Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)

verfasst von: Fleurice Parat, François Holtz, Miloš René, Renat Almeev
Abstract: The equilibrium phase relations of a mafic durbachite (53 wt.% SiO ₂) from the Třebíč pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338-335 Ma), have been determined as a function of temperature (900-1,100°C), pressure (100-200 MPa), and H ₂O activity (1.1-6.1 wt.% H ₂O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni-NiO (NNO) buffer and 2.6 log unit above NNO buffer (ΔNNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3-4 wt.% H ₂O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite-clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.
Organisationseinheit(en): Institut für Mineralogie
Externe Organisation(en): Albert-Ludwigs-Universität Freiburg
Akademie Věd České Republiky (AV ČR)
Typ: Artikel
Journal: Contributions to Mineralogy and Petrology
Band: 159
Seiten: 331-347
Anzahl der Seiten: 17
ISSN: 0010-7999
Publikationsdatum: 04.08.2009
Publikationsstatus: Veröffentlicht
Peer-reviewed: Ja
ASJC Scopus Sachgebiete: Geophysik, Geochemie und Petrologie
Elektronische Version(en): https://doi.org/10.1007/s00410-009-0430-5 (Zugang: Unbekannt)

BibTeX

@article{b1e9e298e4cd40f981ac13d4a98f9585,
title = "Experimental constraints on ultrapotassic magmatism from the Bohemian Massif (durbachite series, Czech Republic)",
abstract = "The equilibrium phase relations of a mafic durbachite (53 wt.% SiO 2) from the T{\v r}eb{\'i}{\v c} pluton, representative of the Variscan ultrapotassic magmatism of the Bohemian Massif (338-335 Ma), have been determined as a function of temperature (900-1,100°C), pressure (100-200 MPa), and H 2O activity (1.1-6.1 wt.% H 2O in the melt). Two oxygen fugacity ranges were investigated: close to the Ni-NiO (NNO) buffer and 2.6 log unit above NNO buffer (ΔNNO + 2.6). At 1,100°C, olivine is the liquidus phase and co-crystallized with phlogopite and augite at 1,000°C for the whole range of investigated pressure and water content in the melt. At 900°C, the mineral assemblage consists of augite and phlogopite, whereas olivine is not stable. The stability field of both alkali feldspar and plagioclase is restricted to low pressure (100 MPa) at nearly water-saturated conditions (<3-4 wt.% H 2O) and T < 900°C. A comparison between experimental products and natural minerals indicates that mafic durbachites have a near-liquidus assemblage of olivine, augite, Ti-rich phlogopite, apatite and zircon, followed by alkali feldspar and plagioclase, similar to the mineral assemblage of minette magma. Natural amphibole, diopside and orthopyroxene were not reproduced experimentally and probably result from sub-solidus reactions, whereas biotite re-equilibrated at low temperature. The crystallization sequence olivine followed by phlogopite and augite reproduces the sequence inferred in many mica-lamprophyre rocks. The similar fractionation trends observed for durbachites and minettes indicate that mafic durbachites are probably the plutonic equivalents of minettes and that K- and Mg-rich magmas in the Bohemian Massif may have been generated from partial melting of a phlogopite-clinopyroxene-bearing metasomatized peridotite. Experimental melt compositions also suggest that felsic durbachites can be generated by simple fractionation of a more mafic parent and mixing with mantle-derived components at mid crustal pressures.",
keywords = "Bohemian Massif, Durbachite, Experimental study, T{\v r}eb{\'i}{\v c}, Ultrapotassic magma",
author = "Fleurice Parat and Fran{\c c}ois Holtz and Milo{\v s} Ren{\'e} and Renat Almeev",
note = "Funding Information: Acknowledgments We thank F. Holub for guiding us in the fields and for the starting sample and M. Burchard for the experiment with piston cylinder. We also thank D. Prelevic and an anonymous reviewer for their constructive comments and J. Hoefs for the editorial handling of this manuscript. This work was supported by the University of Freiburg (Wissenschaftlichen Gesellschaft), the German Science Foundation (DFG) project 436 TSE 113/48/0-1 and the institute research plan of the IRSM CAS CR (A V0Z30460519). Copyright: Copyright 2012 Elsevier B.V., All rights reserved.",
year = "2009",
month = aug,
day = "4",
doi = "10.1007/s00410-009-0430-5",
language = "English",
volume = "159",
pages = "331--347",
journal = "Contributions to Mineralogy and Petrology",
issn = "0010-7999",
publisher = "Springer Verlag",
number = "3",
}