The Tasmanian Geologist - Geological Society of Australia

Association of Volcanology and Chemistry of the Earth’s
Interior) meeting in Reykjavik, Iceland in 2008. In
accordance with our funding policy (see November
newsletter), we reproduce abstracts of the papers
presented by them.
Facies analysis of a partly extrusive, basaltic
submarine cryptodome, Shirahama Group,
Izu Peninsula, Japan
Sarah M. Gordee, J. McPhie & S. R. Allen
University of Tasmania
The physical evolution of modern and ancient
volcanic environments can be best understood through
the essential field technique of facies mapping. Detailed
mapping of volcanic deposits in SW Izu Peninsula, Japan
has provided information that exemplifies the importance
of this method. Two main facies associations were
identified that provide information on the eruption
history and lateral changes in eruption style of a
submarine basaltic volcanic complex
The first facies association comprises massive,
monomictic, jigsaw-fit basalt breccia that grades
vertically and laterally into thick beds of reversely
graded, clast-rotated to chaotic, monomictic basalt
breccia. Clasts are angular with curviplanar margins and
are interpreted to have formed by quench fragmentation.
Jigsaw-fit breccia and associated breccia beds are
interpreted as in situ and resedimented hyaloclastite,
respectively. The succession is overlain and partly
infilled by thin beds of pumiceous sand.
The second facies association comprises
elongate, tightly packed, coherent basalt lobes that grade
into domains of monomictic, jigsaw-fit to chaotic, clastto
matrix-supported basalt breccia with pumiceous sand
matrix. The coherent lobes cross-cut and locally disrupt
the overlying pumiceous sand beds. The clasts in the
sediment-matrix basalt breccia are texturally and
compositionally identical to the coherent lobes and the
facies is interpreted as peperite.
Collectively, the two facies associations are
interpreted to represent the margin of a partly extrusive
submarine cryptodome. In situ and resedimented
hyaloclastite breccias indicate an extrusive, subaqueous
environment and are consistent with expected facies
associations across the quench fragmented carapace and
resedimented flanks of a seafloor basaltic lava dome.
Pumiceous sand infilled the interstices within the breccia
pile to form a sediment-matrix breccia. The facies
association comprising coherent lobes and peperite
indicates an intrusive origin and is interpreted to
represent the subsurface portion of the cryptodome. The
intrusion utilized syn-depositional micro-grabbens within
the pumiceous sand that developed in response to the
expanding magma body beneath the surface, and
interaction of the magma with the wet, unconsolidated
sand produced peperitic domains. Regionally, another
similar facies association at the same stratigraphic
interval includes a thick mound of basaltic fluidal clast
breccia that may represent weakly explosive fire
fountaining at another seafloor vent.
The Luise amphitheatre, Lihir Island, Papua
New Guinea: Caldera, maar crater or
sector-collapse scar?
Jacqueline L. Blackwell 1 , Jocelyn McPhie 1 ,
David R. Cooke 1 , Kirstie A. Simpson 1 ,
Jonathon Rutter 2
1
Australian Research Council Centre for Excellence in
Ore Deposits
2
LGL, Lihir Operations, Papua New Guinea
The 40Moz. Ladolam gold deposit on Lihir
Island, Papua New Guinea occurs in a 3.5 by 4.0 km
seaward-facing, horseshoe-shaped amphitheatre.
Previous workers have postulated that the amphitheatre
is a caldera, a maar-diatreme crater or a volcanic sector
collapse amphitheatre. Correct interpretation of the
amphitheatre underpins understanding of ore formation.
Our analysis of the dimensions and morphology
of the amphitheatre, together with mapping and logging
the bedrock facies in which it occurs, strongly favours
sector collapse of the former Luise volcano. The
amphitheatre is much larger than maar-craters and lacks
the pyroclastic facies typically associated with both
maars and caldera. The bedrock mainly comprises
variable altered trachyandesitic and trachybasaltic lavas
and shallow intrusions, and volcaniclastic facies typical
of the proximal and medial parts of andesitic cone
volcanoes. Furthermore, bathymetry surrounding Lihir
Island has revealed the presence of hummocky
topography and marginal levees extending offshore from
the Luise harbour, interpreted to be a debris avalanche
deposit. A coral-limestone unit that encircles the rest of
the island is absent from the Luise harbour, having been
destroyed by a sector-collapse-generated debris
avalanche.
Sector collapse involving removal of ~ 1 km of
the former Luise volcano is consistent with porphyry –
style alteration being overprinted by epithermal-style
mineralisation and alteration.
GSA Tas. Division Committee (2008-09)
Chairman: Dr Nick Direen
Tel 0413 030612
ndireen@frogtech.com.au
Secretary: Dr Andrew McNeill
C/- CODES, University of Tasmania
Private Bag 79
Hobart 7001
Tel: 03 62262487
Fax: 03 62267662
andrew.mcneill@utas.edu.au
Treasurer: Dr Peter McGoldrick
Committee Members:
Dr Ron Berry
Dr Garry Davidson
Dr Mark Duffett
Mr John Everard (newsletter editor)
jeverard@mrt.tas.gov.au
Dr Jacqueline Halpin
Mr Michael Vicary
Ms Isabella von Lichtan
The Tasmanian Geologist, 10th February 2008