Precambrian geology of the southeastern Llano uplift, Texas
Creators: name: McGehee, Richard V.
Publisher: name: The University of Texas at Austin
Year: 1963
Subjects: subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Texas
subject: Llano Uplift
subject: Stratigraphic geology
subject: Precambrian
subject: Geology--Texas--Llano Uplift
subject: Geology, Stratigraphic--Precambrian

description: In central Texas sedimentary rocks and associated intrusive mafic and silicic igneous rocks underwent a single episode of regional metamorphism about 1,100 million years ago. In the final stages of metamorphism great masses of granite and innumerable small pegmatite bodies were emplaced. A stratigraphic section of metasedimentary rock (constituting the Llano Group), totaling more than 28,000 feet in thickness is present in the southeastern part of Llano County. It includes two formations: the Valley Spring Gneiss, consisting mostly of quartz-feldspar rock, and the overlying Packsaddle Formation, which varies from marble, graphite schist, and hornblende schist in the lower part, through hornblende schist, mica schist, and quartz-feldspar rock in the middle, to dominantly quartz-feldspar rock with some hornblende schist in the upper part. On the basis of the measured sections and geologic mapping, four new members (named Honey Creek, Sandy Creek, Rough Ridge, and Click, from oldest to youngest) are proposed as subdivisions of the Packsaddle. Ortho-amphibolite and granite gneiss are common throughout much of the area. Most of the metamorphosed igneous bodies are sill-like; some may actually have been extrusive sheets. The Coal Creek Serpentine mass, largely enclosed by Big Branch (quartz diorite ortho-) Gneiss is present in the southwestern part of the mapped area. Red Mountain (granite ortho-) Gneiss is also present in a series of sills in the southern part of the area. Rank of regional metamorphism of most of the rocks corresponds to Turner's (Turner and Verhoogen, 1960) almandine-amphibolite facies and Miyashiro's (1961) amphibolite facies of the andalusite-sillimanite type. Andalusite, sillimanite, and cordierite are widespread although rarely abundant in pelitic rocks of the area. Psammitic rocks are composed mostly of quartz, microcline and andesine, with minor mica and hornblende. Original carbonate rocks are commonly represented by calcite-tremolite-diopside marble and by tremolite-diopside rock. Kyanite, staurolite, and ortho-pyroxene are absent. Contact metamorphic minerals include wollastonite, idocrase, garnet, sillimanite, and andalusite. Contact metamorphism near granite and pegmatite bodies probably approached or reached conditions of the pyroxene-hornfels facies. Mineral assemblages show no increase of rank with increasing stratigraphic depth in exposed rocks. They indicate temperature-pressure conditions intermediate between those of regional and of contact metamorphism. After the peak of regional metamorphism potassium metasomatism associated with granite intrusion converted andalusite, sillimanite, and cordierite to sericite and garnet to biotite. Metasomatic effects in some of the amphibolite included addition of sodium to plagioclase and probably to hornblende, conversion of hornblende to chlorite, and scapolitization. Foliation is generally parallel to original bedding and lineation parallel to major fold axes; however, some of the rocks possess an additional foliation, and others have one and even two additional lineations. Major structural elements are the broad regional Babyhead-Indian Flat anticline, which trends northwest-southeast through the area and plunges about 20 degrees to the southeast, and three northwest to north-trending thrust faults. On a small scale many of the rocks are highly deformed by faults, boudinage, and tight, intricate folds. High angle faults are mostly Paleozoic or younger