Town Mountain Granites
Age Range: 1.12 Ga to 1.070 Ga (Billion years ago)
Geochemistry: Metaluminous to slight peraluminous, K2O rich, high Fe/Mg ratios in mafic minerals.
Mineralogy: Microcline phenocrysts, plagioclase, quartz, biotite ± hornblende, titanite, magnetite ± ilmenite, zircon, allanite, ± fluorite
A Gallery of Town Mountain Granite Images is also available.
(I originally wrote this for an AAPG walking tour of the Building Stones of Houston and have been slowly improving it with time.)
The Town Mountain Granite suite (TMG) of the Proterozoic Llano Uplift of central Texas consists of numerous voluminous intrusions with similar characteristics and ages. TMG is generally pink, very coarse grained, porphyritic granite with accompanying pink coarse-grained non-porphyritic granite. Mineralogically, TMG consists primarily of plagioclase feldspar, potassium feldspar (microcline), and quartz with biotite and/or hornblende (The vast majority plot in the granite field of the IUGS classification). Accessory minerals commonly include titanite (sphene), zircon, magnetite and/or ilmenite, and apatite with fluorite, allanite, and/or pyrite being present in some rocks. Texturally the granites are dominated by the large pink microcline feldspar crystals. Differences in the color, shape, or amount of these microcline feldspar crystals are responsible for much of the differing appearance of building stone varieties. Faint to well-developed alignment of these large crystals (magmatic foliation) occurs somewhere in most intrusions. Several common varieties of TMG building stone show this fabric. Some TMG types also show rapakivi (plagioclase rimming microcline crystals) and anti-rapakivi (microcline rimming plagioclase) textures. Other differences in appearance are due to variations in the amount of biotite (black crystals), and the color of quartz (clear to pale yellow to dark gray).
Regionally, the granites intrude multiply-deformed schists, gneisses, and other metamorphic rocks of the Llano Uplift. Most intrusions of TMG have roughly circular to teardrop-shaped surface expressions (Barnes, 1981). The U/Pb zircon ages for TMG are 1133 Ma to 1070 Ma (Roback, unpublished data; Walker, 1992). Most granites are undeformed but high-temperature solid-state deformation is present in at least three plutons (Reed and Helper, 1994).
Chemically the granites are complex. In the past they have been classified as "anorogenic" or "A-type" granites. The granites do show the high Fe/Mg ratios in biotites and hornblendes that are characteristic of A-type granites. In addition some of the granites have fluorite, another characteristic of A-type granites. However, the granites are all metaluminous to slightly peraluminous with none of the peralkaline granites common to A-type suites. Dr. Diane Smith of Trinity University has done trace element analysis of the granites and found that some have chemistries consistent with "within plate granites" and others plot as "synorogenic" or "volcanic arc" granites. The deformation in the granites seems to be related to compressional events, thus ruling out an "anorogenic" classification. However, many people consider A-type chemistries as not being confined to anorogenic granites. Further chemical work, and perhaps clarification of definitions, is necessary before the classification of these granites can be fully understood.
Commercial quarry operations have been underway since 1882 when TMG was quarried to construct the Texas Capitol [see History of Granite Mtn.]. Numerous other buildings in Texas and elsewhere have since been constructed of this stone. Impressive reserves of quality building stone are still present in the Llano region (Barnes and others, 1947; Garner, 1992). In the past gray granites where also quarried in the Llano region, but today quarrying seems, with one exception, to be limited to TMG. Excellent opportunities to observe TMG in its natural habitat are available at the Enchanted Rock State Park north of Fredericksburg, on LCRA land just downstream of the Alvin Wirtz Dam, and at roadside picnic area on FM 1431 just west of Marble Falls.
TMG near you?
Barnes, V.E. (1981) Llano sheet, Geologic Atlas of Texas, 1:250,000. Bureau of Economic Geology Publication, University of Texas at Austin.
Barnes, V.E. , Dawson, R.F., and Parkison, G.A. (1947) Building Stones of Central Texas. University of Texas Publication no. 4246, 198 pgs
Garner, L. Edwin (1992) The dimension stone industry of Texas. Mineral Resource Circular No. 82, Bureau of Economic Geology, Austin, TX, 16 pgs.
Reed, Robert M. and Helper, Mark A. (1994) Evidence for solid-state deformation of ~1.1 Ga "anorogenic" granites in the Llano Uplift, Texas. Geological Society of America Abstracts with Programs, 26, 1, 25.
Walker, N. (1992) Middle Proterozoic geologic evolution of the Llano uplift, Texas: Evidence from U-Pb zircon geochronometry. Geological Society of America Bulletin, 104, 494-504.