USE OF MICROGRANITOID ENCLAVES AS SOLID-STATE STRAIN MARKERS IN DEFORMED GRANITIC ROCK - AN EVALUATION

Microgranitoid enclaves are common in many plutons and are potentially valuable as solid state strain markers in tectonically deformed grani toids. We consider the parameters necessary to assess enclaves as two- dimensional strain markers. These include (i) their primary shapes, (i i) differences in rheology between enclave and host granitoid, (iii) t he magmatic shape fabric ellipse of enclave populations, (iv) the maxi mum fluctuation of enclave long axes developed during magmatic process es, (v) the angle between the magmatic shape fabric ellipse and tecton ic strain ellipse at the onset of tectonic deformation, and (vi) the t ype of strain path undergone by the markers. Enclave populations show characteristic magmatic distributions as shown by plots of the magmati c shape fabric ellipse vs enclave axial fluctuation from several pluto ns in the Sierra Nevada, California, and in the Adamello massif, Italy . The plots of tectonically undeformed and deformed enclaves show near ly complete overlap, creating substantial uncertainties in separating magmatic strain from tectonic strain if only the final enclave axial r atio is considered. Enclave/host strength differences have been modele d using published mineral deformation data and theoretical approaches to simulate deformation of aggregates. Results indicate very similar f low properties of enclaves and felsic host between 500 degrees and 800 degrees C, and possibly at lower temperatures as well, provided that biotite-bearing enclave and host compositions are present. Two-dimensi onal strain modeling of the enclave populations indicates that values of the total shape fabric ellipse as determined from held measurements can be close to the true tectonic strain under some circumstances, bu t most commonly the former substantially over- or underestimates the l atter. If the parameters (i-v) are assessed at each strain site, howev er, and each measured surface subsequently modeled using those paramet ers, over- and underestimates of solid state strain can be greatly red uced or eliminated, and reasonable values for two-dimensional tectonic strain of the granitoid and its enclaves can be obtained. (C) 1998 El sevier Science Ltd. All rights reserved.