Precambrian Rocks of the Llano Region, Texas
Dr. Emilio Mutis-Duplat, Professor of Geology and Chair Department of Science and Mathematics, College of Arts and Sciences, The University of Texas of the Permian Basin, Odessa, Texas
The oldest rocks that crop out in the Llano region of Texas are Precambrian metamorphic rocks belonging to the Llano Group, which is subdivided into three formations: Valley Spring Gneiss (the oldest), Lost Creek Gneiss, and Packsaddle Schist (the youngest). Meta-igneous rocks are common in the three formations, as well as migmatites. Precambrian igneous rocks were intruded both concordantly and discordantly into the metamorphic rocks. Rank of regional metamorphism corresponds to the amphibolite facies and some metasomatism and retrograde metamorphism has been identified.
Valley Spring Gneiss shows the greatest variety in lithologic composition. Quartz-feldspar-mica gneiss, quartz-feldspar-epidote gneiss, quartz-feldspar-amphibole gneiss, and quartzite alternate with mica schist, amphibolite, calc-silicate rock, and marble. The rocks range from very well foliated to poorly foliated. They seem to originally have been a complex mixture of volcanic and intrusive rocks with interlayered sedimentary rocks. The total thickness is unknown, but probable exceeds 20,000 feet.
“The Lost Creek Gneiss is a fine- to medium-grained, nonfoliated to very well foliated quartz-feldspar-hornblende-biotite gneiss grading into augen gneiss and migmatite” (Mutis-Duplat, 1980, p. 226). The precursor rock seems to have been rhyolitic sheets although predominantly arkosic sedimentary rocks are also a possibility. The maximum thickness that has been measured is 3,500 feet.
The youngest Precambrian metasedimentary rocks are those of the Packsaddle Schist. They are composed of quartz-feldspar-hornblende or mica gneiss and schist, quartzite, marble, calc-silicate rock, and graphite schist. The rocks range from well foliated to poorly foliated. The total thickness exceeds 20,000 feet.
Several meta-igneous rocks are very conspicuous. The most important ones are: Coal Creek Serpentine that appears to have originally been an intrusive rock, Big Branch Gneiss, originally a tonalite, and Red Mountain Gneiss, originally a granite.
Migmatites are very common, particularly in the Lost Creek Gneiss. They are not the product of partial melting or anatexis. They show agmatic or breccia structures and occur primarily along the axes of tight folds. “It is believed that during tight folding the Precambrian metamorphic rocks were megascopically fractured, and magma was injected into these fractures producing the migmatites” (Mutis-Duplat, 1978).
The igneous rocks are mostly granitic in composition. They range from very coarse grained such as in the Katemcy and the Enchanted Rock batholiths, to very fine grained aplites. The youngest igneous rock seems to be the so-called “llanite” that is characterized by the presence of red feldspar and blue quartz.
In the southeastern part of the Llano region the Precambrian folds are normal and broad, but those in the northwestern part are very tight and overturned. Most of the faults are Paleozoic normal faults, but a few Precambrian faults have also been identified, including at least three thrust faults, as well as some shear zones.
Mutis-Duplat, Emilio, 1978, Origin of Precambrian migmatites in Purdy Hill quadrangle, Mason County, Texas (abs.): Geological Society of America Abstracts with Programs, v. 10, no. 1, p. 23.
Mutis-Duplat, Emilio, 1980, Lost Creek Gneiss in the Purdy Hill quadrangle, Mason County, Texas: Texas Journal of Science, v. 32, no. 3, p. 223-231.