Biotite from Čierna hora Mountains granitoids (Western Carpathians, Slovakia) and estimation of water contents in granitoid melts

Abstract

Biotite from Čierna hora Mountains granitoids (Western Carpathians, Slovakia) and estimation of water contents in granitoid meltsBiotite is the dominant ferromagnesian mineral in different granites from the Čierna hora Mountains, in the Western Carpathians (Slovakia). A higher content of Fe3+(up to 20 %) is characteristic for the biotites from I-type Sokoľ and Sopotnica granitoid bodies in contrast to the biotites from S-type Ťahanovce granitoids showing decreased Fe3+amount (around 5 %). The Fe/(Fe + Mg) ratio in biotites from the Sokoľ and Sopotnica massifs between 0.47 and 0.54 is rather low with respect to that in biotite from the Ťahanovce [Fe/(Fe + Mg) = 0.55-0.63] and Miklušovce [Fe/(Fe + Mg) = 0.73-0.81] granite body. Water fugacities and contents calculated using Wones' (1981) calibration of biotite stability equation and Burnham's (1994) water dissolution model yield relatively similar values of 4-5 wt. % in remaining melts at 400 MPa and various levels of fo2and activities of annite for magnetite-bearing assemblages. This suggests an effective buffering role of biotite in both oxygen and water fugacities. Comparison of the peraluminosity index (A/CNK) of biotite with the same index in whole-rock shows distinctly higher A/CNK values for biotite indicating its aluminous character and important role as a significant aluminium carrier. The biotite composition indicates that granitoids in the Čierna hora Mts can be primarily derived from the lower crust; their protolith was influenced by mixing and/or assimilation process.