Authors / CoAuthors
Smithies, R.H. | Van Kranendonk, M.J. | Champion, D.C.
Abstract
The lower part of the Pilbara Supergroup records 300 million years of voluminous basaltic magmatism from c. 3.515 to 3.24 Ga. The basalts are divided into two compositionally distinct (high-Ti and low-Ti) but contemporaneous and interbeded types. Compared to the low-Ti basalts, the high-Ti basalts (TiO2>0.8 wt%) have relatively high concentrations of HFSE and REE, are generally more Fe-rich, have very low Al2O3/TiO2 (18.7 - 8.9) and high Gd/Yb ratios (1.12-2.23) - they have affinities with Al-depleted (or Barberton-type) komatiites and komatiitic basalts that formed during high pressure melting in particularly hot mantle plumes. The composition of these basalts, and their source, did not change significantly throughout the 300 m.y. period of basalt eruption. In contrast, low-Ti basalts show distinct secular trends to lower concentrations of incompatible trace elements and lower ratios of La/Sm, La/Gd, La/Yb and Gd/Yb that reflect a source progressively more depleted than NMORB source. Gd/Yb ratios in the younger basalts are as low as 0.67, well below estimates for modern depleted mantle (0.98), and reflect a strongly depleted source. The source for the low-Ti basalts formed from the depleted residue of the plumes that produced the earliest high-Ti basalts. It remained isolated from the convecting asthenosphere throughout the 300 m.y. period of basaltic magmatism, but remelted each time one of several younger plumes (the sources for contemporaneous high-Ti basalts) impinged on the lithospheric mantle. The result was a thick pile of interbedded high- and low-Ti basalt. A complimentary, thick, depleted, and buoyant sub-continental lithospheric mantle (SCLM), developed, in situ from the Low-Ti basalt source, and also by accumulation of the plume source for the high-Ti basalts. These results support models for Palaeoarchaean protocrust formation through extensive mantle plume magmatic events, and models that suggest that the Archaean SCLM formed at the same time as early voluminous mafic magmatism. Basalts in the lower Pilbara Supergroup are typically not highly contaminated by felsic crust. Neither the degree of contamination nor the proportion of contaminated rocks appears to have increased significantly with decreasing age, despite clear evidence that the volume of co-existing felsic material increased significantly over that period. Likewise, there is no evidence for subduction modification of mantle sources until the youngest basalts of the uppermost Pilbara Supergroup erupted at c. 3.0 Ga.
Product Type
nonGeographicDataset
eCat Id
63497
Contact for the resource
Custodian
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Cnr Jerrabomberra Ave and Hindmarsh Dr GPO Box 378
Canberra
ACT
2601
Australia
Keywords
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- External Publication
- ( Theme )
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- geochemistry
- ( Theme )
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- stratigraphy
- ( Theme )
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- plate tectonics
- ( Theme )
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- geology
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- AU-WA
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
Publication Date
2005-04-19T00:00:00
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unknown
Topic Category
geoscientificInformation
Series Information
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Unknown
Parent Information
Extents
[-24.0, -20.0, 119.0, 122.0]
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Spatial Resolution
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