New Zealand Journal of Geology and Geophysics abstracts
Change from calc-alkaline to adakitic magmatism recorded in the
Early Cretaceous Darran Complex, Fiordland, New Zealand
A. M. WANDRES
S. D. WEAVER
D. SHELLEY
J. D. BRADSHAW
Department of Geological Sciences
University of Canterbury
Private Bag 4800
Christchurch, New Zealand
Abstract The Early Cretaceous Darran Complex of the Median
Tectonic Zone near Milford, South Island, New Zealand, consists dominantly of
biotite - two-pyroxene diorites cut by biotite-hornblende microdiorite and
quartz monzodiorite dikes. The host diorite gives a 138 +/- 2.9 Ma SHRIMP
age on zircons, which is interpreted to be the age of igneous crystallisation.
The calc-alkaline geochemistry of the host diorites and the microdiorites is
attributed to melting of a mantle wedge source fluxed by slab-derived fluids.
Ductile deformation (D1) of the host diorite and the microdiorite dikes took
place at mid-upper amphibolite facies conditions, with extension lineations
indicating a top to the NNE sense of shear. A SHRIMP age of 136 +/- 1.9 Ma
on zircons from a quartz monzodiorite dike injected along D1 shears is
statistically indistinguishable from that of the host diorite. This suggests
that D1 was synmagmatic and that subduction, during or shortly after magma
emplacement, was oblique to the Gondwana margin at c. 138 Ma.
The quartz monzodiorite dikes are enriched in Na2O, Al2O3, and Sr, depleted in
Y, and have a distinctly adakitic geochemistry. The change in chemistry from
calc-alkaline magmas to alkali-calcic adakitic magmas reflects the melting of a
mafic, garnet-bearing, essentially plagioclase-free source in the root of a
volcanic arc system.
Keywords geochemistry; diorite; adakite; Median Tectonic
Zone; Darran Complex; Fiordland; Early Cretaceous; oblique subduction;
Gondwana; SHRIMP
New Zealand Journal of Geology and Geophysics, 1998, Vol. 41: 1-14
0028-8306/98/4101-0001 $7.00/0 (c) The Royal Society of New Zealand
1998
PDF file of entire paper: medium quality (4301K); (scanned from paper original: notes about this process)
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