similar source under higher pressures consistent with depths of 25-45 km. Two scenarios accounting for the near-trench position of these latter rocks are envisioned. The first invokes rapid tectonic erosion and changes in sedimentary wedge geometry. The second one postulates that parts of the slab are subducted rapidly to depths of 20-30 km right under the trench.