MICROSTRUCTURAL CONTRAST ACROSS THE INTRUSIVE CONTACT BETWEEN THE PACKSADDLE SCHIST AND THE TOWN MOUNTAIN GRANITE, LLANO UPLIFT, TEXAS

Recent studies of the intrusive contact between the ~1.3 Ga Packsaddle Schist and the ~1.1 Ga Town Mountain Granite have revealed an interesting contrast in microstructures across this boundary. The syn- to post-tectonic Town Mountain Granite intruded the polydeformed and polymetamorphosed forearc basin deposits of the Packsaddle Domain during anorogenic magmatism in the wake of the Grenville Orogeny. The gradational intrusive contact between the units is exposed within the boundaries of Enchanted Rock State Natural Area. Near the contact, the quartz-feldspar matrix of the Packsaddle Schist exhibits an equigranular, polygonal fabric that is consistent with recrystallization. Biotite and/or muscovite in these rocks is moderately to strongly foliated; larger biotite grains are oriented along and parallel to the contact. Chlorite grains are not foliated, and chlorite rims a porphyroblast of what appears to be altered andalusite. These textures suggest late, retrograde growth of chlorite. In contrast, the coarser quartz, microcline, and plagioclase of the Town Mountain Granite are inequigranular and interlobate. The microcline contains exsolution lamellae. Myrmekitic intergrowths of quartz and plagioclase are present between grains of quartz and microcline, which may be consistent with retrograde breakdown of the microcline. The Packsaddle Schist and the Town Mountain Granite experienced very different Proterozoic histories and would be reasonably expected to exhibit microstructural differences. However, the juxtaposition described here presents an opportunity to explore the microstructures of both units along the contact in order to study the influence of the Town Mountain Granite intrusion on the Packsaddle Schist, better integrate their geologic histories, and apply the microstructural context to interpret timing relationships.