Grenville Basement Igneous & Metamorphic Rocks of the Llano UpliftLisa BishopAmber SandersonSavannah Wallace

What? When? Where?• In central and west Texas

exposures of the Grenville Orogeny record 300my 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 (McGehee).

Mosher, 1998

Geologic Map of Texas

utexas.edu

The Rocks

Pelitic Schist

http://uts.cc.utexas.edu/~rmr/images/nonporph.jpg

Granites

http://uts.cc.utexas.edu/~rmr/images/LLmggrn-256.jpgPacksaddle Schist

http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/metamorf/images/Meta01.jpg

Precambrian Units of Llano Uplift • Units of the Llano uplift

include sediments, lavas, and pyroclastics (McGehee, 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 almondine-amphibolite with pyroxene and hornfels near the intrusions (McGehee).

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

McGehee, 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 quartz-feldsparpmica schist

▫ Rough-gray leptite, qtz-feldspar-mica 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 BatholithTown Mountain Granite

Mohr, 1983

References• McGehee, Richard V. 1979. PreCambrian Rocks of the

Southeastern Llano Region, Texas. Bureau of Economic Geology Geological Circular v.79 #3

• Clabaugh, S.E. and McGehee, 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 v36 #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 v82: 361-368.

A Special Thanks to Anna Garrett and Jennifer

Jerkins.