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Petrology of garnet-, spinel-, and sillimanite-bearing granites from the bavarian forest, West Germany

Petrologie von Granat-, Spinell- und Sillimanif-führenden Graniten aus dem Bayerischen Wald, Bundesrepublik Deutschland

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Summary

Samples from 17 small granitic intrusions of late-Variscan age have been investigated for their chemical compositions and petrographic characteristics. All of them contain biotite, 14 cordierite, 6 andalusite, 2 gamet, green spinel, and rare sillimanite. The granites are peraluminous (normative corundum between 1.0 and 3.5 per cent) and highly evolved (DI between 87 and 97). Biotite and garnet in the garnet bearing granites are highly ferrous, MnO in garnet is moderate (3 to 5 per cent), the spinel is essentially hercynite with 2 to 16 per cent ZnO. From textural relationships the sequence of crystallization of the aluminous minerals is deduced as: cordierite 1+biotite 1, reaction of biotite 1 and some cordierite 1 to garnet (+ sillimanite?), rimming of garnet by cordierite 2 and biotite 2, growth of spinel in plagioclase, in cordierite 1, and around sillimanite. The pressure during the magmatic stage was between 2 and 3 kb. The spinel is not a magmatic phase.

Zusammenfassung

Proben von 17 kleinen, spätvariskischen Granitintrusionen wurden chemisch und petrographisch untersucht. Alle 17 enthalten Biotit, 14 Cordierit, 6 Andalusit, 2 Granat, grünen Spinell und Sillimanit. Die Granite sind peralumisch (normativer Korund zwischen 1,0 und 3,5%) und haben hohe Differentiationsindizes (DI zwischen 87 und 97). Der Biotit und der Granat der granatführenden Granite sind reich an Eisen, der Mangangehalt ist mäßig hoch (MnO zwischen 3 und 5%), der Spinell ist im wesentlichen Hercynit mit 2 bis 16% ZnO. Aus den Gefügebeziehungen läßt sich folgende Kristallisationsfolge der Al-haltigen Minerale (ohne Feldspäte) ableiten: Cordierit 1+Biotit 1, Reaktion von Biotit 1 und etwas Cordierit 1 zu Granat (+ Sillimanit?), Säume von Cordierit 2 und Biotit 2 um Granat, Sprossung von grünem Spinell in Plagioklaskernen, Cordierit 1 und um Sillimanit. Während des magmatischen Stadiums lag der Druck zwischen 2 und 3 kb. Der Spinell ist keine magmatische Phase.

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Propach, G., Gillessen, B. Petrology of garnet-, spinel-, and sillimanite-bearing granites from the bavarian forest, West Germany. TMPM Tschermaks Petr. Mitt. 33, 67–75 (1984). https://doi.org/10.1007/BF01082302

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Keywords

  • Corundum
  • Cordierit
  • Sillimanite
  • Andalusite
  • Granitic Intrusion