Microfabrics in a metamorphosed quartzite from Mount McCartney, Montana (work in progress)

Three samples of naturally deformed quartzite conglomerate were collected west of Mount McCartney near Dillon, Montana.  A granitic pluton intruded and metamorphosed the sandstone conglomerates.  I made thin sections of these rocks and analyzed them using a universal stage to obtain Crystallographic Preferred Orientations (CPO).  I plotted these data on equal area stereonets, to infer the deformation mechanisms of the quartzite.  We are currently trying to evaluate the Shape Preferred Orientation (SPO) by using NIH imaging software to calculate the stretch of the grains.

One of the samples was microprobed to identify a fibrous mineral. From the shape it appears to have been fibrous sillimanite, but the original mineral has been replaced by sericite (fine grained white mica).  We infer that sericite replaced fibrous sillimanite, and that the sillimanite indicates a metamorphic temperature of 540-570 C. I orientated the sericite and plotted the data on a rose diagram, which showed that there was a strong orientation parallel to the foliation in the quartzite, illustrating a Shape Preferred Orientation.  This tells us that the sillimanite/sericite was present during and/or after the deformation of the quartz.

The ultimate goal is to use this information to evaluate flow laws for deformed quartz.  We can come up with a flow law for the rocks collected in Montana by using the inferred temperature by the replaced sillimanite, the temperatures associated with the pluton, the strain from using the data collected from stretched cobbles, and the deformation mechanism to be found by using the CPO of the rock samples. (Christian Teyssier, advisor)