Phreatomagmatism at Coombs Hills, Antarctica : magma-water super-volcanism in a wet, failed rift

McClintock, M. K. (Murray Kevin)

The Mawson Formation at Coombs Hills, Antarctica comprises mainly massive tuffbreccia (~80% vol.), includes a high proportion of accidentallithics, has steep contacts with country rock, includes rafts of bedded pyroclastic rock and country rock, and is intruded by and intermingled with dikes, sills and peperite. Massive tuffbreccia deposits are cut by steeply dipping to sub-vertical zones oftuffbreccia of contrasting componentry and grainsize to host rocks, tangled with irregular intrusions and peperite. Dikes or pods of coherent basalt metres in diameter grade, via peperite contacts, into rafts of undisrupted country rock, and into surrounding tephra comprising sedimentary debris mixed with glassy basaltic clasts. The Mawson succession is capped by planar and cross-bedded tuff, lapilli tuff and tuff breccia, which comprises ::::;1 0% of the Mawson overall. Componentry suggests quarrying of country rock from relatively shallow levels (?::::;700 m). Coombs Hills outcrop records non-explosive to explosive magma-water interaction and intrusion of associated hypabyssal complexes within a novel type of phreatomagmatic vent complex that is basaltic, large volume (kilometres3 to tens ofkilometres3 ) and oflow to negative relief. This unusual style of volcanism developed as the precursor to Ferrar LIP magmatism during Middle Jurassic fragmentation of Gondwana, when voluminous rising flood basalts intercepted equally abundant water-saturated Beacon Supergroup sediments and sedimentary rocks. Similar rocks elsewhere in the Transantarctic Mountains, and associated with other flood basalt provinces worldwide (Karoo LIP, Siberian Traps), suggest that this style of basaltic volcanism may be characteristic of environments of combined rifling and flood magmatism. Although Coombs Hills is an atypical vent complex, processes of transport and deposition within the complex were like those in other subaerial phreatomagmatic volcanoes. The bulk of deposits at Coombs Hills are most like those of lower diatremes and diatreme root zones, recording tephra jetting and churning of massive vent-filling tephra by phreatomagmatic explosions. Water-magma interaction was driven by interaction of basalt with water hosted in porous country rock and vent-filling debris. Spatial and temporal heterogeneity in availability of external water led to a variety of eruptive styles at many sites both laterally and vertically within the Coombs Hills crater. Some tephra was deposited by base surge and pyroclastic flow beyond active eruption sites, but broadening of craters via lateral quarrying recycled most of this volcaniclastic rock en masse back into the vent. Quarrying was facilitated by unstable country rock collapsing from vent margins, leading to digestion of collapsed blocks into massive vent fill and adding both mass and further external water to the volcanic system. The fundamental controls oflarge-volume phreatomagmatism at Coombs Hills were: (1) An abundant magma supply associated with emplacement and eruption ofFerrar LIP flood basalts; (2) An equally abundant shallow-level external water hosted in poorly consolidated, porous and permeable Beacon Supergroup country rock, with water supply enhanced by moderate rainfall in a humid climate; and (3) Poorly consolidated, fragile wall rock, which facilitated repeated collapse of vent margins into vents to create a broad shallow vent complex most like a nest of diatremes. Wall rock collapse and subsequent crater growth increased the supply of water-saturated sediment available to fuel eruptions, increased the potential capture of local drainage and reduced the escape of water-saturated tephra from craters, allowing recycling of water as well as pyroclasts. Implications for volcanic processes are ( 1) that recycling of water is probably as ubiquitous as recycling of pyroclasts in phreatomagmatic vents, and allows largevolume phreatomagmatic systems to grow beyond constraints imposed by far-field recharge of external water alone; (2) that the proportion of recycled water increases with vent size; and (3) that peperite is an important indicator of, and participant in, phreatomagmatic explosions, and magma-sediment interaction can promote enhanced explosivity of magma-water interaction.

1 v. (various pagings) : col. ill. (3 folded) ; 30 cm.

2001McClintock