Upper Cretaceous Volcanic Glass from Western Montana

Geology, age, and tectonic setting of the Cretaceous Sliderock Mountain Volcano, Montana

“…observed during the course of our study. Our 40Ar/39Ar dates are similar to conventional K-Ar dates for the Elkhorn Mountains Volcanics and volcanic rocks in the Maudlow basin north and west of the Crazy Mountains Basin (Robinson and Marvin, 1967;Robinson and others, 1968;Tilling and others, 1968;Hanna, 1973;Marvin and others, 1989). Additional isotopic dating is necessary elsewhere throughout the volcanic field in order to more accurately constrain the ages of Late Cretaceous magmatism along the southern margin of the Crazy Mountains Basin and to relate magmatism to the regional tectono-magmatic evolution of the northern Rocky Mountains.…”

Distribution of thorium, uranium, and potassium in igneous rocks of the Boulder batholith region, Montana, and its bearing on radiogenic heat production and heat flow

“…Although the Elkhorn Mountains Volcanics is cut by even the earliest rocks of the Boulder batholith, geologic evidence and K-Ar age data indicate that the Robinson and Marvin, 1967. ) prebatholith volcanism and the onset of batholit1!…”

“…Analysis for major elements using rapid methods described by Shapiro and Brannock (1962) Average ....... 16................. 16a... .............. 192................ 196................ 140................ 12................. 334................ 339................ Average __ . 3.11 Glassy welded tuff, described by Robinson and Marvin (1967).…”

Journal of Research of the U. S. Geological Survey, 1973, volume 1, issue 2

“…After original paleomagnetic studies on the Elkhorn Mountains Volcanics (Colville, 1961;Hanna, 1965Hanna, , 1967 were completed, two pieces of new information about the volcanic field that offered an opportunity to further the paleomagnetic understanding of the rocks were published. First, radiometric dates obtained for welded tuffs and breccias not previously sampled (Robinson and Marvin, 1967;Robinson and others, 1968) provided a firm time base for previous inferences about paleomagnetic field directions and polarities. Second, an aeromagnetic map covering parts of the Elkhorn Mountains volcanic field (Johnson and others, 1965) showed an anomalous zone of field values over basalts that occur within or near an extensive mining district, suggesting that the basalts have anomalous magnetizations.…”