Abstract

The Llano Uplift of central Texas is a gentle structural dome exposing ~1.37 Ga to 1.23 Ga metaigneous and metasedimentary rocks of Grenville affinity intruded by ~1.119 to 1.070 Ga post- to syntectonic granites collectively known as the Town Mountain Granite (TMG). The Lone Grove batholith (LGB), one in a series of TMG plutons, is chemically and texturally zoned consisting of four mappable units: 1) porphyritic coarse-grained, 2) coarse-grained, 3) medium-grained, and 4) fine-grained granite. Principal minerals include perthitic microcline, zoned plagioclase (An 1–20), quartz, biotite, and hornblende, with accessory Fe-Ti oxides, zircon, apatite, titanite, and fluorite. The LGB suite has mineralogical, textural, and chemical characteristics of A-type granites: i.e., 1) accessory fluorite, 2) rapakivi texture, 3) A/CNK 0.93–1.02, 4) K₂O/Na₂O 1.21–1.97, 5) high Fe/Mg, 6) high Sr, Zr, and Y, 7) low Ca, Cr, Ni, Eu, and Zn, and 8) enrichment in Ba, K, LREE, and HREE. Additionally, they are sub-solvus, metaluminous to marginally peraluminous, high-K, calc-alkaline biotite-hornblende granites (Fe/(Fe+Mg)= 0.71–0.91 and 0.78–0.91 for biotite and hornblende, respectively), that display distinct variation trends with increasing silica content.

Variation diagrams suggest that the four mappable units lie along a common liquid line of descent and may represent sequential fractionation products. Estimated crystallization T-P ranges of 750–850° C and 0.2 to 0.5 GPa respectively are based on thermal metamorphic mineral assemblages, normative Q-Ab-Or plots, and Q-Ab-Or-H₂O experimental data (Johannes and Holtz, 1996). The assemblage of titanite + magnetite + quartz suggests crystallization occurred at low f_O2 [confirmed by Fe/(Fe+Mg) vs^[4] Al microprobe analyses of hornblende] and a water content of less than 1.5 wt. % (Wones, 1989). Compared to other Town Mountain-type plutons, the LGB granites display a comparable iron content at similar alkali and silica enrichments. Melting models suggest the LGB evolved from the partial melting of lower crustal rocks of tonalitic composition.

On tectonic discrimination diagrams (e.g., Rb vs Yb+Ta, Nb vs Y) the LGB granites plot in the “within plate” and “syn-collisional granite” fields. However, with few intermediate and virtually no mafic rocks, and no coeval volcanic rocks in the Llano district, the LGB’s tectonic setting, in comparison to its A-type chemistry, remains unclear.