Dual origins for pantellerites, and other puzzles, at Mount Takahe volcano, Marie Byrd Land, West Antarctica
Introduction
Mt. Takahe lies on the north flank of the West Antarctic rift, bordering the Amundsen Sea (Fig. 1). It is partly buried by the West Antarctic ice sheet, which is > 2 km thick at this locality (Drewry, 1983). Its exposed volume of at least 550 km3 is ~ 1.5 × the volume of Mt. Shasta, the largest volcano in the Cascade Range (Christiansen, 1990). Mt. Takahe is one of the youngest of the 2–3 active volcanoes in Marie Byrd Land (MBL), as indicated by 24 K-Ar and 40Ar/39Ar ages, all < 192 ka. It also may be the largest of the 18 basalt-trachyte volcanoes that comprise the MBL province (Fig. 1). Elsewhere in the province, wherever the base of a felsic section is exposed, it is underlain by basaltic rocks (basanite, basalt, hawaiite) that make up ~ 90% of volcano volume (LeMasurier, 2013). If Mt. Takahe has a similar structure, it could be ~ 5000 km3, equivalent to Kilimanjaro, the largest volcano in the East African rift.
This paper is a continuation of a systematic study of 18 large shield volcanoes in MBL. The goal is to understand the evolution of a province with an exceptional range of peralkaline felsic rocks, ranging from phonolite to trachyte to comendite to pantellerite, but a narrow range of basalts. Basanite and ne-hawaiite are the most common basaltic rock at Mt. Takahe and in the province as a whole. Hy-ol-basalt has been found at only 3 localities, described below. However because low-pressure fractionation of transitional basalt is the most widely accepted mechanism for producing pantellerite in other alkaline provinces, this mechanism was carefully considered in a previous study of Ames and Flood Range (AR/FR) pantellerites (LeMasurier et al., 2011). It was rejected for the following reasons. (1) Hy-ol-basalts at all three localities are out of isotopic equilibrium with the AR/FR suite, (2) The 1400 m basement-floored transitional basalt section at Mt. Murphy (Fig. 1) is surmounted by metaluminous ne-trachytes, rather than the pantellerites that might be expected from associations in other provinces. (3) Isotope data provide no basis for crustal contamination as a factor in the origin of AR/FR pantellerites. (4) The most likely contaminants in the AR/FR region are peraluminous granites, which would inhibit the development of peralkalinity. The polybaric fractionation model, described below, has proved to be the most satisfactory explanation of field, geochemical, and petrographic characteristics of peralkaline, Si-oversaturated rocks in the AR/FR, and also in the Executive Committee Range (ECR) (LeMasurier et al., 2003, LeMasurier et al., 2011). The possibility that suitable hy-basalts lie concealed beneath ice cover has always been considered in these studies. It remains a fall-back position, but one we have not had to appeal to because the polybaric model is well supported by experimental studies, and by all the appropriate data collected at AR/FR, and ECR volcanoes, and the data presented here for Mt. Takahe.
All 18 MBL volcanoes are surmounted by trachytes, plus phonolites, or pantellerites, or comendites, and taken as a whole, these have been erupted more or less contemporaneously over the past ~ 14 m.y. (LeMasurier and Thomson, 1990). The general problem in this province is to explain how this diverse suite of felsic rocks has been produced contemporaneously, rather than, for example, sequentially, over the past 14 m.y.
Several anomalies at Mt. Takahe enlarge the dimensions of the problem. This volcano is dominated by qz- trachytes, hy-ol- trachte and ne-trachytes, with subordinate amounts of basanite, hawaiite, and mugearite. Also included is one pantellerite characterized by some of the most extreme incompatible element enrichments on earth, and another that is among the least enriched of pantellerites (Table 1). These extremes are not found elsewhere in MBL.
Two anomalies appear to be unique, or nearly so, in all of West Antarctica (WA). These include low 143Nd/144Nd isotope ratios and high Ba in basaltic rocks, both believed to be related to a source inherited from pre-85 Ma subduction (LeMasurier et al., 2016). The sum of all these characteristics expands the geochemical parameters of the province. Their interpretation complements previous work and refines our understanding of volcanic evolution.
Section snippets
Previous work
Mt. Takahe was first visited and described during the International Geophysical Year (IGY) by the 1957–1958 MBL traverse party (Anderson, 1960). A limited reconnaissance was conducted in 1968; K-Ar dating of samples collected during this season established its late Quaternary age (LeMasurier and Rex, 1990). Since 1960 Mt. Takahe has been proposed frequently as a source of volcanic ash layers in ice cores from the interior of West Antarctica (WA) (LeMasurier, 1972, Kyle et al., 1981, Wilch et
Regional context
The MBL province lies within the West Antarctic rift system (WARS), which is similar in scale to the Basin and Range Province (USA) and East African rifts. The most obvious manifestations of extension in this region are voluminous alkaline volcanism, attenuated continental crust, and horst and graben structure, all of which continue offshore to the Marie Byrd seamounts (Fig. 1), suggesting that the rift has no clear northern boundary (Heinemann et al., 1999, LeMasurier, 2008, LeMasurier and
Physical volcanology
Mt. Takahe is a “trachytic shield volcano”. The term was introduced by Webb and Weaver (1975) to describe volcanoes from the Kenya rift that range up to 50 km in base diameter, are composed mainly of peralkaline trachyte flows and pyroclastic rocks, with minor volumes of alkaline basalt and mugearite, and surface slopes of ~ 5°. The exposed part of Mt. Takahe fits this description closely in morphology, composition and structure (Fig. 2), with a base diameter at ice level of ~ 35 km and surface
Analytical methods
The polar environment provides exceptionally fresh, un-weathered samples. This is reflected in low H2O and CO2 of samples in Table 1, which are original raw data. Glasses are fresh and non-hydrated. Loss of mobile elements from devitrification is unlikely. No devitrified glass has been observed in thin section, probably because the volcano is too young and the environment is cold and dry.
Thirty four new major element, 37 new trace element, and 11 new Sr and Nd isotope analyses are presented in
Mineral compositions
Table 1S, Table 2S, Table 3S, Table 4S present 45 microprobe analyses of minerals and glasses from a hawaiite, a mugearite, two trachytes, and a pantellerite. The tables include analyses of feldspar phenocrysts from all 5 rock samples included in the tables, olivine phenocrysts from 4, pyroxene phenocrysts from 3, plus arfvedsonite (3 samples), Ti-magnetite (2 samples), accessories and glasses from 3 samples, and kaersutite analyses from a websterite nodule in basanite from Mt. Flint (Fig. 1),
Rock compositions
The following is focused largely on felsic rocks. The compositions of basaltic rocks have been described and discussed in detail by LeMasurier et al. (2016) and will only be summarized where needed to provide context for felsic rocks.
Modeling
Mass balance modeling was done using the program “Petrograph” (Petrelli et al., 2005), which employs methods of Stormer and Nicholls (1978) for major element models, and Wood and Fraser (1976) for trace element models. The program uses rock and mineral compositions from the study area. Initial and final magma compositions, and phases to be subtracted, are chosen from among the analyzed samples, as described below. Depths of magma chambers are inferred from experimentally determined stability
Adequacy of fractional crystallization
A fundamental problem posed by the Mt. Takahe suite is the co-existence of qz-trachytes and ne-trachytes, together with basanites and ne-hawaiites, and the fact that K/Ar and 40Ar/39Ar ages indicate that these disparate rock types are coeval (e.g. Table 1, sample localities 11–14). Most commonly among alkaline provinces one finds silica oversaturated felsic rocks associated with hy-basalts (e.g. Pantelleria; White et al., 2009), and phonolites associated with basanites (e.g. Ross Island,
Summary and conclusions
- 1.
Mt. Takahe is unusual among alkaline volcanoes. It is composed mainly of Si-oversaturated and ne-normative trachytes, in nearly equal proportions, associated with basanite and ne-hawaiite. The latter are the only exposed mafic rocks. This suite is somewhat similar to that of the Kerguelen Archipelago, but with no transitional (hy-normative) basalt to serve as the putative parent for oversaturated felsic rocks. It also bears many similarities to the Massif Central (France) alkaline suite, but
Acknowledgements
Field and laboratory work on the geochemistry and petrology of Mt. Takahe volcano has been supported by NSF grants DPP 80-20836 and #0536526 (to WEL), administered by the Office of Polar Programs. WEL thanks the Institute of Arctic and Alpine Research (INSTAAR) for funds to cover publication costs. We are grateful to Gary Ernst for a critical and helpful review of an early draft of the manuscript. WEL gratefully acknowledges a lot of help from Stan Hart, over many years, which has always led to
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