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Mantle reservoirs and mafic magmatism associated with the break-up of Gondwana : the Balleny Plume and the Australian-Antarctic discordance : U-Pb zircon dating of a Proterozoic mafic dyke swarm in the Vestfold Hills, East Antarctica

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Lanyon, R (1994) Mantle reservoirs and mafic magmatism associated with the break-up of Gondwana : the Balleny Plume and the Australian-Antarctic discordance : U-Pb zircon dating of a Proterozoic mafic dyke swarm in the Vestfold Hills, East Antarctica. PhD thesis, University of Tasmania.

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Abstract

Within the recent literature, the isotopic heterogeneity of ocean island basalts (OIB)
is generally ascribed to mixing between any two or more of four isotopically distinct mantle
end-member components - DMM, HIMU, EMI and EMU - the origin and precise location of
which are still the subject of much debate. An attempt is made here to constrain the
geochemical characteristics of these end-member components using literature-derived endmember
OIB data. This has confirmed the presence of consistent trace element differences
and also suggested certain major element distinctions, which are interpreted to reflect
diverse end-member mantle source compositions rather than differences in the pressure
and temperature of melting involved in OIB production.
EMI basalts, which extend to the lowest 143Nd/144Nd values of all OIB, were found
to possess the most distinctive major element characteristics, the latter evident as higher
Si02 and lower FeO*, CaO, TIO2, P2O5 and K2O values than both HIMU and EMU OIB.
However, EMI basalts have the least well defined trace element systematics. In contrast,
HIMU and EMU basalts, previously characterised by the highest Pb isotope ratios and the
highest 87Sr/86Sr values respectively, were found to have overlapping major element
abundances but quite distinctive trace element systematics. HIMU basalts, have the highest
CaO and generally lower LILE/HFSE and LREE/HFSE abundance ratios, intermediate Th/La
and Th/Nb, lower Zr/Nb and higher Nb/Pb and Ce/Pb values than either of the EM
components. EMU basalts extend to the highest K2O and Ti02 values and are characterised
by lower Ba/Th, higher Th/La and higher Th/Nb values than EMI OIB.
Previous workers have recognised a regional HIMU radiogenic isotope and trace
element signature within Cretaceous to Recent volcanics scattered throughout the
dispersed continental fragments of eastern Gondwana and dredged from the TasmanSea -
southwest Pacific Ocean seafloor. This study has ascribed the HIMU nature of continental
volcanism in Tasmania, the South Island and offshore islands of New Zealand, and Marie
Byrd Land and the McMurdo Volcanic Group of West Antarctica to localised and intermittent
periods of tectonically-induced decompression and melting of underplated HIMU material
emplaced at the base of the lithosphere by one or more upwelling plume heads prior to or
coincident withthe onset of continental break-up in this region. Ocean floor HIMU volcanism
in the southern Tasman Sea and southwest Pacific Ocean has been attributed to
lithospheric plate movement over the persisting plume conduits.
Previous plate-tectonic reconstructions have ascribed a 4000 km long cun/ed chain
of seamounts and islands, extending from the western flank of Lord Howe Rise to the
Balleny Islands close to the Ross Sea region of the southwest Pacific Ocean, to movement
of the Indian-Australian and Antarctic Plates over the Balleny Plume with progressive similar latitude to the Balleny Islands, has an homologous isotopic and trace element
signature and therefore appears to be the product of a second and parallel HIMU plume
trace, here termed the Scott Plume. The latter is also held responsible for the enriched
geochemical features of some Macquarie Island basalts. The Balleny and Scott Plumes
appear to have been temporarily trapped by, and to have contaminated the eruptives of, the
Southeast Indian and Pacific-Antarctic spreading ridges respectively. This has resuKed in the
<28 Ma (between AS and the active spreading ridges) seafloor in this region bearing an EMORB
to HIMU-DMM isotopic signature. The fact that samples dredged from virtually zeroage
oceanic crust on the southern flank of the SEIR and northwest of the Balleny Islands,
also bear this distinctive isotopic and trace element signature impliesthat the supply of HIMU
material to this region of the SEIR has continued until very recently.
Significant contamination of Balleny Plume HIMU volcanics by DMM material,
particularly evident in Pb-Pb isotopic space, suggests that the Balleny Plume is a relatively
weak plume which entrained large amounts of depleted upper mantle during upwelling.
Progressive temporal dilution of the HIMU plume component by DMM material may imply
either increasing amounts of upper mantle entrainment, suggesting a gradual weakening of
the plume with time, or progressive preferential melting of entrained material within a
heterogeneous plume. The Balleny Plume also appears to have slightly higher timeintegrated
Th/U than other known HIMU sources, resulting in the most radiogenic Balleny
Plume samples plotting above the HIMU field in 206pb/204pb208pb/204pb space, and
thereby potentially expanding the known range of HIMU isotopic values.
Part 2:
The Southern Ocean between the southern margin of Australia and Wilkes Land in
Antarctica hosts an important section of the global mid-ocean ridge system. Other
researchers have concluded that the Southeast Indian Ridge (SEIR) in this region
experienced a dramatic change in spreading rate at -44.5 Ma, coincident with a major period
of global plate reorganisation. Comparative studies, within the recent literature, of individual
fast and slow spreading ridges have revealed distinct morphological and geochemical
parameters which have been attributed to different underlying magma chamber processes.
Anattempt is made here to discern if the geochemical parameters correlated with spreading
rate in the literature are also evident with a change in spreading rate along a single segment
of mid-ocean ridge. This effort has been concentrated on the chemistry of basalts collected
from four Southern Ocean dredge sites north of the SEIR. Two of these sites were located
on oceanic crust formed during a period of relatively slow seafloor spreading prior to 49 Ma,
whereas the other two correspond to a period of more rapid seafloor spreading after 44.5
Ma. The lack of recognisable and systematic geochemical differences between the SEIR
slow and intermediate spreading rate eruptives suggests that the change in spreading rate at -44.5 Ma, and the inferred change in underlying mantle processes, may not have been
great enough to result in the significant geochemical variations cited in the literature.
Alternatively, the off-axis Southern Ocean basalt database may be insufficient to enable the
detection of such differences.
The Southern Ocean south of the Great Australian Bight is also host to a globally
anomalous region of mid-ocean ridge, known as the Australian-Antarctic Discordance (AAD).
In addition to its recognised morphological and geophysical anomalies, the AAD represents
the current on-axis location of a proposed isotopic boundary between Indian and Pacific
Ocean upper mantle convective regimes, previously defined from analyses of <4 Ma MORB
dredged from within and adjacent to the AAD. An investigation is undertaken here into the
off-axis location of this isotopic boundary, concentrating on radiogenic Pb, Sr and Nd
isotope data for -36-66 Ma seafloor dredged from either side of the northward extrapolation
of the AAD. The results show that >36 Ma seafloor east of the AAD has an Indian Ocean
MORB isotopic signature, thereby implying that the proposed Indian-Pacific Ocean isotopic
boundary does not extend directly north of the ridge towards the southern margin of
Australia. Progressive westward migration of an arcuate-shaped front of Pacific Ocean upper
mantle therefore appears to be a consequence of Australian-Antarctic rifting and Southern
Ocean opening, favouring models of active mantle flow outlined in the recent literature such
as Pacific Ocean basin shrinkage, convergence of hotspot-driven along-axis asthenospheric
flow and/or direct Indian and Pacific Ocean upper mantle convergence.
Part 3:
The Vestfold Hills, one of several Archaean cratonic blocks within the East Antarctic
Shield, comprises a high-grade metamorphic basement complex intruded by at least nine
generations of Early to Middle Proterozoic mafic dykes. Extensive U-Pb ion microprobe
(SHRIMP) analyses of zircons, derived predominantly from late-stage felsic differentiates of
the mafic dykes, provide precise crystallisation ages for several dyke generations. These
new ages enable constraints to be placed on both the history of mafic magmatism in the
Vestfold Hills and the timing of the various interspersed, and already documented,
Proterozoic deformation events. In addition to demonstrating the utility of zircons derived
from felsic late-stage differentiates for the dating of co-genetic mafic dykes, this study also
places doubt on previous whole-rock Rb-Sr dating of mafic dyke suites in this and other
areas of East Antarctica.
207pb/206pb zircon ages of 2241 ±4 Ma and 2238±7 Ma for the Homogeneous and
Mottled Norites, respectively, provide a younger emplacement age for associated Group 2
High-Mg tholeiite dykes than the whole-rock Rb-Sr date (2424172 Ma) originally interpreted
as the age of all high-Mg intrusives in the Vestfold Hills. Zircon ages of 1754116 Ma and
1832172 Ma confirm the previously defined Rb-Sr age ofthe Group 2 Fe-rich tholeiites. Two
later dyke generations, the Group 3 and 4 Fe-rich tholeiites, distinguished on the basis of field orientations and cross-cutting relationships, yield zircon emplacement ages of 1380±7
Maand 1241±5 Ma,which also defineminimum ages fortwo suites of lamprophyre dykes.
Xenocrystic zircons within both felsic segregations and mafic dykes yield zircon ages
of 2478±5 Ma to -2740 Ma, indicating the presence of Archaeancrustal source rocks of this
antiquity beneath the Vestfold Hills.

Item Type: Thesis (PhD)
Keywords: Magmatism, Geology, Stratigraphic, Dikes (Geology)
Copyright Holders: The Author
Copyright Information:

Copyright 1994 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

The preliminary results of Chapters 3 and 4 have been published as: Lanyon R., Varne R. and Crawford A.J., 1993, Tasmanian Tertiary basalts, the Balleny Plume, and opening of the Tasman Sea (southwest Pacific Ocean), Geology, 21, 555-558.

The author identifies chapter 8 as being submitted for publication in Geology as: Lanyon R. and Crawford A.J. (submitted -August 1994) Westward migration of Pacific Ocean upper mantle into the Southern Ocean region between Australia and Antarctica. Geology. An article published under that name appeared in Geology, June 1995, v. 23, no. 6, p. 511–514. The text in the chapter does not appear to significantly resemble the text of the article.

Chapter 9 has been published as: Lanyon R., Black L.P., Seitz H.-M., 1993, U-Pb zircon dating of mafic dykes and its
application to the Proterozoic geological history of the Vestfold Hills, East Antarctica, Contributions to mineralogy and petrology, 115, 184-203. The final publication is available at Springer via http://dx.doi.org/10.1007/BF00321219

Date Deposited: 19 Dec 2014 02:34
Last Modified: 21 Apr 2016 02:00
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