New Zealand Journal of Geology and Geophysics abstracts
Geochemistry, mineralogy, and metamorphic history of kyanite-orthoamphibole-bearing
Alpine Fault mylonite, South Westland, New Zealand
Murray K. McClintock*
Alan F. Cooper
Department of Geology
University of Otago
P.O. Box 56
Dunedin, New Zealand
email: murraym@soest.hawaii.edu
email: alan.cooper@stonebow.otago.ac.nz
*Present address: Department of Geology & Geophysics, School of Ocean
& Earth Science & Technology,
1680 East-West Road, Honolulu 96822, Hawaii, USA.
Abstract Mylonite outcropping in the Alpine Fault
Zone at Makawhio River, South Westland, shows mineralogy unlike rocks found
elsewhere in the Haast Schist. The Makawhio mylonites comprise kyanite-orthoamphibole
hornblendite, garnet amphibolite, chlorite-hornblende schist, kyanite-plagioclase
schist, and talc-tremolite rocks, are characterised by ultrabasic to intermediate
chemistry, show extreme enrichment in Cr and Ni, and collectively include
kyanite, margarite, fuchsite, scapolite, and the first reported occurrence
of gedrite and anthophyllite from the Eastern Province. These unusual mylonites
form a pod-like body enveloped by chlorite-hornblende schist within Torlesse-derived
metasediments. The geochemistry and mineralogy of the Makawhio rocks suggests
an origin as a veined ophiolitic rock metasomatised before or during upper
amphibolite facies Alpine Schist metamorphism and mylonitisation, resulting
in a localised occurrence of rocks atypical of the Haast Schist overall.
These kyanite-bearing, metasomatised mylonites are juxtaposed with quartzofeldspathic
and calc-silicate mylonites, and with amphibolites derived from ocean-floor
basalt, within the Alpine Fault Zone. The Makawhio River kyanite-bearing
mylonites and associated rocks probably represent a tectonised sliver of
the Pounamu Ultramafic Belt ophiolite, which is exposed to the north and
south as part of the imbricated basement to the Torlesse Terrane along the
western margin of the Southern Alps. The range of lithologies and unusual
nature of the mylonites at Makawhio River may also represent an extension
of the Torlesse Caples Terrane boundary incorporated into the plate boundary
zone during oblique convergence and dextral shear on the Alpine Fault.
Keywords Alpine Fault; Haast Schist; metasomatism;
kyanite; gedrite; anthophyllite; Southern Alps
G01025 Received 17 August 2001; accepted 30 October 2002; published 21
March 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 47-62
0028-8306/03/4601-0047 $7.00/0 © The Royal Society of New Zealand
2003
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