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Isotopic constraints on the origin of Appalachian mafic complexes

Shaw, H. F. and Wasserburg, G. J. (1984) Isotopic constraints on the origin of Appalachian mafic complexes. American Journal of Science, 284 (4-5). pp. 319-349. ISSN 0002-9599.

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Isotopic analyses of modern oceanic basalts and ophiolites have shown that both modern and ancient oceanic crust have a characteristic Nd and Sr isotopic signature indicative of derivation from a depleted mantle reservoir. It also appears that the Nd isotopic system is not appreciably disturbed by metamorphism. These isotopic characteristics have been extended to the Pt. Sal, Kings-Kaweah, and Josephine ophiolites of California. We have used these characteristics in an attempt to identify pieces of proto-Atlantic oceanic crust among the mafic and ultramafic rocks of the Appalachians. Sm-Nd mineral isochrons for the Baltimore Mafic Complex, Maryland (BMC) yield an age of 490 ± 20 my which we interpret as the igneous crystallization age. BMC whole rock samples do not define isochrons and have initial isotopic compositions of -6.4 < Є_(Nd)(T) < - 2.2, +51 < Є_(Sr)(T) < + ll5. Є_(Nd)(T) and Є_(Sr)(T) are anti-correlated. This is not the signature of depleted mantle and oceanic crust hut is similar to old continental crust. We propose that the BMC is a mafic continental intrusion, possibly subduction related, which was contaminated with old continental crust during emplacement. Whole rock samples from the Thetford Mines Complex, Qe (TMC), do not define isochrons and have -1.5 < Є_(Nd)(T) < +4.2, + 2.6 < Є_(Sr)(T) < + 114. These data do not in any way reflect the signature of normal oceanic crust. These results are in contrast with geologic relationships that show the TMC to have the characteristics of an ophiolite complex. The TMC is chemically and isotopically similar to a class of other ophiolites that have affinities to modern honinites. The Chunky Gal Amphibolite, North Carolina, Lake Chatuge complex, North Carolina, and Hazen's Notch Amphibolite, Vermont were found to have a depleted mantle signature with +5 < Є_(Nd) (T) < +8 and may he fragments of oceanic crust. The Webster-Addie body, North Carolina, has Є_(Nd)(T)~-1, Є_(Sr)(T)~+30 and is not isotopically similar to oceanic crust or the other North Carolina mafic bodies analyzed. From these isotopic results we infer that Appalachian mafic rocks have diverse origins: some are continental intrusives (BMC), others are probably fragments of oceanic crust (Vermont and North Carolina amphiholites). Future models for the development of the Appalachians must allow for these various origins. The possibility that some onhiolites are not normal oceanic crust but have an origin in a partially continental setting or as anomalous oceanic crust may require further attention.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 1984 American Journal of Science. We are grateful to T. Fenninger and R. Laurent for assistance in the field in Quebec and to T. Cullen, G. Fisher, B. Marsh, and J. Meyers for assistance in Maryland. Discussions with them and with B. Morgan, A. K. Sinha, and B. Hanan were most rewarding. We also thank G. Harper, S. Jacobsen, J. Laird, B. Morgan, H. McSween, K. Misra, J. Paque, and A. K. Sinha for providing samples. As always, D. A. Papanastassiou was an invaluable source of information on chemical and mass spectrometric procedures. The initial formulation of this project grew out of discussions with A. L. Albee on the nature of the mafic rocks of New England. His support and interest are gratefully acknowledged. P. D. Fullagar and an anonymous reviewer provided constructive reviews of this paper. This work was funded by the National Science Foundation Grants PHY79-23638A2 and EAR79-19786 and the National Aeronautics and Space Association Grant NGL 05-002-188.
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NASANGL 05-002-188
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Caltech Division of Geological and Planetary Sciences3851
Lunatic Asylum Lab430
Issue or Number:4-5
Record Number:CaltechAUTHORS:20140305-103135141
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Official Citation: H. F. Shaw and G. J. Wasserburg Isotopic constraints on the origin of Appalachian mafic complexes Am J Sci May 1, 1984 284:319-349; doi:10.2475/ajs.284.4-5.319
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:44154
Deposited By: Ruth Sustaita
Deposited On:05 Mar 2014 19:00
Last Modified:09 Mar 2020 13:19

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