Abstract
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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