Abstract:
Continental collisions are the place where granitic plutons result from the melting of crustal components. Granitic plutons are built up by successive input of magma with a variable composition and hence temperature and chemistry. The intrusion of a new magma batch has consequences on the element mobility in the melt. Diffusion in already formed crystals is limited, due to the short time interval between magma input, and because of the low values of element diffusivity in solids. Because the new magma is generally hotter than the magma chamber, the temperature in the contact zone is modified. It activates diffusion by and modifies its characteristic length for element mobility in the melt. A new intrusion also modifies the partition coefficients, decreasing compatibility and increasing the incompatibility. The change in temperature has also effects on fluid exsolution controlled by crystallization, or second boiling. The present paper examines the intrusion of magma (fel-sic or mafic) into a felsic magma chamber with a time interval of 30 ky. The intrusion of magma with similar composition, hence low (±100d̀C) temperature difference has few effects. The diffusion lengths for elements rarely exceed one order of magnitude. The fluids released by the cooling magma are balanced by their reincorporation into the warming magma. In contrast, the intrusion of mafic magma into felsic magma chamber results in temperature difference that can reach ±300d̀C. It may change the diffusion length up to two orders of magnitude for elements having large activation energy. Partition coefficients also vary by more than one order of magnitude
