The rise of the Brunovistulicum: age, geological, petrological and geochemical character of the Neoproterozoic magmatic rocks of the Central Basic Belt of the Brno Massif
Abstrakt
The Brno Massif is the largest exposed part of the Brunovistulicum (eastern Bohemian Massif) representing Precambrian basement incorporated into the Central European Variscan Belt. Two well-known Cadomian granodiorite complexes of magmatic-arc origin are separated by N-S-trending belt of mafic rocks previously compared to ophiolite.
This so-called Central Basic Belt is formed by a slightly metamorphosed volcanic part (Metabasite Zone) in the east and dominantly plutonic Diorite Zone in the west. Our new geological, geochemical and isotopic data including U-Pb zircon dating reveal two distinct Precambrian magmatic events within the Central Basic Belt preceding the Cadomian arc.
The geochemical signatures of the dominant late Tonian (c. 730Ma) tholeiitic basalts (epsilon Nd725=+7.8 to+6.7) in the Metabasite Zone suggest a direct derivation from a mantle source in an extensional setting. Also, the associated sporadic rhyolitic lavas and tuffs are primitive, showing a short mean crustal residence (epsilon Nd725=+6.0 and + 5.7; TDMNd(.2stg)0.9Ga).
By contrast, the Cryogenian (c. 650Ma) magmatism of the Diorite Zone clearly demonstrates features of a magmatic-arc origin. Rather primitive whole-rock geochemistry and radiogenic Nd isotopic signature (epsilon Nd655 values typically falling between + 7 and + 6) show that this arc was either intraoceanic, or developed on recently accreted, immature mafic crust.
Based on all the available data, three successive tectono-magmatic stages have been identified in the Brno Massif in the Neoproterozoic times (c. 730-600Ma), as products of a single long-lived, multi-stage subduction system spanning nearly full Neoproterozoic supercontinent cycle from the break-up of Rodinia to the assembly of Pannotia.