New Zealand Journal of Geology and Geophysics abstracts
Igneous rocks of the Brook Street Terrane, New Zealand:
implications for Permian tectonics of eastern Gondwana and magma
genesis in modern intra-oceanic volcanic arcs
Carl Spandler1
Kurt Worden2
Richard Arculus1
Steve Eggins3
1 Department of Earth and Marine Sciences
Australian National University
Canberra, Australia 0200
email: carl@geology.anu.edu.au
2 Geoscience Australia
GPO Box 378
Canberra, Australia 2601
3 Research School of Earth Sciences
Australian National University
Canberra, Australia 0200
Abstract The Brook Street Terrane of South Island,
New Zealand, is a remnant of a primitive intra-oceanic arc system of
Permian age. The terrane consists largely of volcanogenic sequences
that contain plagioclase- and clinopyroxene-phyric basalts, high-MgO
ankaramite dikes, and basaltic to andesitic volcaniclastic and
sedimentary rocks. Dacites and rhyolites are relatively rare. Intruding
the sequences are thick dolerite dikes, trondhjemite plutons, and
numerous small cumulate complexes. The cumulate complexes contain
early-formed olivine- and clinopyroxene-rich ultramafic cumulates
overlain by anorthite and hornblende-bearing gabbros. There is
convincing geological evidence to support earlier interpretations of a
direct correlation between the Brook Street Terrane and the Gympie
Terrane of Queensland, and we present geochemical data to support
correlation with the Teremba Terrane of New Caledonia. It is likely
that these terranes are exposed sections of an extensive island-arc
system that was active in the Pacific in Permian times. Dislocation of
the arc probably occurred during accretion to the Gondwana margin and
subsequent Gondwana breakup.
The major and trace element geochemistry of a range
of mafic dikes and flows from along the terrane precludes significant
geochemical variation along the terrane and shows that the majority of
magmas were primitive island-arc tholeiites. The Bluff Complex is an
exception and may have formed in a back-arc or arc-rift environment.
The geochemistry, petrology, and field relations indicate that most of
the intrusive and volcanic rocks are directly related products of upper
crustal magmatic differentiation. Primary magma types include high-MgO
ankaramites and trondhjemites that are suggested to have formed by
partial melting of lower crustal clinopyroxene-rich cumulates and
gabbros, respectively. The parental ankaramites fractionated to form
the bulk of the Brook Street Terrane, including the mafic-ultramafic
cumulates and evolved melts of high-Al basalt to andesite composition.
The Brook Street Terrane is an excellent analogue
for modern island-arc systems and allows for the evaluation of magmatic
processes that operate at the subvolcanic level of arcs. The wide
distribution of ankaramites in the Brook Street Terrane indicates that
parental magmas in island arcs may be more primitive than is currently
recognised. Furthermore, partial melting of arc lower crustal cumulates
before delamination may be crucial to the development of arcs and the
evolution of the continental crust.
Keywords Brook Street Terrane; island arc;
ankaramite; parent magma; magmatic differentiation; crustal melting;
continental crust
G03029; Received 9 September 2003; accepted 2 March 2004; Online
publication date 23 March 2005
New Zealand Journal of Geology & Geophysics, 2005, Vol. 48:
167–183
0028–8306/05/4801–0167 © The Royal Society of New Zealand 2005
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