Structure, emplacement, and tectonic setting of Late Devonian granitoid plutons in the Tepla-Barrandian unit, Bohemian Massif

The tnovice and ista granodiorite-tonalite plutons are small (similar to 27 and similar to 38 km(2), respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Tepla-Barrandian unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated above a subducted slab in a continental margin arc setting. The U-Pb zircon age of the tnovice pluton, newly determined at 375 +/- A 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb-Pb age of 373 +/- A 1 Ma for the ista pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons (CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the tnovice and ista plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic activity from the similar to NW to the similar to SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath the Tepla-Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif.