What does the lack of biotite in granite mean even though there are amphiboles in the modal composition? Should biotite be a petrogenetic mineral in granite magmas?
Daniel,
If you are getting amphiboles, then Fe-Mg and water was available in the magma. What about K-feldspar? Are you getting any? If you don't have K-feldspar or other K-bearing minerals (e.g. muscovite), then the magma could have been deficient in K2O and you are looking at a tonalite kind of rock instead of granite.
Granitic intrusions which contain both amphibole and biotite phenocrysts have crystallized from volatile-rich (H2O, Cl, F) magmas. Many exploration geologists would also call them "fertile" intrusions.
Interesting answers, Thanks. However, there are many K-feldspar in the modal composition and there is no phenocrysts of amphibole or biotite in this granite.
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