Effect of tourmaline on melt fraction and composition of first melts in quartzofeldspathic gneiss
European Journal of Mineralogy Volume 3 Number 3 (1991), p. 527 - 536
published: Jun 14, 1991
manuscript accepted: Jan 8, 1991
manuscript received: Aug 13, 1990
ArtNo. ESP147050303005, Price: 29.00 €
Abstract Experiments were conducted in order to investigate the influence of tourmaline and the effect of boron (B) on amount and composition of partial melts generated during anatexis. Comparative experiments were performed with a peraluminous gneiss and with the same gneiss containing 6 wt% tourmaline. Different amount of H2O were added to both starting materials. Experiments have been conducted at 3 kbar, 730, 750 and 775°C. The breakdown of tourmaline occurs between 750 and 775°C. In runs with low fraction of melt (0.5 to 1.5 wt% added H2O), the run products were melt, newly crystallized cordierite, residual biotite, plagioclase, quartz and alkali feldspar. With increasing melt fraction, alkali feldspar and finally also quartz participate completely in the melt. Except for Al, which is higher in B-rich melts (there is 0.5 to 1 % more normative corundum in B-bearing melts when compared to B-free melts), the compositions of B-bearing and B-free melts are not significantly different. Thus, the presence of B in the first-generated melts does not influence significantly the composition of leucogranitic magmas. In contrast, the breakdown of 6 wt% tourmaline in a quartzofeldspathic gneiss causes a significant increase in the proportion of melt produced.