LATE PERMIAN AGE OF NH4-BEARING ILLITE IN ANTHRACITE FROM EASTERN PENNSYLVANIA - TEMPORAL LIMITS ON COALIFICATION IN THE CENTRAL APPALACHIANS

Authigenic NH4-bearing illites in coal fractures from the Anthracite r egion of eastern Pennsylvania yield individual K/Ar dates of 236-253 M a (+/-8 my.) and a composite isochron of 253 Ma (+/-8 m.y.). Rb/Sr dat a for the same samples indicate an isochron of 260 Ma (+/-44 m.y.), co nsistent with the more precise K/Ar data. The age of NH4-bearing illit e authigenesis should reflect the time at which coal attained anthraci te rank, because mineralogical and geochemical evidence indicates that NH4-bearing illite formed exclusively during anthracite formation (T = 200-275-degrees-C) from organically derived nitrogen and a kaolinite precursor. Similar Late Permian K/Ar dates for both NH4-rich illite a nd K-illite in anthracite fractures and lack of correlation between pe rcent NH4 substitution in illite and the K/Ar date suggest that NH4 do es not influence significantly the closure temperature of illite and, thus, that NH4-rich illite can be used for dating the time of authigen esis. Given the uncertainties in closure temperatures for illite as we ll as the maximum temperatures for anthracite formation, we consider t he oldest K/Ar date of 253 Ma (+/-8 m.y.) to be a minimum age for NH4- illite authigenesis and anthracite formation. Based on the retention o f a detrital age signature in the <0.5-mum size fraction of shale samp les, as well as theoretical and empirical estimates of illite closure temperatures, we estimate the oldest K/Ar date of 253 Ma to be only sl ightly younger (no more than about 10 m.y.) than the time at which coa l-bearing rocks experienced peak temperatures associated with anthraci te formation, for which we suggest an age range of 265-255 Ma. Re-exam ination of syn- and post-folding secondary chemical remanent magnetiza tions in rocks from the central Appalachians indicates a late Early to Late Permian age (260-250 Ma) for Alleghanian deformation in the vici nity of the Anthracite region. The age of folding is similar to the 26 5-255 Ma age range for anthracite formation interpreted from illite da tes and is consistent with previous studies of vitrinite reflectance f abrics in coal, which indicate that anthracite formation occurred prio r to and during folding. Thus, age data in conjunction with paleomagne tic and vitrinite reflectance data allow a time period of 25-35 m.y. f or anthracite formation and Alleghanian deformation, beginning after d eposition of the coal in Late Pennsylvanian or Early Permian time (ca. 290-280 Ma) and concluding by the Late Permian time (ca. 260-250 Ma), significantly longer than previous estimates of less than 15 m.y. for this thermal/tectonic event.