HR: 12:05h
AN: V52A-08 [Abstracts]
TI: High-Magnesium Andesites in south-eastern Papua New Guinea: tectonic control of primitive Arc magmas?
AU: * Smith, I E
EM: ie.smith@auckland.ac.nz
AF: Geology-SGGES, University of Auckland,PB92019, Auckland Mail Centre, Auckland, 1142,
New Zealand
AB:
The late Cenozoic high-K arc-type volcanic association in south-eastern Papua New Guinea developed in an
environment of complex tectonic processes including obduction, subduction, rifting, sea floor spreading and uplift
of metamorphic core complexes. However, although the arc is demonstrably active there is no seismic evidence
for current subduction.. The volcanoes of the association define an arc extending a distance of 250 km from the
Papuan Peninsula south-eastward through the D'Entrecasteaux Islands into the Louisiade Archipelago. Rock
types are predominantly basaltic andesite and andesite but include basalt, dacite and rhyolite. These rocks
constitute a high-K arc-type volcanic suite typical of many subduction related volcanic associations. A sub-set of
the Papuan suite is a group ranging from basalt to dacite which, although comparable in most other aspects of
their geochemical compositions, are higher in MgO, Cr and Ni. These relatively high-Mg rocks (up to 12 wt.
percent MgO at the basaltic end of the spectrum) are less porphyritic and have simple olivine- or clinopyroxene-
dominated phenocryst assemblages compared with the ‘normal' rocks of the association. The ‘normal' rocks are
typical plagioclase phyric arc-type rocks containing augite and hypersthene with or without olivine, hornblende
and biotite. Both high-Mg and ‘normal' rock types are spatially and temporarily interlinked.
The high-Mg rocks are interpreted to represent magmas derived by partial melting of subduction modified mantle
which have risen rapidly from their mantle source. In contrast the ‘normal' low-Mg rocks represent magmas which
were modified by shallow intracrustal processes. In this case the source of the magmas is subduction-modified
mantle rendered fertile by subduction although tapping of this source is not directly linked to active subduction.
The unusual abundance of high-Mg in south-eastern Papua is linked to extensional tectonics which allowed deep
sourced magmas to pass through the crust without significant modification. This model is consistent with the
current tectonic setting in which sea floor spreading of the Woodlark Rise is impinging on the continental crust of
Papua. This explanation supports the hypothesis that high-Mg andesites can represent primitive melts in
subduction settings that have avoided the trap of intracrustal processes.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly