AN EXPERIMENTAL-STUDY ON THE FORMATION OF COMPOSITE INTRUSIONS FROM ZONED MAGMA CHAMBERS

We present a model which accounts for the common, but paradoxical arra ngement of composite intrusions (i.e. silicic core and mafic margins) on the basis of analogue experiments using gelatin and aqueous solutio ns. The present model involves simultaneous flow-out of the upper and lower magmas from a longitudinal crack along the chamber wall. Experim ental results suggest that the mafic magma from the lower layer leaks from the side-wall of the chamber and travels faster than the silicic magma because of its lower viscosity, so that the mafic magma reaches the tip of the crack first. Once the mafic magma reaches the crack tip , then the rate of dyke propagation becomes determined by the viscosit y of the less viscous mafic magma, and so it can advance rapidly. The viscous silicic magma can flow efficiently into the center of the dyke , being lubricated by the mafic magma margins. This model accounts for the common arrangement of composite intrusions and gives an efficient mechanism of flow of viscous silicic magmas.