Geology topics

J. Wooden

Publications and source records attributed to J. Wooden.

10 recordsLinked to original sources

USGS2009

U-Pb zircon ages from the southwestern Karoo Basin, South Africa - Implications for the Permian-Triassic boundary

U-Pb ages determined using sensitive high-resolution ion microprobe-reverse geometry on 205 single-grain zircons from 16 ash beds within submarine fan deposits of the Ecca Group provide the first evidence of a marine Permian-Triassic (P-T) boundary in the Karoo Basin of South Africa. These U-Pb ages provide an objective basis for correlating the deep-marine sediments of the southwest Karoo Basin with fluvial-deltaic deposits in the central and eastern parts of the basin where the P-T boundary is recorded in a diverse macrofauna. Furthermore, these new zircon ages and their correlation imply asymmetric subsidence and variable sedimentation rates across the basin. ?? 2009 Geological Society of America.

Geology↗

USGS2008

Creation of a continent recorded in zircon zoning

We have discovered a robust microcrystalline record of the early genesis of North American lithosphere preserved in the U-Pb age and oxygen isotope zoning of zircons from a lower crustal paragneiss in the Neoarchean Superior province. Detrital igneous zircon cores with ??18O values of 5.1???-7.1??? record creation of primitive to increasingly evolved crust from 2.85 ?? 0.02 Ga to 2.67 ?? 0.02 Ga. Sharp chemical unconformity between cores and higher ??18O (8.4???-10.4???) metamorphic overgrowths as old as 2.66 ?? 0.01 Ga dictates a rapid sequence of arc unroofing, burial of detrital zircons in hydrosphere-altered sediment, and transport to lower crust late in upper plate assembly. The period to 2.58 ?? 0.01 Ga included ???80 m.y. of high-temperature (???700-650 ??C), nearly continuous overgrowth events reflecting stages in maturation of the subjacent mantle root. Huronian continental rifting is recorded by the youngest zircon tip growth at 2512 ?? 8 Ma (??? 600 ??C) signaling magma intraplating and the onset of rigid plate behavior. This >150 m.y. microscopic isotope record in single crystals demonstrates the sluggish volume diffusion of U, Pb, and O in zircon throughout protracted regional metamorphism, and the consequent advances now possible in reconstructing planetary dynamics with zircon zoning. ?? 2008 The Geological Society of America.

Geology↗

USGS2007

How was the Triassic Songpan-Ganzi basin filled? A provenance study

The Triassic Songpan-Ganzi complex comprises >200,000 km2 of 5-15 km thick turbiditic sediments. Although surrounded by several magmatic and orogenic belts, the Triassic high- and ultrahigh-pressure Qinling-Tongbai-Hong'an-Dabie (QTHD) orogen, located several hundred kilometers to the east, was proposed as its major source. Middle to Late Triassic samples from the northern and southern Songpan-Ganzi complex, studied using detrital white mica 40Ar/39Ar ages, Si-in-white mica content, and detrital zircon U/Pb ages, suggest that the northern Songpan-Ganzi deposystem obtained detritus from the north: the north China block, east Kunlun, northern Qaidam, Qilian, and western Qinling; the southern Songpan-Ganzi deposystem was supplied from the northeasterly located Paleozoic QTHD area throughout the Ladinian and received detritus from the Triassic Hong'an-Dabie orogen during the Carnian, indicative of exhumation of the orogen at that time. The QTHD orogen fed the Norian samples in the southeastern southern Songpan-Ganzi deposystem, signifying long drainage channels along the western margin of the south China block. An additional supply from the Emeishan magmatic province and/or the Yidun arc is suggested by the paucity of white mica in the southern Songpan-Ganzi deposystem. Mica ages of Rhaetian sediments from the northwestern Sichuan basin best correlate with those of the Triassic QTHD orogen. Our Si-in-white mica data demonstrate that the high- and ultrahigh-pressure rocks of the Hong'an-Dabie Shan were not exposed in the Middle to Late Triassic.

Tectonics↗

USGS2005

U/Th dating by SHRIMP RG ion-microprobe mass spectrometry using single ion-exchange beads

We present a new analytical method for U-series isotopes using the SHRIMP RG (Sensitive High mass Resolution Ion MicroProbe) mass spectrometer that utilizes the preconcentration of the U-series isotopes from a sample onto a single ion-exchange bead. Ion-microprobe mass spectrometry is capable of producing Th ionization efficiencies in excess of 2%. Analytical precision is typically better than alpha spectroscopy, but not as good as thermal ionization mass spectroscopy (TIMS) and inductively coupled plasma multicollector mass spectrometry (ICP-MS). Like TIMS and ICP-MS the method allows analysis of small samples sizes, but also adds the advantage of rapidity of analysis. A major advantage of ion-microprobe analysis is that U and Th isotopes are analyzed in the same bead, simplifying the process of chemical separation. Analytical time on the instrument is ???60 min per sample, and a single instrument-loading can accommodate 15-20 samples to be analyzed in a 24-h day. An additional advantage is that the method allows multiple reanalyses of the same bead and that samples can be archived for reanalysis at a later time. Because the ion beam excavates a pit only a few ??m deep, the mount can later be repolished and reanalyzed numerous times. The method described of preconcentrating a low concentration sample onto a small conductive substrate to allow ion-microprobe mass spectrometry is potentially applicable to many other systems. Copyright ?? 2005 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS2005

Oxygen isotopic composition and U-Pb discordance in zircon

We have investigated U-Pb discordance and oxygen isotopic composition of zircon using high-spatial resolution ??18O measurement by ion microprobe. ??18O in both concordant and discordant zircon grains provides an indication of the relationship between fluid interaction and discordance. Our results suggest that three characteristics of zircon are interrelated: (1) U-Pb systematics and concomitant age discordance, (2) ??18O and the water-rock interactions implied therein, and (3) zircon texture, as revealed by cathodoluminescence and BSE imaging. A key observation is that U-Pb-disturbed zircons are often also variably depleted in 18O, but the relationship between discordance and ??18O is not systematic. ??18O values of discordant zircons are generally lighter but irregular in their distribution. Textural differences between zircon grains can be correlated with both U-Pb discordance and ??18O. Discordant grains exhibit either a recrystallized, fractured, or strongly zoned CL texture, and are characteristic of 18O depletion. We interpret this to be a result of metamictization, leading to destruction of the zircon lattice and an increased susceptibility to lead loss. Conversely, grains that are concordant have less-expressed zoning and a smoother CL texture and are enriched in 18O. From this it is apparent that various stages of water-rock interaction, as evidenced by systematic variations in ??18O, leave their imprint on both the texture and U-Pb systematics of zircon. Copyright ?? 2005 Elsevier Ltd.

Geochimica et Cosmochimica Acta↗

USGS2004

Spatial and temporal variation of Cenozoic surface elevation in the Great Basin and Sierra Nevada

The surface uplift of mountain belts caused by tectonism plays an important role in determining the long-term climate evolution of the Earth. However, the general lack of information on the paleotopography of mountain belts limits our ability to identify the links and feedbacks between topography, tectonics, and climate change on geologic time-scales. Here, we present a δ 18 O and δD record of authigenic minerals for the northern Great Basin that captures the timing and magnitude of regional surface uplift and subsidence events in the western United States during the Cenozoic. Authigenic calcite, smectite, and chert δ 18 O values suggest the northern Great Basin region experienced ~2km of surface uplift between the middle Eocene and early Oligocene followed by ~1 to 2km of surface subsidence in the southern Great Basin and/or Sierra Nevada since the middle Miocene. These data when combined with previously published work show that the surface uplift history varied in both space and time. Surface uplift migrated from north to south with high elevations in southern British Columbia and northeastern Washington in the middle Eocene and development of surface uplift in north and central Nevada in the Oligocene. This pattern of north to south surface uplift is similar to the timing of magmatism in the western Cordillera, a result that supports tectonic models linking magamtism with removal of mantle lithosphere and/or a subducting slab.

California, Idaho, Nevada, Oregon, Utah↗

USGS2004

U-Pb zircon constraints on the tectonic evolution of southeastern Tibet, Namche Barwa area

The eastern syntaxis of the Himalayas is expressed in the crust as a pronounced southward bend in the orogen. The change in strike of geologic features coincides with the high topography of the Namche Barwa region, the exposure of granulite-grade metamorphic rocks, and a 180-degree bend in the Yalu Tsangpo. We have conducted a geochronologic and geochemical investigation of several suites of granitoids collected from the Namche Barwa massif and subjacent terranes of southeastern Tibet, ranging from cm-scale dikes and sills to larger, outcrop-scale intrusions. U-Pb SHRIMP-RG zircon ages establish at least five magmatic episodes: ∼400 to 500 Ma, ∼120 Ma, 40 to 70 Ma, 18 to 25 Ma, and 3 to 10 Ma . These episodes broadly correlate to spatial patterns in sample localities, as follows: 400 to 500 Ma ages occur in zircon cores collected from within the massif proper; ∼120 Ma granites, related to early Gangdese arc plutonism, are primarily located northeast of Namche Barwa; later (40-70 Ma) Gangdese activity is expressed in granites west of Namche Barwa. 18 to 25 Ma granites occur both along the suture zone west of Gyala Peri, and directly north of Namche Barwa along the area of the Jiali fault zone, and are attributed both to shearing within the Jiali fault zone and to an early Miocene Gangdese Thrust event. Exceptionally young (<10 Ma) zircon ages are clustered near the core of the massif, along the Yalu Tsangpo gorge. Trace-element geochemical data indicates the presence of both fluid-present and fluid absent melts, with a fluid-absent (decompression) melting regime dominating near the core of Namche Barwa.

Tibet↗

USGS2003

Extreme 34S depletions in ZnS at the Mike gold deposit, Carlin Trend, Nevada: Evidence for bacteriogenic supergene sphalerite

We identified submicrometer-sized framboidal sphalerite (ZnS) below the base of supergene oxidation in a Carlin-type gold deposit of Eocene age in Nevada, United States, where the framboidal sphalerite forms a blanket-like body containing >400,000 metric tons of zinc. Framboidal sphalerite <0.1 ??m in diameter, formed in the early Miocene, ranges from <0.1 to 0.35 mol% FeS; the ??34S values range from -25??? to -70???, the lowest values measured in a marine or terrestrial environment. These S isotope data demonstrate the involvement of sulfate-reducing bacteria and provide the first documentation that sphalerite can form significant supergene sulfide-enrichment blankets.

Geology↗

USGS1999

The northern Sacramento Mountains, southwest United States. Part II: Exhumation history and detachment faulting

Thermochronologic and thermobarometric data reveal the timing, distribution and intensity of thermal events associated with detachment faulting in the Sacramento Mountains metamorphic core complex. In the northwest Sacramento Mountains, cooling rates of c. 100°C Ma −1 are associated with Late Cretaceous plutonism followed by cooling of the crust by thermal conduction. Post-Late Cretaceous cooling slowed to c. 1–6°C Ma −1 . Finally, the region records average cooling rates of 38–53°C Ma −1 between c. 20 and 15 Ma. In contrast, the thermal profile of the northeast Sacramento Mountains is dominated by syntectonic Tertiary plutonism followed by very rapid cooling. A granodioritic suite intruded at c. 680°C and c. 3 kbar at c. 20 Ma, records cooling to <100°C by c. 15 Ma. Such rapid cooling and exhumation suggests that unroofing by tectonic denudation was the driving mechanism for the final cooling. The similarity of the miocene cooling profiles between these two areas clearly suggests that the Sacramento Mountains experienced a regional cooling event associated with tectonic unroofing driven by regional Miocene crustal extension. Estimates of the initial angle of the Sacramento Mountains detachment fault using palaeothermal gradients suggest that it was active at a dip of 25°.

Geological Society Special Publication↗

USGS1998

The Whakamaru group ignimbrites, Taupo Volcanic Zone, New Zealand: Evidence for reverse tapping of a zoned silicic magmatic system

The Whakamaru group ignimbrites are widespread voluminous welded ignimbrites which crop out along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand. The ignimbrites have a combined volume exceeding 1000 km 3 , and were erupted from a large caldera in the central TVZ around 340 ka, following a c. 350 ka hiatus in caldera-forming activity in TVZ. Analysis of individual pumice clasts identifies five distinct magma types (rhyolite types A to D, and high alumina basalt) and significant gradients in temperature, water content, and Sr isotopic composition in the pre-eruptive Whakamaru magmatic system. There is a marked variation in mineral assemblage with composition; type A low-silica rhyolite pumices contain plagioclase, quartz, orthopyroxene, hornblende, biotite, and magnetite/ilmenite with distinctive large rounded quartz phenocrysts. High-silica (types B and C) pumices contain quartz (smaller, subhedral phenocrysts), plagioclase, sanidine, biotite, and magnetite/ilmenite. Type D pumices are rich in plagioclase and biotite phenocrysts, and have anomalously high Rb contents (>200 ppm) relative to all other pumice types. Rhyolite types B and C are related to type A magma by a two-stage crystal fractionation process, probably by side wall crystallisation and convective fractionation. The first stage involved 30–40% fractionation of a plagioclase-dominated (sanidine-free) assemblage to produce a type B magma, which in turn underwent fractionation of a plagioclase/quartz/sanidine assemblage to produce the highly evolved, but relatively Ba-depleted, type C magmas. Stratigraphic variations in modal proportions of mineral phases, and calculated Fe–Ti oxide equilibrium temperatures indicate that eruptions commenced with the hottest, least evolved magmas, and more evolved magmas became important at a later stage in the eruption along with a high alumina basalt component. This reverse-zoned sequence precludes simple sequential tapping of a large zoned magma chamber, and indicates a complex magma chamber configuration and/or withdrawal dynamics during eruption. Type D magma, which appears to be unrelated to either types A or B by crystal fractionation, may have formed a separate subjacent chamber that was ruptured and incorporated into the eruption. The Whakamaru magma system provides clear evidence that (less evolved) low silica rhyolites undergo significant fractionation at shallow crustal levels in central TVZ, to produce the generally more evolved rhyolites more commonly erupted at the surface, and suggests large ignimbrite eruptions may tap multiple magma chambers.

Journal of Volcanology and Geothermal Research↗