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Evolution of Neoarchean-Paleoproterozoic basement in the Brunovistulia terrane, S Poland : geological, P-T and geochemical records

Żelaźniewicz Andrzej Ryszard, Jastrzębski Mirosław

Geological Quarterly

2021

Vol. 65, No. 2

art. no. 20

Brunovistulia is a composite terrane of Gondwana descent that eventually was accreted to the SW margin of Baltica, central Europe. It is built of metagneous and metasedimentary rocks that originated mainly between 650 and 550 Ma. However in the Upper Silesian part of Brunovistulia, much older fragments have been drilled, which yielded U-Pb zircon ages between 2.75 and 2.0 Ga. They have been interpreted as an “exotic” constituent of the Brunovistulia superterrane, named the Rzeszotary Terrane. Our geological and geochemical studies of the Rzeszotary borehole cores yielded new data on the composition, provenance and evolution of that terrane. Precursors of the Rzeszotary complex were separated from the depleted mantle prior to or around 3.2-3.0 Ga. At 2.75-2.6 Ga, a juvenile magmatic arc edifice formed, beneath which oceanic lithosphere was subducted. Decompression melting of the mantle brought about tholeiite magmas of IAT/MORB composition with LILE additions. Tonalitic and trondhjemitic precursors of gneisses present today were formed at that time, probably due to partial melting of mantle-derived wet basalts at the base of the island arc. Around 2.0 Ga, the arc collided with an unspecified cratonic mass and was subject to orogenic deformation, metamorphism and migmatization. The entire arc edifice was then strongly shortened and forced down to depths equivalent to ~6-12 kbar where the rocks underwent contractional deformation and metamorphism (~500-700°C). Tonalites and trondhjemites were changed to gneisses, and basites to epidote- and garnet amphibolites. These rocks underwent syntectonic migmatization through the mechanism of segregation/differentiation in the presence of fluids and incipient partial melting. Synmetamorphic shortening of the rock pile, which led to folding and heterogeneous development of shear zones with thrust kinematics, terminated with intrusions of K-granites at 2.0 Ga, being followed by some brittle-ductile deformation of unconstrained timing. The 2.0 Ga event may have been connected with the 2.1-1.8 Ga global amalgamation of the Paleoproterozoic supercontinent of Columbia. Later the future Rzeszotary terrane was detached from the Gondwana mainland, reassembled and eventually, in the Neoproterozoic, it became part of the foreland of the Cadomian Orogen in Central Europe.

Neoarchean to Paleoproterozoic fragments in the Brunovistulia terrane, S Poland : a component of the Columbia Supercontinent?

Żelaźniewicz Andrzej Ryszard, Fanning C. Mark

Geological Quarterly

2020

Vol. 64, No. 1

120--129

The composite terrane of Brunovistulia includes basement of the Upper Silesia Block, southern Poland. In its NE part, the basement is elevated by the Rzeszotary Horst. In the Rzeszotary 2 borehole (Rz2) drilled in the horst, partly migmatized amphibolites, felsic gneisses and granites occur. An Na-plagioclase-phengite-K-feldspar neosome contained zircons that yielded U-Pb SHRIMP ages ~2.75-2.6 Ga (cores and single grains) and ~2.0 Ga (rims and single grains). The older ages are interpreted as the time of origin of the igneous protolith of the migmatized amphibolites. The younger ages recorded metamorphism and migmatization that affected both the magmatic precursor of the amphibolites and accompanying felsic rocks during a contractional tectonic/orogenic event. Migmatization was greatly enhanced by an influx of alkali-bearing fluids which heralded intrusion of late-orogenic unfoliated K-granite in an extensional regime, terminating the 2.0 Ga event. It is proposed that the entire orogenic edifice, of which the Brunovistulian rocks drilled in Rz 2 are a small part, represents fragments of the Columbia Supercontinent that was assembled in the Paleoproterozoic and broken up in the Mesoproterozoic. In Ediacaran times, these fragments became eventually incorporated into the Cadomian orogen in the form of its foreland and contributed to the formation of the composite terrane of Brunovistulia. Such a scenario explains why the U-Pb zircon age spectra in the Rzeszotary terrain differ dramatically from those in the remainder of Brunovistulia, which is thought to be the Cadomian hinterland.