MAGMA MINGLING IN DIKES AND SILLS

Dikes in which two different liquids flowed simultaneously, usually ca lled composite dikes, naturally fall into two classes depending on whi ch lithology forms the contact with the country rock, and hence, which liquid was the first to enter the fracture. In these two kinds of dik es, the structures formed by the mingling of the two liquids differ. D ikes in which the more basic liquid entered first have contacts betwee n the two lithologies that are nearly planar, and parallel to the dike walls, whereas the more basic lithology forms discrete pillows in tho se dikes in which the more silicic liquid entered first. Experiments i ndicate that these pillows probably form from a flow-front instability that develops when a liquid invades another of higher viscosity betwe en two parallel rigid walls. We provide scalings for the critical flow rate for the onset of this instability, the time required for the ins tability to develop, and the wavelength that is selected. These scalin gs are consistent with field observations.