Lu–Hf garnet geochronology applied to plate boundary zones: Insights from the (U)HP terrane exhumed within the Woodlark Rift

Abstract

High-pressure and ultra high-pressure (U)HP metamorphic rocks occur in many of the world's major orogenic belts, suggesting that subduction of continental lithosphere is a geologically important process. Despite the widespread occurrence of these rocks, relatively little is known about the timescales associated with (U)HP metamorphism. This is because most (U)HP terranes are tectonically overprinted and juxtaposed against rocks with a different history. An exception to this are the Late Miocene (U)HP metamorphic rocks found in active metamorphic core complexes (MCC) in the Woodlark Rift of southeastern Papua New Guinea. This region provides a rare opportunity to study the garnet Lu–Hf isotopic record of (U)HP metamorphism in a terrane that is not tectonically overprinted. In order to constrain the timing of garnet growth relative to the history of (U)HP metamorphism and the evolution of the Woodlark Rift, Lu–Hf ages were determined, in conjunction with measurements of Lu and major element zoning, for garnets from three metamorphic rocks. Garnets from the three samples yielded different ages that, instead of recording the spatial and temporal evolution associated with a single metamorphic event, provide information on the timing of three separate plate boundary events. The youngest Lu–Hf age determined was 7.1 ± 0.7 Ma for garnets in a Late Miocene coesite eclogite. The age is interpreted to record the time when a garnet-bearing partial melt of the mantle crystallized within subducted continental lithosphere at (U)HP conditions. The young Lu–Hf age from the coesite eclogite is in contrast to a 68 ± 3.6 Ma Lu–Hf age obtained on large (1–2 cm) garnet porphyroblasts, from within the Pleistocene amphibolite facies shear zone carapace bounding exposures of (U)HP rocks in the D'Entrecasteaux Islands. This older age records the growth of garnet in response to continental subduction and ophiolite obduction in the region north and east of Australia during late Mesozoic/early Cenozoic time. In order to relate the (U)HP metamorphic rocks to the larger-scale tectonic evolution of the Woodlark Rift, a third Lu–Hf age of 11.2 ± 2.1 Ma was determined for dynamically recrystallized garnet from the southeastern margin of the rift. While the tectonic implications of the Lu–Hf age from the southeastern-rifted margin are not fully known, this result suggests that the eastern and western parts of the Woodlark Rift might have a different metamorphic history. The garnet Lu–Hf results demonstrate that prior to rifting, the Woodlark Rift underwent a complex history of metamorphism involving garnet growth and recrystallization. The ability of the Lu–Hf system to date Late Miocene garnet growth, and at the same time preserve Latest Cretaceous garnets within Pleistocene shear zones in the Woodlark Rift demonstrate that Lu–Hf garnet geochronology is an essential component of investigations where the primary objective is documenting and understanding the full range of plate boundary processes recorded in metamorphic minerals.