Physical volcanology and geological relationships of the Jurassic Ferrar Large Igneous Province, Antarctica

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Abstract

The Ferrar Large Igneous Province forms a linear belt for 3500 km along the Transantarctic Mountains, and as a geochemical province extends into southeastern Australasia. The principal components of the Ferrar are: intrusive — Ferrar Dolerite sills and dikes, and Dufek intrusion; pyroclastic — the Prebble, Mawson and Exposure Hill Formations; effusive — the Kirkpatrick Basalt. In terms of the three dimensional architecture of the Ferrar, a range of “facies” can be recognized in each of the principal components. The Ferrar province was initiated with a major episode of phreatomagmatism leading to formation of tephra cones and associated deposits, and near-surface vent structures. Activity switched to predominantly quiet effusion of alternating thick flood basalt flows and thin pahoehoe lobes and flows. Intrusive bodies were emplaced early, given the occurrence of dolerite clasts in tuff–breccias, but most sills were probably intruded after accumulation of extrusive rocks. Pre-existing rift structures played a major role in controlling the transport and distribution of the Ferrar magmas and the apparent centers of extrusive activity. The associated paleohydrology controlled the eruption styles.

Introduction

The Ferrar Large Igneous Province (FLIP) constitutes the major part of the tholeiitic mafic magmatism in Antarctica associated with Gondwana break-up (Elliot, 1992). The FLIP crops out from the Theron Mountains near the Weddell Sea to Horn Bluff in Wilkes Land, a distance of 3500 km (Fig. 1), and is also present in southeastern Australasia (e.g. Tasmania). The Queen Maud Land magmatic province is essentially synchronous with, but geographically separated from, the Ferrar. The former is chemically and isotopically distinct from the Ferrar and comparable to the Karoo Large Igneous Province of southern Africa (Marsh et al., 1997, Luttinen and Furnes, 2000). This province is restricted to northwestern Queen Maud Land except for an outlier in the Theron Mountains where there is spatial overlap with the Ferrar (Fig. 1).

Intrusive rocks of the FLIP comprise the Dufek layered mafic intrusion, and sills and dikes of the Ferrar Dolerite. These were emplaced mainly into sub-horizontal strata of the Devonian Taylor Group and Permian–Triassic Victoria Group (Fig. 2; Collinson et al., 1994; see Elliot and Fleming, 2004, Fig. 3, for sill distribution), which together form the Beacon Supergroup. The extrusive component comprises pyroclastic rocks (Prebble, Mawson and Exposure Hill Formations) and overlying lava flows of the Kirkpatrick Basalt.

The Ferrar province has a coherent geochemistry which is characterized by enriched Sr and Nd initial isotope ratios and trace element patterns with a strong crustal imprint (Fleming et al., 1995 and references therein). Two geochemical types have been recognized (Fleming et al., 1995). The Scarab Peak Chemical Type (SPCT) occurs as the capping flow(s) of the Kirkpatrick Basalt (Fleming et al., 1992, Elliot et al., 1999), and as sills and dikes in the Theron Mountains and Whichaway Nunataks (Leat et al., 2006, Leat, in press). SPCT rocks have evolved iron-rich compositions (SiO2 = 57%; MgO = 2.3%; FeOT = 15.3%) but initial isotope ratios (87Sr/86Sr = 0.7096; ɛNd = −4.3; Elliot et al., 1999) less enriched than most of those of the other chemical type. The SPCT rocks form less than about 10% of the Kirkpatrick lavas and less than 1% of all known Ferrar rocks. The other 99% of the Ferrar tholeiites, which include the Dufek layered intrusion and the Australasian rocks, belong to the Mount Fazio Chemical Type (MFCT) (Fleming et al., 1995, Morrison and Reay, 1995, Molzahn et al., 1996, Antonini et al., 1999). Excluding cumulate rocks, the MFCT dolerites and basalts exhibit a range in chemical compositions (MgO = 9.0–2.5%) and initial isotope ratios (87Sr/86Sr = 0.7087–0.7117; ɛNd =  5.6 to − 4.8) that display coherent patterns of variation. No lavas, except for a few with cumulate crystals, are known that are more mafic than MgO = 7.5%. The MFCT range of compositions can be explained (Fleming et al., 1995) by fractional crystallization plus no more than about 5% crustal contamination of the most mafic Ferrar liquids known, which are aphyric chilled margins of olivine-bearing dolerites (8.5–9% MgO). Petrographically the lavas consist of two pyroxenes, plagioclase, opaques and a glassy to tachylitic to microcrystalline matrix; augite and pigeonite predominate, but orthopyroxene is present in a number of north Victoria Land (NVL) lavas. The geochemistry shows that many are basaltic andesites or andesites (icelandites) rather than basalts sensu stricto, as is the case for other flood basalt fields such as the Columbia River. Dolerites have similar mineralogy although olivine occurs in a few sills and dikes. The Basement Sill in the Dry Valleys, south Victoria Land (SVL) has a conspicuous orthopyroxene-rich layer that formed by emplacement of a thick crystal-rich mush (Marsh, 2004), and a similar tongue occurs locally in the Peneplain Sill.

The age of the Ferrar rocks by U/Pb dating of zircon and baddeleyite from two Ferrar Dolerite sills and the Dufek intrusion is 183.6 ± 1.8 Ma (Encarnación et al., 1996, Minor and Mukasa, 1997). Dating by the 40Ar/39Ar technique on feldspar separates from lavas and sills (Heimann et al., 1994, Fleming et al., 1997) has yielded a slightly younger age of 180 ± 1.8 Ma (recalculated to the constants recommended by Renne et al., 1994, Renne et al., 1998). A similar age has been determined for the Queen Maud Land tholeiites (Zhang et al., 2003), although other workers (Brewer et al., 1992, Duncan et al., 1997) report a slightly older age (184–182 Ma). An age of 180–184 Ma is Late Jurassic (Toarcian) on the revised timescale of Pálfy et al. (2000).

The setting for the Ferrar extrusive rocks was a tectono-magmatic rift which had been initiated in the interval between the end of Victoria Group foreland basin deposition (Collinson et al., 1994) in late Triassic time and onset of flood basalt magmatism. The rift is documented by a sequence of silicic pyroclastic rocks, volcaniclastic strata, and sparse siliciclastic beds, and by the monoclinal structures affecting them (Elliot and Larsen, 1993; Hanson Formation of Elliot, 1996, Elliot, 2000). The age is not well constrained, however a U/Pb SHRIMP date (182.7 ± 1.8 Ma) on zircon from a silicic tuff clast in Prebble pyroclastic rocks is indistinguishable from the age of the FLIP (Elliot et al., 2007). Monoclinal flexuring continued during lava effusion as shown by structures affecting flows in the southern Queen Alexandra Range (Elliot and Larsen, 1993).

Section snippets

Facies architecture

Studies on the Etendeka province led to the development of a model for the three dimensional architecture of flood basalts (Jerram, 2002). This model, based on extensive and excellent exposures in the Etendeka, has been applied to basalts on the Isle of Skye, British Tertiary Igneous Province, and developed with respect to seismic imaging (Single and Jerram, 2004), and to continental flood basalt provinces as a whole (Jerram and Widdowson, 2005). The model provides a convenient framework for

Pyroclastic rocks: Prebble, Mawson, and Exposure Hill formations

At almost all localities where the Kirkpatrick Basalt is exposed, the lavas are underlain by pyroclastic rocks. These rocks are particularly well displayed in the central Transantarctic Mountains, CTM (Hanson and Elliot, 1996) and the Allan Hills–Coombs Hills region of SVL (Elliot and Hanson, 2001, White and McClintock, 2001, Elliot et al., 2005, Reubi et al., 2005, Ross et al., 2008-this issue). Farther north in the Prince Albert Mountains (PAM) (Elliot, 2002), the Deep Freeze Range (Roland

Effusive rocks: Kirkpatrick Basalt

The basalt lavas form mesas, which in NVL are as long as 45 km, and isolated mountains and nunataks that are laterally limited (about 5 km or less). Basalt sequences vary in thickness up to more than 750 m and flows number as many as 41. Individual flow thicknesses range from about 1 m to as much as 230 m. Representative stratigraphic columns are given in Fig. 8.

The pre-eruption surface is exposed neither at Otway Massif and the Grosvenor Mountains immediately to the south, nor over most of PAM

Intrusive rocks: Ferrar Dolerite and Dufek intrusion

The Ferrar Dolerite is the most extensive expression of the FLIP and is discussed in some detail in Elliot and Fleming (2004). The Ferrar Dolerite includes sills and dikes, and larger dolerite masses of uncertain relationships to their country rocks. The Dufek intrusion is the only well exposed large plutonic body.

Discussion

At the end of Permo-Triassic foreland basin deposition, which occurred in a mid to high-latitude setting, a major river system flowed towards Australia parallel to the Panthallassan (paleo-Pacific) active margin (Collinson et al., 1994). This large floodplain was disrupted in Early Jurassic time by rifting and silicic volcanism, with the deposition of silicic tuffs and volcaniclastic strata, clastic sediments bearing volcanic glass shards, and arkoses. The shard-bearing sandstones are known in

Conclusions

The proposal presented by Jerram (2002) for description of flood basalt provinces can be applied to the Ferrar Large Igneous Province, however the greater range of facies recognizable suggests the need for some modification. The pyroclastic facies of Jerram (2002) is divided into four principal facies: a distal extra-vent facies with an epiclastic sub-facies, a proximal vent facies, an intra-vent facies, and an intrusive facies with a clastic dike sub-facies. All four principal facies form

Acknowledgments

Field research on which this paper is based has been supported by a number of grants from the Office of Polar Programs, National Science Foundation, most recently NSF grant OPP 0087919 to DHE and NSF grant OPP 0126106 to THF. An invitation to participate in the Italian Antarctic Program, which enabled examination of the Ferrar rocks in the USARP Range, NVL, is gratefully acknowledged (DHE). Further, the opportunity to participate in the Magmatic Field Workshop in the Dry Valleys in January

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