Boninite-derived amphibolites from the Lanterman-Mariner suture (northern Victoria Land, Antarctica): New geochemical and petrological data

In northern Victoria Land (Antarctica) the Lanterman-Mariner suture separates the inboard Wilson Terrane from the outboard Bowers and Robertson Bay terranes. This boundary is characterized by the presence of discontinuous bodies of mafic to ultramafic rocks with a metamorphic grade ranging from medium-P amphibolite facies (Mountaineer Range) to ultrahigh-P eclogite facies (Lanterman Range). The Dessent Ridge is a narrow tectonic slice occurring along the Lanterman-Mariner suture in the Mountaineer Range. The slice mainly consists of amphibolites and amphibole-rich schists with very peculiar mineral assemblages comprising talc + clinoamphibole + orthoamphibole ± staurolite ± garnet. Talc shows very unusual compositions with significant Al and Na contents linked to the Si− 1□− 1Al+ 1Na+ 1 and Si− 1 Mg− 1Al+ 2 substitutions (i.e. “tschermak talc”). Bulk-rock major, trace-element and REE compositions range from MORB/arc tholeiite to boninite. Along the Lanterman-Mariner suture the presence of metamorphic rocks whose protoliths have a boninitic affinity is here reported for the first time. They attest to the past existence of a subduction zone with an arc–backarc system between the Wilson and the Bowers terranes. Mineral assemblages and the mineral chemistry allow us to document a retrograde P–T-path from a regional medium-P amphibolite (T ≈ 690 °C; P ≈ 0.9 GPa) to greenschist facies, with a typical cooling–unloading path. Amphibolite-facies metamorphism was followed by structural reworking and partial recrystallization under strike-slip shear deformation. The rocks most affected by shearing document that this deformation developed under the transition amphibolite–greenschist facies, although local higher temperature conditions can be ascribed to shear heating. Comparison with other mafic–ultramafic rocks from the Lanterman-Mariner suture suggests that, from the regional amphibolite-facies metamorphism onwards, the area extending from the Lanterman Range to the Mountaineer Range, including the Dessent Ridge, underwent a nearly synchronous tectono-metamorphic evolution. The Lanterman-Mariner suture resulted from the accretion of an arc/backarc system to the palaeo-Pacific continental margin of Gondwana during the Ross Orogen (Cambro–Ordovician), a typical subduction–accretion orogen.