Abstract:
Many genetic models, including those dealing with the relationship between ore content and siliceous magmas, rely heavily on the idea that the crystallization of granitic intrusives leaves (at depth) chambers of residual melt that is rich in certain ore elements, as well as fluorine, lithium and, especially, water. For example, an investigation of melt inclusions in minerals of liquidus composition from granites of about 40 granite intrusions in various parts of the USSR indicates that melts that are intruded at meso- and hypabyssal depths generally contain a maximum of 2.5 to 3.0 percent of water in the initial stage of crystallization and are thus undersaturated in water. In view of the spatial and temporal proximity of these injective phases and the fact that they are concentrated in areas that are contained within the geophysical boundaries of the pluton, the data appear to constitute direct evidence that residual melts with high water content can actually separate during the crystallization of large intrusive bodies. Based on geological data, we estimate that these residual chambers had volumes in the tens of cubic kilometers.