Formation of a spatter-rich pyroclastic density current deposit in a Neogene sequence of trachytic-mafic igneous rocks at Mason Spur, Erebus volcanic province, Antarctica

Abstract

Erosion has revealed a remarkable section through the heart of a volcanic island, Mason Spur, in the southwestern Ross Sea, Antarctica, including an unusually well exposed section of caldera fill. The near-continuous exposure, 10 km laterally and >1 km vertically, cuts through Cenozoic alkalic volcanic rocks of the Erebus volcanic province (McMurdo Volcanic Group) and permits the study of an ancient volcanic succession that is rarely available due to subsequent burial or erosion. The caldera filling sequence includes an unusual trachytic spatter-rich lapilli tuff (ignimbrite) facies that is particularly striking because of the presence of abundant black fluidal, dense juvenile spatter clasts of trachytic obsidian up to 2m long supported in a pale cream-coloured pumiceous lapilli tuff matrix. Field mapping indicates that the deposit is an ignimbrite and, together with petrological considerations, it is suggested that mixing of dense spatter and pumiceous lapilli tuff in the investigated deposit occurred during emplacement, not necessarily in the same vent, with the mixed fragmental material emplaced as a pyroclastic density current. Liquid water was not initially present but a steam phase was probably generated during transport and may represent water ingested during passage of the current as it passed over either wet ground (possibly puddled), a stream, shallow lake or possibly snow. Well exposed caldera interiors are uncommon and that at Mason Spur is helping understand eruption dynamics associated with a complex large island volcano. The results of our study should help to elucidate interpretations of other, less well exposed pyroclastic density current deposits elsewhere in Antarctica and globally.

Eruptive history of Mason Spur, a Miocene—Pleistocene polygenetic volcanic complex in southern Victoria Land, West Antarctic Rift System, Antarctica

“…TAM-transantarctic mountains (stippled ornament) comprehensive reassessment of the volcanic evolution to be made. An important result of our study is the discovery of a thick caldera-filling sequence of ignimbrites, reported initially by Martin et al (2018). Similar deposits are very rare and much less voluminous elsewhere in the WARS and our study thus fills a significant gap in our knowledge of the eruptive and structural evolution of the large polygenetic WARS volcanoes.…”

“…Mason Spur represents one of the early stages of volcanism of the Mount Morning Volcanic Field. Previous geological studies of Mason Spur were a PhD project by Wright-Grassham (1987; a brief summary is given by Wright and Kyle 1990) and papers by Martin et al ( , 2013Martin et al ( , 2018. The study by Wright-Grassham (1987) was reconnaissance in nature and investigated Mount Discovery, Mount Morning and Minna Bluff in addition to Mason Spur.…”

“…Obsidian is also locally prominent, particularly in a spectacular deposit at Windscoop Bluff. That deposit has been interpreted as a spatter-rich pyroclastic density current deposit, which formed in unusual circumstances, i.e., co-eruption of ignimbrite and trachytic spatter in separate vents and mingling of the different products during transport (Martin et al 2018).…”

“…Fine and coarse breccias are often prevalent in MSP1. They are formed of monomict, dense, fine, and aphanitic trachyte lava clasts that are texturally and compositionally identical to the massive hypabyssal intrusions (Martin et al 2018). There is also a textural gradation from jigsaw-fractured trachyte to orthobreccia or rare parabreccia, sometimes within the same intrusion although, in general, large outcrop areas are simply formed of massive breccia.…”

“…The abundant hypabyssal intrusions (see next section) and pervasive hydrothermal alteration in the ignimbrites and breccias might suggest that MSP1 is a subvolcanic (hypabyssal) complex. However, the association of abundant clastic dykes, deformation of (unconsolidated) ignimbrites and widespread massive brecciated intrusions, together with the large volume implied by the extensive thick outcrop and its pervasive alteration, indicate that unit MSP1 is probably a caldera fill (assumed by Martin et al 2018). The deformation resulted from one or more caldera collapse events, and is also evidenced by the variable, often very steep dips (> 60°) of relict stratified ignimbrites present.…”