2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 8:00 AM-4:45 PM

Modeling Fractional Crystallization In the Mesoproterozoic Lone Grove Batholith, Llano Uplift, Central Texas

GRAY, Walt1, SMITH, Robert K.2 and GIBBS, Tyson R.2, (1)University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, (2)Department of Geological Sciences, University of Texas at San Antonio, San Antonio, TX 78249, Walter.Gray@utsa.edu

The Llano Uplift of central Texas is a structural dome exposing ~1.370 to 1.230 Ga (Grenville) metaigneous and metasedimentary rocks intruded by ~1.119 to 1.070 Ga post- to syntectonic granites collectively known as the Town Mountain Granite (TMG). The Lone Grove batholith is one of a number of chemically and texturally zoned TMG plutons comprised of a central fine-grained unit surrounded by medium-grained, coarse-grained, and porphyritic coarse-grained units. New major-element, trace-element, and mineral geochemistry data reveals silica enrichment trends from the outer to inner units. This trend is most prevalent on major-element variation diagrams, but trace-element vs. SiO2 plots display considerable scatter. Although the major element trends are suggestive of fractional crystallization along a common liquid line of descent, scatter in the trace-element data may suggest that other processes such as mixing, assimilation, or multiple lines of descent may have been involved in the evolution of the batholith; i.e., the four textural units. As an aid in resolving this uncertainty, fractional crystallization was evaluated using a weighted least squares modeling approach. The first step in the analysis was the development of a Fortran-based linear least squares computer code. Mineral and major-element data (microprobe and whole rock analyses, respectively) were then input to the code and predictions made as to the proportions of minerals fractioned during compositional changes from the outer to inner units. Using the predicted mineral proportions, bulk distribution coefficients were calculated and used to predict trace-element concentrations. The predicted concentrations were compared to measured values and the validity of the fractional crystallization hypothesis evaluated.