Glimmerite Veins in High-pressure Rocks from Lower Austria - Clues for the Origin of High-K Magmas in Orogenic Belts

Harry Becker Department of Terrestrial Magnetism, The Carnegie Institution of Washington,

5241 Broad Branch Road, N.W., Washington D.C., 20015

Thomas Wenzel Institut fuer Geowissenschaften, FB 22, LE Mineralogie,

Johannes-Gutenberg-Universitaet, D-55099 Mainz

In the southern Bohemian massif glimmerite veins occur in orogenic peridotites (Hedlik and Zemann, 1951) and, more rarely, in associated high-pressure granulites. The latter have been metamorphosed and exhumed during Carboniferous crustal thickening. The glimmerites are medium to coarse grained, and consist of more than 90 wt. % phlogopite, up to 9 wt. % apatite and trace amounts of rutile, graphite and zircon. Locally, the veins contain abundant magnesite. The high K2O/Na2O (13-40), K2O (4-9.5 wt. %), and MgO (15-27 wt. %) indicate the ultrapotassic nature of the glimmerites. The glimmerites have very high LILE (e. g., 270-880 ppm Rb), LREE, P, and F contents, as well as high Cr (up to 1200 ppm), and Ni (up to 660 ppm) abundances. REE patterns are strongly fractionated with Lan=70-1000 and Lun=1-20. In primitive mantle normalized concentration diagrams, U and Th are less enriched relative to other LILE and the LREE. Normalized abundances of Nb and Ta are relatively high but somewhat lower than those of U and Th. Except in one sample that contains abundant zircon, Zr and Hf are strongly depleted relative to the MREE. Major and trace element compositions of the glimmerites are difficult to explain by partial melting of the nearby felsic granulites and gneisses. The granulites should be much stronger depleted in Rb and Ba if they represent residues of partial melting. Complementary to the granulites, the glimmerites have high concentrations of Cs, U, and Th, and low Rb/Cs, and Th/U. In addition, initial Sr-Nd isotopic compositions of glimmerites and granulites are overlapping, and both rock types have negative Sr and Eu anomalies, indicating a genetic relationship. We propose, that the glimmerites crystallized from a SiO2-Al2O3-K2O rich fluid, deliberated during the high-temperature, high-pressure breakdown of F-rich biotite in the granulites (T>800-900°C ? at 1.6 to 1.8 GPa (Carswell and O'Brian, 1993). High abundances of P in the glimmerites indicate that significant amounts of apatite or monazite must have dissolved in the fluid. We suggest, that the high MgO, Cr, and Ni abundances in the glimmerites are not a primary feature, but the result of later interaction of the fluid with the peridotites. As indicated by the glimmerites, high-K fluids, containing sufficient amounts of F and P, are capable to provide the surplus of LILE and LREE that distinguishes high-K magmas from common calcalkaline magmas. In collision and subduction zones high-K fluids deliberated from subducted sediments may freeze or cause partial melting in a variety of lithologies. Frozen high-K fluids and their host rocks may undergo partial melting during lithospheric delamination and orogenic collapse, resulting in the formation of diverse high-K magmas such as shoshonites/monzonites, durbachites, leucitites, lamproites and high-K lamprophyres.


Carswell, D.A. & O'Brien, P.J., J. Petrol. 34, 427-459 (1993).

Hedlik, A. & Zemann, J., Tschermaks Mineral. Petrogr. Mitt. 2, 407-416 (1951).