Prograde P-T evolution and episodic zircon growths of barroisite eclogites in the Lanterman Range, northern Victoria Land, Antarctica

We studied foliated barroisite eclogites affected by minor retrogression in the Lanterman Range, northern Victoria Land, Antarctica, which occupy an only minor amount among the mafic boudins in the study area. Three stages of prograde metamorphism were unveiled in the eclogites, defining two distinctive P-T trajectories, M₁-M₂ and M₃. Relic minerals in garnet porphyroblasts suggest that initial prograde assemblages (M₁) consist of garnet + omphacite + barroisite/Mg-pargasite + epidote + phengite + paragonite + rutile/titanite + quartz; and subsequent assemblages (M₂) of garnet + omphacite + barroisite + phengite + rutile ± quartz. The inclusion-rich inner part of garnet porphyroblasts preserves a bell-shaped Mn profile of the M₁, whereas the inclusion-poor outer segment of the M₂ is typified by outward decrease of Ca/Mg and X_Fe (= Fe²⁺/(Fe²⁺ + Mg)) value. A pseudosection modeling employing the fractionated bulk-rock composition suggests that the eclogites have initially evolved from 15-20 kbar and 520-570 °C (M₁) to 22-25 kbar and 630-680 °C (M₂). The latter is in accordance with P-T condition estimated from two independent geothermobarometers; the garnet-clinopyroxene-phengite ( 25 ± 3 kbar and 660 ± 100 °C) and Zr-in-rutile ( 650-700 °C at 22-27 kbar). The second segment (M_3A-M_3B) of the prograde P-T path is recorded in the overgrowth of grossular-rich garnet rims, which permit us to estimate the P-T condition at 26 ± 3 kbar and 720 ± 80 °C. Zircon crystals separated from a barroisite eclogite (sample E-1a) are clearly resolved into three zones by cathodoluminescence (CL) imaging. The SHRIMP U-Pb dating on the texturally-distinct dark-CL mantle and bright-CL rim domains with low Th/U ratios as well as bright-CL neoblasts of zircon yielded the weighted mean ages of 514 ± 7 Ma (tσ) and 499 ± 9 Ma (tσ), representing the times of metamorphic overgrowths and/or neoblast growths during the M₁-M₂ and M₃, respectively. An average burial rate for 15 Ma duration is too low (< 2 mm·a^-1) for cold subduction ( 5-10 °C·km^-1), invoking an exhumation stage between two prograde segments, or burial events. Taken together, we suggest that two discrete phases of subduction are recorded in the suture-zone eclogites of the Antarctic Cambrian-Ordovician Ross Orogen.