Geochemistry of Glimmerite Veins in Peridotites from Lower Austria-Implications for the Origin of K-rich Magmas in Collision Zones

Abstract

In the southern Bohemian massif (central Europe), veins of medium- to coarse-grained glimmerites occur in peridotite massifs and at the contact with large bodies of felsic granulites. Crystallization of the glimmerites and metamorphism of the granulites were coeval at 335-340 Ma. The glimmerites contain phlogopite and minor amounts of apatit and graphite, and are ultrapotassic, with high concentrations of large ion lithophile elements, light rare earth elements (LREE), P, F, Cr and Ni, the last indicating reaction o the metasomatic agent with the peridotites. Furthermore, the glimmerites show strongly fractionated REE patterns, negative anomalies of Nb, Ta and Ti, and, with one exception, very low Zr and Hf abundances. The absence of feldspars and low Na2O and CaO abundances in the glimmerites rule out that the glimmerites precipitated from a melt. The felsic granulites have low Th, U and Cs abundances, and high Rb/Cs, complementary to the glimmerites. Both rock types overlap in initial Sr-Nd isotopic compositions and show negative Eu anomalies. The chemical evidence and the high F abundances in prograde biotite inclusions in granulite garnets indicate that the glimmerites may have crystallize from a fluid, liberated during the high-temperature, high-pressure breakdown of F-rich phlogopite in the felsic granulites. The fluid must have contained HF, H2O, and carbon species as major volatile components. The minor occurrence of mica-poor dolomite veins with glimmerites suggests that liquid immiscibility may have played a role in the formation of these rocks. Glimmerites probably represent potential precursors to potassic magmas in collision zones.