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A Geochronologic Study of a Granite Pluton from the Llano Uplift, Texas

Zartman, R. E. and Wasserburg, G. J. (1962) A Geochronologic Study of a Granite Pluton from the Llano Uplift, Texas. Journal of Geophysical Research, 67 (4). p. 1664. ISSN 0148-0227. doi:10.1029/JZ067i004p01627.

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Granite and related pegmatite and aplite from several localities within a pluton from the Llano uplift, Texas, are being studied geochronologically. Biotite, muscovite, hornblende, microcline, plagioclase, quartz, apatite, and fluorite have been analyzed by refined chemical and mass spectrometric methods in order to determine the consistency in ages between various minerals and between different localities within an individual pluton. Field and petrologic evidence suggests that this intrusive has had a simple history of emplacement and no later metamorphism. Quadruplicate analyses on a master biotite yield K-Ar and Rb-Sr ages reproducible to 1 per cent. In most cases K-Ar determinations on biotites, muscovites, and hornblendes and Rb-Sr determinations on biotites, muscovites, and microclines give ages which fall within a 5 per cent spread. A half-life of 1.307 X 10^9 years and a branching ratio of 0.124 are used to calculate the K-Ar ages, and Rb-Sr ages are calculated with a half-life of 50 X 10^9 years. The average age of the pluton thus determined is 1060 m.y. K-Ar determinations on microclines give ages which are 5 to 15 per cent lower. One plagioclase from the granite gives a K-Ar age of 920 m.y. Two suites of biotites, one from pegmatites and one from a border facies of the granite, give anomalously low Rb-Sr ages. The pegmatitic biotite also has a somewhat low K-Ar age; however, the biotite from the granite gives a normal age by this method, as do coexisting microclines from both these rocks by the Rb-Sr method. Geologic evidence suggests that meteoric or hydrothermal fluids may have been responsible for this age discrepancy. Ages determined on a gneiss, a pegmatite cutting the gneiss, and a granite porphyry all give results approximately equal to those of the granite. No evidence of a significantly older basement rock or a younger igneous or metamorphic event in the area has been obtained to date.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1962 by the American Geophysical Union. Article first published online: 7 Dec 2012.
Issue or Number:4
Record Number:CaltechAUTHORS:20141015-103634277
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Official Citation:Abstracts of the papers presented at the First Western National Meeting, American Geophysical Union, Los Angeles, California, December 27–29, 1961, J. Geophys. Res., 67(4), 1627–1664, doi:10.1029/JZ067i004p01627
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:50401
Deposited By: Ruth Sustaita
Deposited On:15 Oct 2014 18:18
Last Modified:10 Nov 2021 18:55

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