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Micro structural and mineralogical evidence for limited involvement of magma mixing in the petrogenesis of a Hercynian high-K calc-alkaline intrusion: the Kozárovice granodiorite, Central Bohemian Pluton, Czech Republic

Published online by Cambridge University Press:  03 November 2011

Vojtěch Janoušek ,
D. R. Bowes ,
Colin J. R. Braithwaite  and
Graeme Rogers
Vojtěch Janoušek
Affiliation:
Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic.e-mail:janousek@cgu.cz
D. R. Bowes
Affiliation:
Division of Earth Sciences, University of Glasgow, Glasgow G12 8QQ, Scotland, UK.
Colin J. R. Braithwaite
Affiliation:
Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland, UK.
Graeme Rogers
Affiliation:
Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland, UK.
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Abstract

Textural and mineralogical features in the high-K calc-alkaline Kozárovice granodiorite (Hercynian Central Bohemian Pluton, Bohemian Massif) and associated small quartz monzonite masses imply that mixing between acid (granodioritic) and basic (monzonitic/monzogabbroic) magmas was locally petrogenetically significant.

Net veining, with acicular apatite and numerous lath-shaped plagioclase crystals present in the quartz monzonite, and abundant mafic microgranular enclaves (MME) in the granodiorite, indicate that as the monzonitic magma was injected into the granodioritic magma chamber, it rapidly cooled and was partly disintegrated by the melt already present. Evidence from cathodoluminescence suggests that the two magmas exchanged early-formed plagioclase crystals. In the quartz monzonite, granodiorite-derived crystals were overgrown by narrow calcic zones, followed by broad, normally zoned sodic rims. In the granodiorite, plagioclase crystals with calcic cores overgrown by normally zoned sodic rims are interpreted as xenocrysts from the monzonite. After thermal adjustment, crystallisation of the monzonitic magma ceased relatively slowly, forming quartz and K-feldspar oikocrysts.

Although the whole-rock geochemistry of the quartz monzonite and the MME support magma mixing, major- and trace-element based modelling of the host granodiorite has previously indicated an origin dominated by assimilation and fractional crystallisation. Magma mixing therefore seems to represent a local modifying influence rather than the primary petrogenetic process.

Keywords

Bohemian Massifcathodoluminescence microscopyhybridisationmafic microgranular enclavesmineral chemistryplagioclaseresorptionwhole-rock geochemistry
Type
Research Article
Information
Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 91 , Issue 1-2: Fourth Hutton Symposium: The Origin of Granites and Related Rocks , 2000 , pp. 15 - 26
Copyright
Copyright © Royal Society of Edinburgh 2000

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