Reference Number: 9085
GeoREF Number: 2011-012663
Publication Year: 2011
Abstract: "Geochronologic data from the southern margins of the Colorado Plateau (western United States) show an inboard radial migration of Neogene basaltic magmatism. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. Strongly asthenospheric alkali basalt (epsilon (sub Nd) >4) appeared on the western plateau margin ca. 5 Ma, on the southeastern margin at 7 Ma, and is lacking from the plateau's other margins. Tomographic data suggest that low-velocity mantle underlies almost all recent (younger than 1 Ma) basaltic volcanism in a ring around much of the Colorado Plateau at a depth of 80 km. The combined isotopic and tomographic data indicate that the low-velocity mantle is asthenosphere along the western and southeastern margins of the plateau, but modified lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infiltrating and replacing lithospheric mantle, especially where Proterozoic boundaries exist. This model explains (1) the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, (2) the inboard sweep of Neogene magmatism, and (3) isotopic evidence that much (but not all) of the low-velocity mantle is asthenospheric. These data support models that ongoing uplift of the edges of the Colorado Plateau is driven by mantle processes."
Descriptors: "absolute age, alkali basalts, alkaline earth metals, Ar/Ar, Arizona, asthenosphere, basalts, basanite, body waves, Cenozoic, Colorado Plateau, continental lithosphere, cosmogenic elements, dates, elastic waves, erosion, geochemistry, geochronology, Grand Canyon, He-3, helium, igneous rocks, isotope ratios, isotopes, lithosphere, low-velocity zones, magmatism, mantle, metals, Mohave County Arizona, neodymium, Neogene, noble gases, plateaus, rare earths, seismic waves, Sr-87/Sr-86, stable isotopes, strontium, teleseismic signals, Tertiary, tomography, Uinkaret volcanic field, United States, upper mantle, upwelling, velocity structure, volcanic fields, volcanic rocks, volcanism"
Classification: "18 Solid-earth geophysics, 03 Geochronology"
Source: "Geology (Boulder), vol. 39, pp. 27-30"
Contributors:Mahood, Gail A; Ring, Joshua H; Manganelli, Simone; McWilliams, Michael O
Reference Number: 8941
GeoREF Number: 2010-029009
Publication Year: 2010
Abstract: "We undertook a (super 40) Ar/ (super 39) Ar study of young mafic and silicic lavas at Mammoth Mountain and the Long Valley caldera (east-central California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29- and 41 ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafic lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence ( approximately 190-160 ka), the Mammoth sequence ( approximately 120-58 ka), the northwest caldera sequence ( approximately 41-29 ka), and the Inyo chain sequence ( approximately 9 ka-present). In each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162+ or -2 ka and 99+ or -1 ka for bracketing mafic lava flows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999)."
Descriptors: "absolute age, acidic composition, Ar/Ar, basaltic composition, calderas, California, Caso Diablo Till, Cenozoic, clastic sediments, dacites, dates, eruptions, glaciation, igneous rocks, lava, Long Valley Caldera, mafic composition, magma chambers, magmas, Mammoth Mountain, Mono County California, periodicity, Pleistocene, Quaternary, sediments, till, trachytes, United States, upper Quaternary, volcanic features, volcanic rocks, volcanism, volcanoe"
Classification: "24: Quaternary geology, 03: Geochronology"
Source: Geological Society of America Bulletin122.3-4 (March 2010): 396-407
Contributors:Davis, Jesse W.
Reference Number: 9042
GeoREF Number: 2010-900999
Publication Year: 2010
Descriptors: "absolute age, Ar/Ar, Bayo Canyon Basalt, Cenozoic, dates, distribution, eruptions, Guaje Canyon Basalt, igneous rocks, lava flows, lower Miocene, Miocene, Neogene, New Mexico, North America, north-central New Mexico, Oligocene, Pajarito Plateau, Paleogene, periodicity, pyroclastics, Rio Grande Rift, Tertiary, Tschicoma Formation, United States, upper Oligocene, USGS, volcanic rocks, volcanism"
Contributors:Memeti, Valbone; Paterson, Scott; Matzel, Jennifer; Mundil, Roland; Okaya, David
Reference Number: 9084
GeoREF Number: 2010-098069
Publication Year: 2010
Abstract: "Precise chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, super(40)Ar/ super(39)Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10-60 km super(2) bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between 60.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward."
Descriptors: Age distribution Ages Biotite Cathodoluminescence Chambers Compositions Contacts Continuous Contrasts Cores Cross Sections Crystallization Crystals Dikes Direction Forming Fractionation Fractions Grades Grain sizes Granite Gray Scale Growth Horizontal Indexes Intrusion Lakes Lobes Location Magma Magma chambers Mapping Materials Military Personnel Minerals Modeling Models Moving Occurrences Plots Plutons Potential Pulses Recycling Relationships Rock Shades Sheets Sliding Small Solidus Square Texture Thermal Two-Dimensional Vertical Zircon Zones Zoning
Source: "Bulletin of the Geological Society of America. Vol. 122, no. 11-12, pp. 1912-1931. Nov 2010."
Contributors:Jones, James V III; Rogers, Steven A; Connelly, James N
Reference Number: 8645
GeoREF Number: 2010-080681
Publication Year: 2010
Abstract: "We report new U-Pb zircon ages for four previously undated Proterozoic granitoid intrusions exposed in the southern Sawatch Range, central Colorado. Coarse-grained to K-feldspar megacrystic granite (Henry Mountain granite) exposed along Taylor Canyon 20 km north of Gunnison, Colorado, crystallized at 1697 + or - 7 Ma. It cuts across high-temperature deformational fabrics in metavolcanic and metasedimentary country rocks, thus bracketing at least one Paleoproterozoic tectonic event locally. This granite also contains a well-developed northeast-striking, subvertical foliation that postdates emplacement. The three other intrusions all yielded Mesoproterozoic ages. The Monarch Pass pluton, comprising coarse- to medium-grained granodiorite exposed 50 km east of Gunnison, crystallized at 1447 + or - 9 Ma. It cuts across well-developed fabrics in metavolcanic host rocks and contains a widespread biotite foliation. Coarse-grained to K-feldspar megacrystic granite (Horsethief granite) exposed 5-10 km northwest of Taylor Park Reservoir was emplaced at 1437 + or - 5 Ma, and it is locally deformed. Fine-grained, muscovite-biotite granite (Taylor River granite) that cuts across the southwestern part of the Henry Mountain Pluton crystallized at 1428 + or - 23 Ma. A subvertical, northeast-striking biotite foliation cuts across the contact between these two intrusions, suggesting that northwest-directed subhorizontal shortening occurred locally during the Mesoproterozoic. These ages provide new opportunities to constrain the age of tectonism in central Colorado and to further understand the Proterozoic tectonic evolution of southern Laurentia."
Classification: "05A Igneous and metamorphic petrology, 03 Geochronology"
Source: Contributions to Geology 45.1 (2010): 1-22
Contributors:Tappa, Michael J
Reference Number: 8910
GeoREF Number: 2010-075164
Publication Year: 2009
Abstract: "A compelling new model for caldera evolution challenges the standard depiction of large-scale volcanism. Here, I test these competing models by establishing the temporal and chemical relationship between ignimbrite and potential cogenetic plutons related to the Questa caldera, Latir volcanic field, New Mexico."
Descriptors: "absolute age, calderas, Cenozoic, chemical composition, crystal fractionation, eruptions, Holocene, igneous rocks, ignimbrite, intrusions, Latir volcanic field, New Mexico, numerical models, plutons, pyroclastics, Quaternary, Questa Caldera, Taos County New Mexico, trace elements, U/Pb, United States, volcanic features, volcanic rocks, volcanism, volcanoes"
Classification: "24: Quaternary geology, 05A: Igneous and metamorphic petrology"
Contributors:Brueseke, Matthew E.; Hart, William K.; Heizler, Matthew T.