New Zealand Journal of Geology and Geophysics abstracts
A Carboniferous 40Ar/39Ar amphibole emplacement age for the Au-bearing Sams Creek alkali-feldspar granite dike, west Nelson, New Zealand
A. J. Tulloch
GNS Science
Dunedin Research Centre
Private Bag 1930
Dunedin, New Zealand
W. J. Dunlap
Research School of Earth Sciences
Australian National University
Mills Rd, Canberra, ACT 0200
Australia
Abstract A peralkaline alkali-feldspar granite
dike associated with Au-sulfide mineralisation at Sams Creek has
previously yielded inconsistent Triassic K-Ar ages, and has thwarted
attempts at U-Pb zircon dating. We report here a disturbed 40Ar/39Ar
step heating spectrum and both single and multi-grain laser fusion
analyses of amphibole, from which we interpret an age of 319 ±
8 Ma (MSWD 0.05) for dike emplacement. Although the sample is a
complex mix of arfvedsonite and late-stage riebeckite of possibly
widely disparate ages, the age gradient is best explained by
significant argon loss in a relatively low metamorphic grade setting
from amphiboles, which are less retentive of radiogenic argon than most
common amphiboles. The youngest step heating and laser ages of c.
≤180 Ma indicates that disturbance of the Ar system occurred in
the Late Jurassic to Early Cretaceous, probably due to the thermal
effects of the c. 170–105 Ma Median Batholith and
satellite plutons. The c. 319 Ma emplacement age is
consistent with correlation to the widely distributed Foulwind Suite
(294–320 Ma) of peralkaline and A-type granites. The age
helps to constrain the timing of at least some of the movement on the
Devil River Fault which crosscuts the dike, and leads to the suggestion
that north–south folding and greenschist facies metamorphism
which affects both dike and host rocks may be related to mid-Jurassic
to Early Cretaceous subduction.
Keywords Sams Creek dike; 40Ar/39Ar; arfvedsonite; geochronology; argon diffusivity; Nelson
G05034; Received 20 July 2005; accepted 15 March 2006; Online publication date 12 May 2006
New Zealand Journal of Geology & Geophysics, 2006, Vol. 49: 233–240
0028–8306/06/4902–0233 © The Royal Society of New Zealand 2006
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