Grenville Basement Igneous & Metamorphic Rocks of the Llano Uplift

Grenville Basement Igneous &
Metamorphic Rocks of the Llano
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
Mosher, 1998
Geologic Map of Texas
The Rocks
Packsaddle Schist
Pelitic Schist
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
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
• 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
The Nonconformity of Town Mountain
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
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
• Lower contact contains quartzfeldspar schist
• Contains 4 formations (oldest to
▫ 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
▫ Click-hornblende schist, underlain
by leptite and qtz-feldspar-mica
• 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
• 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:
A Special Thanks to Anna
Garrett and Jennifer Jerkins.

similar documents