Slate Ordovician

Analysis and Paragenetk History of Anthraxolite in Ordovician Slate, Eastern Pennsylvania... 

The coal-like material has been analyzed and identified as "anthraxolite, a term introduced by Chapman in 1871 (6) for anomalous anthracite-like carbon found in veinlets and cavities in Ordovician slates of Ontario and Quebec. The name has since been applied by others to similar veinlets and masses of anthracite-like material found in anomalous host rocks. 

The display of dates in Fig. 4.9 also demonstrates that the whole-rock K-Ar dates of the Priestley Schist are lower than those of the Priestley Slate and that they overlap the K-Ar and Rb-Sr dates of biotite of the Granite Harbor Intrusives of the Terra-Nova-Bay area. This observation confirms the hypothesis that the rocks of the Priestley Schist were outgassed during the magmatic activity that accompanied the Ross Orogeny. The Granite Harbor Intrusives and the Priestley Schist subsequently cooled at about the same rate until the Middle Ordovician at about 450 Ma. 

Fig. 4.20 The whole-rock K-Ar dates of slates in the Bowers Terrane vary widely from less than 470 Ma to greater than 330 Ma. The numerical values of these dates depend on several factors, including the extent of outgassing of inherited Ar, possible contact metamorphic effects, and the retentivity of slate for radiogenic Ar at low temperature at or below the surface of the Earth. Therefore, the sedimentary and volcanic rocks of the Bowers Group were probably deposited more than about 470 million years ago prior to the start of the Middle Ordovician Epoch (Data from Adams et al. 1982 Adams 2(X)6)...

The whole-rock K-Ar dates of two samples of slate and one of siltstone of the Leap Year Group in Appendix 4.S.4.3 range from 448 9 (Late Ordovician) to 421 3 Ma (Late Silurian) (Adams et al. 1982 Adams 2006). These dates shown Fig. 4.20 are also younger than the probable Cambrian depositional age of the rocks. 

Adams (1986) dated a large suite of samples of fine-grained pelitic rocks (well cleaved slates) from the Swanson and Mackay mountains and from Mt. Passel in the Ford Ranges of MBL by the K-Ar method. These dates in Fig. 15.13a have a distinctly bimodal distribution. The older suite reflects the low-grade metamorphism and deformation of the Swanson Formation during the Late Ordovician. The younger suite records the time when the rocks were heated by the intrusion of Late Devonian granites. Part B of Fig. 15.13 contains the whole-rock K-Ar dates of slates... 

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