Twenty-six new age determinations by the fission-track method establish a chronology for volcanism, intrusion, and mineralization in the Thomas Range, Keg Mountain, and Desert Mountain, in western Utah. The fission-track ages confirm D. R. Shawe's three-fold classification of igneous rocks. The oldest group of rocks consists of flows, agglomerates, and some ash-flow tuffs that were deposited 38-39 m.y. ago. The middle group contains widespread rhyolitic ash-flow tuffs that originated from local volcanic centers 30-32 m.y. ago. A dike emplaced along the probable ring-fracture zone of the Keg caldera about 31 m.y. ago indicates that caldera collapse occurred soon after eruption of ash-flow tuffs. Rocks believed to belong-to the middle group also were intruded by the quartz monzonite of Desert Mountain 27-30 m.y. ago. Little or no igneous activity took place within the next 20 m.y., during which time the region was strongly broken by Basin-and-Range faulting. Rhyolites of the youngest group were extruded 8-10 m.y. ago in the Keg Mountain area and 6-7 m.y. ago in the Thomas Range. The beryllium-fluorite mineralization at Spor Mountain occurred after the rhyolitic volcanism in the Thomas Range.

Utah↗

USGS1975

Age provinces in the basement rocks of Liberia

Fourteen new Rb-Sr ages and 17 new K-Ar ages, when combined with published ages, confirm the previous subdivision of the basement rocks of Liberia into 3 age provinces. The Liberian age province was metamorphosed and intruded 2,700 m.y. ago, but the data suggest that many of the rocks of this province are probably considerably older. The Eburnean age province was metamorphosed approximately 2,150 m.y. ago. Rocks of the Pan-African age province were originally part of the Liberian province, but underwent a relatively intense metamorphism 500 m.y. ago. The K-Ar age patterns suggest that the Pan-African province was uplifted relative to the Liberian province sometime after 500 m.y. ago, but fission-track ages of apatites indicate that any such relative vertical displacements must have occurred prior to 120 m.y. ago.

Journal of Research of the U.S. Geological Survey↗

USGS1973

Radiometric ages of Kodiak Seamount and Giacomini Guyot, Gulf of Alaska: Implications for Circum-Pacific tectonics

Kodiak Seamount and Giacomini Guyot have been dated at 22.6 ± 1.1 and 19.9 ± 1.0 [2σ (standard deviation)] × 10 6 years, respectively. Concordant whole-rock and plagioclase potassium-argon dates and fission-track apatite ages demonstrate that significant quantities of excess radiogenic 40 Ar are not present in the dated samples. These seamounts are the northwesternmost edifices of the Pratt-Welker chain, which cuts obliquely across magnetic anomaly patterns in an older northeastern Pacific sea floor. The older of the two dated seamounts is in the Aleutian Trench, apparently about to be subducted. If one assumes that seamounts are generated by plate motion over a fixed hot spot in the mantle, a Pacific-plate motion of 6.6 centimeters per year during early Miocene time may be calculated.

Alaska↗

USGS1971

Age of emplacement of Riley County, Kansas, kimberlites and a possible minimum age for the Dakota Sandstone

Field evidence suggests that the kimberlites of Riley County, Kansas, were emplaced into Lower Permian rocks in post-Dakota Sandstone time. The Dakota Sandstone in Kansas is thought to be earliest Late Cretaceous in age; thus the maximum age of emplacement of the kimberlites is approximately 100 ± 20 m.y. K-Ar dates on chloritized biotite and phlogopites from the kimberlites range from 112 ± 6 m.y. to 380 ± 40 m.y.; the dates earlier than 100 m.y. (6 of 7 samples) are attributed to either the xenocrystalline nature of the chlorites and/or excess Ar and low temperature of intrusion. Fission track dates from two apatites from granite xenoliths are 115 ± 12 m.y. and 123 ± 12 m.y.; these dates reflect cooling at about 120 m.y. ago, which may place a maximum age of emplacement on the kimberlites.

Kansas↗