| dc.description.abstract | At greater than 3000 km long, and 500 km wide, the East Australian volcanic chain is one of the largest
intraplate volcanic regions in the world and is represented by over 50 individual provinces, which erupted
during the period 84{0 Ma. Some of these provinces show clear age-progression north to south from 34-6Ma
(Cohen et al., 2013; Wellman et al., 1970), whilst others show no age-progression at all. O shore to the east
lie the Tasmantid and Lord Howe volcanic chains which formed coevally with the age-progressive continental
volcanoes, 56{7 Ma, and 28{7 Ma, respectively (Kalnins et al., 2015; Seton et al., 2019a; Mortimer et al., 2010;
Quilty, 1993; McDougall and Duncan, 1988). Broad patterns in age, lithosphere thickness, major element
concentrations, trace element concentrations, isotope ratios, and volume have allowed the identi cation of
three distinct forms of intraplate volcanism operating across the three chains. These include: (1) oceanic,
age-progressive volcanism from a deep, EMI-type mantle plume; (2) continental, age-progressive volcanism
from the same plume mixed with melts from the metasomatised subcontinental lithospheric mantle (SCLM);
and (3) continental, non-age-progressive volcanism related to passive melting of the depleted mantle and the
metasomatised SCLM.
A deep plume source is particularly evident through analysis of Tasmantid samples which lie along the
extinct Coral and Tasman Sea spreading ridges. As seafloor spreading ceased around 52 Ma, the Tasmantid
seamounts were emplaced onto progressively older and thicker oceanic lithosphere. Ratios of any incompatible
element (Nb, Sr, Th, LREE, P) versus any garnet-compatible element (Y, V, Sc, HREE) increase moving
south through the seamount chain, in accordance with this increase in lithosphere thickness. These changes
are adequately modeled by melting of a deep, garnet-lherzolite source, where the degree of melting varies along
the chain and is controlled by lithosphere age and thickness. Isotopic results from the Tasmantids show that
increasing degrees of melting correlate with greater isotopic enrichment (high 87Sr/86Sr and 206Pb/204Pb,
and low "Nd) and lower Nb/La. This suggests the source of the Tasmantids is a deep, EMI-like plume, with
relatively low Nb compared to other OIBs.
On the continent, the most extreme HFSE and LILE enrichments are found in the age-progressive leucitite
samples, which lie on the thickest continental lithosphere. These are most likely produced from melting of
the SCLM, which has been previously enriched through metasomatism. SCLM involvement is evident from
K>Na, which cannot be achieved through partial melting of normal mantle or fractional crystallisation alone,
and phlogopite phenocrysts, up to 5mm long, coupled with Th-U depletions, and Rb, Ba, K enrichments,
which probably reflect melting of a phlogopite-rich source i.e., not the convecting mantle. On a plot of Nb/Y
versus Zr/Y all continental, age-progressive provinces fit well with mixing between the same SCLM source as
the leucitites and the EMI-plume source of the Tasmantids. Other factors pointing towards SCLM involvement
in the age-progressive continental volcanoes include the fact that the highest enrichments are only observed
in continental samples and that there are good correlations between the most extreme enrichments and the
most primitive continental samples containing lithospheric xenoliths and plagioclase megacrysts.
Volume estimates of each province have been calculated using a number of methods that include data from
multispectral satellites, boreholes, magnetic surveys, geological maps, and high-resolution digital elevation
models. A comparison of these volumes with each other and also with geochemical results, suggest that
the lava fields consistently represent lower volume, small degree melts and the central volcanoes consistently
represent higher volume, high degree melts. The Tasmantid seamounts represent higher degrees of melting
still, which is to be expected due to their location on thinner oceanic lithosphere.
The results of this thesis show that age-progressive intraplate volcanism in eastern Australia can be well
explained by mixing of a two-component source; a deep, EMI-type mantle plume, and a varyingly metasomatised
SCLM. Sr-Nd-Pb isotope data for all non-age-progressive, continental provinces t with mixing between
the depleted mantle and components of EMI, EMII and HIMU reservoirs. Such observations are most easily
explained by passive melting of the upper mantle once again mixed with melts from the metasomatised SCLM. | en |
