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A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York

Silver, Leon T. (1968) A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York. Memoir - New York State Museum and Science Service . pp. 233-251. ISSN 0548-8265.

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Cogenetic uranium-lead isotope systems in zircons have been analyzed for five rock samples from the anorthositic domes of the Adirondack highlands and compared with data from a cogenetic suite in a charnockitic gneiss from the Ticonderoga dome in the eastern Adirondacks. Textural and structural arguments applied to the zircons and rocks from the anorthosite suite indicate that some of the zircon are metamorphic and reveal that conditions producing granulite facies metamorphism exited in different parts of the anorthosite complex at discretely different times during the interval from 1020 to 1100 m.y. ago. The Ticonderoga gneiss sample yields an interpreted zircon age of 1130 ± 10 m.y. Zircon characteristics argue this to be the age of magmatic crystallization of the host rock. Extensive studies have confirmed this age as the prevailing age in many large bodies of charnockite (or syenite) gneiss which are peripheral to the anorthosite domes. Petrological arguments of persistent spatial association and compositional gradation between the anort11osite and a suite ranging from norite to mangerite, syenite, quartz syenite and granite, imply that all of these rock are comagmatic; this leads to the conclusion that the age of the anorthosite itself is 1130 m.y. No evidence has been found for any older ages in any of the Adirondack orthogneisses to support the hypothesis of a pre-"Grenville" basement. The mounting evidence for almost precise age contemporaneity between the Adirondack anorthosite-syenite complex and the Duluth anorthositic gabbro-granophyre complex should provoke a close comparative petrologic and structural scrutiny of these two great stratiform masses.

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Additional Information:© 1968 New York State Museum. The author is indebted to numerous individuals for geologic guidance and assistance in the field aspects of this work, particularly Matt Walton in the eastern Adirondacks, Dirk de Waard and W. D. Romey in the central highlands, the late Brian Davis in the northern highlands, Robert Hargraves in the northwest highlands, A. E. J. Engel in the northwest lowlands, and Y. W. Isachsen throughout the entire Adirondack Mountains. For helping the writer focus on the problems of the age and origin of the anorthosite massif, each of these individuals and especially my colleague, H. P. Taylor, Jr., together with the writings of A. F. Buddington and R. Balk, deserve commendation without the onus of responsibility. Professor Donald Potter and enthusiastic students from Hamilton College cheerfully helped the author move mountains of rock, as did the New York State Geological Survey. Joseph Mayer, Ollie Shields, and Mrs. Cathy Savin helped with the laborious mineral separations at various stages, and Mrs. Geraldine Baenteli was a mainstay in the chemistry and mass spectrometry throughout this research. This work was supported by NSF grant No. GP 931, and AEC research contract AT (04-3)-427 for which this is publication CALT 427-17.
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Atomic Energy CommissionAT (04-3)-427
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Caltech Division of Geological Sciences1490
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:61403
Deposited By: Tony Diaz
Deposited On:22 Oct 2015 18:11
Last Modified:03 Oct 2019 09:07

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