A Geochronologie Study of the Lone Grove Pluton from the Llano Uplift, Texas

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A field and petrologic study of the Lone Grove granitic pluton and surrounding rocks from the Llano Uplift, Texas, suggests that this area has been involved in a single orogenic cycle with no later general metamorphism. Samples of granite, aplite, pegmatite, rhyolite, and metamorphic rocks were investigated in order to determine the precision in Rb-Sr and K-Ar ages between various minerals and different localities. Refined chemical and mass spectro-metric methods are capable of yielding ages on most highly radiogenic minerals to an analytical precision of ± 1 ½ per cent or better. Most of the ages from these rocks show a spread commensurate with the experimental error. The average Rb-Sr age on microclines, muscovites, and biotites is 1020 million years and the average K-Ar age on muscovites, biotites, and hornblendes is 1045 million years (Rb87, λβ = 1·47× 10−11 yr−1; K40, λβ = 4.72 × 10−10 yr−1, and λε = 0–585 × 10−10 yr−1). A total rock Rb-Sr age on one of the granites gives no indication of being older than those of the constituent minerals. The only rock to show a real age difference is a rhyolite porphyry, which gives an average Rb-Sr microcline age of 920 million years. A metasedimentary gneiss having a total rock Rb-Sr age of 1110 million years may contain some radiogenic strontium from an earlier history. K-Ar determinations on several microcline and plagioclases give ages which are 5–20 per cent low relative to the other minerals, presumably due to argon diffusion from the feldspar. Anomalously low Rb-Sr ages occur on several fresh biotites from pegmatites and granite. Evidence is presented for strontium or rubidium migration in these rocks although the exact nature of the process is not known. Also somewhat low K-Ar ages 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 apparent age from such surface alteration is biotite, by the Rb-Sr method. The Sr87/Sr88 ratio of the original strontium incorporated into the minerals of the granite is determined on several minerals having low Rb/Sr ratios and is found to be 0.0843±0–002 (normalized to Sr86/Sr88 = 0.1194). A discussion of the distribution of rubidium, potassium, and normal strontium throughout the pluton is given and partitioning factors for the rubidium to potassium concentrations between different mineral species are calculated.