Ultrahigh‐pressure metamorphism and exhumation of garnet‐bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Abstract

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo-Pacific margin of Gondwana. A narrow and discontinuous high- to ultrahigh-pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet-bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet-bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium-grained garnet + olivine + low-Al orthopyroxene + clinopyroxene, whereas finer-grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg-chlorite ± serpentine ± talc. A high-temperature (∼950 °C), spinel-bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet-bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity-driven exhumation

Antarctica and supercontinent evolution: historical perspectives, recent advances and unresolved issues

“…The restriction of HP-UHP metamorphism to younger ages has been proposed to reflect a change in the thermal regime of subduction to 'cold' subduction, in which a strong mantle lithosphere facilitates focused downwards and upwards flow of material in subduction channels (Brown 2007;Gerya et al 2008;Sizova et al 2010). The only example of HP -UHP metamorphism in East Antarctica is that reported from the Lanterman Range of northern Victoria Land (Palmeri et al 2007). Like other HP -UHP areas, it is no older than Ediacaran -Cambrian and can be linked to subduction-accretion along an active ocean-continent margin.…”

“…Compilation of major metamorphic belts of the world in terms of their apparent thermal gradient (dT/dz) at peak metamorphic conditions and age, modified from Brown (2006Brown ( , 2007. Thermal gradients are divided into three groups: (1) HP-UHP: high-pressure and ultrahigh-pressure metamorphic belts characteristic of modern oceanic subduction zones, marked by grey circles, with the larger blue circle for the East Antarctic example in the Lanterman Range, northern Victoria Land (Palmeri et al 2007); (2) E-HPG: eclogite and high-pressure granulites characteristic of continental subduction in collisional orogens, marked by grey squares, with three larger green squares for East Antarctic examples from the Miller Range of the central Transantarctic Mountains (Peacock & Goodge 1995), the Shackleton Range (Schmädicke & Will 2006) and the Grove Mountains (Liu et al 2009a); (3) G-UHT: granulite and ultrahigh-temperature metamorphic belts characteristic of back-arc regions and long-lived orogenic plateaus, marked by grey diamonds, with larger orange diamonds for the many granulites and UHT areas in East Antarctica (Harley 2003). Vertical grey time bands are indicative of the four main phases of supercontinent formation, from Gondwana and Rodinia back to Nuna and Kenorland.…”

Ultrahigh‐pressure metamorphism at the Lanterman Range (northern Victoria Land, Antarctica)

“…The BMC consists of micaschists and gneisses affected by medium-to high-grade metamorphism with an intermediate pressure character (Grew and Sandiford, 1984;Talarico et al, 1998). This review focuses on the GHMC, a lithotectonic unit characterized by abundant mafic and ultramafic rocks, including well-preserved eclogites and garnet-bearing ultramafic rocks (Ricci et al, 1996;Palmeri et al, 2007), which occur as lenses and pods (centimetric to metric) within a metasedimentary sequence of dominant gneisses and minor quartzites (Figure 3). Mafic rocks including eclogites show variable geochemical affinities including E-MORB, T-MORB and orogenic calc-alkaline features, suggesting different mantle sources (Di Vincenzo et al, 1997;Ghiribelli, 2000;Rocchi et al, 2003).…”

“…Eclogite-facies rocks along the Palaeozoic active margin of Gondwana are rare (Foster et al, 2005;Goodge, 2007). They have been described in Northern Victoria Land (Antarctica), western Tasmania and south-eastern Australia (Di Vincenzo et al, 1997;Turner et al, 1998;Meffre et al, 2000;Och et al, 2003;Palmeri et al, 2007 and reference therein ;. Although, there is a similar age (% 500 Ma) for HP/UHP metamorphism (Di Vincenzo et al, 1997;Watanabe et al, 1997;Meffre et al, 2000), strong differences are evident in terms of P-T conditions of metamorphic peaks and dP/dT (Watanabe et al, 1997;.…”

Petrology of the eclogites from western Tasmania: Insights into the Cambro-Ordovician evolution of the paleo-Pacific margin of Gondwana

“…Eclogite facies rocks along the paleo-Pacific margin of Gondwana are rare. They have been described in Northern Victoria Land (Antarctica), western Tasmania and southeastern Australia, and they formed at an early Paleozoic convergent plate boundary close to the eastern margin of Gondwana (Turner et al, 1998;Palmeri et al, 2007 and reference therein ;Och et al, 2003).…”