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 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
Granites
Packsaddle Schist
http://uts.cc.utexas.edu/~rmr/images/LLmggrn-256.jpg
http://facweb.bhc.edu/academics/science/harwoodr/geol101/labs/metamorf/images/Meta01.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 (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 almondineamphibolite 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 quartzfeldspar 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
• 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.