The role of eclogite in the rift-related metasomatism and Cenozoic magmatism of Northern Victoria Land, Antarctica Massimiliano Melchiorre a, ⁎, Massimo Coltorti a , Costanza Bonadiman a , Barbara Faccini a , Suzanne Y. O'Reilly b , Norman J. Pearson b a Department of Earth Science, Ferrara University, via Saragat 1, 44123 Ferrara, Italy b GEMOC, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia abstract article info Article history: Received 20 April 2010 Accepted 25 November 2010 Available online 6 December 2010 Keywords: Mantle peridotite xenoliths Eclogites Lithospheric mantle West Antarctic Rift System Re/Os systematics Sr, Nd, and Hf isotopic analyses of separated clinopyroxenes and in situ Re–Os isotopic analysis of sulphides in mantle-peridotite xenoliths from Baker Rocks (BR) and Greene Point (GP), less than 100 km apart in Northern Victoria Land (NVL), Antarctica, provide further constraints on the evolution of the sub-continental lithospheric mantle beneath NVL and suggest that eclogitic reservoirs may have played a role in the metasomatism and magmatism of the area. Most of the BR sulphides have radiogenic 187 Os/ 188 Os (0.1318–0.379 with 187 Re/ 188 Os ratios between 0.46 and 3.3), while unradiogenic 187 Os/ 188 Os characterizes the GP suite (0.1068–0.1279 with 187 Re/ 188 Os ratios from 0.0002 to 0.045). In BR silicates 87 Sr/ 86 Sr varies between 0.70296 and 0.70488, 143 Nd/ 144 Nd lies within a narrow range (0.51271–0.51296), and 176 Hf/ 177 Hf ranges from 0.28300 to 0.28337. Clinopyroxenes from GP have similar 87 Sr/ 86 Sr (0.70277 to 0.70434), 143 Nd/ 144 Nd between 0.51261 and 0.51347, and 176 Hf/ 177 Hf between 0.28332 and 0.28519. Notwithstanding the rather limited number of sulphides in the GP suite, Os model ages for BR and GP largely overlap. A histogram of T RD (Time of Rhenium Depletion) model ages shows peaks at 3.0–3.3 Ga, 2.3 Ga, 1.3– 1.4 Ga, 0.9–1.1 Ga, 580–620 Ma and 120 Ma (the youngest being recorded only at BR). These ages fit reasonably well with the events that affected Antarctica during its geological evolution. The highly radiogenic Os found in the BR xenoliths can be explained through mixing with an eclogitic lithotype. Mafic magmas, now eclogite, may have been introduced into the sublithospheric NVL mantle during the Ross Orogeny (550–600 Ma) or older subduction events and reactivated during the opening of the West Antarctic Rift System. The mixing component will depend on the age of the subducted material. About 15% of recycled Archean material would be necessary in order to account for the most radiogenic Os-isotope values. Based on Hf systematics the most radiogenic Hf in the GP clinopyroxenes could also be explained by adding between 35 and 20% eclogite. However in this locality the possible presence of garnet in the peridotitic domains and successive re-equilibration in the spinel stability field, could also account for the highly radiogenic Hf in the clinopyroxene values. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Extensive petrological investigations on both mantle xenoliths and lavas have been carried out on rocks from several localities of Northern Victoria Land (NVL, Antarctica). Most of the studies on mantle xenoliths have focussed on major- and trace-element compositions and on thermobarometric calculations. Petrological studies of lavas have been carried out with the aim of defining the nature of their mantle sources. Berg et al. (1989) investigated the thermobarometric conditions of a suite of granulites hosted in the Cenozoic alkaline volcanic rocks from McMurdo Sound and proposed a geotherm, which was later confirmed by Beccaluva et al. (1991), indicating the thermobaro- metric conditions consistent with a dynamic rift. Some further constraints on the nature and evolution of the mantle beneath this region have been proposed by Coltorti et al. (2004), who explained the amphibole in mantle xenoliths from Baker Rocks as a reaction product between undersaturated alkaline-silicatic metasomatic fluids and pre-existing clinopyroxene. Perinelli et al. (2006), in a study of a suite of anhydrous mantle xenoliths entrained in the Cenozoic volcanics of Greene Point, concluded that some portions of the lithospheric mantle originated in the garnet stability field and subsequently equilibrated in the spinel facies. They also measured the isotopic composition of oxygen and suggested a possible influence of partial melting on oxygen isotope composition. Cenozoic basalts from Northern Victoria Land have recently been described in detail and isotopically characterized by Nardini et al. Lithos 124 (2011) 319–330 ⁎ Corresponding author. E-mail address: mlcmsm1@unife.it (M. Melchiorre). 0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2010.11.012 Contents lists available at ScienceDirect Lithos journal homepage: www.elsevier.com/locate/lithos