New Zealand Journal of Geology and Geophysics abstracts
Reassessment of the origin of the Dun Mountain Ophiolite, New Zealand:
Nd-isotopic and geochemical evolution of magma suites
W. J. SIVELL
Division of Earth Sciences
The University of New England
Armidale 2351, Australia
M. T. MCCULLOCH
Research School of Earth Sciences
The Australian National University
Canberra 0200, Australia
Abstract Magmatic suites with contrasting isotopic and
geochemical compositions, sequentially emplaced in different tectonic regimes,
comprise the Dun Mountain Ophiolite Belt (DMOB), New Zealand. At D'Urville
Island, the northernmost exposure of the DMOB, earliest erupted (stage 1)
pillow basalts have [[epsilon]]Nd(T) = +6.3 to +7.5, and are incompatible
element enriched, like basalts from geochemically anomalous ridge segments.
Overlying stage 2 basalts (sheeted flows) show a narrow range of
[[epsilon]]Nd(T) = +8.3 +/- 0.2, with chemical characteristics of
depleted back-arc basin basalts. These rocks are intruded by mafic to silicic
stage 3 magmas, which have high uniform initial
143Nd/144Nd ratios ([[epsilon]]Nd(T) =
+9.3 +/- 0.2) over a wide range of 147Sm/144Nd
values (yielding a precise Early Pemian Nd-isotope age of
278 +/- 4 Ma (MSWD = 0.48)). Stage 3 magmas show pronounced
subduction-related geochemical signatures similar to island arc tholeiites
(IAT) from immature arcs. They are closely analogous to some (boninite)-IAT
magmas which characterise "infant arc" eruptive activity in forearc basins of
present-day Western Pacific island arc systems. A wide range of stage 3
magma compositions, ranging from near-primary basaltic dikes (Mg# = 74) to
extremely fractionated silicic plagiogranites with uniformly very depleted
isotopic ratios, is consistent with slow spreading rates which gave rise to
polybaric, closed-system fractionation of magmas and periodic chamber
abandonment. Some stage 3 rocks with SiO2 levels in the andesite range
have low-TiO2 contents and high Mg#, and may be fractionated equivalents of
boninites. High [[epsilon]]Nd(T) values of stage 3 magmas indicate a lack
of subducted sediment with inherited crustal residence signatures, and reflect
the extent of supra-subduction zone (SSZ) mantle wedge depletion. DMOB
stage 3 magmas may represent forearc magmatism that was the precursor to
normal subduction-related volcanism established by c. 265 Ma in the
Brook Street Arc and derived from a SSZ mantle source with identical Nd-isotope
characteristics. Less isotopically depleted stage 1 and 2 basalts (and
rocks of the Lee River Group in general) may represent fragments of
pre-existing mafic ocean crust sensu stricto, as suggested by
significantly older Nd-isotopic ages for the basaltic seafloor volcanic
component of the DMOB. Upon initiation of subduction, pre-existing
geochemically anomalous (possibly back-arc basin) ocean crust (stage 1 and
2 magmas) was intruded by the "infant arc" (stage 3) magmas, as proposed
for older seafloor remnants in forearc regimes of the Izu-Bonin-Mariana and
Tonga arc systems.
Keywords ophiolite; Permian; New Zealand; geochemistry;
Nd-isotopes; subduction; forearc basin
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43:
133-146
0028-8306/00/4302-0133 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1700K); (scanned from paper original: notes about this process)
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