DUCTILE DEFORMATION IN GRANITES AND CONTACT METAMORPHIC ROCKS, LLANO UPLIFT, CENTRAL TEXAS
REED, Robert M., Department of Geological Sciences, University of Texas, Austin, TX 78712 (E-mail address)
Proterozoic (Grenvillian) granites in the Llano Uplift have previously been described as "anorogenic" and a late-stage low-P "static" metamorphism is thought to be related to thermal effects of their emplacement. However, recent work has shown that deformation in and around some of these granites was synchronous with their intrusion.
The Wolf Mountain intrusion (WMI), a chevron-shaped granitic body, occurs in the hinge of a SE-plunging syncline. Foliation in the granite is parallel to both the axial plane of this syncline and to the dominant wall rock foliation (image). The foliation in the granite is partly magmatic, but it has been overprinted by solid-state deformation, including high-T recrystallization of feldspars and quartz. This deformation can also be seen in contact-metamorphosed pelitic country rocks. Sillimanite and andalusite in the Llano Uplift formed during the late, low-P "static" metamorphism. Near the WMI, prismatic sillimanite pseudomorphs randomly oriented andalusite crystals. Prismatic sillimanite also occurs as lineated and foliated crystals parallel to the axial plane of the syncline. Fibrous sillimanite (fibrolite) defines intrafolial folds. These relations are best explained by a pre-emplacement thermal pulse causing growth of unaligned andalusite crystals followed by sillimanite growth during deformation-related emplacement.
The Midway Sill (MS) intrudes along the contact between two major lithological units. The MS is more deformed than the WMI, and should be called an augen gneiss. Foliation within the MS trends WNW and dips SW with a shallow SE-plunging stretching lineation. Kinematic indicators show a combination of right-lateral and thrust motion. In the country rock, dikes related to the MS are folded and boudinaged.
The foliation seen in both of these intrusions is parallel to and seemingly related to foliation in the country rock. Deformation and intrusion were synchronous, at least for these two intrusions, which suggests that they are late synorogenic, not anorogenic. Some minerals formed in the low-P "static" metamorphism are related to deformation, particularly near the granitic intrusions in this study.
