Age Determinations Rb-Sr Method

The rocks of the Patuxent/Hannah Ridge formations have been difficult to date by the whole-rock Rb-Sr 

Nevertheless, Eastin (1970) attempted to measure the ages of the siliciclastic sedimentary basement rocks and of the igneous rocks that are associated with it by the whole-rock Rb-Sr method using rock samples provided by Dwight L. Schmidt of the US Geological Survey. Eastin s data and brief descriptions of the samples are compiled in Appendices 8.5.5 and 8.5.6 and his interpretations are illustrated by diagrams included in the text. All of the dates calculated by Eastin (1970) have been recalculated to the modern value of the decay constant of Rb (k = 1.42 x 10 year ) and therefore supersede dates that were quoted previously by Faure et al. (1968, 1977). 

Eastin (1970) analyzed 11 samples of laminated siltstone, slate, and subgraywacke of the Patuxant Formation that were collected by D.L. Schmidt from outcrops in the Patuxent Range that had not been alteredbycontactmetamorphism. Nevertheless, the data points representing these samples scatter far beyond analytical error above and below line A in Fig. 8.9 that was fitted by Eastin (1970) to all of the samples by the method of York (1966, 1967). Line B is defined only by samples 437 and 444, whereas line C is constrained by samples 439, 441, 436, 440, and 446. The dates and initial Sr/ Sr ratios that Eastin (1970) derived from these straight lines are  

Eastin (1970) concluded that these Rb-Sr dates were reset by the last metamorphic event that affected the rocks of the Patuxent Range and that none of the dates represent the time of deposition these sedimentary rocks. 

Another way to determine the age of the Patuxent Formation is to date the felsic lava flows that are interbedded with the sedimentary rocks of the Patuxent Formation in the Schmidt and Williams hills. However, the Rb-Sr data for eight samples of felsic igneous rocks 

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