50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...
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Flow systems are influenced by structural<br />
permeability and modulated by stress cycling<br />
which accompanies intermittent rupturing on<br />
faults, with coupled changes in fault-fracture<br />
permeability.<br />
The NZPB thus provides a world-class natural<br />
laboratory where dynamic flow systems are<br />
accessible to investigation by geological,<br />
geochemical and geophysical techniques, both<br />
onshore and offshore, and by numerical modelling.<br />
Questions to be addressed for each of the tectonic<br />
domains include the fluid sources, the rates of flow,<br />
the degree of water–rock interaction along flow<br />
paths, the influence of structural permeability on<br />
fluid redistribution, and the total fluid budget. A<br />
Discussion Meeting and Workshop is proposed for<br />
late 2006, hosted by GNS Science at Avalon in<br />
Lower Hutt, to evaluate our present state of<br />
knowledge, define areas for future research, and<br />
devise research strategies to enhance understanding<br />
in this field of ever-increasing importance.<br />
ORAL<br />
SILICIC MAGMAS IN SUBDUCTION<br />
SYSTEMS<br />
Ian E M Smith<br />
Department of Geology, University of Auckland,<br />
PB92019, Auckland, New Zealand<br />
(ie.smith*auckland.ac.nz)<br />
A widely accepted paradigm is that continental<br />
crust is exclusively generated in continental<br />
subduction systems where large volume silicic<br />
magmas are produced by crustal anatexis.<br />
However, work in oceanic arcs suggests that this<br />
general conception is not valid. Although oceanic<br />
subduction systems mainly produce magma ranging<br />
from basaltic to basaltic andesite, silicic magmas<br />
can be locally abundant. For example, a feature of<br />
the Tonga-Kermadec oceanic arc is the abundance<br />
of silicic compositions typically associated with<br />
calderas comparable in scale to those of continental<br />
settings. In the thermal evolution of an oceanic arc<br />
system the processes of underplating, together with<br />
the continuous magmatic (and thermal) flux, can<br />
generate a crustal thickness in which dehydration<br />
melting of under plated arc material generates felsic<br />
magmas.<br />
Oceanic arcs evolve through the following four<br />
stages:<br />
1. Generation of subduction-related basaltic<br />
magmas initiates a volcanic system on oceanic<br />
crust. Under-plating magmas cool and<br />
crystallize and hydrothermal convection<br />
develops through the lower crust. At the top of<br />
the system relatively fractionated magmas<br />
begin to build a volcanic edifice.<br />
2. During arc infancy (0.5-1.0 my) heat is<br />
transferred by convection to the lower arc<br />
crust. Hydration of crust through reaction of<br />
pyroxene + olivine –bearing lithologies with<br />
hydrous fluids produces amphibole. Surficial<br />
eruption continues with eruption of basaltandesite<br />
magmas.<br />
3. A stage of arc adolescence (1-2 Ma)<br />
commences as the temperature of a significant<br />
volume of lower crust approaches the<br />
amphibole-saturated solidus at 850-950 o C.<br />
Dehydration melting fluxes the crust and melt<br />
volumes of a few km 3 to tens of km 3 are<br />
rapidly generated. A 20-30% melt fraction<br />
segregates from a granulitic residue and<br />
ascends to upper levels in the system.<br />
Triggering mechanisms include episodic<br />
transfer of extensional strain into the crust or a<br />
pulse of magma associated with a major<br />
recharge event. Felsic magmatic eruptive<br />
4.<br />
activity may be interspersed with continuing<br />
basaltic-andesitic activity.<br />
Arc maturity (>3 Ma) sees a continuation of<br />
basaltic to andesitic activity. The lower crust<br />
having undergone dehydration melting is now<br />
anhydrous granulite significantly below its<br />
solidus temperature and it acts as a thermal<br />
insulator preventing convection of<br />
hydrothermal fluids. Further anatexis can only<br />
occur if appropriate source materials remain to<br />
participate in the hydration-dehydration cycle.<br />
Work in andesitic volcanic systems shows that the<br />
composition of erupted magmas varies<br />
systematically with time a feature that is interpreted<br />
as increasing involvement of a crustal component<br />
as the isotherm rises in response to magmatic flux.<br />
By analogy with the proposed model for oceanic<br />
arcs rhyolitic magmatic systems in continental,<br />
subduction linked settings represent the end<br />
member state of this evolutionary spectrum of<br />
petrological processes where the crustal component<br />
has come to dominate.<br />
ORAL<br />
TRENDS IN RHYOLITE GEOCHEMISTRY,<br />
MINERALOGY, AND MAGMA STORAGE<br />
DURING THE LAST 50 KYR AT OKATAINA<br />
AND TAUPO VOLCANIC CENTRES<br />
Victoria C. Smith 1 ,P.Shane 1 &I.A.Nairn 2<br />
1 Department of Geology, The University of<br />
Auckland, Private Bag 92019, Auckland.<br />
2 45 Summit Road, Rotorua RD5.<br />
(v.smith*auckland.ac.nz)<br />
During the past 50 kyr, rhyolitic volcanism at OVC<br />
and TVC caldera centres has been more frequent<br />
50 th <strong>Kaikoura</strong>05 -80- <strong>Kaikoura</strong> <strong>2005</strong>