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

Abstract

Prograde P-T-t paths of eclogites are often ambiguous owing to high variance of mineral assemblages, large uncertainty in isotopic age determinations and/or variable degree of retrograde equilibration. We investigated these issues using the barroisite eclogites from the Lanterman Range, northern Victoria Land, Antarctica, which are relatively uncommon but free of retrogression. These eclogites revealed three stages of prograde metamorphism, defining two distinctive P-T trajectories, M1-2 and M-3. Inclusion minerals in garnet porphyroblasts suggest that initial prograde assemblages (M-1) consist of garnet+omphacite+barroisite/Mg-pargasite+epidote+phengite+paragonite+rutile/titanite+quartz, and subsequent M-2 assemblages of garnet+omphacite+barroisite+phengite+rutile +/- quartz. The inclusion-rich inner part of garnet porphyroblasts preserves a bell-shaped Mn profile of the M-1, whereas the inclusion-poor outer part (M-2) is typified by the outward decrease in Ca/Mg and X-Fe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk-rock composition suggests that the eclogites have initially evolved from similar to 15 to 20 kbar and 520-570 degrees C (M-1) to similar to 22-25 kbar and 630-650 degrees C (M-2). The latter is in accordance with P-T conditions estimated from two independent geothermobarometers: the garnet-clinopyroxene-phengite (similar to 25 +/- 3 kbar and 660 +/- 100 degrees C) and Zr-in-rutile (similar to 650-700 degrees C at 22-27 kbar). The second segment (M3A-B) of prograde P-T path is recorded in the grossular-rich overgrowth rim of garnet. Apart from disequilibrium growth of the M-3A garnet, ubiquitous overgrowth of the M-3B garnet permits us to estimate the P-T conditions at similar to 26 +/- 3 kbar and 720 +/- 80 degrees C. The cathodoluminescence (CL) imaging of zircon grains separated from a barroisite eclogite revealed three distinct zones with bright rim, dark mantle and moderately dark core. Eclogitic phases such as garnet, omphacite, epidote and rutile are present as fine-grained inclusions in the mantle and rim of zircon, in contrast to their absence in the core. The sensitive high-resolution ion microprobe U-Pb dating on metamorphic mantle domains and neoblasts yielded a weighted mean Pb-206/U-238 age of 515 +/- 4 Ma (t sigma), representing the time of the M-2 stage. On the other hand, overgrowth rims as well as bright-CL neoblasts of zircon were dated at 498 +/- 11 Ma (t sigma), corresponding to the M-3. Average burial rates estimated from the M-2 and M-3 ages are too low (<2 mm/year) for cold subduction regime (similar to 5-10 degrees C/km), suggesting that an exhumation stage intervened between two prograde segments of P-T path. Thus, the P-T-t evolution of barroisite eclogites is typified by two discrete episodes with an c. 15 Ma gap during the middle Cambrian subduction of the Antarctic Ross Orogeny.