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 M3. Inclusion 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 M2 assemblages 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 part (M2) is typified by the outward decrease in Ca/Mg and XFe (=Fe2+/(Fe2++Mg)) values. A pseudosection modelling employing fractionated bulk‐rock composition suggests that the eclogites have initially evolved from ~15 to 20 kbar and 520–570°C (M1) to ~22–25 kbar and 630–650°C (M2). The latter is in accordance with P–T conditions 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 (M3A–B) of prograde P–T path is recorded in the grossular‐rich overgrowth rim of garnet. Apart from disequilibrium growth of the M3A garnet, ubiquitous overgrowth of the M3B garnet permits us to estimate the P–T conditions at ~26 ± 3 kbar and 720 ± 80°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 206Pb/238U age of 515 ± 4 Ma (tσ), representing the time of the M2 stage. On the other hand, overgrowth rims as well as bright‐CL neoblasts of zircon were dated at 498 ± 11 Ma (tσ), corresponding to the M3. Average burial rates estimated from the M2 and M3 ages are too low (<2 mm/year) for cold subduction regime (~5–10°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.

Construction of P–T–t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

“…Mineral inclusions in and core-to-rim compositional zoning of garnet are robust recorders of prograde metamorphism for low-and medium-temperature HP eclogites, which have been well documented by previous studies (e.g. Kim et al, 2019;Wang et al, 2018;Wei, Yang, et al, 2009). The garnet in the Tongbai eclogites is characterized by a weakly bell-shaped X sps (X represents the proportion of the garnet end-member) distribution, which is interpreted as petrographic evidence for the growth zoning of garnet (Spear, 1995).…”

Middle Cambrian slope deposits in northern Victoria Land, Antarctica: Fingerprinting small carbonate platforms dominated by grainy carbonates and microbial reefs

“…10). These are supported by the stratigraphic position of the group and existence of a coesitebearing eclogite (~498 Ma) in western margin of the Bowers Terrane (Federico et al, 2009;Rossetti et al, 2011;Kim et al, 2019). The upper Furongian to Lower Ordovician Leap Year Group consists mostly of continental facies which was envisaged as a result of deposition in the tectonic inversion of the basin, caused by possible continental collision and rapid exhumation reflected by occurrence of the retrogressed amphibolite facies (490 to 486 Ma) (Di Vincenzo et al, 1997;Federico et al, 2006Federico et al, , 2009Rocchi et al, 2011;Rossetti et al, 2011).…”

Experimental study of metamorphic reactions and dehydration processes at the blueschist–eclogite transition during warm subduction

“…Regarding barroisite, an open question is its P–T environment of formation. There are two views about the metamorphic stage at which barroisite forms: as part of a prograde and even peak metamorphic assemblage coexisting with the eclogite phases garnet and omphacite (Chatterjee & Ghose, 2010; Di Vincenzo, Horton, & Palmeri, 2016; Gómez‐Pugnaire et al., 1997; Kim et al., 2019; Weller et al., 2015; Yokoyama, Brothers, & Black, 1986) versus formation at the epidote–amphibolite facies as the product of later decompression (Bucher, 2005; Caby et al., 2008; Kabir & Takasu, 2016; Li, Klemd, Gao, & Meyer, 2012; Lombardo, Rolfo, & Compagnoni, 2000; Miyamoto, Enami, Tsuboi, & Yokoyama, 2007; Okay & Whitney, 2010). Singh, Pant, Saikia, and Kundu (2013) made a systematic investigation of the role of amphiboles during the metamorphism of UHP rock in the Tso Morari Complex, northwest Himalayas.…”

“…This observation is consistent with both previous petrological analysis T A B L E 6 Experimental conditions and results on reaction 4 using winchite as the starting amphibole of natural samples and experimental studies of metabasaltic rocks. Barroisite is a common amphibole in eclogites in warm subduction zones, including those from the Nerkau Complex in Northern Urals (Gómez-Pugnaire, Karsten, & Sánchez-Vizcaíno, 1997), the Sanbagawa metamorphic belt in southwest Japan (Weller, Wallis, Aoya, & Nagaya, 2015), Akeyazi metamorphic terrane of Western Tianshan in China (Li, Klemd, Gao, & John, 2016) and the Lanterman Range of northern Victoria Land in Antarctica (Kim, Kim, Cho, & Lee, 2019). Experimental work at the stability field of blueschist and eclogite, starting with natural basaltic rock or glass, also showed a compositional change of amphibole from sodic glaucophane to sodic-calcic barroisite (Forneris & Holloway, 2004;Liu et al, 1996;Molina & Poli, 2000).…”