Correlation of Ordovician and Silurian Formations of Eastern Montana

Correlation charts made from electric log data show the relationship of Ordovician and Silurian strata in eastern Montana. The Silurian sediments terminate westward in eastern Montana, but the Ordovician sediments are present westward into central Montana

Potassium-argon dating : principles, techniques, and applications to geochronology /

Bibliography: p. [227]-240

Distribution of Ordovician and Devonian metamorphism in mafic and pelitic schists from northern Vermont

^(40)Ar/^(39) Ar range from 471 to 439 my and confirm that Ordovician metamorphism is recorded from the Canadian border south to the Precambrian core of the Green Mountain Anticlinorium (GMA). Medium-high pressure (P) facies series metamorphism in the north and medium-P metamorphism to the south. Similar metamorphic ages and compatible estimates of T and P indicate mafic and pelitic rocks were metamorphosed together. Medium-P Devonian metamorphism has overprinted the Ordovician metamorphism in northern Vermont east and probably west of the GMA axis, resulting in conventional K/Ar, ^(40)Ar/^(39)Ar total fusion and plateau ages of 386 to 355 my on muscovite, biotite, and amphibole. --Modified journal abstract

Conventional and 40Ar/39Ar K-Ar ages of volcanic rocks at DSDP Leg 55 Holes

Conventional K-Ar, 40Ar/39Ar total fusion, and 40Ar/39Ar incremental heating data on hawaiite and tholeiitic basalt samples from Ojin (Site 430), alkalic basalt samples from Nintoku (Site 432), and alkalic and tholeiitic basalt samples from Suiko (Site 433) seamounts in the Emperor Seamount chain give the following best ages for these volcanoes: Ojin = 55.2 ± 0.7 m.y., Nintoku = 56.2 ± 0.6 m.y., and Suiko = 64.7 ± 1.1 m.y. These new data bring to 27 the number of dated volcanoes in the Hawaiian-Emperor volcanic chain. The new dates prove that the age progression from Kilauea Volcano on Hawaii (0 m.y.) through the Hawaiian-Emperor bend (- 43 m.y.) to Koko Seamount (48.1 m.y.) in the southernmost Emperor Seamounts continues more than halfway up the Emperor chain to Suiko Seamount. The age versus distance data for the Hawaiian-Emperor chain are consistent with the kinematic hot-spot hypothesis, which predicts that the volcanoes are progressively older west and north away from the active volcanoes of Kilauea and Mauna Loa. The data are consistent with an average volcanic propagation velocity of either 8 cm/year from Suiko to Kilauea or of 6 cm/year from Suiko to Midway followed by a velocity of 9 cm/year from Midway to Kilauea, but it appears that the change in direction that formed the Hawaiian- Emperor bend probably was not accompanied by a major change in velocity

Geochemistry and ages at DSDP Hole 19-192A

Conventional K-Ar and 40Ar/39Ar age data on altered basalts from DSDP Hole 192A on Meiji Guyot, Emperor Seamount chain, indicate a minimum age for the volcano of 61.9 ± 5.0 m.y. The K-Ar data are consistent with the early Maestrichtian date of the overlying sediments, but do not provide either a positive or negative test of the hypothesis that Meiji is older than Emperor volcanoes to the south. The most prominent alteration affecting the age measurements is potassium metasomatism, particularly of feldspar phenocrysts. The K-Ar apparent ages of feldspar separates from the Meiji basalts show that more than half of the potassium metasomatism occurred within the last 25 m.y. or so, and that if the potassium replacement rate has been constant, then the alteration of the Meiji basalts did not begin for 10 to 20 m.y. after the volcano formed