Sunday, 5 October 2008
George R. Brown Convention Center, Exhibit Hall E
A unique geologic section east of Lake Buchanan in the NE-most Llano Uplift provides new insights into the regional geologic and orogenic development of the Grenvillian Llano Mobile Belt (LMB) at the southern margin of Laurentia about 1.35-1.0 Ga ago. It shows a 4.9 km thick, tectonically stacked sequence of Mesoproterozoic gneisses with minor schists and granitoids, based on recent geologic mapping of a 52 km2
area at scale 1:10,000. A total of 31 Precambrian lithologic mapping units are distinguished. Lithotectonic domain units are comprised of heterogeneous, pink and pink-gray felsic gneisses of Valley Spring (VSD) domain affinity and variable hornblende, pyroxene, and graphite gneisses and schists of Packsaddle (PD) domain character. They show in geologic section an alternating pattern from structural bottom to top: VSD-PD-VSD-VSD-PD. Each unit is composed of a non-repetitive section of distinct gneiss and schist layers parallel to the regionally dominant foliation S3 striking E-W at 45 degrees S average dip.
The polyphase deformation history includes five deformational events (D1-D5) dominated by folding and shearing. These can be related to three facies of regional dynamothermal metamorphism, from oldest to youngest: eclogite facies (Me), transitional lower granulite to amphibolite facies (Mt), and amphibolite facies with three phases distinguished (Ma3, Ma4, Ma5). They illustrate a clockwise P-T-t path of development and regional metamorphic retrogression after the initial high-pressure (HP/MT) metamorphism. Correlation with orogenic uplifting and exhumation is indicated that followed earlier continental collision, subduction, and tectonic emplacement.