AR-40 AR-39 WHOLE-ROCK DATA CONSTRAINTS ON ACADIAN DIAGENESIS AND ALLEGHANIAN CLEAVAGE IN THE MARTINSBURG FORMATION, EASTERN PENNSYLVANIA/

A comparison of Ar-40/Ar-39 age spectra of whole-rock mudstone and sla te samples from the Ordovician Martinsburg Formation at Lehigh Gap, Pe nnsylvania, and stratigraphic and thermal constraints support an Alleg hanian age for regional slaty cleavage and a late Acadian age for diag enesis in these rocks. Age spectra from mudstones have a sigmoidal sha pe, with slopes that climb steeply from apparent Mesozoic ages to inte rmediate saddle regions with Devonian apparent ages, and then climb st eeply again to Late Proterozoic apparent ages. The steps with these ol dest apparent ages are interpreted to be dominated by Late Proterozoic detrital muscovite. The saddle region of the mudstone samples gives v ery Late Silurian to earliest Devonian ages, which are maximum ages of diagenetic micas and which eliminate a Taconic age for the cleavage. The ages of the saddle regions of the slate samples containing cleavag e-forming muscovite is <similar to 375. This is the maximum age of thi s mica and requires an Alleghanian age for the cleavage. These age con straints are supported by ages of individual mica components calculate d with knowledge of the total gas ages and mass fractions of the micas and by predictions from thermal modeling. We conclude that the Taconi c orogeny in the Martinsburg Formation in eastern Pennsylvania was a v ery mild event. Not only is the cleavage in these rocks not Taconic in age, but even the mild (similar to 100C) diagenetic growth of illite was Silurian or younger. Thus the Taconic event in these rocks is limi ted to loading of less than about 3 km.