Radioactive elements in collisional and within-plate Sodic-Potassic Granitoids: Accumulation levels and metallogenic significance
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Abstract The Belomorian Belt in the White Sea region was until recently considered the oldest core of the Baltic Shield but is now recognized to be essentially Late Archaean. This study reports U-Pb zircon ages and whole-rock Nd-isotopic results for aluminous metasedimentary gneisses and tonalitic-trondhjemitic-granodioritic intrusions. Metamorphic zircons from the aluminous gneisses date the earliest high-grade metamorphism to about 2.86 Ga. This was a low- to moderate-pressure granulite-facies event. Multigrain fractions of detrital zircons from these gneisses preserve little memory of protolith ages, but the whole-rock Sm-Nd DM-model ages are below 3.0 Ga which excludes dominantly Early to Middle Archaean detritus sources and indicates a short crustal prehistory. For the tonalitic-trondhjemitic intrusive rocks, U-Pb zircon and Sm-Nd model ages define two generations formed c. 2.8 and 2.74–2.72 Ga ago. The oldest events in the Belomorian Belt can thus be correlated with Late Archaean developments in the adjoining Karelian Province further southwest in the Baltic Shield, but occurred at an active continental margin. This supports a Late Archaean accretional/collisional relationship between these two crustal segments.
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There is an opinion that Paleoproterozoic magmatism in the Karelian craton of the Baltic Shield is linked solely with the influence of mantle plumes. On the basis of petrogeochemical data on magmatic formations from local geological sections in the southern part of the Karelian Craton, the author infers a similar mantle source for magmatic rocks of all Paleoproterozoic age levels, from Sumian to Ludikovian. The melting that occurred in the lithospheric mantle rules out the plume source for the considered Paleoproterozoic magmatic formations.
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The continental mass of Australia (Fig. 8.1) is made up of four great structural units, to which we may add a fifth represented in the fringing island arcs to the north and east. The oldest and most extensive unit is the Precambrian craton which comprises most of the central and western parts of the continent where Precambrian rocks are exposed beneath an intermittent Phanerozoic cover.
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Riphean
Radiometric dating
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