Major element and isotopic studies on the James Run/Port Deposit association, Maryland: tectonic analogues and Taconic deformation

Abstract

The gneissic Port Deposit pluton and associated James Run metavolcanics bear a fault relation with the Wissahickon metasediments of the Glenarm terrane of the northeastern Maryland Piedmont, USA. Bulk chemical and mineralogical data classify the meta-igneous rocks as a bimodal tholeiite-dacite/tonalite suite with trondhjemitic affinities and indicate that subsolidus alteration during dynamic metamorphism was minor. Comparison of these data with other tectonic environments suggests their generation by partial fusion of amphibolitic basement.

Previous studies have dated magmatic events in the Maryland Piedmont at 520 Ma, 420 Ma, and 300 Ma (Sinha et al., 1980). The U-Pb zircon upper intercept age for the Port Deposit is 525 ± 10 Ma, the Rb-Sr whole rock isochron age is 467 ± 21 Ma, and Rb-Sr model biotite ages are 400 and 300 Ma. Regional and theoretical considerations indicate the 525 zircon age may represent the time of igneous consolidation. Detailed petrographic observations in the Port Deposit indicate the Rb-Sr whole rock age represents the time of deformation and development of gneissic textures. The 467 Middle Ordovician age of deformation for the Port Deposit is interpreted as representative of the Taconic orogeny for the central Appalachian Piedmont; Rb-Sr whole rock systematics were upset, but concurrent anatexis of felsic basement is not locally evident. The fault contact between the Port Deposit and easternmost facies of the Wissahickon places an upper age limit of 467 Ma for that particular facies. Rb-Sr model biotite ages are interpreted as metamorphic cooling phenomenon and reflect the 420 and 300 Ma thermal highs.