Microgranitic (mafic) enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape. In order to constrain the conditions under which such enclaves were generated and deformed by their host pluton, we apply drop deformation theory to model their distortion. Our model takes into account the change in viscosity of both the host and enclave with temperature, compositon and the amount of suspended solids in enclave/host systems, and it assumes that simple shear flow predominates in the plutonic system. We focus on the deformation history of enclaves over the time period in which they remain liquid and are either more or less viscous than their granitic host.

We find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic enclaves: those with silica contents near those of the host granite are highly unstable. This is in resonable accord with field observations of the abundance of enclaves of different chemistry. Most deformation of microgranitic enclaves is likely to occur at relatively high temperatures, probably between 950 ¿C and 1050 ¿C, and therefore, the enclaves record magmatic strain of the host over only a limited temperature-time range in the host's cooling history. Furthermore, the observation of apparent deformation of these enclaves in a liquid regime coupled with our calculated dependences of enclave deformation on flow velocity implies that magmatic flow velocities are likely to be below 10 m/yr in enclave-bearing plutonic systems. ¿American Geophysical Union 1994