124 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



front." From this line westward the sedimentary rocks are non-tectonites. 

 The other zones have been described in previous parts of this chapter. 



Age Determinations by Radioactivity 



The first isotope age determinations on the minerals of the crystalline 

 Piedmont were published in 1941 (Goodman and Evans), and since then 

 methods and calculations have been refined, new methods developed, 

 and a fair number of presumably reliable dates have been determined. 



Two groups of ages are now fairly well established, namely, one 

 ranging from 1000 to 1100 m.y. and one ranging from 250 to 390 m.y. 

 References to all significant dates may be found in recent publications by 

 Tilton et al. (1958), Hurley et al. (1958, 1959), and Carr and Kulp 

 ( 1957 ) . An abstract by Kulp et al. ( 1957 ) is significant for ages in the 

 southern Piedmont. 



The older ages (1000-1100 m.y.) come principally from zircon sub- 



U238 U235 Pb207 , Th232 



jected to , , , and analyses. 



Pb206 Pb207 Pb206 Pb208 



Three of the mantled domes in the Baltimore area (zircon from the 

 Baltimore gneiss ) , two gneisses from Bear Mountain, New York, a gneiss 

 from Shenandoah National Park, Virginia, and two gneisses from 

 Hibernia. New York, were sampled and the zircons run. The results range 

 from 1030 to 1170. Rubidium-strontium age measurements were also 

 made on microcline from the three Baltimore gneiss domes and a value 

 is fixed for one at 1200 plus 100 or minus 200 m.y. and for another at 

 about 1040 m.y. It is concluded by Tilton et al. ( 1958 ) that the zircon 

 and microcline ages record a 1000-1100-m.y. crystallization in the Pied- 

 mont. 



Now, from the same specimens of Baltimore gneiss from which the 



iv i_ • j i.. • i. K40 j Rb87 

 zircon and microcline ages were obtained, biotite by and — 



5 y A40 Sr87 



analyses gave ages of 305-339 m.y. For the older and younger dates of 

 the same rock two interpretations can be thought of: 



(1) The gneiss was crystallized or recrystallized 1000-1100 m.y. ago (2) 

 The gneiss was originally a clastic sediment metamorphosed 300-350 m.y. ago, 



and the zircon and microcline were relict detrital grains eroded from a terrain 

 1000-1100 m.y. old. The first interpretation is favored, chiefly because of the 

 non-clastic character of the microcline grains. Their irregular shapes, with deli- 

 cate projections and interlocking contacts with other minerals, were clearly 

 formed during crystallization of the gneiss. Possible detrital origin for the 

 zircon cannot be excluded, although if this were the case a greater age than 

 that of the microcline might be expected. It is concluded that the microcline 

 and the zircon probably record a 1000-1100 m.y. crystallization in the Balti- 

 more gneiss, while biotite records a second crystallization 300-350 m.y. ago. It 

 should be noted that these conclusions allow either a sedimentary or igneous 

 origin for the gneiss (Tilton et al., 1958). 



Kulp et al. (1957) report a granite from eastern Georgia about 250 

 m.y. old. They also give "apparent ages" of 320-370 m.y. for the "meta- 

 morphic series" in western Virginia and North Carolina as well as the 

 pegmatite swarms in the Spruce Pine and Bryson City districts of North 

 Carolina. 



In New England a number of radioactivity age measurements have 

 been made on plutons where the intrusive relations to well-dated fos- 

 siliferous strata are visible, and it is concluded that the Devonian period 

 began approximately 400 m.y. ago and ended slightly less than 250 m.y. 

 ago. These data will be presented in Chapter 11 on New England. It 

 appears, therefore, that the recrystallization and plutonism (tectonism) 

 in the Piedmont province ran its course during the Devonian period. This 

 is younger than the Taconian orogeny of New England and the Maritime 

 provinces which, from angular unconformities, is dated as late Ordo- 

 vician. The Acadian orogeny is generally regarded as having occurred 

 during the upper half of Devonian time, so the dates over 300 m.y. seem 

 too old for it, unless extended by definition. 



SUMMARY OF OROGENIC HISTORY 



The major lines of evidence of orogeny in the Appalachian mountain 

 system come from the sedimentary domains, the structures and structural 

 relations, metamorphism, plutonism, and isotope age determinations. 

 These have all been reviewed, and now may be integrated and the fol- 

 lowing conclusions reached. 



1. An orogeny occurred along the Atlantic margin of the United 

 States south of New York City in which previously existing rocks were 





