Isotopic Constraints on the Origin of the Baltimore Mafic Complex
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AbstractIsotopic analyses of modern ocean basalts and ophiolites have shown that both modern and ancient ocean crust have a characteristic Nd and Sr isotope signature indicative of derivation from a depleted mantle reservoir. It also appears that the Nd 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 isotope compositions of -6.4 < εNd(T) < -2.2, +51 < εSr(T) < +115. εNd(T) and εSr(T) are anti-correlated. This is not the signature of depleted mantle and oceanic crust but is similar to old continental crust. We propose that the BMS 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 < εNd(T) < +4.2, +2.6 < εSr(T) < +114. These data do not in any way reflect the signature of normal oceanic crust. These results are in contrast with geological 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 boninites. 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 < εNd(T) < +8 and may be fragments of oceanic crust. The Webster-Addie body, North Carolina, has εNd(T) ~ -1, and εSr(T) ~ +30 and is not isotopically similar to oceanic crust or other North Carolina mafic bodies analyzed. From these isotopic results we infer that Appalachian mafic rocks have diverse origins: some are continental intrusions (BMC), others are probably fragments of oceanic crust (Vermont and North Carolina amphibolites). Future models for the development of the Appalachians must allow for these various origins. The possibility that some ophiolites are not normal ocean crust but have an origin in a partially continental setting or as anomalous ocean crust may require further attention. AccessDownload - large file (2.89 Mb) |