Download The 2.69 GA Paringa basalts: Crustal recycling into the

0393-000226
The 2.69 GA Paringa basalts: Crustal recycling into the asthenosphere source
Corresponding author: Nuru M. Said, University of Western Australia, [email protected]
Co-author:
Campbell T. McCuaig, University of Western Australia, [email protected]
Depleted and enriched Paringa basalts form the uppermost part of the Upper Basalt Unit of the
Kambalda Sequence in the 2720-2680 Ma Kalgoorlie Terrane, and provide a window into
compositionally and isotopically heterogeneous asthenosphere. Depleted examples of the basalts
are compositionally restricted with Mg# between 61 and 57, and Ni between 166 and 73 ppm. On
REE and primitive mantle normalized diagrams they are characterised by: (1) near-flat REE
patterns with a small range of LREE depletion (La/Smn 1.01-0.76); (2) no Ce nor Eu anomalies;
(3) Nb/Th ratios of 8.7-12, variably greater than the primitive mantle value of 8; and (4) no
negative primitive-mantle normalised P or Ti anomalies relative to neighbouring REE. They have
compositions similar to Neoarchean tholeiitic basalts associated with komatiites, and εNd 2.7 Ga
values of +1.3 to + 2.2, in keeping with the majority of crustally uncontaminated Neoarchean
basalts and komatiites. Enriched counterparts of the Paringa Basalt are compositionally varied,
with Mg# 76-53 and Ni 391-73 ppm. On REE and primitive- mantle normalized diagrams, they
feature: (1) systematically fractionated LREE (La/Smn=2.1-3.1); (2) HREE mostly ~1, with
outliers to 1.6; (3) no Ce nor Eu anomalies; (4) low Nb/Th ratios of 0.7-1.6 ; and (5) strong negative
primitive- mantle normalised P and Ti anomalies. Epsilon Nd values are -1.7 to -4.4; and there is
no correlation of εNd with indices of crustal contamination such as La/Smn or Nb/Th ratios. This
basaltic suite has previously been interpreted as either boninitic or crustally contaminated
komatiite. Assimilation-fractional crystallisation (AFC) modeling was conducted using a
komatiite as parental liquid. Contaminants used in this modeling were local trondhjemite-tonalitedacite (TTD) Black Flag Group (BFG), average high-Ca granites of the Yilgarn craton, and
average upper-, middle-, or lower continental crust. None of the average Archean crustal
compositions match the AFC trends, nor does the BFG. High-Ca granites do match the trends, as
does average modern upper crustal composition but the latter have depleted mantle Nd-isotope
compositions. There are numerous problems with a model of assimilation of continental crust in
the genesis of the enriched Paringa basalts, including coherent REE patterns over a range of Mg#,
and Nb-, P-, and Ti-anomalies that do not increase with Th or La/Smn. Enriched Paringa basalts
are compositionally distinct from Mg-rich continental flood basalts that feature greater contents of
Ti, and other incompatible elements, with fractionated HREE, or boninites, picrites, or medium-K
basalts from intraoceanic arcs. Rather, these basalts are interpreted to result from recycling of older
continental crust into the mantle source of the plume from which the Kambalda Sequence
komatiites and basalts erupted. The two distinct compositional and isotopic types record a
heterogeneous mantle plume, possibly erupted at a rifted craton margin. This will enhance our
understanding of the processes involved in metal and fluid transfer from Earth’s mantle to crust at
craton margins or/and plate boundaries, and has implications for the generation of ore deposits in
such environments.