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

Citation

Zartman, Robert Eugene (1963) A Geochronological Study of the Lone Grove Pluton from the Llano Uplift, Texas. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SXVC-R234. https://resolver.caltech.edu/CaltechTHESIS:11202012-153218336

Abstract

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, λ_β=1.47x10^(-11)yr^(-1); K⁴⁰, λ_β=4.72x10^(-10)yr^(-1) and λ_e= 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 average Rb⁸⁷-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 rock Rb⁸⁷-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.

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