GEMOC ARC National Key Centre

S. Graham¹, D. D. Lambert² and S. R. Shee³.

1 sgraham@laurel.ocs.mq.edu.au, GEMOC Key Centre Macquarie University Sydney 2109 Australia,
2 VIEPS Monash University 3168 Australia,
3 De Beers Australia Limited 60 Wilson Street South Yarra 3141 Australia.
 

Recent technical success by Stockdale Prospecting Limited (De Beers Australia Limited) in the eastern Archean Yilgarn Craton (Western Australia) has revealed the full extent of a previously little known alkaline ultramafic-carbonatite province centered on the Leonora region.  Alkaline ultramafic rocks occur scattered throughout an area of ~45,000 km², with carbonatites and melnoites (ultramafic lamprophyres) common in the east and kimberlites common in the west.  Previous geochronological studies estimate magmatic activity between 2020-2060 Ma, making this the oldest alkaline ultramafic province known.  The age and petrographic spectrum offered by these rocks make it an excellent region in which to assess: 1) ancient mantle plume signatures, and 2) the kimberlite-melnoite-carbonatite relationship.  We have obtained geochemical and isotopic data from peridotite, macrocryst Cr-spinel, groundmass magnetite and whole rock samples to examine these issues.

Major Element Geochemistry: MgO contents range from <5 wt% in the carbonatites to 5-15 wt% in the melnoites and 15-25 wt% in the kimberlites. K₂O, CaO and MnO are negatively correlated with MgO, a trend resulting from the increased proportion of lithospheric mantle peridotite in kimberlites vs melnoites. Surprisingly Fe₂O₃ is positively correlated with MgO, a feature that may indicate the lithospheric mantle is Fe-rich, or more oxidised, compared to other cratons.
Trace Element Geochemistry: Ni and Cr are enriched in the kimberlites compared to the melnoites and are correlated with MgO. The samples are typically enriched in the incompatible elements.  However, the samples, irrespective of MgO content, show minimal variation in incompatible element ratios.
Whole rock - magnetite Re-Os geochemistry: Re/Os ratios in kimberlites and melnoites are all super-chondritic (Re/Os = 0.4 - 46). Groundmass magnetite has lower Re/Os ratios than its corresponding host and the Re/Os ratios of carbonatite groundmass magnetite are higher those of the other magnetites.  One macrocryst spinel separate has a high Re/Os ratio, suggestive of a cognate origin.
Peridotite-Cr-spinel Re-Os geochemistry: Re/Os ratios for 2 peridotites are divergent.  One sample has a high Re/Os ratio (1.1) and the other a low Re/Os ratio (0.15).  Geochemically, the minerals in the high Re/Os peridotite show this sample was part of a metasomatic vein, whereas the low Re/Os ratio peridotite is a lherzolite.  Cr-spinel from one kimberlite yields a Re/Os (0.07) ratio that is high for this mineral species.

Isotope Geochemistry: Kimberlites, melnoites and carbonatites have low ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.09 - 0.11) and initial eNd values yield a limited spread (2020 Ma εNd = 0.3 - 1.6).  The Sm/Nd ratios and initial eNd values are surprisingly uniform given the petrographic and geochemical diversity present.  Nd depleted mantle model ages range from 2380 - 2450 Ma.

Peridotites and kimberlites are correlated on a Re-Os isochron diagram and define an age significantly older than emplacement (2472 ± 99 Ma, γOs(i) = 0.0 ± 5.6, MSWD = 18). Oxides and melnoites are also correlated, but in contrast yield an emplacement age (2024 ± 15 Ma, γOs(i) = 3.3 ± 3.2, MSWD = 2.3).  Given the vast geographic range of this igneous province these are remarkable results and suggest, among other things, that magmatism occurred contemporaneously.
Conclusions: Major and compatible trace element data demonstrate a trend of increasing kimberlitic affinity from east to west throughout the province.  However, incompatible element ratios, and initial eNd values, are independent of MgO, Ni or Cr concentrations.  Hence these data link all of the rocks within the province to a common metasomatised upper mantle source.

Re-Os isotope data yield 2 results.  The kimberlite and peridotite isochron (2472 Ma) is likely a mixing-line, between enriched and depleted mantle endmembers. Secondly, the emplacement age isochron (2024 Ma) has a radiogenic initial γOs value that is comparable to ancient mantle plume systems [1, 2].
Overlying location data on a deep-seismic image (www.rses.anu.edu.au/seismology/skippy97) reveals some interesting correlations. Kimberlites are only found in the west, above fast wave speed regions in the deep (up to 200 km) mantle.  In the east, where melnoites and carbonatites occur, wave speeds are slower at similar mantle depths.  Alkaline syenites (2300-2500 Ma) are also only found in the east.

We therefore propose a comprehensive geodynamic model. Alkaline syenite emplacement resulted from asthenospheric mantle activity that either refertilised, or de-laminated Archaean lithospheric mantle. All Nd and many, including the metasomatised nodule, Os model ages overlap this event.   The average 2024 Ma γOs value of the Re-Os mixing line (2472 Ma isochron) samples is similar to, but higher than the γOs value of the emplacement isochron.  These data may reflect the mixing of metasomatised mantle and a mantle plume at 2024 Ma.  Alternatively, it may also suggest that no mixing occurred and the distinctive Nd and Os plume signature is linked only to the melting of metasomatised upper mantle.
 

References:

[1] Shirey, S. B. (1997) Can.JES, 34, 489-503.
[2] Walker, R. J. et al., (1997) GCA, 61, 3145-3160.