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Dmitry Zozulya, Geological Institute Kola Science Centre RAS (Russian Federation)
Tamara Bayanova, Geological Institute Kola Science Centre RAS (Russian Federation)
Nelson Eby, University of Massachusetts Lowell (United States)
Kare Kullerud, University of Tromso (Norway)
Erling Ravna, University of Tromso (Norway)
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Archean alkaline complexes are extremely rare but are of particular interest because the magmas have a mantle origin. Given their high Sr and Nd contents, crustal contamination has only a minor impact on the mantle-derived isotopic signatures. Therefore, they can provide valuable information on the isotopic composition of the subcontinental mantle and geodynamics in Archean. The previously reported Late Archean alkaline complexes (2.7-2.6 Ga) from the Canadian and Australian shields belong to the potassic series, are depleted in LIL and HFS elements, and are related to subduction, thereby forming in a compressive tectonic environment and having a depleted mantle source. The absence of Archean sodic alkaline complexes forming in extensional environments was mainly ascribed to the absence of metasomatic processes in the mantle and lower lithosphere (Blichert-Toft et al., 1996).
Greenland Archean alkaline complexes (2698-2664 Ma syn- to post-kinematic pyroxenites, hornblendites, norites and diorites, monzodiorites, monzonites, syenites, nephelenitic rocks and carbonatites) are found in the Skjoldungen province in the SE Nain craton (Blichert-Toft et al., 1995). The Baltic examples are the 2610 Ma Siilinjarvi carbonatite-glimmerite intrusion, SW Karelian craton, and the Keivy alkaline province (2670-2654 Ma anorogenic peralkaline granites, syenogranites, syenites and massif-type anorthosites; 2610 Ma OIB-like nepheline syenites and essexites), Kola craton. The 2695 Ma Mikkelvik nepheline syenite stock was recently identified in the West Tromso Archean basement, Northern Norway.
The alkaline rocks mostly show high contents of some incompatible elements (REE, Zr, Y, Nb, and Rb, high Ga/Al (for granite) and low Y/Nb and Yb/Ta (for syenite) ratios) characteristic of an enriched mantle source. Near chondritic and negative eNd (0 to -5) and near chondritic and positive eSr (0 to +50) suggest the mantle source is transitional between the chondritic and enriched mantle (EM2) reservoirs. From the similar geochemical signatures it is suggested that the Archean alkaline magmatism from Greenland, the Baltic and Northern Norway resulted from plume development in a sub-lithospheric mantle having enriched characteristics due to subduction. The observed differences in geochemical features are in accordance with a sequence of magmatic events during the plume development: 2.70-2.66 Ga - initiation, near chondritic and slightly enriched reservoir due to subducted and recycled oceanic crust, mafic shoshonitic parental magma; 2.65-2.61 Ga - evolved enriched reservoir, OIB-like and Na-rich parental magma.
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