The name Darrough Felsite was originally assigned to a body of fine-grained quartzofeldspathic igneous rock probably greater than 3 km thick that is exposed continuously over 100 km 2 in the southern Toiyabe Range of central Nevada. The Darrough was supposed to consist mostly of intrusive rocks of probable Permian age. Reexamination of parts of the Darrough Felsite indicates that it is Tertiary and has no genetic affiliation to pre-Tertiary sedimentary and volcanic rocks of the Toiyabe Range. The Darrough constitutes an enormously thick succession of crystal-lithic tuff, tuff breccia, crystal tuff, and volcanogenic sedimentary breccia and sandstone. The tuffaceous rocks are strongly compacted where ash particles are the dominant constituent but are much less compacted where lithic fragments and crystals constitute a rigid framework. Much of the Darrough Felsite may have undergone extensive siliciflcation, probably as a postcompaction event. Contact relations of the Darrough Felsite indicate that it is younger than pre-Tertiary rocks of the southern Toiyabe Range and the Ophir pluton, here dated at 53.9 m.y. (K-Ar, biotite). Three K-Ar ages of minerals from tuff of the Darrough are 26.1, 22.7, and 22.3 m.y., suggesting a middle Tertiary age. A large dike intruding the Darrough. previously assigned a Jurassic age, gives a K-Ar-biotite age of 29.4 m.y. We tentatively interpret the Darrough Felsite to represent an accumulation of ash and breccia flows in a volcanotectonic depression.

Nevada↗

USGS1975

A high 87Sr 86Sr mantle source for low alkali tholeiite, northern Great Basin

Olivine tholeiites, the youngest Tertiary units (about 8–11 m.y. old) at five widely spaced localities in northeastern Nevada, are geologically related to the basalts of the Snake River Plain, Idaho, to the north and are similar in major element and alkali chemistry to mid-ocean ridge basalts (MORB) and island arc tholeiites. The measured K (1250–3350 ppm), Rb (1·9–6·2 ppm) and Sr (140–240 ppm) concentrations overlap the range reported for MORB. Three of the five samples have low, unfractionated rare earth element (REE) patterns, the other two show moderate light-REE enrichment. Barium concentration is high and variable (100–780 ppm) and does not correlate with the other LIL elements. The rocks have 87 Sr / 86 Sr = 0·7052–0·7076, considerably higher than MORB (~0·702–0·703). These samples are chemically distinct (i.e. less alkalic) from the olivine tholeiites from the adjacent Snake River Plain, but their Sr isotopic compositions are similar. They contain Sr that is distinctly more radiogenic than the basalts from the adjacent Great Basin. About 10 b.y. would be required for the mean measured Rb/Sr (~ 0·02) of these samples to generate, in a closed system, the radiogenic Sr they contain. The low alkali content of these basalts makes crustal contamination an unlikely mechanism. If the magma is uncontaminated, the time-averaged Rb/Sr of the source material must have been ~0·04. A significant decrease in Rb/Sr of the source material (a factor 2̆) thus most probably occurred in the relatively recent (1̌0 9 yr) past. Such a decrease of Rb/Sr in the mantle could accompany alkali depletion produced by an episode of partial melting and magma extraction. In contrast, low 87 Sr 86 Sr "> 87Sr86Sr ratios indicate that the source material of the mid-ocean ridge basalts may have been depleted early in the Earth's history.

Geochimica et Cosmochimica Acta↗

USGS1975

The Oligocene volcanic center at Eureka, Nevada

A volcanic center covering an area of about 80 km 2 near Eureka, Nev., and active in the early Oligocene, is characterized by rhyolitic, rhyodacitic. and andesitic pyroclastic rocks, lava flows, and shallow intrusive bodies. These rocks were emplaced as intertonguing and interpenetrative units during a 5-m.y. interval; most of the volcanism was in the last 3 million years of this period (36 to 33 m.y. ago).

Nevada↗

USGS1974

Thin and layered subcontinental crust of the Great Basin western North America inherited from Paleozoic marginal ocean basins?

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton.

western North America↗

USGS1973

Age and correlation of the Windermere Group in northeastern Washington

Greenstone of basaltic composition forms the middle part of the Windermere System in southern British Columbia and the correlative Windermere Group in northeastern Washington. The greenstone, together with the rest of the Windermere in this region, is highly sheared, altered, and metamorphosed, except for a small exposed mass of relatively unaffected rocks near the community of Chewelah, Washington. K-Ar ages on whole rocks and mineral separates from the Chewelah area indicate the greenstone was probably extruded between 827 and 918 m.y. ago.

Washington↗

USGS1973

Preliminary geologic map of the Austin Quadrangle, Lander County, Nevada

Explore the source record for details and available documents.

Miscellaneous Field Studies Map↗

USGS1972

Paleomagnetic correlations and Potassium-Argon dating of Middle Tertiary ash-flow sheets in the eastern Great Basin, Nevada and Utah

Directions of natural remanent magnetization are used to identify and correlate individual cooling units in the middle Tertiary ash-flow province in central and eastern Nevada and western Utah. Potassium-argon dating indicates that the minimum time between eruptions of individual but genetically related ash-flow cooling units is on the order of 0.8 m.y. As this interval is long in comparison with the secular variation of the direction of the geomagnetic field, in a given volcanic province the direction of natural thermoremanent magnetization is a unique characteristic of each cooling unit. Ash-flow sheets investigated include the Stone Cabin Formation, the tuff of Pancake Summit, the Windous Butte Formation, the Needles Range Formation, the Bates Mountain Tuff, and the tuff of Clipper Gap, of Oliogcene to early Miocene age. The original areas of individual ash-flow cooling units are as great as 8,000 km 2 ; the volumes, 1,300 km 3 . The paleomagnetic correlations, made over distances up to 200 km, confirm most of the previously made lithologic correlations and allow more accurate delineation of single cooling units. These correlations are particularly useful in those parts of the Basin and Range province where the Tertiary stratigraphic section consists mainly of ash-flow sheets, and the outcrops are confined to mountain ranges that are separated by alluvium-filled valleys.

Nevada, Utah↗