Deep fluids and magmatism:The deep processes beneath the Tengchong volcano group
深部流体与岩浆活动:兼论腾冲火山群的深部过程

The deep fluids intensely affect many geological processes in their initiation and development. However, their behaviors are poorly understood till now. The fluid is the most important among the factors affecting magmatism. The fluid has higher activity, and its solubility in melt increases with pressure. These suggest that the magma system must be an open dynamic system, because both input and output of fluid dramatically affect the whole behavior of a magma system. The fluid affects the magma system mainly by changing the melt viscosity, also changing the average density and liquidus and solidus temperatures of the magma. Injecting a few of fluid induces the melt viscosity to change in several orders of magnitude. This dramatic change further leads to quickly decrease the friction between the magma and its conduit rocks. Therefore, the ascent rate of magma should be also increased in several orders of magnitude. When the magma reaches the depth where the fluid separates to different phases, the behavior of the magma system will be more difficult to be expected. Conversely, the fluid escape will cause the magma system to be changed toward the opposite direction. The degassed magma will be detained at the deeper depth due to increasing viscosity and density. It is important to indicate that the lost fluid to the conduit may weaken the mechanical properties of the upper lying substrata, and hence improves the ascent condition in the conduit. Therefore, if the ascent magma may obtained continued recharge of deep fluids, the amount of which is at least equal to the lost, the magma will ascent in a more and more rapid rate. Accordingly, the magma system is a complex dynamic system, and magmatism is a non-equilibrium and non-linear process. These analytical results disaccord to the traditional petrology, but are consistent with the volcanological observations and metallogenic researches. The phenocryst-assemble texture and the syn-magmatic metasomatic texture can be seen commonly in the volcanic rocks of the Tengchong volcano group. The phenocryst-assemble texture suggests that the magma has a brief stopover at some depth before it erupted to the surface. The mineral assemblages indicate different depths where the magmas stay. The syn-magmatic metasomatic texture suggests that the chamber activation depends on injection of the deep fluids. Therefore, it is necessary to pay more attention to the deep fluid process beneath the chamber in volcano monitoring. The close incorporation of monitoring the fluid processes beneath and above the chamber may be a new approach of the volcano monitoring.