Ultrapotassic dykes in the Moldanubian Zone and their significance for understanding of the post-collisional mantle dynamics during Variscan orogeny in the Bohemian Massif

Highlights

• Dyke swarm of ultrapotassic rocks occurs along border of Moldanubian Zone.

• Composition of dykes changes from mafic vaugnerite to syenite.

• The original magma was formed by minimum partial mantle melting.

• Syenite shows signature of fractionation and crustal assimilation.

Abstract

We report mineral textures, geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the Western part of the Moldanubian Zone at contact to the Teplá-Barrandian Block. The dykes have orientation almost perpendicular to the NNE–SSW trending Central Bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites. Based on mineral compositions and geochemistry, the dykes show close relations to Mg-K syenite plutons in the Moldanubian Zone. The two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine-grained matrix that consists of K-feldspar, phlogopite with small amounts of clinopyroxene and accessory quartz, apatite, titanite and sulphides of Fe, Cu, Ni. The syenite porphyry dyke cross cuts the vaugnerite. It contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs.

The vaugnerite and syenite porphyry have high K₂O (6 to 7 wt.%) and mg-number (0.66–0.74), but low CaO and Na₂O contents. The vaugnerite is markedly rich in P₂O₅ (> 2 wt.%) and comparing to syenite porphyry has higher amount of mantle-incompatible elements (e.g. Rb, Cs, Ba, Pb, Th, U), V and Cr. ID-TIMS analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338.59 ± 0.68 and 337.87 ± 0.21 Ma, respectively. Mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent. Formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post-collisional mantle dynamics initiated by slab break-off and asthenosphere upwelling into the Moldanubian accretionary complex during the Variscan Orogeny.

Introduction

The Moldanubian Zone of the Bohemian Massif (Fig. 1) is known for the presence of ultrapotassic igneous (Foley et al., 1987) rocks that form plutons of Mg-K syenite (durbachite) and dykes of vaugnerite, minette, syenite porphyry, kersantite, lamproites (Holub, 1997, Krmíček et al., 2011). This K-rich magmatism is considered as an indicator of evolving mantle dynamics that occurred along the European Variscan orogenic belt (Fig. 1a, Abdelfadil et al., 2014, Couzinié et al., 2014, Finger et al., 2007, Gerdes et al., 2003, Janoušek and Holub, 2007, Krmíček et al., 2016, Štemprok et al., 2014, von Raumer et al., 2013, Zeitlhofer et al., 2016). In the Moldanubian Zone of the Bohemian Massif, the ultrapotassic rocks occur mostly along two NNE–SSW-orientated belts (Fig. 1b) and the origin of their formation is the subject of controversy. According to Janoušek and Holub (2007), the ultrapotassic magma derived from anomalous mantle sources contaminated by crustal material and their formation was related to subduction of the Saxothuringian plate beneath the Teplá-Barrandian Block (the Teplá suture in Fig. 1a) and it was coeval with the granulite facies metamorphism in the Moldanubian Zone (Schulmann et al., 2014). Relations of lamproites dykes and ultrapotassic plutons in the Moldanubian Zone to the subduction of Rhenohercynian oceanic basin (Rhenohercynian suture in Fig. 1a) are assumed by Kroner and Romer (2013) and Krmíček et al. (2016). According to von Raumer et al. (2014), the ultrapotassic rocks along the whole Variscan orogeny (Eastern Alps, Alpine External Massifs with those of Corsica, the French Central Massif, Black Forest, Vosges and the Bohemian Massif, Fig. 1a) were created by slab windows and/or the sinking of the subducted Rhenohecynian slab into the mantle. Available geochronological dating shows wide range of ages of this magmatism that includes ages of 293–343 Ma from the Bohemina Massif (Holub et al., 1997, Kusiak et al., 2010, Verner et al., 2008, von Seckendorf et al., 2004), 305 Ma from vaugnerite in the French Central Massif (Couzinié et al., 2014) and 265 Ma from lamprophyre in the Central Iberian Zone (Scarrow et al., 2011). Based on recent results of petrological study (Faryad et al., 2013, Faryad et al., 2015), the main processes of metamorphism and formation of igneous rocks in the Moldanubian Zone were operated by subduction and subsequent collision of the Moldanubian plate beneath the Teplá-Barrandian Block (the Moldanubian suture in Fig. 1a).

In this work, we present detailed petrology, geochemistry and age data of two dykes of vaugnerite and syenite porphyry that occur within migmatized gneisses of the Monotonous unit in the Moldanubian Zone. Field relations show that the vaugnerite dyke is relatively older than the syenite porphyry but both cross cut the older granite apophyses which follow metamorphic fabrics in the gneisses and are parallel to the SSW–NNE directed Central Bohemian Plutonic Complex (Fig. 1b). By combination of mineral textures and compositional variations in minerals and bulk rock chemistry we attempt to clarify the origin and source of the ultrapotassic magma and analyse subsequent processes of its fractionation and assimilation with crustal material during magma ascendance. The results of this work are used to discuss the relations of ultrapotassic magmatism to mantle dynamics during post-subduction history and granulite facies metamorphism in the Moldanubian Zone.