Constraints on the thermal history of the Blue Ridge in northernmost Virginia: [sup 40]Ar/[sup 39]Ar age dating results

[sup 40]Ar/[sup 39]Ar age dating techniques have been used to analyze hornblende, phlogopite, white mica, biotite, and K-feldspar mineral separates from Middle and Late Proterozoic crystalline basement and from Late Proterozoic-Early Cambrian cover rocks in the Blue Ridge anticlinorium. Regionally, both cover and basement rocks contain a variably developed overprint fabric, the Blue Ridge-South Mountain cleavage, commonly defined by white mica. Petrographic analysis indicates a maximum metamorphic overprint of lower greenschist facies that locally produces biotite. [sup 40]Ar/[sup 39]Ar age spectra of hornblende from crystalline basement units suggest ages ranging from about 1 Ga to about 920 Ma. [sup 40]Ar/[sup 39]Ar age spectra of most cleavage-defining white mica are sigmoidal in shape suggesting the presence of multiple generations of white mica, the last of which probably grew below the blocking temperature for retention of argon in muscovite ([approximately]350 C). Interpretation of these complex spectra suggest that most of the cleavage-defining white mica may have formed below [approximately]350 C during the Late Paleozoic Alleghanian orogeny. However, these complex age spectra, together with age spectra from cleavage-defining phlogopite from a marble in the Fauquier Formation and biotite in the Late Proterozoic Robertson River Igneous Suite suggest the existence of an earlier episodemore » of white mica growth of Middle Paleozoic age that has not been thermally reset or totally recrystallized. High temperature release steps can be interpreted to represent times of cooling through closure during the Middle Paleozoic. Low-temperature steps of these same age spectra suggest final closure or partial resetting of the samples at about 200 Ma or less and are interpreted to represent the effects of Mesozoic rifting.« less