New U-Pb dating of zircons separated from felsic volcanic rocks of the Patuxent and Gambacorta formations from the Pensacola Mountains in the Transantarctic Mountains, Antarctica, yields earliest Ordovician ages of 500 ± 8 and 501 ± 3 Ma respectively. The dated felsic volcanic rock of the Gorecki Felsite Member of the Patuxent Formation is important tectonically, as the felsite, together with mafic volcanic rocks, were previously considered to provide key evidence of a Neoproterozoic rifting event prior to separation of Laurentia from East Antarctica. The new data do not necessarily refute this event but indicate a previously unrecognized early Ordovician period of bimodal magmatism and extension along the Transantaractic Mountains.

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