Lithofacies and eruptive conditions of the southernmost volcanoes in the world (87° S)

Neogene volcanic centres are uncommon in the Transantarctic Mountains but at least three basaltic examples occur within 300 km of South Pole, above 2200 m asl and inland of the margin of the West Antarctic Rift System. They are the southernmost volcanoes on Earth and have yielded Early—mid Miocene isotopic ages. Two of the centres, at Mt Early and Sheridan Bluff, have been examined. The centre at Mt Early is unequivocally glaciovolcanic. It formed a tall monogenetic volcanic edifice at least 1 km high and > 1.5 km in diameter. It erupted under significantly thicker-than-modern ice, which was probably a fast-moving ice stream at the eruptive site and resulted in a distinctive constructive architecture and lithofacies. It is the first described example of a glaciovolcano erupted beneath an ice stream. The characteristics of the second centre at Sheridan Bluff indicate that it was also a monogenetic volcano but with a shield-like profile, originally c. 6 km in basal diameter but just c. 400 m high. It probably erupted in a substantial pluvial lake in an ice-poor or ice-free environment. The strongly contrasting eruptive settings now identified by the volcanic sequences at both centres examined testify to a highly dynamic Antarctic Ice Sheet during the Early—mid Miocene.

Chapter 5.3b Mount Early and Sheridan Bluff: petrology

This study discusses the petrological and geochemical features of two monogenetic Miocene volcanoes, Mount Early and Sheridan Bluff, which are the above-ice expressions of Earth's southernmost volcanic field located at c. 87° S on the East Antarctic Craton. Their geochemistry is compared to basalts from the West Antarctic Rift System to test affiliation and resolve mantle sources and cause of melting beneath East Antarctica. Basaltic lavas and dykes are olivine-phyric and comprise alkaline (hawaiite and mugearite) and subalkaline (tholeiite) types. Trace element abundances and ratios (e.g. La/Yb, Nb/Y, Zr/Y) of alkaline compositions resemble basalts from the West Antarctic rift and ocean islands (OIB), while tholeiites are relatively depleted and approach the concentrations levels of enriched mid-ocean ridge basalt (E-MORB). The magmas evolved by fractional crystallization with contamination by crust; however, neither process can adequately explain the contemporaneous eruption of hawaiite and tholeiite at Sheridan Bluff. Our preferred scenario is that primary magmas of each type were produced by different degrees of partial melting from a compositionally similar mantle source. The nearly simultaneous generation of lower degrees of melting to produce alkaline types and higher degrees of melting forming tholeiite was most likely to have been facilitated by the detachment and dehydration of metasomatized mantle lithosphere.

Chapter 5.3a Mount Early and Sheridan Bluff: volcanology

Two small monogenetic volcanoes are exposed at Mount Early and Sheridan Bluff, in the upper reaches of Scott Glacier. In addition, the presence of abundant fresh volcanic detritus in moraines at two other localities suggests further associated volcanism, now obscured by the modern Antarctic ice sheet. One of those occurrences has been attributed to a small subglacial volcano only c. 200 km from South Pole, making it the southernmost volcano in the world. All of the volcanic outcrops in the Scott Glacier region are grouped in a newly defined Upper Scott Glacier Volcanic Field, which is part of the McMurdo Volcanic Group (Western Ross Supergroup). The volcanism is early Miocene in age (c. 25–16 Ma), and the combination of tholeiitic and alkaline mafic compositions differs from the more voluminous alkaline volcanism in the West Antarctic Rift System. The Mount Early volcano was erupted subglacially, when the contemporary ice was considerably thicker than present. By contrast, lithologies associated with the southernmost volcano, currently covered by 1.5 km of modern ice, indicate that it was erupted when any associated ice was either much thinner or absent. The eruptive setting for Sheridan Bluff is uncertain and is still being investigated.

Thermochronology as a key to deciphering controls on landscape evolution in northern Victoria Land (Transantarctic Mountains)

<p>Northern Victoria Land constitutes the Pacific terminus of the Transantarctic Mountains (TAM) on the western shoulder of the Cenozoic West Antarctic Rift System. It is characterised by a distinct morphological transition from an elevated peneplain that dominates throughout most of the TAM to a strongly undulating relief with prominent narrow crests and alpine peaks. This contrast is associated with a lithological change from high-grade metamorphics and granitoids to low-grade metasedimentary rocks that contain only few scattered igneous bodies.</p><p>New high-resolution thermochronological data (fission-track and (U-Th-Sm)/He) from more than 60 locations in the Southern Cross Mountains and Mountaineer Range of northern Victoria Land provide the basis for studying regional exhumation and uplift with particular focus on the establishment of landscape contrasts. In an integrated approach, differences in topography are examined with respect to regional and local controls including tectonics, lithology and climate to identify differential trends and quantify the morphological evolution of the TAM and West Antarctic Rift System.</p><p> </p><p>Two coastal profiles covering 2 to 3 km in elevation reveal apatite fission track ages from 23 to 45 Ma with mean track lengths of 13.3 – 14.7 μm. Corresponding (U-Th-Sm)/He apatite and zircon data range between 19 – 32 Ma and 24 – 27 Ma, respectively. The dates show distinctive spatial trends of increasing ages from north to south and at greater distance to the coast whereby younger cooling ages correlate with stronger terrain segmentation and higher topographic relief.</p><p>Thermal history modelling of the combined data indicates that accelerated cooling commencing at 35 Ma proceeded at progressively higher rates reaching >25°C/Ma in late stages. This cooling episode continued until at least 20 Ma and refers to exhumation from burial depths of more than 5 km, clearly exceeding the calculated overburden on adjacent crustal blocks to the south. Although rapid upper lithospheric cooling is a generic feature of northern Victoria Land, the current data demonstrates that Cenozoic exhumation dynamics were highly differential. Understanding these patterns requires thorough balancing of structural against isostatic factors, lithological against climate parameters and focussed local incision against large-scale denudation and levelling processes.</p>

Marie Byrd Land lithospheric mantle: A review of the xenolith record

The Marie Byrd Land (MBL) lithospheric mantle xenolith record comprises over 100 samples from a range of localities spanning both major crustal terranes that comprise MBL: Ross and Amundsen provinces. Coarse granular to porphyroclastic in texture, the xenoliths are predominantly Type I spinel-bearing lherzolites to harburgites, but include rare dunite and pyroxenite examples. Garnet is absent and no hydrous phases, such as amphibole or mica, have been reported to date, although traces of apatite may be present. Characterisation of the lithospheric mantle composition and its evolution however, is hampered by patchy and uneven geochemical analyses across the xenolith suite. Nonetheless, a picture emerges of a heterogeneous lithosphere beneath both Ross and Amundsen Provinces. Previously published and new data reported here are consistent with samples ranging from variably cryptically metasomatised residua from variable (10 - 25%) degrees of partial melt extraction to refertilised compositions. Limited isotopic data point to a complex history, providing evidence for both ancient Proterozoic lithospheric mantle and preservation of Ordovician events. The Sr-Nd-Pb composition of the sampled lithospheric mantle overlaps the common low-µ isotopic endmember identified in Cenozoic magmatism from MBL and the wider West Antarctic Rift System.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5309814