|
Titel |
Isotopic characterisation of the sub-continental lithospheric mantle beneath Zealandia, a rifted fragment of Gondwana |
VerfasserIn |
Tod E. Waight, James M. Scott, Quinten H. A. van der Meer |
Konferenz |
EGU General Assembly 2013
|
Medientyp |
Artikel
|
Sprache |
Englisch
|
Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 15 (2013) |
Datensatznummer |
250079357
|
|
|
|
Zusammenfassung |
The greater New Zealand region, known as Zealandia, represents an amalgamation of crustal
fragments accreted to the paleo-Pacific Gondwana margin and which underwent significant
thinning during the subsequent split from Australia and Antarctica in the mid-Cretaceous
following opening of the Tasman Sea and the Southern Ocean. We present Sr, Nd and Pb
isotopes and laser ablation trace element data for a comprehensive suite of clinopyroxene
separates from spinel peridotite xenoliths (lherzolite to harzburgite) from the sub-continental
lithospheric mantle across southern New Zealand. These xenoliths were transported to the
surface in intra-plate alkaline volcanics that erupted across the region in the Eocene and
Miocene (33-10 m.y.a.). Most of the volcanic suites have similar geochemical and
isotopic properties that indicate melting of an OIB-like mantle source in the garnet
stability zone and that contained a HIMU component. The volcanics have tapped
two adjacent but chemically contrasting upper mantle domains: a fertile eastern
domain and an extremely depleted western domain. Both domains underlie Mesozoic
metasedimentary crust. Radiogenic isotope compositions of the clinopyroxene have 87Sr/86Sr
between 0.7023 to 0.7035, 143Nd/144Nd between 0.5128 and 0.5132 (corresponding to
ÉNd between +3 and +13) with a few samples extending to even more depleted
compositions, 206Pb/204 Pb between ca. 19.5 to 21.5 and 208Pb/204 Pb between ca.
38.5 to 40.5. No correlations are observed between isotopic composition, age or
geographical separation. These isotopic compositions indicate that the sub-continental
lithospheric mantle under southern New Zealand has a regionally distinct and pervasive
FOZO to HIMU – like signature. The isotopic signatures are also similar to those of
the alkaline magmas that transported the xenoliths and suggest that most of the
HIMU signature observed in the volcanics could be derived from a major source
component in the sub-continental lithospheric mantle. Trace element abundances in
clinopyroxene are highly heterogeneous and vary from LREE-enriched, relatively flat
and MORB-like, strongly LREE-depleted, through to patterns displaying evidence
for depletion and subsequent re-enrichment. These variations occur throughout
the region and also between different xenoliths from a single eruption site. There
are no clear correlations between REE characteristics and isotopic composition
suggesting that much of the depletion and re-enrichment is relatively recent. A
broad scatter of increasing 143Nd/144Nd with increasing Sm/Nd, plotting broadly
between 150-350 Ma isochrons, may provide some constraints on these events. |
|
|
|
|
|