New Zealand Journal of Geology & Geophysics abstracts
Plutonic rocks of the Median Batholith in southwest Fiordland, New
Zealand: field relations, geochemistry, and correlation
A. H. Allibone*
School of Earth Science
James Cook University
Townsville Q4811, Australia
rodinian@msn.com
I. M. Turnbull
A. J. Tulloch
GNS Science
Private Bag 1930
Dunedin 9054, New Zealand
A. F. Cooper
Department of Geology
University of Otago
PO Box 56
Dunedin 9054, New Zealand
*Present address: Rodinian Pty Ltd, PO Box 1804,
Fyshwick, ACT 2609, Australia.
Abstract This paper provides a first
description of all
major plutonic rock units between Resolution Island and Lake Poteriteri
in southwest Fiordland. Plutonic rocks, of which c. 95% are
granitoids, comprise c. 60% of the basement in southwest
Fiordland. Approximately 50% of the plutonic rocks were emplaced
between c. 355 and 348 Ma, 5% at c. 164 Ma, 25% between
c. 140 and 125 Ma, and 20% between c. 125 and 110 Ma. These
episodes of plutonism occurred in response to terrane amalgamation,
continental thickening, and subduction along the convergent margin of
Gondwana. Correlatives of Devonian plutonic rocks which occur in Nelson
are absent from the area described here.
A wide variety of plutonic rocks were emplaced at
c. 355–348
Ma.
These include relatively small plutons of K- and Rb-rich gabbro-diorite
and members of at least three distinct suites of granitoids. Plutons of
two-mica ± garnet granodiorite, granite, and minor tonalite
share affinities with the S-type Ridge Suite and are the most
widespread c. 355–348 Ma old granitoids in southern Fiordland.
Plutons rich in Ca, Fe and Zr, depleted in K and Na, and containing
quartz diorite, tonalite, and minor granodiorite with the unusual
assemblage red-brown biotite, garnet ± hornblende ±
clinopyroxene also occur widely in southern Fiordland. These plutons
are similar to peraluminous A-type granitoids, indicating A as well as
I and S-type plutonism occurred in the Western Province at this time.
The Newton River and Mt Evans Plutons
have no correlatives amongst c. 355–348 Ma granitoids in southern
Fiordland, but their chemistry is similar to that of the older Karamea
Suite.
Three regional-scale metasedimentary units—locally
fossiliferous
Fanny Bay Group Buller Terrane rocks in southern Fiordland, Edgecumbe
and Cameron Group Takaka Terrane rocks in south-central Fiordland, and
undifferentiated Deep Cove Gneiss high-grade metasedimentary rocks of
western Fiordland—are all stitched by c. 355–348 Ma old plutons,
indicating they have been in close proximity since at least
c. 355–348 Ma. In south-central Fiordland, c. 355–348 Ma old
plutons cut across fabrics defined by upper amphibolite facies mineral
assemblages, indicating low pressure/high temperature metamorphism in
this area before this time.
The c. 164 Ma old leucocratic Lake Mike Granite is a
unique
pluton in southwest Fiordland with no obvious correlatives. Plutons
emplaced between c. 140 and 125 Ma are similar to the Rahu Suite,
although isotopic data are required to confirm this correlation. Rahu
Suite plutonism may therefore have begun by c. 140 Ma, rather than
c. 120 Ma as previously suggested. Plutons emplaced between
c. 125 and 110 Ma have high Sr/Y ratios comparable with the
Separation Point Suite. They occur in both an outboard location around
Lake Poteriteri and an inboard location around the western end of Dusky
Sound. The c. 115 Ma two-mica garnet granites of the Anchor
Island Intrusives #2 probably formed by partial melting of adjacent
ortho- and paragneisses, indicating that upper amphibolite facies
metamorphism in western Dusky Sound occurred during the Early
Cretaceous.
The Dusky Fault does not pass directly out to the coast
through
outer Dusky Sound as previously mapped. Instead it merges with the
major northeast-striking Lake Fraser Fault at Cascade Cove, which
crosses the outer coast near West Cape. The Last Cove Fault is a minor
structure which cannot be traced beyond Last Cove rather than a major
fault of regional extent as has been previously suggested.
Keywords Fiordland; Anchor Island, Tower Intrusives; Bald
Peaks, Big, Brothers, Cascade, Fannin, Five Fingers, Grace Burn,
Houseroof, Mouat, Indian Island, Jeanie, Lake 773, Mid Poteriteri, Mt
Evans, Newton River, Red Head, Spot 59, Treble Mountain, Prices,
Widgeon, Trevaccoon Plutons; Lake Mike, Revolver, North Port, Lake Monk
Granites; Staircase Tonalite; Thundercleft Quartz Diorite; Only Islands
Diorite; dikes; migmatites; gabbro; diorite; tonalite; granodiorite;
granite; Ridge, Tobin, Separation Point, Rahu Suites; Median Batholith;
Dusky, Lake Fraser Fault; new lithologic names
G07002; Online publication date 21 September 2007; Received 19 February
2007; accepted 30 July 2007
New Zealand Journal of Geology & Geophysics, 2007, Vol. 50:
283–314
0028–8306/07/5004–0283 © The Royal Society of New Zealand 2007
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