MICROGRANITIC ENCLAVE SHAPES AND MAGMATIC STRAIN HISTORIES - CONSTRAINTS FROM DROP DEFORMATION-THEORY

Microgranitic (mafic) enclaves are common components of plutonic bodie s and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape. In order t o constrain the conditions under which such enclaves were generated an d deformed by their host pluton, we apply drop deformation theory to m odel their distortion. Our model takes into account the change in visc osity of both the host and enclave with temperature, composition and t he amount of suspended solids in enclave/host systems, and it assumes that simple shear flow predominates in the plutonic system. We focus o n the deformational history of enclaves over the time period in which they remain liquid and are either more or less viscous than their gran itic host. We find that the most stable enclaves with respect to being broken up by their entraining flow are generally the most mafic encla ves: those with silica contents near those of the host granite are hig hly unstable. This is in reasonable accord with field observations of the abundance of enclaves of different chemistry. Most deformation of microgranitic enclaves is likely to occur at relatively high temperatu res, probably between 950 degrees C and 1050 degrees C, and therefore, the enclaves record magmatic strain of the host over only a limited t emperature-time range in the host's cooling history. Furthermore, the observation of apparent deformation of these enclaves in a liquid regi me coupled with our calculated dependences of enclave deformation on f low velocity implies that magmatic flow velocities are likely to be be low 10 m/yr in enclave-bearing plutonic systems.