Origin, bending and anatexis of a Variscan magmatic arc: An odyssey of the Nasavrky Plutonic Complex (Bohemian Massif)

In order to elucidate the nature and timing of dominant magmatic and tectonic processes behind the formation and subsequent disruption of Variscan magmatic arcs, we investigated the Nasavrky Plutonic Complex in the central Bohemian Massif. Using petrology, U–Pb zircon and titanite dating, zircon Hf isotopic compositions, with whole-rock geochemical and Sr–Nd isotopic data, we have identified four magmatic phases. The first two, producing mainly amphibole-bearing tonalites (Nasavrky suite, NaS; 347 ± 2 Ma) and gabbros–quartz diorites (Mafic suite, MfS: c. 343–341 Ma), were related to a continental arc. Post-collisional granodiorites (Skuteč suite, SkS; 337 ± 2 Ma) intruded during the third phase. These three suites originated by vigorous interactions between hydrous mafic and felsic melts derived from strongly depleted, subduction-modified mantle and immature metasediments, probably metagreywackes of the Cadomian arc. Lastly, subvolcanic biotite granites (Křižanovice suite, KrS; c. 324 Ma) of the fourth phase formed by partial melting of the oldest arc-related plutonic rocks. The arc-related magmas along the contact between the Moldanubian and Teplá–Barrandian domains (Peri-Moldanubian Arc; c. 355–340 Ma) were emplaced during the subduction- and collision-related horizontal shortening followed by down-dip shearing, the latter linked to the exhumation of the Moldanubian middle to lower crust. The following kinematic switch, associated with dextral strike-slip movements, led to fragmentation, displacement and rotation of the arc belt along the strike-slip Elbe Zone. The arc belt was finally intruded by post-collisional suites, forming (ultra-) potassic plutons (c. 337–335) and essentially undeformed, anatectic (S- and I-type granitic) plutons (c. 328–323 Ma). Taken together, the principal disruption of the primary Variscan orogenic zoning, fragmentation and rotation of individual crustal blocks into their current configuration reflect an overall transition from subduction–collision-related frontal convergence to dextral transcurrent tectonics at c. 340–325 Ma.