Acta Musei Moraviae

Peraluminous granites in the south part Stražek Moldanubicum – example of the evolution peraluminous granitic melt

Peraluminous muscovite-biotite granites sometimes with tourmaline form small bodies arranged along the Ořechov fault in the south part the Strážek Moldanubicum. According whole rocks geochemistry and petrology these granites can be interpreted as the product of partial melting of the metapelite protolith. Geochemical fractionation of the relatively primitive granite melts leads to enrichment in Rb together with X_Fe and depletion of the Ca, Ba, Sr. Simultaneously melt fractionation is manifested as a change in chemical composition of the rocks-forming minerals as tourmaline, K-feldspar, plagioclase and biotite. Tourmaline occurs in four distinct types based on the textural position in the rocks. Euhedral to subhedral, oscillatory zoned tourmaline (type I) grows directly from granitic melt. Euhedral tourmaline (type II) grows from residual pegmatite melt. Subhedral to anhedral tourmaline (type III) crystallizes from the residual boron reach melt. Hydrothermal tourmaline (type IV) fills small ruptures in the granites. There were distinguished three main substitution vectors, which controls tourmaline chemical composition during granite evolution: ^XNa₁^YFe²⁺₁^WF₁^X◻_-1^YAl_-1^W(OH)_-1, ^X◻₁^W(OH)₁^XNa_-1^WO_-1, ^YMg ^YFe_-1. The compositional trends of the tourmaline coupled with its textural features provide evidence of multiple oversaturation boron during crystallization of peraluminous granitic melt.