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 granitoids. We consider the parameters necessary to assess enclaves as two-dimensional strain markers. These include (i) their primary shapes, (ii) differences in rheology between enclave and host granitoid, (iii) the magmatic shape fabric ellipse of enclave populations, (iv) the maximum fluctuation of enclave long axes developed during magmatic processes, (v) the angle between the magmatic shape fabric ellipse and tectonic strain ellipse at the onset of tectonic deformation, and (vi) the type of strain path undergone by the markers. e populations show characteristic magmatic distributions as shown by plots of the magmatic shape fabric ellipse vs enclave axial fluctuation from several plutons in the Sierra Nevada, California, and in the Adamello massif, Italy. The plots of tectonically undeformed and deformed enclaves show nearly complete overlap, creating substantial uncertainties in separating magmatic strain from tectonic strain if only the final enclave axial ratio is considered. Enclave/host strength differences have been modeled using published mineral deformation data and theoretical approaches to simulate deformation of aggregates. Results indicate very similar flow properties of enclaves and felsic host between 500° and 800°C, and possibly at lower temperatures as well, provided that biotite-bearing enclave and host compositions are present. mensional strain modeling of the enclave populations indicates that values of the total shape fabric ellipse as determined from field measurements can be close to the true tectonic strain under some circumstances, but most commonly the former substantially over- or underestimates the latter. If the parameters (i–v) are assessed at each strain site, however, and each measured surface subsequently modeled using those parameters, over- and underestimates of solid state strain can be greatly reduced or eliminated, and reasonable values for two-dimensional tectonic strain of the granitoid and its enclaves can be obtained.