A detailed geological study of the Lone Grove granitic pluton and surrounding rocks from the Llano Uplift, Texas, was made. Field and petrologic evidence suggests that this intrusive has had a simple history of emplacement with no later metamorphism. Samples of granite, aplite, pegmatite, rhyolite, and metamorphic rocks were investigated geochronologically in order to determine the consistency in Rb87–Sr87 and K40–Ar40 ages between various minerals and between different localities. Refined chemical and mass spectrometric methods of analysis are described and the limits of precision are established. It is found possible to determine ages on most highly radiogenic minerals to a precision of ± 1 1/2% or better. All ages were calculated with the following decay constants: Rb87, λβ=1.47x10(-11)yr(-1); K40, λβ=4.72x10(-10)yr(-1) and λe= 0.585x10(-10)yr(-1). The distribution of ages on most of the minerals from these rocks show a spread commensurate with the experimental error. The averageRb87-Srv age on microclines,muscovites and biotites is 1015 million years and the average K40–Ar40 age on muscovites, biotites, and hornblendes is 1045 million years. A total rockRb87-Sr87 age on one of the granites gives no indication of having an older age than the constituent minerals. The only rock to show an apparently real different age is a rhyolite porphyry,which gives an average Rb87-Sr87 microcline age of 910 million years. K40-Ar40 determinations on several microclines and plagioclases give ages which are 5-20% too low, presumably due to argon diffusion from the feldspar. Anomalously low Rb87–Sr87 ages occur on several biotites from fresh pegmatites and granite. Evidence is presented for radiogenic strontium migration from these rocks although the exact nature of the process is not known. Somewhat low K40–Ar40 ages also are obtained on the pegmatitic biotites. A study of the effects of weathering on the geochronologic systems is made on two obviously altered granites. The only mineral to suffer any decrease in age from such surface alteration is biotite by the Rb87-Sr87 method. The isotopic composition of the common strontium incorporated into the minerals of the granite is determined on several minerals having low Rb/Sr ratios. The Sr87/Sr88 ratio is found to be 0.0843 ± 0.0002 (normalized to make Sr866/Sr88 = 0.1194). A discussion of the distribution of rubidium, potassium, and normal strontium throughout the pluton is given and several partitioning factors for the rubidium to potassium concentrations between different mineral species are calculated.
