New Zealand Journal of Geology and Geophysics abstracts
Metasedimentary, granitoid, and gabbroic rocks from central Stewart Island, New
Zealand
A. H. ALLIBONE
Etheridge Henley Williams Geoscience Consultants
P.O. Box 250
West Deakin, ACT 2600, Australia
A. J. TULLOCH
Institute of Geological & Nuclear Sciences
Private Bag 1930
Dunedin, New Zealand
Abstract A NNE-NE trending strip, 3-8 km wide, extending
from the Freshwater valley across Mt Rakeahua, Table Hill, and Mt Allen to the
northern end of the Tin Range was mapped at a scale of 1:12 500 to locate
and investigate the boundary between the Median Tectonic Zone (MTZ) and Western
Province on Stewart Island.
A NNE-trending fault, herein termed the Escarpment Fault, separates
predominantly ductily deformed rocks on its south side from essentially
undeformed rocks to the north.
North of the Escarpment Fault, a small (2-3 km2) pluton of
alkali-feldspar granite (Freds Camp) intrudes gabbroic rocks tentatively
considered to be associated with gabbro/anorthosite/diorite of the Rakeahua
pluton, centred on Mt Rakeahua. Both units were subsequently intruded by I-type
biotite granite of the South West Arm pluton.
South of the Escarpment Fault the oldest intrusions are biotite
tonalite-granite orthogneisses (Ridge and Table Hill plutons)
intercalated with the sillimanite-cordierite-bearing Pegasus Group
metasedimentary rocks, considered to represent the Western Province. They
contain titanite, allanite, and magmatic epidote-bearing assemblages, implying
affinities with I-type granitoids. These older granitoids have been affected by
at least three phases of ductile deformation. Immediately south of the
Escarpment Fault, the Escarpment pluton (hornblende, biotite quartz
monzonite-quartz monzodiorite) only exhibits effects of the third phase of
deformation. Minor gabbroic intrusives concordant with with the S3 fabric
intrude the Pegasus Group and intercalated orthogneisses. Plutons of two-mica,
garnet +/- cordierite granite (Blaikies and Knob) and younger
biotite-titanite-magmatic epidote granite (Campsite) cut fabrics associated
with the third phase of ductile deformation.
Preliminary U-Pb dating indicates Devonian-Carboniferous, Carboniferous,
Jurassic, and Early Cretaceous emplacement ages for Ridge Orthogneiss, Freds
Camp pluton, South West Arm pluton, and Blaikies pluton, respectively.
South West Arm and Rakeahua plutons are provisionally correlated with the MTZ,
which extends the (south) western boundary of the MTZ at least 12 km
further southwest from the Freshwater faults. Correlation of gabbroic rocks
south of the Escarpment Fault with the MTZ would imply an autochthonous rather
than allochthonous relationship of the latter to the Western Province.
Keywords metasedimentary rocks; psammitic schist; pelitic
schist; cordierite; sillimanite; Pegasus Group; granitoid rocks; garnet;
muscovite; gabbro; diorite; anorthosite; Rakeahua Batholith; Median Tectonic
Zone; Western Province; mylonite; Paterson Group; Stewart Island
New Zealand Journal of Geology and Geophysics, 1997, Vol. 40: 53-68
0028-8306/97/4001--0053 $2.50/0 (c) The Royal Society of New Zealand
1997
PDF file of entire paper: medium quality (4377K); (scanned from paper original: notes about this process)
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