Robert M. Reed
(formerly of) Department of Geological Sciences
The
University of Texas
Austin, TX 78712
E-mail address
An area of Grenville-age metamorphic and igneous rocks exposed in central Texas. Rocks are poly-deformed (up to 5 phases of folding) and poly-metamorphosed. The most recent metamorphism is a low P event with andalusite and/or sillimanite which is thought to be associated with intrusion of voluminous granite plutons.
[further information] [map of the Llano Uplift]
A) Coarse-grained pink porphyritic granites (Town Mountain Granite) in large oval or dike-like bodies, (~30% of Llano Uplift, Johnson and others, 1976)
B) Fine to medium-grained pink or gray granites in irregular bodies (~16 % of Llano Uplift, Johnson and others, 1976)
Age Range: 1.13 Ga to 1.070 Ga
Geochemistry: Metaluminous to slight peraluminous, K2O rich, high Fe/Mg ratios in mafic minerals.
Mineralogy: Microcline phenocrysts with plagioclase, quartz, biotite +/- hornblende, titanite, magnetite +/- ilmenite, zircon, allanite, +/- fluorite
Classification: Town Mountain Granites have been previously considered to be anorogenic.
[further information]
