Geochemistry of glimmerite veins in peridotites from lower Austria - Implications for the origin of K-rich magmas in collision zones

In the southern Bohemian massif (central Europe), veins of medium to coarse -grained glimmerites occur in peridotite massifs and at the contact with la ,ge bodies of felsic granulites. Crystallization of the glimmerites and met amorphism of the granulites were coeval at 335-340 Ma. The glimmerites cont ain phlogopite and minor amounts of apatite and graphite, and are ultrapota ssic, with high concentrations of large ion lithophile elements, light rare earth elements (LREE), P, F, Cr and Ni, the last indicating reaction of th e metasomatic agent with the peridotites. Furthermore, the glimmerites slow strongly fractionated REE patterns, negative anomalies of Nb, la and Ti, a nd, with one exception, very law Zr and HS abundances. The absence of felds pars and low Na2O and CaO abundances in the glimmerites rule out that the g limmerites precipitated from a melt. The felsic granulites have low nz, U a nd Cs abundances, and high Rb/Cs, complementary to the glimmerites. Both ro ck types overlap in initial Sr-Nd isotopic: compositions and show negative Eu anomalies. The chemical evidence and the high F abundances in prograde b iotite inclusions in granulite garnets indicate that the glimmerites may ha ve crystallized from a fluid: liberated during the high-temperature, high-p ressure breakdown of F-rich phlogopite in the felsic granulites. The fluid must have contained HF, H2O, and carbon species as major volatile component s. The minor occurrence of mica-poor dolomite veins with glimmerites sugges ts that liquid immiscibility may have played a role in the formation of the se rocks. Glimmerites probably represent potential precursors to potassic m agmas in collision zones.