Geology topics

Joseph L. Wooden

Publications and source records attributed to Joseph L. Wooden.

At least 19 recordsLinked to original sources

USGS2019

Characterization of the rhyolite of Bodie Hills and 40Ar/39Ar intercalibration with Ar mineral standards

The rhyolite of Bodie Hills (California) is characterized compositionally and the geochronology of selected phases is studied. Sanidine (BHs) from the rhyolite is well suited as a 40 Ar/ 39 Ar reference material with high K/Ca and radiogenic yield. Intercalibration with GA1550 biotite from the Dromedary igneous complex (New South Wales, Australia) yields an age of 9.7946 ± 0.0031 Ma for BHs relative to an age of 98.79 ± 0.54 Ma for GA1550 and a calibration factor ( R BHs/GA1550 ) of 0.096719 ± 0.000032. BHs is also intercalibrated with sanidines of the Taylor Creek Rhyolite (TCs; R TCs/BHs of 2.90874 ± 0.00067), Fish Canyon Tuff (FCs; R FCs/BHs of 2.88339 ± 0.00088), and rhyolite of Alder Creek (ACs; R ACs/BHs of 0.12028 ± 0.00024). These calibration factors yield ages of 28.344 ± 0.011 Ma, 28.099 ± 0.013 Ma, and 1.1809 ± 0.0024 Ma for TCs, FCs, and ACs, respectively, relative to GA1550. Full propagation of errors increases these uncertainties and that of BHs to ±0.9% of their ages. Calibration of BHs using the astronomically tuned age of FCs determined by Kuiper et al. (2008) yields an age of 9.8295 ± 0.0036 Ma. Stepwise heating of BHs reveals the same small, progressive increase in age across the age spectrum reported for FCs, ACs, and other potassium feldspars (e.g., Foland and Xu, 1990; Phillips et al., 2017). This increase is consistent with mass fractionation of argon during step heating and favors use of single-step fusion ages of all sanidines for monitor and calibration purposes.

Chemical Geology↗

USGS2018

Trace element characterisation of MAD‐559 zircon reference material for ion microprobe analysis

We document the composition of a natural zircon gemstone sourced from Madagascar, MAD‐559 – a new reference material for calibrating trace element mass fractions in zircon measured by SIMS. The composition of MAD‐559 was quantified by calibration relative to the well‐documented zircon reference material 91500, for which we compiled existing published data (Mg, Al, Y, rare earth elements, Hf, U, Th) and performed new measurements to characterise the mass fraction of less commonly measured elements (Li, Be, B, F, Na, P, K, Ca, Sc, Ti, Fe, Nb). Measurement results of SL13, CZ3 and MAD‐1 zircons and NIST SRM glasses were performed as quality control materials to test measurement bias and repeatability. We show the intermediate precision for most trace element measurement results of MAD‐559 to be between ± 3% and ± 5% RSD based on 139 measurements by SIMS on twenty‐five individual polished zircon chips measured during a 24‐h period, as well as repeat measurements performed over five separate analytical sessions. Trace element mass fractions were also measured by LA‐ICP‐MS in two different laboratories, and major element compositions measured by electron microprobe, to compare with results measured by SIMS. Based on laser Raman and hyperspectral cathodoluminescence spectroscopy, we show MAD‐559 to have high crystal disorder due to radiation damage relative to crystalline zircon (e.g., SL13 and 91500 zircon). Although the high cumulative alpha dose of MAD‐559 zircon makes it a poor reference material for geochronology, the consistency of the trace element mass fraction results measured in multiple sessions and by various measurement methods shows that it is an ideal reference material for microanalytical trace element mass fraction quantification of zircon.

Geostandards and Geoanalytical Research↗

USGS2017

U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America. Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation. Our detailed zircon and whole-rock data support the following three interpretations: (1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the southern portion of the Sawatch and Mosquito ranges. (2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present regionally. Inherited zircon components of Mazatzal-age were found as cores in some 1.4-Ga Sawatch and Mosquito Range zircons, indicating the likelihood of a relatively local source. These combined data suggest the possibility that all were produced within a continental-margin magmatic arc created as a result of southward-migrating (slab rollback?), north-dipping subduction to the south of the region. (3) Widespread Mesoproterozoic plutonism—with emplacement at various depths and exhibiting bimodal geochemistry—is recognized in 16 different samples. An older group of predominantly peraluminous, yet magnesian granitoids (e.g., granodiorite of Sayers, granite of Taylor River, and the St. Kevin Granite) were emplaced between ca. 1,450 and 1,425 Ma. These geochemical parameters suggest moderate degrees of partial melting in a low-pressure environment. Three younger metaluminous, but ferroan plutons (diorite of Grottos, diorite of Mount Elbert, and granodiorite of Mount Harvard), probably represent a final magmatic pulse at ca. 1,416 Ma. A comprehensive treatment of zircon REE and whole-rock trace-element behavior from Proterozoic rocks is scarce. Discriminant U/Yb versus Y diagrams using zircon data show that the Sawatch and Mosquito plutons are of continental origin, not oceanic. Additional bivariate diagrams incorporating cation ratio combinations of Gd, Ce, Yb, U, Th, Hf, and Eu offer refined insight into differences in fractionation trends and depth of magma generation for the various plutons. These interpretations, on the basis of zircon trace-element data, are mirrored in the whole-rock geochemistry data.

Colorado↗

USGS2014

Proterozoic geochronological links between the Farewell, Kilbuck, and Arctic Alaska terranes

New U-Pb igneous and detrital zircon ages reveal that despite being separated by younger orogens, three of Alaska’s terranes that contain Precambrian rocks—Farewell, Kilbuck, and Arctic Alaska—are related. The Farewell and Kilbuck terranes can be linked by felsic magmatism at ca. 850 Ma and by abundant detrital zircons in the Farewell that overlap the ca. 2010–2085 Ma age range of granitoids in the Kilbuck. The Farewell and Arctic Alaska terranes have already been linked via correlative Neoproterozoic to Devonian carbonate platform deposits that share nearly identical faunas of mixed Siberian and Laurentian affinity. New igneous ages strengthen these ties. Specifically, 988, 979, and 979 Ma metafelsites in the Farewell terrane are close in age to a 971 Ma granitic orthogneiss in the Arctic Alaska terrane. Likewise, 852, 850, 845, and 837 Ma granitic orthogneisses, metafelsite, and rhyolite in the Farewell terrane are similar to the reported 874 to 848 Ma age range of metarhyolites in the Arctic Alaska terrane. The Kilbuck and Arctic Alaska terranes have been previously linked on the basis of provenance: detrital zircons from the Carboniferous Nuka Formation in the Arctic Alaska terrane range from 2013 to 2078 Ma, overlapping the age of Kilbuck granitoids. A new 849 Ma age of a Kilbuck granitoid strengthens the proposed connection. Among the other new results from Kilbuck terrane is a 2085 Ma zircon from a granitoid that now stands as the oldest tightly dated rock in Alaska. We conclude that the Kilbuck, Farewell, and Arctic Alaska terranes were not independent entities with unique geologic histories but instead are related pieces of the circum-Arctic tectonic puzzle.

Alaska↗

USGS2014

U-Pb zircon geochronology of plutonism in the northern Peninsular Ranges batholith, southern California: Implications for the Late Cretaceous tectonic evolution of southern California

Utilizing both sensitive high-resolution ion microprobe (SHRIMP) and conventional isotope dilution–thermal ionization mass spectrometry (ID-TIMS) methods, crystallization and/or emplacement ages have been obtained for a suite of Cretaceous intermediate-composition plutonic samples collected along a roughly E-W–trending traverse through the northern Peninsular Ranges batholith. Previously noted petrologic, mineralogic, and textural differences delineated four major zonations from west to east and raised the need for detailed geochemical and isotopic work. U-Pb zircon geochronology establishes that these zonations are essentially temporally separate. Mean 206 Pb/ 238 U ages date the three older zones from west to east at 126–107 Ma, 107–98 Ma, and 98–91 Ma. Despite petrologic differences, a relatively smooth progression of magmatism is seen from west to east. A fourth zone is defined by magmatism at ca. 85 Ma, which represents emplacement of deeper-level plutons east of the Eastern Peninsular Ranges mylonite zone in an allochthonous thrust sheet in the northeastern Peninsular Ranges batholith. The age data presented here differ slightly from those presented in earlier work for similar rocks exposed across the middle and southern portions of the Peninsular Ranges batholith in that our data define a relatively smooth progression of magmatism from west to east, and that the transition from western-type to eastern-type plutonism is interpreted to have occurred at ca. 98 Ma and not at ca. 105 Ma. The progressive involvement of older crustal components in the enrichment of eastern Peninsular Ranges batholith–type magma sources is documented by the occurrence of Proterozoic zircon inheritance within samples of the eastern part of the batholith.

Book chapter↗

USGS2014

Pb-Sr-Nd-O isotopic characterization of Mesozoic rocks throughout the northern end of the Peninsular Ranges batholith: Isotopic evidence for the magmatic evolution of oceanic arc–continental margin accretion during the Late Cretaceous of southern California

Within the duration of the U.S. Geological Survey (USGS)–based Southern California Areal Mapping Project (SCAMP), many samples from the northern Peninsular Ranges batholith were studied for their whole-rock radioisotopic systematics (rubidium-strontium [Rb-Sr], uranium-thorium-lead [U-Th-Pb], and samarium-neodymium [Sm-Nd]), as well as oxygen (O), a stable isotope. The results of three main studies are presented separately, but here we combine them (>400 analyses) to produce a very complete Pb-Sr-Nd-O isotopic profile of an arc-continent collisional zone—perhaps the most complete in the world. In addition, because many of these samples have U-Pb zircon as well as argon mineral age determinations, we have good control of the timing for Pb-Sr-Nd-O isotopic variations. The ages and isotopic variations help to delineate at least four zones across the batholith from west to east—an older western zone (126–108 Ma), a transitional zone (111–93 Ma), an eastern zone (94–91 Ma), and a much younger allochthonous thrust sheet (ca. 84 Ma), which is the upper plate of the Eastern Peninsular Ranges mylonite zone. Average initial 87 Sr/ 86 Sr (Sr i ), initial 206 Pb/ 204 Pb ( 206 Pb i ), initial 208 Pb/ 204 Pb (average 208 Pb i ), initial epsilon Nd (average ε Ndi ), and δ 18 O signatures range from 0.704, 18.787, 38.445, +3.1, and 4.0‰–9.0‰, respectively, in the westernmost zone, to 0.7071, 19.199, 38.777, −5, and 9‰–12‰, respectively, in the easternmost zone. The older western zone is therefore the more chemically and isotopically juvenile, characterized mostly by values that are slightly displaced from a mantle array at ca. 115 Ma, and similar to some modern island-arc signatures. In contrast, the isotopic signatures in the eastern zones indicate significant amounts of crustal involvement in the magmatic plumbing of those plutons. These isotopic signatures confirm previously published results that interpreted the Peninsular Ranges batholith as a progressively contaminated magmatic arc. The Peninsular Ranges batholith magmatic arc was initially an oceanic arc built on Panthalassan lithosphere that eventually evolved into a continental margin magmatic arc collision zone, eventually overriding North American cratonic lithosphere. Our Pb-Sr-Nd data further suggest that the western arc rocks represent a nearshore or inboard oceanic arc, as they exhibit isotopic signatures that are more enriched than typical mid-ocean-ridge basalt (MORB). Isotopic signatures from the central zone are transitional and indicate that enriched crustal magma sources were becoming involved in the northern Peninsular Ranges batholith magmatic plumbing. As the oceanic arc–continental margin collision progressed, a mixture of oceanic mantle and continental magmatic sources transpired. Magmatic production in the northern Peninsular Ranges batholith moved eastward and continued to tap enriched crustal magmatic sources. Similar modeling has been previously proposed for two other western margin magmatic arcs, the Sierra Nevada batholith of central California and the Idaho batholith. Calculated initial Nd signatures at ca. 100 Ma for Permian–Jurassic and Proterozoic basement rocks from the nearby San Gabriel Mountains and possible source areas along the southwestern Laurentian margin of southern California, southwestern Arizona, and northern Sonora strongly suggest their involvement with deep crustal magma mixing beneath the eastern zones of the Peninsular Ranges batholith, as well as farther east in continental lithospheric zones. Last, several samples from the allochthonous, easternmost upper-plate zone, which are considerably younger (ca. 84 Ma) than any of the rocks from the northern Peninsular Ranges batholith proper, have even more enriched average Sr i , 206 Pb i , 208 Pb i , and ε Ndi signatures of 0.7079, 19.344, 38.881, and −6.6, respectively, indicative of the most-evolved magma sources in the northern Peninsular Ranges batholith and similar to radioisotopic values for rocks from the nearby Transverse Ranges, suggesting a genetic connection between the two.

Book chapter↗

USGS2014

Preliminary geologic map of Black Canyon and surrounding region, Nevada and Arizona

Thermal springs in Black Canyon of the Colorado River, downstream of Hoover Dam, are important recreational, ecological, and scenic features of the Lake Mead National Recreation Area. This report presents the results from a U.S. Geological Survey study of the geologic framework of the springs. The study was conducted in cooperation with the National Park Service and funded by both the National Park Service and National Cooperative Geologic Mapping Program of the U.S. Geological Survey. The report has two parts: A, a 1:48,000-scale geologic map created from existing geologic maps and augmented by new geologic mapping and geochronology; and B, an interpretive report that presents results based on a collection of fault kinematic data near springs within Black Canyon and construction of 1:100,000-scale geologic cross sections that extend across the western Lake Mead region. Exposures in Black Canyon are mostly of Miocene volcanic rocks, underlain by crystalline basement composed of Miocene plutonic rocks or Proterozoic metamorphic rocks. The rocks are variably tilted and highly faulted. Faults strike northwest to northeast and include normal and strike-slip faults. Spring discharge occurs along faults intruded by dacite dikes and plugs; weeping walls and seeps extend away from the faults in highly fractured rock or relatively porous volcanic breccias, or both. Results of kinematic analysis of fault data collected along tributaries to the Colorado River indicate two episodes of deformation, consistent with earlier studies. The earlier episode formed during east-northeast-directed extension, and the later during east-southeast-directed extension. At the northern end of the study area, pre-existing fault blocks that formed during the first episode were rotated counterclockwise along the left-lateral Lake Mead Fault System. The resulting fault pattern forms a complex arrangement that provides both barriers and pathways for groundwater movement within and around Black Canyon. Regional cross sections in this report show that thick Paleozoic carbonate aquifer rocks of east-central Nevada do not extend into the Black Canyon area and generally are terminated to the south at a major tectonic boundary defined by the northeast-striking Lake Mead Fault System and the northwest-striking Las Vegas Valley shear zone. Faults to the west of Black Canyon strike dominantly north-south and form a complicated pattern that may inhibit easterly groundwater movement from Eldorado Valley. To the east of Black Canyon, crystalline Proterozoic rocks locally overlain by Tertiary volcanic rocks in the Black Mountains are bounded by steep north-south normal faults. These faults may also inhibit westerly groundwater movement from Detrital Valley toward Black Canyon. Finally, the cross sections show clearly that Proterozoic basement rocks and (or) Tertiary plutonic rocks are shallow in the Black Canyon area (at the surface to a few hundred meters depth) and are cut by several major faults that discharge most of the springs in the Black Canyon. Therefore, the faults most likely provide groundwater pathways to sufficient depths that the groundwater is heated to the observed temperatures of up to 55 °C.

Arizona, Nevada↗

USGS2013

Eocene extension in Idaho generated massive sediment floods into Franciscan trench and into Tyee, Great Valley, and Green River basins

The Franciscan Complex accretionary prism was assembled during an ∼165-m.y.-long period of subduction of Pacific Ocean plates beneath the western margin of the North American plate. In such fossil subduction complexes, it is generally difficult to reconstruct details of the accretion of continent-derived sediments and to evaluate the factors that controlled accretion. New detrital zircon U-Pb ages indicate that much of the major Coastal belt subunit of the Franciscan Complex represents a massive, relatively brief, surge of near-trench deposition and accretion during Eocene time (ca. 53–49 Ma). Sediments were sourced mainly from the distant Idaho Batholith region rather than the nearby Sierra Nevada. Idaho detritus also fed the Great Valley forearc basin of California (ca. 53–37 Ma), the Tyee forearc basin of coastal Oregon (49 to ca. 36 Ma), and the greater Green River lake basin of Wyoming (50–47 Ma). Plutonism in the Idaho Batholith spanned 98–53 Ma in a contractional setting; it was abruptly superseded by major extension in the Bitterroot, Anaconda, Clearwater, and Priest River metamorphic core complexes (53–40 Ma) and by major volcanism in the Challis volcanic field (51–43 Ma). This extensional tectonism apparently deformed and uplifted a broad region, shedding voluminous sediments toward depocenters to the west and southeast. In the Franciscan Coastal belt, the major increase in sediment input apparently triggered a pulse of massive accretion, a pulse ultimately controlled by continental tectonism far within the interior of the North American plate, rather than by some tectonic event along the plate boundary itself.

California, Idaho, Oregon, Wyoming↗

USGS2013

Geochronologic and geochemical data from Mesozoic rocks in the Black Mountain area northeast of Victorville, San Bernardino County, California

We present geochronologic and geochemical data for Mesozoic rocks in the Black Mountain area northeast of Victorville, California, to supplement previous geologic mapping. These data, together with previously published results, limit the depositional age of the sedimentary Fairview Valley Formation to Early Jurassic, refine the ages and chemical compositions of selected units in the overlying Jurassic Sidewinder Volcanics and of related intrusive units, and limit the age of some post-Sidewinder faulting in the Black Mountain area to a brief interval in the Late Jurassic. The new information contributes to a more complete understanding of the Mesozoic magmatic and tectonic evolution of the western Mojave Desert and surrounding regions.

California↗

USGS2013

Geologic map of the Topock 7.5’ quadrangle, Arizona and California

The Topock quadrangle exposes a structurally complex part of the Colorado River extensional corridor and also exposes deposits that record landscape evolution during the history of the Colorado River. Paleoproterozoic gneisses and Mesoproterozoic granitoids and intrusive sheets are exposed through tilted cross-sectional thicknesses of many kilometers. Intruding them are a series of Mesozoic to Tertiary igneous rocks including dismembered parts of the Late Cretaceous Chemehuevi Mountains Plutonic Suite. Plutons of this suite in Arizona, if structurally restored for Miocene extension, formed cupolas capping the Chemehuevi Mountains batholith in California. Thick (1–3 km) Miocene sections of volcanic rocks, sedimentary breccias, conglomerate, and sandstone rest nonconformably on the Proterozoic rocks and record the structural and depositional evolution of the Colorado River extensional corridor. Four major Miocene low-angle normal faults and a steep block-bounding fault that developed during this episode divide the deformed rocks of the quadrangle into major structural plates and tilted blocks in and east of the Chemehuevi Mountains core complex. The low-angle faults attenuate crustal section, superposing supracrustal and upper crustal rocks against gneisses and granitoids originally from deeper crustal levels. The transverse block-bounding Gold Dome Fault Zone juxtaposes two large hanging-wall blocks, each tilted 90°, and the fault zone splays at its tip into folds in layered Miocene rocks. A synfaulting intrusion occupies the triangular zone where the folded strata detached from an inside corner along this fault between the tilt blocks. Post-extensional upper Miocene to Quaternary strata, locally deformed, record post-extensional landscape evolution, including several Pliocene and younger aggradational episodes in the Colorado River valley and intervening degradation episodes. The aggradational sequences include (1) the Bouse Formation, (2) fluvial deposits correlated with the alluvium of Bullhead City, (3) the younger fluvial boulder conglomerate of Bat Cave Wash, (4) the fluvial Chemehuevi Formation and related valley-margin deposits, and (5) fluvial Holocene deposits under the river and the valley floor. These fluvial records of Colorado River deposition are interspersed with piedmont alluvial fan deposits of several ages.

Arizona, California↗

USGS2011

Source region of a granite batholith: Evidence from lower crustal xenoliths and inherited accessory minerals

Like many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images. Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T ≥ 750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive). The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.

California↗

USGS2011

Peninsular terrane basement ages recorded by Paleozoic and Paleoproterozoic zircon in gabbro xenoliths and andesite from Redoubt volcano, Alaska

Historically Sactive Redoubt volcano is an Aleutian arc basalt-to-dacite cone constructed upon the Jurassic–Early Tertiary Alaska–Aleutian Range batholith. The batholith intrudes the Peninsular tectonostratigraphic terrane, which is considered to have developed on oceanic basement and to have accreted to North America, possibly in Late Jurassic time. Xenoliths in Redoubt magmas have been thought to be modern cumulate gabbros and fragments of the batholith. However, new sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages for zircon from gabbro xenoliths from a late Pleistocene pyroclastic deposit are dominated by much older, ca. 310 Ma Pennsylvanian and ca. 1865 Ma Paleoproterozoic grains. Zircon age distributions and trace-element concentrations indicate that the ca. 310 Ma zircons date gabbroic intrusive rocks, and the ca. 1865 Ma zircons also are likely from igneous rocks in or beneath Peninsular terrane basement. The trace-element data imply that four of five Cretaceous–Paleocene zircons, and Pennsylvanian low-U, low-Th zircons in one sample, grew from metamorphic or hydrothermal fluids. Textural evidence of xenocrysts and a dominant population of ca. 1865 Ma zircon in juvenile crystal-rich andesite from the same pyroclastic deposit show that this basement has been assimilated by Redoubt magma. Equilibration temperatures and oxygen fugacities indicated by Fe-Ti–oxide minerals in the gabbros and crystal-rich andesite suggest sources near the margins of the Redoubt magmatic system, most likely in the magma accumulation and storage region currently outlined by seismicity and magma petrology at ∼4–10 km below sea level. Additionally, a partially melted gabbro from the 1990 eruption contains zircon with U-Pb ages between ca. 620 Ma and ca. 1705 Ma, as well as one zircon with a U-Th disequilibrium model age of 0 ka. The zircon ages demonstrate that Pennsylvanian, and probably Paleoproterozoic, igneous rocks exist in, or possibly beneath, Peninsular terrane basement. Discovery of Pennsylvanian gabbro similar in age to Skolai arc plutons 500 km to the northeast indicates that the Peninsular terrane, along with the Wrangellia and Alexander terranes, has been part of the Wrangellia composite terrane since at least Pennsylvanian time. Moreover, the zircon data suggest that a Paleoproterozoic continental fragment may be present in the mid-to-upper crust in southern Alaska.

Alaska↗

USGS2011

Ages and sources of components of Zn-Pb, Cu, precious metal, and platinum group element deposits in the goodsprings district, Clark County, Nevada

The Goodsprings district, Clark County, Nevada, includes zinc-dominant carbonate replacement deposits of probable late Paleozoic age, and lead-dominant carbonate replacement deposits, copper ± precious metal-platinum group element (PGE) deposits, and gold ± silver deposits that are spatially associated with Late Triassic porphyritic intrusions. The district encompasses ~500 km 2 although the distribution of all deposits has been laterally condensed by late Mesozoic crustal contraction. Zinc, Pb, and Cu production from about 90 deposits was ~160,000 metric tons (t) (Zn > Pb >> Cu), 2.1 million ounces (Moz) Ag, 0.09 Moz Au, and small amounts of PGEs—Co, V, Hg, Sb, Ni, Mo, Mn, Ir, and U—were also recovered. Zinc-dominant carbonate replacement deposits (Zn > Pb; Ag ± Cu) resemble Mississippi Valley Type (MVT) Zn-Pb deposits in that they occur in karst and fault breccias in Mississippian limestone where the southern margin of the regional late Paleozoic foreland basin adjoins Proterozoic crystalline rocks of the craton. They consist of calcite, dolomite, sphalerite, and galena with variably positive S isotope compositions ( δ 34 S values range from 2.5–13‰), and highly radiogenic Pb isotope compositions ( 206 Pb/ 204 Pb >19), typical of MVT deposits above crystalline Precambrian basement. These deposits may have formed when southward flow of saline fluids, derived from basinal and older sedimentary rocks, encountered thinner strata and pinch-outs against the craton, forcing fluid mixing and mineral precipitation in karst and fault breccias. Lead-dominant carbonate replacement deposits (Pb > Zn, Ag ± Cu ± Au) occur among other deposit types, often near porphyritic intrusions. They generally contain higher concentrations of precious metals than zinc-dominant deposits and relatively abundant iron oxides after pyrite. They share characteristics with copper ± precious metal-PGE and gold ± silver deposits including fine-grained quartz replacement of carbonate minerals in ore breccias and relatively low S and Pb isotope values ( δ 34 S values vary from 0–~4‰; 206 Pb/ 204 Pb <18.5). Copper ± precious metal-PGE deposits (Cu, Co, Ag, Au, Pd, and Pt) consist of Cu carbonate minerals (after chalcocite and chalcopyrite) and fine-grained quartz that have replaced breccia clasts and margins of fissures in Paleozoic limestones and dolomites near porphyritic intrusions. Gold ± silver deposits occur along contacts and within small-volume stocks and dikes of feldspar porphyry, one textural variety of porphyritic intrusions. Lead isotope compositions of copper ± precious metal-PGE, gold ± silver, and lead-dominant carbonate replacement deposits are similar to those of Mojave crust plutons, indicating derivation of Pb from 1.7 Ga crystalline basement or from Late Proterozoic siliciclastic sedimentary rocks derived from 1.7 Ga crystalline basement. Four texturally and modally distinctive porphyritic intrusions are exposed largely in the central part of the district: feldspar quartz porphyry, plagioclase quartz porphyry, feldspar biotite quartz porphyry, and feldspar porphyry. Intrusions consist of 64 to 70 percent SiO 2 and variable K 2 O/Na 2 O (0.14–5.33) that reflect proportions of K-feldspar and albite phenocrysts and megacrysts as well as partial alteration to K-mica; quartz and biotite phenocrysts are present in several subtypes. Albite may have formed during emplacement of magma in brine-saturated basinal strata, whereas hydrothermal alteration of matrix, phenocrystic, and megacrystic feldspar and biotite to K-mica, pyrite, and other hydrothermal minerals occurred during and after intrusion emplacement. Small volumes of garnet-diopside-quartz and retrograde epidote-mica-amphibole skarn have replaced carbonate rocks adjacent to one intrusion subtype (feldspar-quartz porphyry), but alteration of carbonate rocks at intrusion contacts elsewhere is inconspicuous. Uranium-lead ages of igneous zircons vary inconsistently from ~ 180 to 230 Ma and are too imprecise to distinguish age differences among intrusion subtypes; most ages are 210 to 225 Ma, yielding a mean of 217 ± 1 Ma. K-Ar and 40 Ar/ 39 Ar ages of magmatic (plagioclase, biotite) and hydrothermal (K-mica) minerals span a similar range (183–227 Ma), demonstrating broadly contemporaneous intrusion emplacement and hydrothermal alteration but allowing for multiple Late Triassic magmatic-hydrothermal events. Imprecision and range of isotopic ages may have resulted from burial beneath Mesozoic and Tertiary strata and multiple intrusion of magmas, causing thermal disturbance to Ar systems and Pb loss from zircons in intrusions. Separate late Paleozoic (zinc-dominant carbonate replacement deposits) and Late Triassic (all other deposits) mineralizing events are supported by form, distribution, and host rocks of metal deposits, by hydrothermal mineral assemblages, isotope compositions, metal abundances, and metal diversity, and by small intrusion volumes. These characteristics collectively distinguish the Goodsprings district from larger intrusion related carbonate replacement districts in the western United States. They can be used to evaluate proximity to unexposed porphyritic intrusions associated with PGE and gold ± silver mineralization.

Nevada↗

USGS2010

Rapid middle Miocene extension and unroofing of the southern Ruby Mountains, Nevada

Paleozoic rocks in the northern Ruby Mountains were metamorphosed during Mesozoic crustal shortening and Cenozoic magmatism, but equivalent strata in the southern Ruby Mountains were never buried deeper than stratigraphic depths prior to exhumation in the footwall of a west dipping brittle normal fault. In the southern Ruby Mountains, Miocene sedimentary rocks in the hanging wall of this fault date from 15.2 to 11.6 Ma and contain abundant detritus from the Paleozoic section. Apatite fission track and (U-Th)/He samples of the Eocene Harrison Pass pluton record rapid cooling that peaked ca. 17–15 Ma, while apatite fission track data from Jurassic plutons east and west of the southern Ruby Mountains indicate near-surface temperatures (<60°C) since the Cretaceous. We interpret these data to record rapid unroofing of the southern Ruby Mountains during slip on the west dipping brittle detachment between 17–16 and 10–12 Ma, followed by minor high-angle faulting. We interpret published Oligocene to early Miocene K-Ar biotite and zircon fission track dates from the Harrison Pass pluton to be partially reset rather than to directly record fault slip. Our new data, together with published data on the distribution and composition of Miocene basin fill, suggest that rapid middle Miocene slip took place on the west dipping brittle detachment that bounds the Ruby Mountains and East Humboldt Range for 150 km along strike. This fault was thus active during a period of rapid extension (ca. 17–15 to 12–10 Ma) documented widely across the northern Basin and Range Province.

Nevada↗

USGS2010

Episodic intrusion, internal differentiation, and hydrothermal alteration of the Miocene Tatoosh intrusive suite south of Mount Rainier, Washington

The Miocene Tatoosh intrusive suite south of Mount Rainier is composed of three broadly granodioritic plutons that are manifestations of ancestral Cascades arc magmatism. Tatoosh intrusive suite plutons have individually diagnostic characteristics, including texture, mineralogy, and geochemistry, and apparently lack internal contacts. New ion-microprobe U-Pb zircon ages indicate crystallization of the Stevens pluton ca. 19.2 Ma, Reflection-Pyramid pluton ca. 18.5 Ma, and Nisqually pluton ca. 17.5 Ma. The Stevens pluton includes rare, statistically distinct ca. 20.1 Ma zircon antecrysts. Wide-ranging zircon rare earth element (REE), Hf, U, and Th concentrations suggest late crystallization from variably evolved residual liquids. Zircon Eu/Eu*–Hf covariation is distinct for each of the Reflection-Pyramid, Nisqually, and Stevens plutons. Although most Tatoosh intrusive suite rocks have been affected by weak hydrothermal alteration, and sparse mineralized veins cut some of these rocks, significant base or precious metal mineralization is absent. At the time of shallow emplacement, each of these magma bodies was largely homogeneous in bulk composition and petrographic features, but, prior to final solidification, each of the Tatoosh intrusive suite plutons developed internal compositional variation. Geochemical and petrographic trends within each pluton are most consistent with differential loss of residual melt, possibly represented by late aplite dikes or erupted as rhyolite, from crystal-rich magma. Crystal-rich magma that formed each pluton evidently accumulated in reservoirs below the present level of exposure and then intruded to a shallow depth. Assembled by episodic intrusion, the Tatoosh intrusive suite may be representative of midsized composite plutonic complexes beneath arc volcanoes.

Washington↗

USGS2010

Dissolution-reprecipitation of igneous zircon in mid-ocean ridge gabbro, Atlantis Bank, Southwest Indian Ridge

Zircons recovered from oceanic gabbro exposed on Atlantis Bank, Southwest Indian Ridge, typically display oscillatory and sector zoning consistent with igneous crystallization from mafic magmas. In one rock (of twenty investigated), weak-oscillatory-zonation patterns are overprinted by secondary textural features characterized by mottled, convoluted and wavy internal zonation patterns that are frequently associated with secondary micron- to submicron-scale micro-porosity. These zircons are hosted in a felsic vein that intruded an oxide gabbro, both of which are cross-cut by monomineralic amphibole- and quartz-rich veinlets. Zircons with weak-oscillatory-zonation patterns record a weighted-average 206 Pb/ 238 U age of 12.76 ± 0.20 Ma (mswd = 1.5), and have high trace element concentrations [e.g., ΣREEs (∼ 0.4–2.2 wt.%), Y (∼ 0.6–2.8 wt.%), P (∼ 0.4–0.9 wt.%)], and Th/U (0.1–0.5). These zircons are anomalously old (≥1 Myr) relative to the magnetic age for this portion of oceanic crust (11.75 Ma). In contrast, zircons with non-igneous, secondary textures have a younger weighted-average 206 Pb/ 238 U age of 12.00 ± 0.16 Ma (mswd = 1.7), and have lower trace element concentrations [e.g., ΣREEs (∼ 0.2–0.8 wt.%), Y (∼ 0.3–1.0 wt.%), P (∼ 0.1–0.3 wt.%)], and slightly lower Th/U (0.1–0.3). The weighted-average age of these zircons is similar to the magnetic anomaly age, and other 206 Pb/ 238 U ages of nearby rocks. We do not observe a correlation between crystallographic misorientation, internal texture, or trace element chemistry. We suggest that the decrease in trace element concentrations associated with the development of non-igneous alteration textures is attributed to the purging of non-essential structural constituent cations from the zircon crystal lattice at amphibolite-facies conditions. The mechanism of alteration/re-equilibration was likely an interface-coupled dissolution–reprecipitation processes that affected pre-existing, anomalously old zircons during shallow-level magmatic construction of Atlantis Bank at ∼ 12.0 Ma.

Chemical Geology↗

USGS2010

Thermomagmatic evolution of Mesoproterozoic crust in the Blue Ridge of SW Virginia and NW North Carolina: Evidence from U-Pb geochronology and zircon geothermometry

New geologic mapping, petrology, and U-Pb geochronology indicate that Mesoproterozoic crust near Mount Rogers consists of felsic to mafic meta-igneous rocks emplaced over 260 m.y. The oldest rocks are compositionally diverse and migmatitic, whereas younger granitoids are porphyritic to porphyroclastic. Cathodoluminescence imaging indicates that zircon from four representative units preserves textural evidence of multiple episodes of growth, including domains of igneous, metamorphic, and inherited origin. Sensitive high-resolution ion microprobe (SHRIMP) trace-element analyses indicate that metamorphic zircon is characterized by lower Th/U, higher Yb/Gd, and lower overall rare earth element (REE) concentrations than igneous zircon. SHRIMP U-Pb isotopic analyses of zircon define three episodes of magmatism: 1327 ± 7 Ma, 1180–1155 Ma, and 1061 ± 5 Ma. Crustal recycling is recorded by inherited igneous cores of 1.33–1.29 Ga age in 1161 ± 7 Ma meta-monzogranite. Overlapping ages of igneous and metamorphic crystallization indicate that plutons of ca. 1170 and 1060 Ma age were emplaced during episodes of regional heating. Local development of hornblende + plagioclase + quartz ± clinopyroxene indicates that prograde metamorphism at 1170–1145 Ma and 1060–1020 Ma reached upper-amphibolite-facies conditions, with temperatures estimated using Ti-in-zircon geothermometry at ~740 ± 40 °C during both episodes. The chemical composition of 1327 ± 7 Ma orthogranofels from migmatite preserves the first evidence of arc-generated rocks in the Blue Ridge, indicating a subduction-related environment that may have been comparable to similar-age systems in inliers of the Northern Appalachians and the Composite Arc belt of Canada. Granitic magmatism at 1180–1155 Ma and ca. 1060 Ma near Mount Rogers was contemporaneous with anorthosite-mangerite-charnockite-granite (AMCG) plutonism in the Northern Appalachian inliers and Canadian Grenville Province. Metamorphism at ca. 1160 and 1060 Ma correlates temporally with the Shawinigan orogeny and Ottawan phase of the Grenvillian orogeny, respectively, suggesting that the Blue Ridge was part of Rodinia dating back to ca. 1180 Ma.

North Carolina;Virginia↗

USGS2010

Reconnaissance geochronology of tuffs in the Miocene Barstow Formation: Implications for basin evolution and tectonics in the central Mojave Desert

Early to middle Miocene lacustrine strata of the Barstow Formation are well dated in just a few places, limiting our ability to infer basin evolution and regional tectonics. At the type section in the Mud Hills, previous studies have shown that the lacustrine interval of the Barstow Formation is between ~16.3 Ma and ~13.4 Ma. Elsewhere, lake beds of the Barstow Formation have yielded vertebrate fossils showing the Hemingfordian/Barstovian transition at ~16 Ma but are otherwise poorly dated. In an attempt to clarify the age and depositional environments of the lake deposits, we are mapping the Barstow Formation and dating zircons from interbedded tuffs, as well as testing ash-flow tuffs for the distinctive remanent magnetization direction of the widespread Peach Spring Tuff. Thus far, our new U-Pb zircon ages indicate that the Barstow lake beds contain tuff beds as old as 19.1 Ma and as young as 15.3 Ma. At Harvard Hill, Barstow lake beds contain a thick tuff dated at 18.7 Ma. On the basis of zircon ages, mineralogy, zircon chemistry, and paleomagnetic results, we consider the thick tuff to be a lacustrine facies of the Peach Spring Tuff. We have identified the Peach Spring Tuff by similar methods at eight localities over a broad area, providing a timeline for several fluvial and lacustrine sections. The new dates indicate that long-lived lacustrine systems originated before 19 Ma and persisted to at least 15 Ma. The onset of lacustrine conditions predates the Peach Spring Tuff in most Barstow Formation sections and may be older than 19.5 Ma in some places. The new data indicate that the central Mojave Desert contained narrow to broad lake basins during and after extension, and that Barstow lacustrine deposits did not exclusively postdate extensional tectonics. At present, it is unclear whether several separate, small lake basins coexisted during the early to middle Miocene, or if instead several small early Miocene basins gradually coalesced over about 6 million years to form one or two large middle Miocene lake basins.

California↗