Petrology and geochronology of eclogites from the Lanterman Range, Antarctica

The mafic eclogites of the Lanterman Range are the first record of a well-preserved high-pressure assemblage from the Pacific end of the Transantarctic Mountains. They occur among pods and lenses (from <1 to ~30m in size) of mafic and ultramafic metamorphic rocks that constitute a narrow zone intercalated with amphibolite-facies metasediments. This zone extends along the faulted contact between the Wilson Terrane and the Bowers Terrane, in northern Victoria Land. Most of the amphibolites and retrogressed eclogites analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm-Nd whole-rock data indicate a Neoproterozoic age, most probably around 700-750 Ma. For this group of metabasites a tectonic setting of an incipient ocean basin is proposed on geological and geochemical grounds. In contrast, the well-preserved eclogites are characterized by strong enrichment in more incompatible elements and pronounced negative Ta and Nb anomalies in MORB-normalized element patterns. Geochemical and Nd isotope data suggest that they are derived from a different mantle source with an enriched signature. The age of the protolith of the well-preserved eclogites, however, remains unconstrained. In the well-preserved eclogite samples the reaction textures testify to three main metamorphic stages: (1) an eclogite facies stage, (2) a medium-pressure amphibolite facies stage, and (3) a low-pressure amphibolite facies stage. The high-pressure event occurred at temperatures of up to ~850°C based on garnet and omphacite thermometry and at a minimum pressure of ~15 kbar based on the jadeitic content of omphacites. Internal Sm-Nd isochrons from two well-preserved eclogites are 500 ± 5 Ma (rutile, clinopyroxene, amphibole, whole rock and garnet) and 492 ± 3 Ma (rutile, clinopyroxene, whole rock and garnet). Rutile-whole-rock 238U-206Pb ages (~500 Ma) overlap the range of the Sm-Nd mineral ages. The inferred P-T path, the microtextural features and the overlap of the Sm-Nd garnet ages with the range of the 238U-206Pb rutile-whole-rock ages indicate fast cooling and suggest that the time of the high-pressure event was ~500 Ma. These data place both the formation and exhumation of eclogite within a convergent plate margin setting, thus documenting the subduction-accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land.