THE LONE GROVE BATHOLITH, LLANO UPLIFT, CENTRAL TEXAS: A MESOPROTEROZOIC A-TYPE GRANITE (YES), ANOROGENIC (PROBABLY NOT)
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 750-850o 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 sphene + magnetite + quartz suggests crystallization occurred at low fO2 [confirmed by Fe/(Fe+Mg) vs  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 LGBs tectonic setting, in comparison to its A-type chemistry, remains unclear.