The East European Craton (Baltica) at 1.6-1.4 Ga: Continuing supercontinent agglomeration or break-up?
|Location||International Geological Congress,oslo 2008|
|Holding Date||08 October 2008|
The 1.6-1.0 Ga Mesoproterozoic evolution of the East European Craton (EEC) was strikingly different in its western and eastern parts (Bogdanova et. al., 2008, Prec. Res. 160). Intense rifting and the formation of deep sedimentary basins and aulacogens characterized the east, while accretionary and collisional geodynamics dominated the west. New precise isotopic ages confirm that before the 1.1-1.0 Ga Sveconorwegian orogeny the entire western EEC was affected by strong lithospheric deformation caused by the 1.50-1.45 Ga Danopolonian orogeny (DPO).
Depending on the distance from its frontal zone of convergence in the south, the DPO’ expressions varied. In the south, above a NNE-dipping strong seismic reflector in the upper mantle (Krawczyk et al., 2002, Tectphys., 360), the upper crust was marked by southward thrusting. This was accompanied by syntectonic emplacement of high-K, ferroan, semi-A type granitoids, as well as by metamorphism and migmatization. Two major Danopolonian stages can be distinguished in the Blekinge-Bornholm Province, where granitoid magmatism was initially accommodated in NW shear zones and later succeeded by the formation of migmatites and leucocratic, partly peraluminous igneous suites related to southward thrusting. All the granitoids have eNd (1.45 Ga) between -0.1 and -1.9 (Johansson and Zarins, in press), indicating Palaeoproterozoic sources.
Ca. 1.50-1.42 Ga migmatization of Paleoproterozoic rocks during the "Hallandian" metamorphic event has also been identified within the Eastern Segment of the Sveconorwegian orogen (Möller et al., 2007, J. Metam. Geol., 25). To the north, more bimodal, extensional magmatism was active, producing a number of intracratonic mafic and granitoid, often rapakivi-type intrusions, and creating volcano-sedimentary basins.
Altogether, the DPO comprised subduction of oceanic crust and formation of an active continental margin along the southwestern edge of the EEC, followed by fast changes of directions of plate motions and collision with a continent or an island arc. Assumedly, these can have been Amazonian/South American terranes, which have Mesoproterozoic evolutions resembling that in the EEC; but their paleogeographic positions at 1.5-1.4 Ga have not yet been determined.
Palaeomagnetically, Laurentia, Siberia, North China (?), Amazonia (?) and the EEC (Baltica) at 1. 5-1.4 Ga preserved their Palaeoproterozoic interrelationships (Pesonen et al. 2003, Tectphys., 375). Similar palaeogeography prevailed in 1.0 Ga Rodinia until its final break-up at ca. 650-600 Ma (Li et al., 2008, Prec. Res., 160).
Some remaining general questions are: (1) Was there a single supercontinent like Rodinia or several continental assemblies in the Palaeoproterozoic? (2) Does rifting like that in the eastern EEC evidence a superplume event at 1.6-1.4 Ga and the break-up of a supercontinent or was it a consequence of plate motions? (3) Did Rodinia evolve under different deep-Earth conditions than its predecessor/s?