PETROLOGY AND GEOCHRONOLOGY OF ECLOGITES FROM THE LANTERMAN RANGE, ANTARCTICA

The mafic eclogites of the Lanterman Range are the first record of a w ell-preserved high-pressure assemblage from the Pacific end of the Tra nsantarctic Mountains. They occur among pods and lenses (from <1 to si milar to 30m in size) of mafic and ultramafic metamorphic rocks that c onstitute a narrow zone intercalated with amphibolite-facies metasedim ents. This zone extends along the faulted contact between the Wilson T errane and the Bowers Terrane, in northern Victoria Land. Most of the amphibolites and retrogressed eclogites analysed have geochemical comp ositions that resemble transitional to E-type mid-ocean ridge basalt ( MORB) and Nd isotope data indicative of differentiation from a deplete d mantle source. The age of the igneous precursors is not well determi ned, but Sm-Nd whole-rock data indicate a Neoproterozoic age, most pro bably around 700-750 la. For this group of metabasites a tectonic sett ing of an incipient ocean basin is proposed on geological and geochemi cal grounds. In contract, the well-preserved eclogites are characteriz ed by strong enrichment in more incompatible elements and pronounced n egative Ta and Nb anomalies in MORE-normalized element patterns. Geoch emical and Nd isotope data suggest that they are derived from a differ ent mantle source with an enriched signature. The age of the protolith of the well-preserved eclogites, however, remains unconstrained. In t he well-preserved eclogite samples the reaction textures testify to th ree main metamorphic stages: (Ij an eclogite facies stage, (2) a mediu m-pressure amphibolite facies stage, and (3) a low-pressure amphibolit e facies stage. The high-pressure event occurred at temperatures of up to similar to 850 degrees C bared on garnet and omphacite thermometry and at a minimum pressure of similar to 15 kbar bared on the jadeitic content of omphacites. Internal Sm-Nd isochrons from two well-preserv ed 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 U-238-(206)pb ages (similar to 500 Ma) overlap the range of the Sm-Nd mineral ages. The inferred P-T path, th e microtextural features and the overlap of the Sm-Nd garnet ages with the range of the U-238-Pb-206 rutile-whole-rock ages indicate fast co oling and suggest that the time of the high-pressure event war similar to 500 Ma. These data place both the formation and exhumation of eclo gite within a convergent plate margin setting, thus documenting the su bduction-accretional nature of the early Palaeozoic Ross Orogen in nor thern Victoria Land.