Plagioclase crystallisation in a granodioritic melt and its petrogenetic implications for the origin of the A‐type granite in East Junggar, NW China

Crystallisation experiments were performed on a natural I‐type granodiorite from East Junggar of Xinjiang, NW China, at 1.0 and 1.5 GPa in the temperature range between 700 and 875 °C and for various water contents to investigate the crystallisation behaviour of plagioclase in granodioritic melts. Experimental results indicate that the optimal conditions for plagioclase to crystallise in this granodioritic melt are approximately 875 °C, 1.0 GPa, and ~10 wt% water content. All the experimental feldspars have ternary composition because of the high pressure in this study, with Or contents increasing while An contents decreasing with the increase of pressure. They consist of Or‐rich plagioclase at 1.0 GPa, passing to An‐rich alkali feldspars at 1.5 GPa. The ternary feldspar plays a role in keeping the total amount of alkali in the residual melt relatively constant. After plagioclase‐dominant crystallisation, the residual melt shows characteristics similar to the A‐type granites from East Junggar, implying that the fractional crystallisation of the I‐type granodioritic melt at high pressure may be one of the important processes generating the A‐type magmas in East Junggar. Mass balance calculation and trace element modelling show that the fractionally crystallised phase assemblage and proportion could be plagioclase (40.2 wt%) + orthopyroxene (6.3 wt%) + hornblende (6.3 wt%). The experimentally determined plagioclase crystal growth rates range from 1.17 × 10 −8 to 2.70 × 10 −8  mm/s. By comparing the crystal growth rate with the crystal size in natural rocks, the timescale of the crystallisation and differentiation of the I‐type granodioritic magma that is parental to the A‐type granitic magma in East Junggar was estimated to be several hundred years, in excellent agreement with the result from a simple numerical model.