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| Titel |
Kosmin granodiorite and its variable enclaves |
| VerfasserIn |
Joanna Gruszczyńska, Anna Pietranik |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250040079
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| Zusammenfassung |
| Kośmin granodiorite represents Variscan magmatic activity in Poland, Sudetes. It is ca. 335
Ma and can be correlated with a similar magmatic episode in Czech Republic, Germany
and Austria [1]. The geochemical data such as high contents of MgO and K2O,
enrichment in LILE, high ratios of LREE/HREE, enrichment in Cr and Ni and
low Na2O contents are similar to other K-rich intrusions in the Bohemian Massif
[1].
Kośmin granodiorite contains variable dioritic, sjenitic and monzodioritic enclaves which
potentially record processes of magma generation and differentiation typical for potassic
magmatism.
Three analyzed samples of the host granodiorite have similar composition (58 wt.
% SiO2, 5Â wt. % MgO, 4 wt. % K2O, 43 ppm Ni, 470 ppm Sr, 9 La/LuN). The
mineral composition is dominated by K-feldspar and plagioclase, with subordinate
hornblende, biotite, quartz and zircon and apatite as accessory minerals. An in
plagioclase varies from 33 to 42% and average SrO content in plagioclase is ca. 650-850
ppm.
The enclaves in the granodiorite can be divided into 3 groups according to their
mineralogy and chemical composition:
(1) Monzodiorites are the dominating type and are characterized by a range of chemical
compositions (50-55 wt. % SiO2, 6-10 wt. % MgO, 4-5 wt. % K2O, 30-200 ppm Ni, 300-450
ppm Sr, 5-11 La/LuN). They consist of plagioclase, pyroxene, amphibole, biotite, K-feldspar,
quartz and often contain variable amounts of plagioclase phenocrysts. The phenocrysts have
variable An contents from 32 to 70%, but the highest An is observed in thin (ca. 100 μm)
patchy zones close to the plagioclase rims. The SrO contents in plagioclase are ca. 900 ppm
and are sometimes higher in the patchy zones up to 1100 ppm. The structure and
chemical composition of phenocrysts is consistent with their crystallization in the
host granodioritic magma and later transport and resorption in the monzodioritic
magma.
(2) Syenites comprise ca. 10% of enclaves and are characterized by very high K2O
contents up to 9 wt. %, high Ba up to 3500 ppm and positive Eu anomaly ca. 1,6. They are
dominated by K-feldspar and biotite with minor amounts of hornblende and plagioclase. An
content in plagioclase is low 31 to 41% and SrO content is 670 ppm, lower than that in the
monzodioritic plagioclase.
(3) Cumulates represented so far by two enclaves are characterized by high MgO contents
(11 wt. %) and low SiO2 contents (48 wt. %). The major minerals in the cumulates are
pyroxene, hornblende, biotite and feldspar. Plagioclase contains 30-43% An and SrO contents
are ca. 650 ppm.
Chemical composition of whole-rock samples, composition of individual minerals and
microstructures observed in plagioclase are consistent with the enclaves being represented by
either co-mingled magmas (monzodiorites) or entrained material (syenites and cumulates,
possibly from the source region).
The work was financed by grant no. 4185/PB/ING-08 by the Polish Ministry of Science
and Higher Education
[1] Wenzel et al., Internationa Journal of Earth Sciences (Geol. Rundsch.) 1997, Vol. 86,
No 3, pp. 556-570 |
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