CONSTRAINTS IN THE THERMAL HISTORY OF THE POTOMAC AND WESTMINSTER TERRANES IN MARYLAND, VIRGINIA, AND WASHINGTON D.C.: EXTRACTION OF USEFUL AGES FROM COMPLEX 40AR/39AR AGE SPECTRA

New ⁴⁰Ar/³⁹Ar data reveal ages and thermal discontinuities that identify mapped and unmapped fault boundaries in the Potomac and Westminster terranes in northern Virginia and southern Maryland. The rocks of the Potomac terrane near Washington D.C. have been previously subdivided into three units: the Mather Gorge, Sykesville, and Laurel Formations. In the Mather Gorge Formation, three metamorphic thermal domains are newly identified. The Blockhouse Point and Bear Island domains are separated by an unmapped Late Devonian fault. ⁴⁰Ar/³⁹Ar cooling ages of hornblende in the Bear Island domain are Late Ordovician (~475 Ma), reflecting cooling from Taconic metamorphism while the Blockhouse Point domain was first metamorphosed in the Devonian. ⁴⁰Ar/³⁹Ar data from muscovites in a third (eastern) domain of the Mather Gorge Formation, the Stubblefield Falls domain, record Devonian thrusting of the Sykesville Formation on the Plummers Island fault over this domain. Hornblende cooling ages in the Sykesville and Laurel Formations are Early Devonian (~400 Ma), reflecting cooling from Taconic intrusives (Aleinikoff et al., 2002) and Acadian metamorphism. The age of retrograde and overprinting muscovite in phyllonites from domain-bounding faults are late Devonian (Acadian) and late Pennsylvanian (Alleghanian) marking the time of assembly of these domains and subsequent movement on the Plummers Island fault. Net vertical motion between the Bear Island domain of the Mather Gorge complex and the Sykesville Formation across the Plummers Island fault is east-side-up. Zircon fission track cooling ages demonstrate thermal equilibrium across the Potomac terrane in the early Permian.

In contrast, the Westminster terrane records early Paleozoic events. It is separated from the Potomac terrane, to the east, by the Pleasant Grove fault. ⁴⁰Ar/³⁹Ar data of muscovite concentrates from an E-W traverse in the Westminster terrane indicate muscovite growth in the Middle Devonian in its eastern part and Silurian muscovite growth in its western part. The lack of a large age discontinuity across the Pleasant Grove fault is surprising. Significant Alleghanian reactivation and retrogression of fabrics appears to be restricted to the Mather Gorge, Sykesville and Laurel Formations of the Potomac terrane.