The Lithosphere

Magmatism of the McMurdo Volcanic Group

Principal Investigator: Alan Cooper
Organisation: University of Otago

What we do and why we do it:
In the 2004/2005 and 2005/2006 seasons, field parties from the Geology Department, University of Otago, joined in 2004 by geologists from the University of Waikato, have been producing a geological map of the Mt Morning volcano, showing the distribution of lavas and pyroclastic (explosively fragmented) volcanic rocks, identifying the type of magma responsible and, from their orientation and other features, the likely mechanism of their formation. From existing radiometric dates we know that Mt Morning is the oldest and longest lived of the southern Victoria Land volcanic centres. Our aim, through laboratory analysis of collected samples, is to determine the chronological sequence of volcanic eruptions and trace the change in composition of the magmas as the volcano evolves. Many of the rising magmas within the volcano break off fragments of the immediately underlying basement to the volcano, and more deep seated rocks from the source areas of magmas within the Earth's mantle, and carry them to the surface. By investigating these accidental fragments we hope to be able to reconstruct a geological cross section of the crust and underlying mantle in southern Victoria Land.

Some things we've found out so far:
Volcanism started nearly 18 million years ago in a marginally marine setting, but Mt Morning soon evolved into a subaerial volcanic cone. There has been a large variety of magmas erupted throughout its history, early activity was dominated by trachytes or rhyolites (but with contemporaneous basalt as well), but the most recent cinder-cones and craters on the northern flanks of the volcano are basaltic. These cones are not appreciably eroded and are therefore interpreted to be very young, overlapping in terms of age with the currently active Mt Erebus, farther north. A previously dated phonolite lava flow from Mt Morning has been dated at 1.15 million years.

To the north of Mt Morning, in the Ross Sea are fault-bounded sedimentary basins that have been accumulating detritus over the Tertiary period. Some of that detritus will have been contributed from explosive volcanism on Mt Morning and other volcanic centres, other material will have been eroded from the growing volcanoes by fluvial, and in colder climes, glacial activity. A knowledge of the geological sequence on Mt Morning will therefore help in the interpretation of sedimentary cores, to be retrieved from these sedimentary basins in the forthcoming ANDRILL programme.

The northern slopes of Mt Morning are built across a fault zone, possibly a strand of the major fault zone that separates the Transantarctic Mountains from the stretched and thinned lithosphere of the Ross Embayment. This fault zone may well have effectively controlled the site of the volcano, acting as a conduit along which magma rose to high level.

Basaltic rocks of Mt Morning are rich in fragments from the deep crust and mantle, including some very rare pieces of mantle that have been extensively altered by exchange with deep-seated magma or H2O/CO2-rich fluid. Investigation of these rocks will enable us to build up a regional picture of the geological evolution of southern Victoria Land extending from the surface to depths of 10s of km.

This research contributes to the ANDRILL project.

Recent publications:
Read, S.E. Cooper, A.F. Walker, N. W. Geochemistry and U-Pb geochronology of the Neoproterozoic-Cambrian Koettlitz Glacier Alkaline Province, Royal Society Range, Transantarctic Mountains, Antarctica. Royal Scoiety of New Zealand Bulletin 35: 143-151, 2002.

Mellish, S.D. Cooper, A.F. Walker, N. W.The Panorama Pluton:  a composite gabbro-monzodiorite early Ross Orogeny intrusion in southern Victoria Land, Antarctica. Royal Scoiety of New Zealand Bulletin 35: 129-141, 2002.

Cox, S.C. Parkinson, D.L. Allibone, A.H. Cooper, A.F. Isotopic character of Cambro-Ordovician plutonism, southern Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics 43: 501-520, 2000.