MAFIC MICROGRANITOID ENCLAVES: STRAIN MARKERS, FABRIC ELEMENTS OR BLOBS OF DECEPTION?

Lack of good strain markers in plutons and the presence of widely distributed and elliptical mafic microgranitoid enclaves make them attractive for magmatic strain analyses. While the use of enclaves as strain markers has been challenged (Williams and Tobisch, 1994) they continue to be used to quantify magmatic strains. Mafic enclaves behave differently than markers typically used in strain analyses (e.g. ooids, volcanic clasts), thus interpretation of enclave shapes, orientations, and spatial distributions must account for: temporal and spatial variations in formation of single enclaves and enclave populations, initial shape of enclaves, and the effect of enclave/host viscosity contrasts. Initial enclave shapes are typically far from spherical and we have noted many field examples where mafic dikes and sheets were dismembered, forming enclaves that have large axial ratios, preferred orientations and internal mineral fabrics. This is problematic when attempting to quantify magmatic strains because removing these primary fabric effects is difficult in plutons, due to the fact that the mode and timing of enclave formation is typically unknown. We have also examined many cases where enclaves do not record total strain. Key examples include: 1) Mafic enclaves that show internal mineral fabrics that are typically inconsistent between enclaves in a population, often are not parallel to the host magmatic fabric, and do not necessarily parallel the long dimension of elongate enclaves; and 2) Elongate mafic enclaves that define a fabric in a pluton that is not parallel to the host mineral fabric. Both of these examples (and others) illustrate the fact that mafic enclaves are not passive markers in plutons, that they do not record total magmatic strain, but do record only increments of strain during a complex rheological evolution. Our observations strongly suggest that mafic enclaves undergo complex histories, including internal strain at high temperatures, rigid rotation through much of the hosts' crystallization history, and possibly late internal strain as host magmas approach their solidus.