Igneous and Metamorphic U-Pb ages from the Baltimore Mafic Complex, Maryland Piedmont.

by Sinha A.K., Hanan B.B., and Wayne D.M.

in Sinha A.K., Whalen J.B., and Hogan J.P. eds.,

The Nature of Magmatism in the Appalachian Orogeny,

Geological Society of America Memoir 191, p. 279 - 286.(1997)

Abstract

The Baltimore mafic complex in the central part of the Appalachian orogen has been interpreted to have originated as a sub-arc plutonic complex. New U-Pb ages of zircons separated from four different bulk compositions yield both crystallization and metamorphic ages. A crystallization age of 489 Ma is obtained on rocks with no modal pyroxenes, and a metamorphic age of 453 Ma with an inherited zircon age of ~2 Ga is defined by zircons separated from more mafic lithologies with abundant pyroxenes. Our data suggests that pyroxenes, when involved in the process of amphibolitization, provide sufficient zirconium to form metamorphic zircons. The inherited zircon component cannot be readily identified through chemical mapping of zircons. Whole-rock Pb isotope data with elevated ²⁰⁷Pb / ²⁰⁴Pb isotopic rations support the model of crustal interaction that probably provided the inherited zircons.

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