Grenville Basement Igneous Metamorphic Rocks of the Llano

Grenville Basement Igneous & Metamorphic Rocks of the Llano Uplift Lisa Bishop Amber Sanderson Savannah Wallace

What? When? Where? • In central and west Texas exposures of the Grenville Orogeny record 300 my of tectonic activity and metamorphism along the Laurentian margin. • The Llano uplift represents a structural dome that is precambrian in age. • On a regional scale it would look like an island of igneous rock surrounded by earlier sedimentary structures (Mc. Gehee). Mosher, 1998

Geologic Map of Texas utexas. edu

The Rocks Granites Packsaddle Schist http: //uts. cc. utexas. edu/~rmr/images/LLmggrn-256. jpg http: //facweb. bhc. edu/academics/science/harwoodr/geol 101/labs/metamorf/images/Meta 01. jpg Pelitic Schist http: //uts. cc. utexas. edu/~rmr/images/nonporph. jpg

Precambrian Units of Llano Uplift • Units of the Llano uplift include sediments, lavas, and pyroclastics (Mc. Gehee, 1979). • Rock History includes metamorphism, folding, faulting, and intrusion. • Total Thickness: 28, 000 ft • Major Units: ▫ Valley Spring Gneiss ▫ Packsaddle Schist ▫ Town Mountain Granite http: //www. tpwd. state. tx. us/backgrounds/images/erock_bg. jpg

Geologic Map of Llano Uplift Mosher, 2008

Metamorphic and Tectonic Origins • High Temperature-Low Pressure metamorphism produced by Convergent Subduction at margin. • Uplift and Retrotransport produced by later subduction of continental crust (Mosher, 2008). • Upwelling of Asthenosphere produced granite intrusions (Mosher, 2008). • Facies are predominatley almondineamphibolite with pyroxene and hornfels near the intrusions (Mc. Gehee).

Tectonic Evolution • Llano is a part of the southern margin of Laurentia • This area is a part of the Grenville orogenic belt that extends from Texas to Scandinavia. • A continent-continent collision occurred inducing metamorphism and pushing up existing rock. • A shear zone is developed between the Valley Springs and Packsaddle Formations. • Cambrian seas deposited sediment in the area that was later eroded. • The basement rock was then uplifted. Mosher, 2008

Structural Elements • Llano Uplift is a broad structural dome in central Texas with 2 to 3 km of structural relief. (Mosher, 1998) • During orogeny the area is folded into long anticlines and synclines. • Thrust faults are present • Metamorphism in the area occurred at high temperatures but at a moderate depth. • Foliations are parallel to original bedding • Lineations are parallel to major fold axes

Subsurface View of Llano Uplift • The sills intruded before metamorphism was complete. • Granite intrusions pushed into the surrounding rock to create tighter folds. • 3 major faults span the area http: //hlmn. 281. com/march/uplift. htm

The Nonconformity of Town Mountain Granite Allison, 2014

Llano Supergroup • Precambrian (2. 5 Ga - 542 Ma) • Igneous to Metamorphic Rocks • 3 major Groups that cover 75% of the area ▫ Valley Spring Gneiss ▫ Packsaddle Schist ▫ Town Mountain Granite • The Packsaddle Schist Group is subdivided into 4 unique lithological formation ▫ ▫ Honey Formation Sandy Formation Rough Ridge Formation Click Formation

Llano Supergroup Mc. Gehee, 1979

Valley Spring Gneiss • Core of the Babyhead anticline • Metamorphosed intrusive igneous rocks • Paragenesis is the primary rock type • Alternating layers of pink quartz-feldspar gneiss and biotite gneiss • Contains altered limestones • Gradational upper contact with the Packsaddle Schist containing biotite and amphibole schist • Thickness totals 8, 480 ft. Mohr, 1983

Packsaddle Schist • Dominantly feldspathic amphibole and biotite schists that are dark in color • Lower contact contains quartz-feldspar schist • Contains 4 formations (oldest to youngest) ▫ Honey- graphite, hornblende, muscovite schist and marble ▫ Sandy-alternating units of hornblende schist, and quartzfeldsparpmica schist ▫ Rough-gray leptite, qtz-feldsparmica schist, and biotite-cordierite gneiss ▫ Click-hornblende schist, underlain by leptite and qtz-feldspar-mica schist • Thickness totals 20, 00 ft. Mohr, 1983

Town Mountain Granite • Formed due to a magmatic intrusion • Coarse-grained, pink, quartz-plagioclase-microcline rock, in part porphyritic with large microcline phenocrysts. • Occurs in plutons up to 13 mi in size • Xenoliths comprised of metamorphic minerals (tourmaline, wollastonite, idocrase, andalusite, sillimanite, cordierite, muscovite, and biotite) from the Valley Spring Gneiss and Packsaddle Schist due to intrusive magma • Gradational lower contact with the Packsaddle Schist • Nonconformity located between the Town Mountain Granite and the Packsaddle Schist

Enchanted Rock Batholith Town Mountain Granite Mohr, 1983

References • Mc. Gehee, Richard V. 1979. Pre. Cambrian Rocks of the Southeastern Llano Region, Texas. Bureau of Economic Geology Geological Circular v. 79 #3 • Clabaugh, S. E. and Mc. Gehee, R. V. *See Wayne for further info. • Mosher, S. Levine, J. S. F. and Carlson, W. D. 2008. Mesoproterozoic plate tectonics: A collisional model for the Grenville-aged orogenic belt in the Llano uplift, central Texas. The Geological Society of America v 36 #1: 55 -58. • Mosher, Sharon. 1998. Tectonic evolution of the southern Laurentian Grenville orogenic belt. GSA Bulletin v 110 #11: 1357 -1375. • Barnes, Virgil E. 1988. The Precambrian of Central Texas. Geological Society of America Centennial Field Guide South-Central Section v 82: 361 -368.

A Special Thanks to Anna Garrett and Jennifer Jerkins.