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Geology topics

J. L. Wooden

Publications and source records attributed to J. L. Wooden.

At least 73 records · Page 4Linked to original sources
USGS2001

Post-caldera volcanism: In situ measurement of U-Pb age and oxygen isotope ratio in Pleistocene zircons from Yellowstone caldera

The Yellowstone Plateau volcanic field, the site of some of the largest known silicic volcanic eruptions, is the present location of NE-migrating hotspot volcanic activity. Most volcanic rocks in the Yellowstone caldera (0.6 Ma), which formed in response to the climactic eruption of 1000 km3 of Lava Creek Tuff (LCT), have unusually low oxygen isotope ratios. Ion microprobe analysis of both U-Pb age and ??18O in zircons from these low-??18O lavas reveals evidence of complex inheritance and remelting. A majority of analyzed zircons from low-??18O lavas erupted inside the Yellowstone caldera have cores that range in age from 2.4 to 0.7 Ma, significantly older than their eruption ages (0.5-0.4 Ma). These ages and the high-??18O cores indicate that these lavas are largely derived from nearly total remelting of normal-??18O Huckleberry Ridge Tuff (HRT) and other pre-LCT volcanic rocks. A post-HRT low-??18O lava shows similar inheritance of HRT-age zircons. The recycling of volcanic rocks by shallow remelting can change the water content and eruptive potential of magma. This newly proposed mechanism of intracaldera volcanism is best studied by combining in situ analysis of oxygen and U-Pb isotope ratios of individual crystals. ?? 2001 Elsevier Science B.V. All rights reserved.

Earth and Planetary Science Letters↗
USGS2001

Implications of SHRIMP and microstructural data on the age and kinematics of shearing in the Asir terrane, southern Arabian Shield, Saudi Arabia

The Asir terrane consists of north-trending belts of variably metamorphosed volcanic, sedimentary, and plutonic rocks that are cut by numerous shear zones (Fig. 1). Previous workers interpreted the shear zones as sutures, structures that modify earlier sutures, or structures that define the margins of tectonic belts across which there are significant lithologic differences and along which there may have been major transposition (Frisch and Al-Shanti, 1977; Greenwood et al., 1982; Brown et al., 1989). SHRIMP data from zircons (Table 1) and sense-of-shear data recently acquired from selected shear zones in the terrane help to constrain the minimum ages and kinematics of these shearing events and lead to an overall model of terrane assembly that is more complex than previously proposed.

Gondwana Research↗
USGS2001

Geology, geochronology, geochemistry, and Pb-isotopic compositions of Proterozoic rocks, Poachie region, west-central Arizona — A study of the east boundary of the Proterozoic Mojave crustal province

The Poachie region at the south edge of the Colorado Plateau transition zone in western Arizona is underlain by early and middle Proterozoic plutonic rocks locally separated by screens of amphibolite-facies early Proterozoic metamorphic rocks and is in the eastern part of the Mojave crustal province, as shown by 35 Pb-isotope analyses of rocks and feldspars. Previously published mapping, 10 U-Pb zircon ages, 55 rock analyses, and other studies in the larger region of the transition zone lead to interpretation of the history and origin of the Proterozoic crust.

Arizona↗
USGS2000

Stanford-USGS shrimp-RG ion microprobe: A new approach to determining the distribution of trace elements in coal

The distribution of Cr and other trace metals of environmental interest in a range of widely used U.S. coals was investigated using the Stanford-USGS SHRIMP-RG ion microprobe . Using the oxygen ion source, concentrations of Cr (11 to 176 ppm), V (23 to 248 ppm), Mn (2 to 149 ppm), Ni (2 to 30 ppm), and 13 other elements were determined in illite/smectite, a group of clay minerals commonly present in coal. The results confirm previous indirect or semi-quantitative determinations indicating illite/smectite to be an important host of these metals. Calibration was achieved using doped aluminosilicate-glass synthetic standards and glasses prepared from USGS rock standards. Grains for analysis were identified optically, and confirmed by 1) precursory electron microprobe analysis and wavelength-dispersive compositional mapping, and 2) SHRIMP-RG major element data obtained concurrently with trace element results. Follow-up investigations will focus on the distribution of As and other elements that are more effectively ionized with the cesium primary beam currently being tested.

ACS Division of Fuel Chemistry, Preprints↗
USGS1999

Lead isotope compositions of Late Cretaceous and early Tertiary igneous rocks and sulfide minerals in Arizona: Implications for the sources of plutons and metals in porphyry copper deposits

Porphyry copper deposits in Arizona are genetically associated with Late Cretaceous and early Tertiary igneous complexes that consist of older intermediate volcanic rocks and younger intermediate to felsic intrusions. The igneous complexes and their associated porphyry copper deposits were emplaced into an Early Proterozoic basement characterized by different rocks, geologic histories, and isotopic compositions. Lead isotope compositions of the Proterozoic basement rocks define, from northwest to southeast, the Mojave, central Arizona, and southeastern Arizona provinces. Porphyry copper deposits are present in each Pb isotope province. Lead isotope compositions of Late Cretaceous and early Tertiary plutons, together with those of sulfide minerals in porphyry copper deposits and of Proterozoic country rocks, place important constraints on genesis of the magmatic suites and the porphyry copper deposits themselves. The range of age-corrected Pb isotope compositions of plutons in 12 Late Cretaceous and early Tertiary igneous complexes is 206 Pb/ 204 Pb = 17.34 to 22.66, 207 Pb/ 204 Pb = 15.43 to 15.96, and 208 Pb/ 204 Pb = 37.19 to 40.33. These Pb isotope compositions and calculated model Th/U are similar to those of the Proterozoic rocks in which the plutons were emplaced, thereby indicating that Pb in the younger rocks and ore deposits was inherited from the basement rocks and their sources. No Pb isotope differences distinguish Late Cretaceous and early Tertiary igneous complexes that contain large economic porphyry copper deposits from less rich or smaller deposits that have not been considered economic for mining. Lead isotope compositions of Late Cretaceous and early Tertiary plutons and sulfide minerals from 30 metallic mineral districts, furthermore, require that the southeastern Arizona Pb province be divided into two subprovinces. The northern subprovince has generally lower 206 Pb/ 204 Pb and higher model Th/U, and the southern subprovince has higher 206 Pb/ 204 Pb and lower model Th/U. These Pb isotope differences are inferred to result from differences in their respective post-1.7 Ga magmatic histories. Throughout Arizona, Pb isotope compositions of Late Cretaceous and early Tertiary plutons and associated sulfide minerals are distinct from those of Jurassic plutons and also middle Tertiary igneous rocks and sulfide minerals. These differences most likely reflect changes in tectonic setting and magmatic sources. Within Late Cretaceous and early Tertiary igneous complexes that host economic porphyry copper deposits, there is commonly a decrease in Pb isotope composition from older to younger plutons. This decrease in Pb isotope values with time suggests an increasing involvement of crust with lower U/Pb than average crust in the source(s) of Late Cretaceous and early Tertiary magmas. Lead isotope compositions of the youngest porphyries in the igneous complexes are similar to those in most sulfide minerals within the associated porphyry copper deposit. This Pb isotope similarity argues for a genetic link between them. However, not all Pb in the sulfide minerals in porphyry copper deposits is magmatically derived. Some sulfide minerals, particularly those that are late stage, or distal to the main orebody, or in Proterozoic or Paleozoic rocks, have elevated Pb isotope compositions displaced toward the gross average Pb isotope composition of the local country rocks. The more radiogenic isotopic compositions argue for a contribution of Pb from those rocks at the site of ore deposition. Combining the Pb isotope data with available geochemical, isotopic, and petrologic data suggests derivation of the young porphyry copper-related plutons, most of their Pb, and other metals from a hybridized lower continental crustal source. Because of the likely involvement of subduction-related mantle-derived basaltic magma in the hybridized lower crustal source, an indiscernible mantle contribution is probable in the porphyry magmas. Clearly, in addition, Pb was contributed from the local country rocks. This is most evident in sulfide minerals in veins that are late stage, hosted in Proterozoic gneiss, and/or peripheral to the porphyry copper deposit.

Economic Geology↗
USGS1998

Early Archean crust in the northern Wyoming province: Evidence from U-Pb ages of detrital zircons

U–Pb ages of individual detrital and metamorphic zircons from 12 Archean metasedimentary rocks, including quartzites, from the Beartooth, Ruby, and Tobacco Root uplifts of the northern Wyoming province indicate that they were deposited between 2.7 and 3.2 Ga. Younger, metamorphic zircons are found as overgrowths and new grains in some samples, and yield ages between 2.7 and 1.9 Ga. They are, however, much less abundant than detrital grains, which constitute >75% of the 355 grains analyzed. The majority of the detrital grains have ages between 3.2 and 3.4 Ga; none are younger than 2.9 Ga. Grains with 207 Pb/ 206 Pb ages between 3.4 and 4.0 Ga constituted approximately 15% of all grains with analyses within 10% of concordia, but are concentrated in samples from the eastern Beartooth Mountains. Comparison of the average of the Pb–Pb ages of individual zircons within 10% of concordia with previously published Lu–Hf chondritic model ages for some individual samples suggests that the age distribution recorded by the U–Pb system in these zircons has not been significantly disturbed by pre- or post-depositional Pb-loss. Collectively, these data suggest that the individual metasedimentary rocks did not completely share a common provenance and that a major crust-forming cycle occurred 3.2 to 3.4 Ga. In conjunction with previously published U–Th–Pb whole-rock data, these results suggest that rocks with a relatively high proportion of >3.4 Ga grains may have had crust of comparable age in their provenance.

Precambrian Research↗
USGS1997

Triassic plutonism in southern California: Southward younging of arc initiation along a truncated continental margin

Earliest Cordilleran magmatism in the southwestern United States is recorded by a belt of Triassic plutons that intrude Proterozoic basement of the Mojave crustal province and its cratonal/miogeoclinal cover. The belt extends from the western Mojave Desert through the Transverse Ranges to the Colorado River trough. Triassic plutons are predominantly alkali-calcic, Fe- and Sr-enriched quartz monzodiorites and monzonites. The northern part of the belt is composed of two older plutonic suites (241–231 Ma) which are high K to shoshonitic; the southern part of the belt is a younger (218–213 Ma), sodic-alkalic suite. The plutonic record in southern California suggests a short-lived, southward younging continental margin arc setting for emplacement of Triassic plutons, superimposed on a continental margin modified by sinistral transform faulting. Triassic plutonism in this region was followed by a magmatic lull prior to the onset of voluminous Middle to Late Jurassic Cordilleran arc magmatism.

California↗
USGS1996

Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: Evidence from U-Pb systematics in zircon

The Beartooth-Bighorn magmatic zone (BBMZ) and the Montana metasedimentary province (MMP) are two major subprovinces of the Archean Wyoming province. In the northwestern Beartooth Mountains, these subprovinces are separated by a structurally, lithologically and metamorphically complex assemblage of lithotectonic units that include: (1) a strongly deformed complex of trondhjemitic gneiss and interlayered amphibolites; and (2) an amphibolite facies mafic unit that occurs in a nappe that structurally overlies the gneiss complex. Zircons from a trondhjemitic blastomylonite in the gneiss complex yield concordant U-Pb ages of 3.5 Ga, establishing it as the oldest rock yet documented in the Wyoming province. Two younger events are also recorded by zircons in this rock: (1) an apparently protracted period of high-grade metamorphism and/or intrusion of additional magmas at ∼ 3.25 Ga; and (2) growth of hydrothermal zircon at ∼ 2.55 Ga, apparently associated with ductile deformation that immediately preceded structural emplacement of the gneiss. Although this latter event appears confined to areas along the BBMZ-MMP boundary, evidence of ∼ 3.25 Ga igneous activity is found in the overlying amphibolite (3.24 Ga) and throughout the MMP. These data suggest that this boundary first developed as a major intracratonic zone of displacement at or before 3.25 Ga. The limited occurrences of 2.8 Ga magmatic activity in the MMP suggest that it had a controlling influence on late Archean magmatism as well.

Precambrian Research↗
USGS1995

Sources of the Early Cretaceous plutons in the Turtle and West Riverside Mountains, California

Ages and initial isotopic ratios of Early Cretaceous (˜100 Ma) plutons of the Cordilleran Interior in the southern Turtle and West Riverside mountains distinguish them from Late Cretaceous plutons in surrounding ranges in the eastern Mojave Desert. Furthermore, the studied plutons have isotopic and geochemical characteristics more similar to plutons of Cretaceous age in the coastal batholiths (Peninsular Ranges and Sierra Nevada) than to most Mesozoic plutons in the Cordilleran Interior. The studied plutons are calcic, in contrast to the mostly cak-alkaline Mesozoic plutons of the eastern Mojave Desert. Distinctive isotopic signatures of the granitoids include lower initial 87 Sr/ 86 Sr of 0⋅705–0⋅710, δ 18 O of +6⋅3 to +7⋅7‰, 208 Pb/ 204 Pb of 38⋅3–39⋅5, and higher ε Nd of −3⋅86 to −9⋅60 than the Late Cretaceous plutons in the region. The distinctive characteristics of these Early Cretaceous plutons are probably both location and time specific and result from: (1) emplacement in a cold, untapped ‘Mojave-type’ Proterozoic upper crust, (2) a significant component of basaltic magmas partially melted from the asthenosphere or subcontinental lithosphere and (3) a magmatic component derived from Proterozoic, mafic, lower crust. They interacted less with their crustal hosts than did the later, more voluminous Late Cretaceous plutons.

California↗
USGS1995

Jurassic thrusting of Precambrian basement over Paleozoic cover in the Clipper Mountains, southeastern California

The Clipper Mountains in the eastern Mojave Desert expose evidence of Jurassic plutonic intrusion along what was an active thrust at the east fringe of the exposed Cordilleran Jurassic magmatic arc. This event occurred during a period of widespread arc magmatism and intra-arc thrusting in the Cordillera related to subduction under the west edge of North America. Jurassic plutons in the eastern Mojave Desert are compositionally more diverse and more K 2 O-rich than Cretaceous plutons. Late-kinematic intrusion of the Jurassic Goldhammer pluton, exposed in the Clipper Mountains, was along an active ductile thrust fault that put Proterozoic basement gneiss over Paleozoic strata by the time of intrusion. U-Pb geochronology and hornblende geobarometry are interpreted to indicate that the pluton was emplaced at 161 ± 10 Ma at a pressure approximately 0.46 GPa or more. This pressure corresponds to approximately a 17-km depth or more, at least 13 km greater than inferred stratigraphic overburden (2–4 km) at the time of intrusion. The excess we attribute to pre-intrusion tectonic burial from overthrusting of the observed allochthon of Proterozoic basement and (or) from earlier Mesozoic overthrusting. Ductile deformation continued along the observed thrust system during intrusion of the Goldhammer pluton. Fabrics in the pluton and country-rock record ductile shearing that was partly top westward but mostly top-eastward; the shearing began before or during the intrusion and continued during and after intrusion. The Jurassic burial history in the Clipper Mountains parallels that in adjacent ranges to the east, but contrasts with that in ranges to the south and west where exposed Paleozoic rocks were at colder and shallower crustal levels in Jurassic time. The tectonic record in the Clipper Mountains suggests large crustal thickening and topographic uplift that would be expected to leave a sedimentary record in Jurassic basins.

GSA Special Papers↗
USGS1994

Petrogenesis of the highly potassic 1.42 Ga Barrel Spring pluton, southeastern California, with implications for mid-Proterozoic magma genesis in the southwestern USA

Syenites from the Barrel Spring pluton were emplaced in the Early Proterozoic Mojave crustal provine of southeastern California at 1.42 Ga. All rocks, even the most mafic, are highly enriched in incompatible elements (e.g. K 2 O 4–12 wt%, Rb 170–370 ppm, Th 12–120 ppm, La 350–1500xchondrite, La/Yb n 35–100). Elemental compositions require an incompatible element-rich but mafic (or ultramafic) source. Trace element models establish two plausible sources for Barrel Spring magmas: (1) LREE enriched garnet websterite with accessory apatite±rutile (enriched lithospheric mantle), and (2) garnet amphibolite or garnet-hornblende granulite with enriched alkali basalt composition, also with accessory apatite±rutile (mafic lower crust). Nd and Pb isotopic ratios do not distinguish a crust vs mantle source, but eliminate local Mojave province crust as the principal one, and indicate that generation of the enriched source occurred several hundred million years before emplacement of the Barrel Spring pluton. 1.40–1.44 Ga potassic granites are common in southeastern California, suggesting a genetic link between the Barrel Spring pluton and the granites; however, although the same thermal regime was probably responsible for producing both the granitic and syentic magmas, elemental and isotopic compositions preclude a close relationship. Isotopic similarity of the Barrel Spring pluton to 1.40–1.44 Ga granites emplaced in the Central Arizona crustal province to the east may imply that a common component was present in the lithosphere of these generally distinct regions.

California↗
USGS1994

Anatexis, hybridization and the modification of ancient crust: Mesozoic plutonism in the Old Woman Mountains area, California

A compositionally expanded array of granitic (s.l.) magmas intruded the > 2 Ga crust of the Old Woman Mountains area between 160 and 70 Ma. These magmas were emplaced near the eastern (inland) edge of the Jurassic/Cretaceous arcs of western North America, in an area where magma flux, especially during the Jurassic, was considerably lower than to the west. The Jurassic intrusives and over half of the Cretaceous intrusives are predominantly metaluminous and variable in composition; a major Cretaceous suite comprises only peraluminous monzogranite. Only the Jurassic intrusions show clear evidence for the presence of mafic liquids. All units, including the most mafic rocks, reveal isotopic evidence for a significant crustal component. However, none of the Mesozoic intrusives matches in isotopic composition either average pre-intrusion crust or any major unit of the exposed crust. Elemental inconsistencies also preclude closed system derivation from exposed crust. Emplacement of these magmas, which doubled the volume of the mid- to upper crust, did not dramatically change its elemental composition. It did, however, affect its Nd and especially Sr isotopic composition and modify some of the distinctive aspects of the elemental chemistry. We propose that Jurassic magmatism was open-system, with a major influx of mantle-derived mafic magma interacting strongly with the ancient crust. Mesozoic crustal thickening may have led to closed-system crustal melting by the Late Cretaceous, but the deep crust had been profoundly modified by earlier Mesozoic hybridization so that crustal melts did not simply reflect the original crustal composition. The clear evidence for a crustal component in magmas of the Old Woman Mountains area may not indicate any fundamental differences from the processes at work elsewhere in this or other magmatic arcs where the role of pre-existing crust is less certain. Rather, a compositionally distinctive, very old crust may simply have yielded a more readily identifiable crustal fingerprint. The same processes that were involved here-mafic magma influx, hybridization, and remelting of hybridized crust-are likely to be typical of arc settings. ?? 1994.

LITHOS↗
USGS1994

Multiple isotopic components in Quaternary volcanic rocks of the Cascade Arc near Crater lake, Oregon

Quaternary lavas and pyroclastic rocks of Mount Mazama, Crater lake caldera, and the surrounding area have variable Sr, Nd, and Pb isotopic compositions. High-alumina olivine tholeiites have 87Ar/86Ar ratios of 0.70346-0.70364; basaltic andesite, 0.70349-0.70372; shoshonitic basaltic andesite, 0.70374-0.70388; and andesite, 0.70324-0.70383. Dacites of Mount Mazama have 87Sr/ 86Sr ratios of 0.70348-0.70373. Most rhyodacites converge on 0.7037. Andesitic to mafic-cumulate scoriae of the climatic eruption, and enclaves in pre-climactic rhyodacites, cluster in two groups but show nearly the entire 87Sr/86Sr range of the data set, confirming previously suggested introduction of diverse parental magmas into the growing climactic chamber. Magma evolution is described.

Oregon↗
USGS1993

Monazite paragenesis and U-Pb systematics in rocks of the eastern Mojave Desert, California, U.S.A.: implications for thermochronometry

Studies of the paragenesis and U-Pb systematics of monazite in rocks from the eastern Mojave Desert, California, corroborate its potential usefulness as a prograde thermochronometer and in dating granite inheritance. Unmetamorphosed Latham Shale and its equivalents at grades ranging from greenschist to upper amphibolite facies are virtually identical in composition. Monazite is absent in the shale and low-grade schists, but it is abundant in schists at staurolite and higher grades. Lower-grade schists instead include minute Th- and Ce-oxides and unidentified Ce-poor LREE-phosphates that apparently are lower-temperature precursors to monazite. Thus monazite originates when the pelite passes through lower-amphibolite-facies conditions. Monazites from three Upper Cretaceous granites yield ages that are strongly discordant. Upper intercepts of 1.6-1.7 Ga are similar to those defined by U-Pb data for coexisting zircons and coincide with a period of copious magmatism in the Mojave crust. As the host Upper Cretaceous granitic magmas were all above 700??C, effective closure of the restitic monazites to Pb loss must be well in excess of this temperature. U-Pb compositions of monazite from Proterozoic granitoids and schist also indicate high Pb retentivity. Taken together, these studies support the suggestion that monazite can be an effective prograde thermochronometer. At least in pelites, it is not usually retained as a detrital mineral, but rather forms during moderate-temperature metamorphism. Its U-Pb system should not be reset by subsequent higher-grade metamorphism. ?? 1993.

Chemical Geology↗
USGS1993

Lead isotopic evidence for the origin of Paleo- and Mesoproterozoic rocks of the Colorado Province, U.S.A.

Lead isotopic ratios of K-feldspars and whole-rocks from 1.7- and 1.4-Ga plutonic rocks of the Colorado Province are relatively non-radiogenic for 207Pb 204Pb, plotting below the average crust model curve of Stacey and Kramers (1975), indicating that the terrane was derived primarily from juvenile, mantle material. Slightly more radiogenic ratios in the northern part of the terrane, near the Archean Wyoming Province, suggest minor inclusion of an older component. The data from 1.7-Ga plutons plot in a broad field suggesting two episodes of re-equilibration with whole-rock Pb, probably related to heating events in the Mesoproterozoic (1.4 Ga) and Cretaceous (70 Ma). Possible differences in calculated whole-rock Th U, coupled with slight Pb isotopic variations, along the north-south transect suggest either a terrane boundary through central Colorado (near Salida and Gunnison), or fundamental differences in source rocks (metasedimentary vs. metavolcanic). UPb analyses of multigrain splits of detrital zircons from quartzites throughout the Colorado Province have failed to identify Archean detritus. The oldest 207Pb 206Pb ages found (in two samples of quartzite from northern Colorado) are about 2.0 Ga (perhaps derived from rocks of the Trans-Hudson orogen), in contrast to 2.75-Ga detrital zircon in a Paleoproterozoic quartzite from the southern part of the Wyoming Province. While we are not yet able to discern if these ages are true provenance ages or mixtures of Archean and Paleoproterozoic components, the absence of easily recognizable Archean zircons supports other isotopic data and a conclusion that most of the Paleoproterozoic crust of the Colorado Province was ultimately derived from a juvenile (at 1.8 Ga) mantle reservoir. ?? 1993.

Precambrian Research↗
USGS1992

Geochemical evolution of Jurassic diorites from the Bristol Lake region, California, USA, and the role of assimilation

Late Jurassic dioritic plutons from the Bristol Lake region of the eastern Mojave Desert share several geochemical attributes with high-alumina basalts, continental hawaiite basalts, and high-K are andesites including: high K2O concentrations; high Al2O3 (16-19 weight %); elevated Zr/TiO2; LREE (light-rare-earth-element) enrichment (La/YbCN=6.3-13.3); and high Nb. Pearce element ratio analysis supported by petrographic relations demonstrates that P, Hf, and Zr were conserved during differentiation. Abundances of conserved elements suggest that dioritic plutons from neighboring ranges were derived from similar parental melts. In the most voluminous suite, correlated variations in elemental concentrations and (87Sr/86Sr)i indicate differentiation by fractional crystallization of hornblende and plagioclase combined with assimilation of a component characterized by abundant radiogenic Sr. Levenberg-Marquardt and Monte Carlo techniques were used to obtain optimal solutions to non-linear inverse models for fractional crystallization-assimilation processes. Results show that the assimilated material was chemically analogous to lower crustal mafic granulites and that the mass ratio of contaminant to parental magma was on the order of 0.1. Lack of enrichment in 18O with differentiation is consistent with the model results. Elemental concentrations and O, Sr, and Nd isotopic data point to a hydrous REE-enriched subcontinental lithospheric source similar to that which produced some Cenozoic continental hawaiites from the southern Cordillera. Isotopic compositions of associated granitoids suggest that partial melting of this subcontinental lithosphere may have been an important process in the development of the Late Jurassic plutonic arc of the eastern Mojave Desert. ?? 1992 Springer-Verlag.

Contributions to Mineralogy and Petrology↗