Geological evolution of the Coombs–Allan Hills area, Ferrar large igneous province, Antarctica: Debris avalanches, mafic pyroclastic density currents, phreatocauldrons

Abstract

The Jurassic Ferrar large igneous province of Antarctica comprises igneous intrusions, flood lavas, and mafic volcaniclastic deposits (now lithified). The latter rocks are particularly diverse and well-exposed in the Coombs-Allan Hills area of South Victoria Land, where they are assigned to the Mawson Formation. In this paper we use these rocks in conjunction with the pre-Ferrar sedimentary rocks (Beacon Supergroup) and the lavas themselves (Kirkpatrick Basalt) to reconstruct the geomorphological and geological evolution of the landscape. In the Early Jurassic, the surface of the region was an alluvial plain, with perhaps 1 km of mostly continental siliciclastic sediments underlying it. After the fall of silicic ash from an unknown but probably distal source, mafic magmatism of the Ferrar province began. The oldest record of this event at Allan Hills is a ≤180 m-thick debris avalanche deposit (member m 1 of the Mawson Formation) which contains globular domains of mafic igneous rock. These domains are inferred to represent dismembered Ferrar intrusions emplaced in the source area of the debris avalanche; shallow emplacement of Ferrar magmas caused a slope failure that mobilized the uppermost Beacon Supergroup, and the silicic ash deposits, into a pre-existing valley or basin.The period which followed ('Mawson time') was the main stage for explosive eruptions in the Ferrar province, and several cubic kilometres of both new magma and sedimentary rock were fragmented over many years. Phreatomagmatic explosions were the dominant fragmentation mechanism, with magma-water interaction taking place in both sedimentary aquifers and existing vents filled by volcaniclastic debris. At Coombs Hills, a vent complex or 'phreatocauldron' was formed by coalescence of diatreme-like structures; at Allan Hills, member m 2 of the Mawson Formation consists mostly of thick, coarse-grained, poorly sorted layers inferred to represent the lithified deposits of pyroclastic density currents. Meanwhile at Carapace Nunatak, mafic clasts were mixed with detrital material to form the Carapace Sandstone in alluvial and eventually lacustrine environments.Eruptions then became largely effusive, producing hundreds of metres of flood lavas that covered the landscape ('Kirkpatrick time'). In places, lava flowed into ephemeral lakes to form pillow-palagonite breccias (base of sequence, Carapace Nunatak) or pillow lavas (top of sequence, Coombs Hills). Several generations of Ferrrar intrusions were emplaced during the course of these events; at least three can be distinguished based on field relations. New geochemical data indicates that for the Ferrar province, magma involved in the explosive eruptions had the same major element composition as that which produced shallow intrusions and lavas. We also note the possibility that flood lavas were fed by plugs cross-cutting the Mawson Formation at Coombs Hills, rather than by major dikes extending to the surface. Finally, we infer that eruption plumes were limited to the troposphere and that direct...

Maar-diatreme volcanoes: A review

“…Kött et al, 1995 report a rhyolitic maar), including kimberlite, various ultramafic melts (Hopi Buttes volcanic field -Williams, 1936;Missouri River Breaks -Hearn, 1968), carbonatite (Gross Brukkaros volcanic field: Kurszlaukis and Lorenz, 1997;Lorenz and Kurszlaukis, 1997), through alkali basalts (Lorenz, 1970;1973;Leys, 1983;Nemeth et al, 2001;2003), to at least tholeiitic basaltic andesite (Coombs Hills: Ross et al, 2008a). These magmas have a wide range of viscosities and inferred volatile contents, but there are few systematic differences in diatreme deposits or structures beyond those of the juvenile particles themselves (e.g., juvenile fragment shape, vesicularity, crystallinity).…”

“…They are also research targets in studies of monogenetic volcanic fields, including those undergoing assessments of volcanic risk (Lorenz, 2007). Identification of maardiatreme deposits precursory to flood-basalt eruptions has added larger-volume eruptions to the family, as well as information from superb Antarctic exposure at Coombs Hills and Allan Hills, sites which are part of the Jurassic Ferrar Large Igneous Province (White and McClintock, 2001;McClintock and White, 2006;Ross and White, 2006;Ross et al, 2008a).…”

The importance of the transport system in shaping the growth and form of kimberlite volcanoes

“…1e). In one example, a quarter of a tephra rim is now enclosed in ventfilling rocks, implying that the small edifice (original radius ~200 m) was "eaten" from below by a laterally propagating diatreme, and partly subsided into it (Ross et al, 2008a;Fig. 1f).…”

Quantification of vesicle characteristics in some diatreme-filling deposits, and the explosivity levels of magma–water interactions within diatremes

“…The other major component of the Large Igneous Province is the Ferrar Dolerite, a series of sills and dikes which invade the older sedimentary rocks of the Beacon Supergroup (Elliot and Fleming, 2008). Lavas in the Kirkpatrick Basalt, and 'basaltic' (lithic and juvenile) fragments in the volcaniclastic deposits, are mostly tholeiitic basaltic andesite (Ross et al, 2008a, and references therein), but here clasts, lavas and intrusions are nevertheless referred to as 'basaltic', for simplicity and in keeping with historical usage.…”

“…1b), the Mawson Formation is similar to the m 2 rocks at Allan Hills in texture and composition (Ross, 2005), but much of the formation is non-bedded. Minor layered volcaniclastic rocks occur at high elevations (McClintock and White, 2006;Ross et al, 2008a) and wide clastic dikes cross-cut primary volcaniclastic rocks (Ross and White, 2005b). The non-bedded portion of the Mawson Formation at Coombs Hills is interpreted as the infill of a diatreme complex, of mostly phreatomagmatic origin (White and McClintock, 2001;McClintock and White, 2006;Ross and White, 2006).…”