50thKaikoura05 -1- Kaikoura 2005 CHARACTERISATION OF NEW ...

Flow systems are influenced by structural
permeability and modulated by stress cycling
which accompanies intermittent rupturing on
faults, with coupled changes in fault-fracture
permeability.
The NZPB thus provides a world-class natural
laboratory where dynamic flow systems are
accessible to investigation by geological,
geochemical and geophysical techniques, both
onshore and offshore, and by numerical modelling.
Questions to be addressed for each of the tectonic
domains include the fluid sources, the rates of flow,
the degree of water–rock interaction along flow
paths, the influence of structural permeability on
fluid redistribution, and the total fluid budget. A
Discussion Meeting and Workshop is proposed for
late 2006, hosted by GNS Science at Avalon in
Lower Hutt, to evaluate our present state of
knowledge, define areas for future research, and
devise research strategies to enhance understanding
in this field of ever-increasing importance.
ORAL
SILICIC MAGMAS IN SUBDUCTION
SYSTEMS
Ian E M Smith
Department of Geology, University of Auckland,
PB92019, Auckland, New Zealand
(ie.smith*auckland.ac.nz)
A widely accepted paradigm is that continental
crust is exclusively generated in continental
subduction systems where large volume silicic
magmas are produced by crustal anatexis.
However, work in oceanic arcs suggests that this
general conception is not valid. Although oceanic
subduction systems mainly produce magma ranging
from basaltic to basaltic andesite, silicic magmas
can be locally abundant. For example, a feature of
the Tonga-Kermadec oceanic arc is the abundance
of silicic compositions typically associated with
calderas comparable in scale to those of continental
settings. In the thermal evolution of an oceanic arc
system the processes of underplating, together with
the continuous magmatic (and thermal) flux, can
generate a crustal thickness in which dehydration
melting of under plated arc material generates felsic
magmas.
Oceanic arcs evolve through the following four
stages:
1. Generation of subduction-related basaltic
magmas initiates a volcanic system on oceanic
crust. Under-plating magmas cool and
crystallize and hydrothermal convection
develops through the lower crust. At the top of
the system relatively fractionated magmas
begin to build a volcanic edifice.
2. During arc infancy (0.5-1.0 my) heat is
transferred by convection to the lower arc
crust. Hydration of crust through reaction of
pyroxene + olivine –bearing lithologies with
hydrous fluids produces amphibole. Surficial
eruption continues with eruption of basaltandesite
magmas.
3. A stage of arc adolescence (1-2 Ma)
commences as the temperature of a significant
volume of lower crust approaches the
amphibole-saturated solidus at 850-950 o C.
Dehydration melting fluxes the crust and melt
volumes of a few km 3 to tens of km 3 are
rapidly generated. A 20-30% melt fraction
segregates from a granulitic residue and
ascends to upper levels in the system.
Triggering mechanisms include episodic
transfer of extensional strain into the crust or a
pulse of magma associated with a major
recharge event. Felsic magmatic eruptive
4.
activity may be interspersed with continuing
basaltic-andesitic activity.
Arc maturity (>3 Ma) sees a continuation of
basaltic to andesitic activity. The lower crust
having undergone dehydration melting is now
anhydrous granulite significantly below its
solidus temperature and it acts as a thermal
insulator preventing convection of
hydrothermal fluids. Further anatexis can only
occur if appropriate source materials remain to
participate in the hydration-dehydration cycle.
Work in andesitic volcanic systems shows that the
composition of erupted magmas varies
systematically with time a feature that is interpreted
as increasing involvement of a crustal component
as the isotherm rises in response to magmatic flux.
By analogy with the proposed model for oceanic
arcs rhyolitic magmatic systems in continental,
subduction linked settings represent the end
member state of this evolutionary spectrum of
petrological processes where the crustal component
has come to dominate.
ORAL
TRENDS IN RHYOLITE GEOCHEMISTRY,
MINERALOGY, AND MAGMA STORAGE
DURING THE LAST 50 KYR AT OKATAINA
AND TAUPO VOLCANIC CENTRES
Victoria C. Smith 1 ,P.Shane 1 &I.A.Nairn 2
1 Department of Geology, The University of
Auckland, Private Bag 92019, Auckland.
2 45 Summit Road, Rotorua RD5.
(v.smith*auckland.ac.nz)
During the past 50 kyr, rhyolitic volcanism at OVC
and TVC caldera centres has been more frequent
50 th Kaikoura05 -80- Kaikoura 2005