The causal link between HP-HT metamorphism and ultrapotassic magmatism in collisional orogens: case study from the Moldanubian Zone of the Bohemian Massif

Abstract

In the Moldanubian Zone of the Bohemian Massif occur Variscan granulites and ultrapotassic magmatites (amphibole-bearing durbachitic suite and two-pyroxene syenitoids) in a close spatial and temporal relationship. The protolith to the most widespread felsic garnet-kyanite-mesoperthite granulites, which equilibrated at P >1.5 GPa and Tc. 1000°C, was analogous to meta-igneous rocks occurring in the Saxothuringian Zone of the NW Bohemian Massif. For the most basic ultrapotassic rocks, the high contents of Cr and Ni as well as high mg# point to derivation from an olivine-rich source (i.e. a mantle peridotite). On the other hand, elevated concentrations of U, Th, light rare earth elements (LREE) and large ion lithophile elements (LILE), pronounced depletion in Ti, Nb and Ta as well as high K 2 O/Na 2 O and Rb/Sr ratios apparently contradict the mantle origin. This dual geochemical character and crustal-like isotopic compositions require melting of anomalous lithospheric mantle sources, metasomatized and contaminated by mature crustal material. Both Moldanubian granulites and ultrapotassic rocks show mutually complementary depletions and enrichments in some trace elements (Cs, Rb, Th, U and Pb). Late Devonian-Early Carboniferous Andean-type subduction is thought to have resulted in continental collision and HP metamorphism of upper crustal, largely meta-igneous lithologies. The subduction of the mature crustal material caused direct contamination and metasomatism in the overlying lithospheric mantle wedge. Shortly after the granulite-facies metamorphic peak (at c . 340 Ma) and the slab break off, these metasomatized and contaminated mantle domains were melted by advected heat from the invading asthenosphere, generating ultrapotassic intrusions, closely related to the granulite occurrences in space and time. This tectonic, petrological and geochemical model is outlined.