Role of water on fractionation processes and trace element distribution in A-type granites: the example of the Baga- Gazriin Chuluu granites (Central Mongolia)

摘要

Major and trace element geochemistry and experimental data are combined to discuss the crystallization of a ferroan granite, the Baga-Gazriin Chuluu pluton (Mongolia). The granite samples have nearly eutectic compositions and the evolution of the major element composition of the granites is controlled by concomitant crystallization of quartz, alkali feldspar and plagioclase from the beginning of crystallization. The crystallization of the tectosilicate phases occurred at ~810 °C, implying low water concentrations in the rhyolitic silicate melts (3 wt % H_2O or less). Fractionation processes occurred mainly in the temperature range 810-760 °C. The absence of fractionation at lower temperature is explained by the high crystal content of water-poor magma below 760 °C. Considering that most of the crystallization of the water-poor magma may occur under fluid-absent conditions, the classical explanation for the lanthanide tetrad effect observed in high-Si granites has to be reconsidered.