David E. Broxton

Episodic caldera volcanism in the Miocene southwestern Nevada volcanic field: Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magmatism and extension

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion 40 Ar/ 39 Ar isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of ∼0.3% (1σ), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km 3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intra-caldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paint-brush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climactic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space

Distribution and chemistry of diagenetic minerals at Yucca Mountains, Nye County, Nevada

Yucca Mountain is being studied as a potential site in southern Nevada for an underground, high-level nuclear waste repository. A major consideration for selecting this site is the presence of abundant zeolites in Miocene ash-flow tuffs underlying the region. Beneath Yucca Mountain four diagenetic mineral zones have been recognized that become progressively less hydrous with depth.Zone I, the shallowest zone, is 375–584 m thick in the central part of Yucca Mountain, but 170 m thick to the north. Zone I contains vitric tuffs that consist of unaltered volcanic glass and minor smectite, opal, heulandite, and Ca-rich clinoptilolite. Zone II thins south to north from 700 to 480 m and is characterized by complete replacement of volcanic glass by clinoptilolite with and without mordenite, and by lesser amounts of opal, K-feldspar, quartz, and smectite. Zone III thins south to north from 400 to 98 m thick and consists of analcime, K-feldspar, quartz, and minor calcite and smectite. Heulandite occurs locally at the top of zone III in the eastern part of Yucca Mountain. Zone IV occurs in the deepest structural levels of the volcanic pile and is characterized by albite, K-feldspar, quartz, and minor calcite and smectite.Clinoptilolite and heulandites in zone I have uniform Ca-rich compositions (60–90 mole % Ca) and Si:Al ratios that are mainly between 4.0 and 4.6. In contrast, clinoptilolites deeper in the volcanic sequence have highly variable compositions that vary vertically and laterally. Deeper clinoptilolites in the eastern part of Yucca Mountain are calcic-potassic and tend to become more calcium-rich with depth. Clinoptilolites at equivalent stratigraphic levels on the western side of Yucca Mountain have sodic-potassic compositions and tend to become more sodium-rich with depth. Despite their differences in exchangeable cation compositions these two deeper compositional suites have similar Si:Al ratios, generally between 4.4 and 5.0. Analcimes have nearly pure end-member compositions, typical of these minerals formed by diagenetic alteration of siliceous volcanic glass; however, K-feldspars are Si-rich compared to the ideal feldspar formula.Bulk-rock contents of Si, Na, K, Ca, and Mg of zeolitic tuffs generally differ significantly from stratigraphically equivalent vitric tuffs, suggesting that zeolite diagenesis took place in an open chemical system. Both the whole rock and the clinoptilolite are relatively rich in Ca and Mg in the eastern part of Yucca Mountain and rich in Na in the western part. The Ca- and Mg-rich compositions of the zeolitized tuffs in the eastern part of Yucca Mountain may be due to cation exchange by the sorptive minerals with ground water partially derived from underlying Paleozoic carbonate aquifers.Diagenetic zones become thinner and occur at stratigraphically higher levels from south to north across Yucca Mountain, probably due to a higher geothermal gradient in the northern part of the area. The diagenetic zones were established when the geothermal gradient was greater than it is today, probably during the thermal event associated with the development of the Timber Mountain-Oasis Valley caldera complex north of Yucca Mountain.

Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera, Complex, SW Nevada: implications for the origin and evolution of large-volume silicic magma bodies

Nd, Sr and O isotopic data were obtained from silicic ash-flow tuffs and lavas at the Tertiary age (16–9 Ma) Timber (Mountain/Oasis Valley volcanic center (TMOV) in southern Nevada, to assess models for the origin and evolution of the large-volume silicic magma bodies generated in this region. The large-volume (>900 km3), chemically-zoned, Topopah Spring (TS) and Tiva Canyon (TC) members of the Paintbrush Tuff, and the Rainier Mesa (RM) and Ammonia Tanks (AT) members of the younger Timber Mountain Tuff all have internal Nd and Sr isotopic zonations. In each tuff, high-silica rhyolites have lower initialɛNd values (∼1ɛNd unit), higher87Sr/86Sr, and lower Nd and Sr contents, than cocrupted trachytes. The TS, TC, and RM members have similarɛNd values for high-silica rhyolites (-11.7 to -11.2) and trachytes (-10.5 to -10.7), but the younger AT member has a higherɛNd for both compositional types (-10.3 and -9.4). Oxygen isotope data confirm that the TC and AT members were derived from lowɛNd magmas. The internal Sr and Nd isotopic variations in each tuff are interpreted to be the result of the incorporation of 20–40% (by mass) wall-rock into magmas that were injected into the upper crust. The lowɛNd magmas most likely formed via the incorporation of lowδ18O, hydrothermally-altered, wall-rock. Small-volume rhyolite lavas and ash-flow tuffs have similar isotopic characteristics to the large-volume ash-flow tuffs, but lavas erupted from extracaldera vents may have interacted with higherδ18O crustal rocks peripheral to the main magma chamber(s). Andesitic lavas from the 13–14 Ma Wahmonie/Salyer volcanic center southeast of the TMOV have lowɛNd (-13.2 to -13.8) and are considered on the basis of textural evidence to be mixtures of basaltic composition magmas and large proportions (70–80%) of anatectic crustal melts. A similar process may have occurred early in the magmatic history of the TMOV. The large-volume rhyolites may represent a mature stage of magmatism after repeated injection of basaltic magmas, crustal melting, and volcanism cleared sufficient space in the upper crust for large magma bodies to accumulate and differentiate. The TMOV rhyolites and 0–10 Ma old basalts that erupted in southern Nevada all have similar Nd and Sr isotopic compositions, which suggests that silicic and mafic magmatism at the TMOV were genetically related. The distinctive isotopic compositions of the AT member may reflect temporal changes in the isotopic compositions of basaltic magmas entering the upper crust, possibly as a result of increasing “basification” of a lower crustal magma source by repeated injection of mantle-derived mafic magmas.

Redistribution of Pb and other volatile trace metals during eruption, devitrification, and vapor-phase crystallization of the Bandelier Tuff, New Mexico

A diverse suite of micron-scale minerals was deposited from vapor during eruption and post-emplacement crystallization of the Bandelier Tuff, New Mexico. The mineral suite is rich in sulfides, oxides, and chlorides of both common and rare metals (e.g., Fe, Pb, Bi, Cu, Ag, Re), and oxides and silicates of incompatible elements (e.g., P, Zr, Y, Nb, Ba and LREE). Minerals preserved in glassy samples grew from magmatic vapor trapped during emplacement, or from vapor migrating along contacts with more impermeable rocks; minerals observed in devitrified samples also grew from crystallization of glass and vapor liberated during this process. In devitrified samples, mafic silicate phenocrysts were partially replaced by an assemblage dominated by smectite and hematite. The syn- to post-eruptive mineral assemblage observed in upper Bandelier Tuff (UBT) samples bears striking similarity to those deposited by cooling gases near active volcanic vents. However, several differences exist: (1) the mineral suite in the UBT is disseminated throughout the unit, and formed over a broad temperature range (> 700 to < 150 °C) at higher rock:gas ratios; (2) the highly evolved composition of the UBT yielded a greater abundance of minerals rich in incompatible elements compared to sublimates from less evolved volcanoes; and (3) the UBT has suffered over 1 million years of post-emplacement exposure, which resulted in solution (or local re-precipitation in fractures) of soluble compounds such as halite, sylvite, and gypsum. Pb was enriched toward the roof of the UBT magma body due to its affinity for the melt and vapor phases relative to crystals (Bulk Dpb < 0.2). Micron-scale Pb minerals appear to have grown from vapor exsolved during eruption, as well as vapor liberated during later devitrification. Additional Pb was scavenged by smectite and hematite that probably formed during the later stages of the devitrification and cooling process. Up to ten-fold increases in Pb concentrations are seen in zones of fumarolic concentration in the UBT, however, most bulk tuff samples have Pb values that appear to preserve magmatic values, indicating only very local trace-metal redistribution. The concentration of Pb and other heavy metals in micron-scale mineral coatings in porous tuff indicates that these metals could be readily mobilized and transported by acidic groundwaters or hydrothermal fluids, and thus locally concentrated into ore-grade deposits in long-lived systems.

Mineralogy and temporal relations of coexisting authigenic minerals in altered silicic tuffs and their utility as potential low-temperature dateable minerals

Abstract Coexisting fine-grained (0.1–20 μm) authigenic silicate minerals separated from altered tuffs in Miocene and Plio-Pleistocene lacustrine deposits were characterized petrographically and using X-ray powder diffraction. The authigenic minerals are dominated by clinoptilolite, erionite, phillipsite, K-feldspar, silica, calcite, smectite, and randomly interstratified illite/smectite. Minor accessories of opal-CT, cristobalite, and barite are present with the major alteration minerals. Authigenic minerals from altered tuffs were dated using the K Ar method to evaluate the utility of these minerals for determining the time of alteration in low-temperature diagenetic environments. The eruption ages of some of these zeolite-rich tuffs were determined using the 40 Ar 39 Ar method on single sanidine and plagioclase minerals. The K Ar isotopic ages of the fine-grained K-feldspar show minimal variation compared with results from the clinoptilolite separates. The isotopic ages from the authigenic K-feldspar (15-13.8 Ma) and some of the zeolites (16.-6.7 Ma) are similar to the eruption ages of the tuffs and indicate early alteration. Despite their open-framework structure, zeolites apparently can retain part or all of their radiogenic argon under favorable conditions (e.g., saturated environment). How much of the radiogenic argon is retained is estimated from the isotopic ages of other coexisting secondary minerals that are commonly dated by the K Ar method. Although zeolite isotopic ages should be interpreted with caution, they may be useful to constrain temporal relations of low-temperature diagenetic processes when used in conjunction with other dateable minerals.

High-potassium intrusive rocks of the Crandall ring-dike complex, Absaroka Mountains, Wyoming

The Crandall ring-dike complex has a gabbro-diorite core intruded by a shoshonitic ring dike. Both have been cut by granular and porphyritic quartz monzonite dikes. The most abundant rock is medium-grained diorite. In contrast, the ring-dike shoshonite has large phenocrysts of plagioclase and augite in a microcrystalline ground-mass of biotite, sanidine, plagioclase, amphibole, magnetite-ilmenite, and variable amounts of glass. Texturally, the plagioclase and pyroxene appear out of equilibrium with the surrounding matrix in both the ring dike and the intrusive core rocks. The gabbro, diorite, and porphyritic quartz monzonite are characterized by plagioclases with bimodal core and rim compositions, indicating that one group is possibly xenocrystic. Trends on variation diagrams for both minor and trace elements cannot be traced from the diorite to the quartz monzonite. On variation diagrams, the shoshonite rocks have the widest scatter of data points, which can be explained by the addition of plagioclase and augite phenocrysts. Geochemical modeling is successful in demonstrating that the gabbro can be formed as a cumulate from the diorite by fractional crystallisation but is unsuccessful in relating the diorite and the quartz monzonite by this process. The strongly positive Eu anomaly in the gabbro supports the cumulate origin; the diorite and shoshonite have chemical and textural signatures which strongly indicate contamination by plagioclase and pyroxene accumulation. Even the porphyritic quartz monzonite has experienced plagioclase accumulation. The nature of the uncontaminated magma is uncertain. Chemically, the diorite is a plutonic equivalent of shoshonite, and support is given to the hypothesis that shoshonites are formed by contamination with plagioclase and pyroxene (Prostka, 1973).

Structure of the Tshirege Member of the Bandelier Tuff at Mesita del Buey, Technical Area 54, Los Alamos National Laboratory

The geological structure of the Tshirege Member of the Bandelier Tuff at Mesita del Buey, Technical Area 54, was examined using precise surveying of the contact between tuff units Iv and 2 for 3.5 km along the north wall of Pajarito Canyon and 0.6 km along the north wall of a tributary to Caiiada del Buey. Estimated structure contours on this contact indicate typical strikes of N40E to N70E along this part of Mesita del Buey, although the apparent stfike of the tuff is E-W at the western part of the survey. Typical dips are 1.OO to 2.0o to the east or southeast, with an estimated maximum dip of 3.2o near the west end of Material Disposal AreaG. Thirty seven faults with vertical displacements of 5 to 65 cm were observedin outcrops along the Pajarito Canyon traverse, and, due to the incomplete exposure of the contact between unit lV and unit 2, many more faults of this magnitude undoubtedly exist. The faults have a wide range in strike and have either down-to-the-west or down-to-the-east components of offset, although about 65% of the observed displacement is down-to-the-west or northwest. The general absence of larger-scale offsets or inflections along the contact between units lV and 2 in areas where the small-scale faults were observed suggests that they are not associated with major fault zones. Instead, these faults may record distributed secondary deformation across the Pajarito Plateau associated with large earthquakes on the main Pajarito fault zone 8 to 11 km to the west, or perhaps earthquakes on other faults in the region. The survey data also suggest that a 150 to 250 m wide zone of greater magnitude faulting is present near the west end of the traverse associated with a horst-and-graben structure displaying about 1.5 to 3.5 m of offset on individual faults, although the total amount of offset across this structure and its orientation are not known.