Petrologic evolution of divergent peralkaline magmas from the Silent Canyon Caldera Complex, Southwestern Nevada Volcanic Field

Abstract

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km3 and 200 km3, respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65–69% SiO2). Low‐silica comendites (69–73% SiO2) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17–1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75–76% SiO2) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0–1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low‐silica comendite; above 73% SiO2, compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.

Timing and effect of detachment-related potassium metasomatism on 40Ar/39Ar ages from the Windous Butte Formation, Grant Range, Nevada

“…Major oxide analyses in table 4 were obtained by X-ray fluorescence techniques in the analytical laboratories of the U.S. Geological Survey (USGS) in Denver, Colo.; analytical methods, accuracy, and precision are described by Taggart and others (1987). Trace-element content (table 4) was determined by energy-dispersive X-ray spectroscopy (Elsass and duB ray, 1982) using 109 Cd and 241 Am sources; accuracy and precision of these analyses are described by Sawyer and Sargent (1989). Major-oxide and trace-element analyses in table 2 were obtained by combined INAA (instrumental neutron activation analysis) and ICP (inductively coupled plasma mass spectrometry) methods.…”

The CHESHIRE Migration Experiment A Summary Report

“…northeast, and the younger Area 20 caldera, to extensive than elsewhere beneath Pahute Mesa, the southwest (Sawyer et al 1994, andSargent 1989). The Timber Mountain caldera complex to the south (Figure 3 (Blankennagel and Weir 1973).…”

“…The entire stratigraphic and lithologic package is offset by younger high-angle normal faults. Recent summaries of the geology and hydrogeology of this volcanic field are contained in Laczniak et al (1996);Sawyer et al (1994);Ferguson et al (1994); Noble et al (1991); Byers et al (1989);and Sawyer and Sargent (1989). Two major buried caIderas of the southwestern Nevada volcanic field, which together form the Silent Canyon caldera complex, are located in the subsurface of the Pahute Mesa underground test area (Figure 3).…”

Geology in the Vicinity of the TYBO and BENHAM Underground Nuclear Tests, Pahute Mesa, Nevada Test Site

“…Chipera, 1988 Bish andChipera, 1989 1.4 Geologic and Hydrogeologic Setting The study area is located within the buried Silent Canyon caldera complex of (Figure 1-1). The caldera complex consists of at least two partially nested calderas, the Grouse Canyon and younger Area 20 calderas (Ferguson et al, 1994;Sawyer and Sargent, 1989). These calderas were formed and then subsequently filled by voluminous eruptions of almost entirely rhyolitic tuff and lava between approximately 13 and 14 million years ago.…”