Source compositions and melting temperatures of the main granitic suites from the Moldanubian Batholith

Abstract

The granitoids of the Moldanubian Batholith (Moldanubian Zone of the Bohemian Massif) are represented by three main suites: I- to I/S-type Weinsberg biotite granites–granodiorites, S-type Eisgarn two-mica granites and I/S-type Freistadt biotite granites–granodiorites. As shown by major-element data and zircon with monazite saturation thermometry, partial melting of metapelites likely produced most of the Eisgarn suite (Mrákotín/Číměř varieties) at c. 775–825 ºC. Parental melts of the Weinsberg and Freistadt suites could have been generated by partial melting of a metagreywackes–metabasalts mixture at c. 740–940ºC. This study confirms that the CaO/Na2O ratio could be used for source assignment of S-type granite melts, whereas the CaO/(FeO + MgO + TiO2) ratio is more suitable for I/S-type granites. The observed systematic positive shift of Al2O3/TiO2 temperatures compared with zircon and monazite saturation temperatures in all granitoids of the Moldanubian Batholith (up to 118 ºC) may be explained by accumulation of ilmenite in the S-type Eisgarn suite. In the I- and I/S-granite types of the Weinsberg and Freistadt suites the Al2O3/TiO2 ratio is controlled by the accumulation of ilmenite and/or titanite. Consequently, the Al2O3/TiO2 ratio may be used as a thermometer only with caution.