CONSTRAINTS ON DEPOSITIONAL AGES OF MARYLAND PIEDMONT ROCKS FROM DETRITAL ZIRCON U/PB DATING

The depositional ages of most Piedmont rocks in the central Appalachians are poorly known principally due to (1) a lack of fossils and (2) a dearth of igneous rocks datable by radiometric means. Dates of intrusive rocks limit the minimum possible depositional age of most eastern Piedmont rocks, but the maximum depositional age of these rocks is poorly constrained. Detrital zircon age signatures of Piedmont rocks, as well as good constraints on depositional ages, are important for stratigraphic correlations across the Appalachians and for better understanding the Neoproterozoic to early Paleozoic tectonic evolution of the central Appalachian region. These signatures are particularly important for correlation with terranes to the east that were buried by younger coastal plain sedimentary rocks because typically only small pieces of the buried rocks are available for sampling, and U/Pb ages of zircons from these fragments provide critical clues about their tectonic affinity. In order to determine the detrital zircon age signatures and to place constraints on the maximum depositional ages of Piedmont meta-clastic rocks in Maryland, we obtained approximately 800 new U/Pb ages of detrital zircons from 7 samples. All samples are dominated by ca. 1000-1600 Ma zircons. Samples from the Blockhouse Point Domain of the Mather Gorge Formation and formations to the west contain only ca. 1000-1600 Ma zircons. In contrast, samples from the Bear Island Domain of the Mather Gorge Formation and units to the east additionally contain a few Paleoproterozoic and Archean zircons as well as a few ca. 550 Ma grains. These data suggest that (1) the Mather Gorge Formation as presently defined contains rocks that received detritus from different sources and (2) a major stratigraphic and/or structural boundary lies between the Blockhouse Point and Bear Island Domains of the Mather Gorge Formation. This boundary also separates meta-clastic rocks to the east that contain intrusions from metasedimentary rocks to the west without intrusions as well as domains that cooled at different times in the Paleozoic. Thus this boundary may be more significant tectonically than recognized heretofore.