Petrology of the welded tuff of Devine Canyon, southeastern Oregon

Abstract

The welded tuff of Devine Canyon, the most widespread ashflow tuff in southeastern Oregon, is centered around the Harney Basin lowland south of Burns; it originally covered about 7,200 square miles and had a volume of over 47 cubic miles. Its age is early Pliocene, about 9 million years. Thickness ranges from a few feet to over 100 feet. Most sections under 35 feet thick are entirely vitric; thicker sections are devitrified but have a vitric base. The distribution of the tuff suggests that the source area lies buried beneath Quaternary sediments in the northwest part of the Harney Basin lowland. Downwarping of this lowland may be related to the expulsion of magma from beneath to form the Devine Canyon and other welded tuff sheets. Phenocrysts are mostly alkali feldspar (Or42Ab5s) and quartz with minor iron-rich pyroxene. Alkali feldspar phenocryst content ranges from 1 to 29 percent and increases markedly upwards in most sections. Analyses of 14 samples :show that the devitrified tuff is uniform in composition. Unlike the welded tuffs of southern Nevada, samples that are rich in phenocrysts are similar in composition to samples that are not; this fact• indicates that phenocrysts crystallized in the magma chamber without significant crystal settling. A water pressure of between 800 and 1,800 bars during phenocryst crystallization is indicated by comparison with the experimental data of Tuttle and Bowen. Resorption of phenocrysts at all levels in the magma indicates reduction in water pressure prior to ash-flow eruption, probably by venting of water vapor plus minor air-fall tuff. Eruption onto a very flat plain produced an exceptionally widespread but relatively thin tuff sheet.

The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

“…The post-collisional magmatism evolution from high-K subalkaline to shoshonitic and eventually to sodic mildly alkaline observed in southernmost Brazil (Nardi and Bonin 1991, Bitencourt and Nardi 1993, 2000, Gastal and Lafon 1998, Wildner et al 2002, Waichel et al 2000, Sommer et al 2005 is referred elsewhere as in Snowdonia and Parys Mountain associations in the British Caledonides (Leat et al 1986), Devine Canyon Tuff -USA (Greene 1973) and Miocene post-collisional volcanism of the Eastern Rif in Morocco (El Bakkali et al 1998). This evolution is characterized by development of shear belts associated to predominantly granitic magmatism and strike-slip basins where a typical and voluminous volcanism and associated sedimentation was developed and preserved, as envisaged by Bonin (2004) for the evolution of the Alpine Belt since the end of Variscan orogeny in Europe.…”

Bimodal volcanism of the High Lava Plains and Northwestern Basin and Range of Oregon: Distribution and tectonic implications of age‐progressive rhyolites

“…Streck and Grunder [1995] calculated the volume of the RST at 280 km 3 DRE. The PCT and DCT (Figure 3) are estimated to be 100-150 and 200-300 km 3 respectively [Green, 1973]. We chose the higher end of Green's estimate for the DCT as its extent and thickness are at least as great as those for the RST.…”

“…[4] Since the original reconnaissance mapping surveys of Russell [1884Russell [ , 1905, there has been persistent interest in the igneous rocks and tectonics of the HLP-NWBR region [e.g., Williams, 1935;Green, 1973;Wells, 1979;Carlson and Hart, 1987;Draper, 1991;Johnson and Ciancanelli, 1984;Johnson and Grunder, 2000;Jordan et al, 2004;Boschmann, 2012]. Much of the work over the past three decades is contained in university student theses with a few specific areas receiving concentrated study (e.g., Glass Buttes).…”

The Geology of the Castle Rock Area, Grant, Harney and Malheur Counties, Oregon

“…The welded tuff of the Drewsey Formation has been radiometrically dated at 9.2 ± 0.6 million years in age (Greene, 1973). The Drewsey Formation has produced Hemphillian mammalian fauna, which indicates that it may be corre- (Lowry, 1968).…”