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Joseph L. Wooden

Publications and source records attributed to Joseph L. Wooden.

At least 37 records · Page 2Linked to original sources
USGS2010

Stratigraphy, age, and depositional setting of the Miocene Barstow Formation at Harvard Hill, central Mojave Desert, California

New detailed geologic mapping and geochronology of the Barstow Formation at Harvard Hill, 30 km east of Barstow, CA, help to constrain Miocene paleogeography and tectonics of the central Mojave Desert. A northern strand of the Quaternary ENE-striking, sinistral Manix fault divides the Barstow Formation at Harvard Hill into two distinct lithologic assemblages. Strata north of the fault consist of: a green rhyolitic tuff, informally named the Shamrock tuff; lacustrine sandstone; partially silicified thin-bedded to massive limestone; and alluvial sandstone to pebble conglomerate. Strata south of the fault consist of: lacustrine siltstone and sandstone; a rhyolitic tuff dated at 19.1 Ma (U-Pb); rock-avalanche breccia deposits; partially silicified well-bedded to massive limestone; and alluvial sandstone and conglomerate. Our U-Pb zircon dating of the Shamrock tuff by SHRIMP-RG yields a peak probability age of 18.7 ± 0.1 Ma. Distinctive outcrop characteristics, mineralogy, remanent magnetization, and zircon geochemistry (Th/U) suggest that the Shamrock tuff represents a lacustrine facies of the regionally extensive Peach Spring Tuff (PST). Here we compare zircon age and geochemical analyses from the Shamrock tuff with those of the PST at Stoddard Wash and provide new insight into the age of zircon crystallization in the PST rhyolite. Results of our field studies show that Miocene strata at Harvard Hill mostly accumulated in a lacustrine environment, although depositional environments varied from a relatively deep lake to a very shallow lake or even onshore setting. Rock-avalanche breccias and alluvial deposits near the base of the exposed section indicate proximity to a steep basin margin and detrital studies suggest a southern source for coarse-grained deposits; therefore, we may infer a southern basin-margin setting at Harvard Hill during the early Miocene. Our geochronology demonstrates that deposition of the Barstow Formation at Harvard Hill extended from before ~19.1 Ma until well after ~18.7 Ma, similar to timing of Barstow Formation lake deposition in the Calico Mountains but at least 3 million years older than comparable lacustrine facies in the Mud Hills type section. These observations are consistent with either of two paleogeographic models: westward transgression of lacustrine environments within a single large basin, or sequential development of geographically distinct eastern and western sub-basins.

California↗
USGS2009

On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ≈ 0.7, a SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La) N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

Contributions to Mineralogy and Petrology↗
USGS2008

Protracted construction of gabbroic crust at a slow spreading ridge: Constraints from 206 Pb/ 238 U zircon ages from Atlantis Massif and IODP Hole U1309D (30°N, MAR)

Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages of 24 samples from oceanic crust recovered in Integrated Ocean Drilling Program (IODP) Hole U1309D and from the surface of Atlantis Massif, Mid-Atlantic Ridge (MAR) (30°N) document a protracted history of accretion in the footwall to an oceanic detachment fault. Ages for 18 samples of evolved Fe-Ti oxide gabbro and felsic dikes collected 40–1415 m below seafloor in U1309D yield a weighted mean of 1.20 ± 0.03 Ma (mean square of weighted deviates = 7.1). However, the ages range from 1.08 ± 0.07 Ma and 1.28 ± 0.05 Ma indicating crustal construction occurred over a minimum of 100–200 ka. The zircon ages, along with petrologic observations, indicate at least 2 major periods of intrusive activity with age peaks separated by 70 ka. The oldest ages are observed below 600 mbsf, an observation inconsistent with models requiring constant depth melt intrusion beneath a detachment fault. The data are most consistent with a “multiple sill” model whereby sills intrude at random depths below the ridge axis over a length scale greater than 1.4 km. Zircon ages from broadly spaced samples collected along the southern ridge of Atlantis Massif yield a detachment fault slip rate of 28.7 ± 6.7 mm/a and imply significant asymmetric plate spreading (up to 100% on the North American plate) for at least 200 ka during core complex formation.

Geochemistry, Geophysics, Geosystems↗
USGS2008

Magmatism and tectonics in a tilted crustal section through a continental arc, eastern Transverse Ranges and southern Mojave Desert

This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.

Arizona, California↗
USGS2006

Detrital zircon provenance of the Late Triassic Songpan-Ganzi complex: Sedimentary record of collision of the North and South China blocks

Using detrital zircon geochronology, turbidite deposystems fed from distinct sediment sources can be distinguished within the Songpan-Ganzi complex, a collapsed Middle to Late Triassic turbidite basin of central China. A southern Songpan-Ganzi deposystem initially was sourced solely by erosion of the Qinling-Dabie orogen during early Late Triassic time, then by Qinling-Dabie orogen, North China block, and South China block sources during middle to late Late Triassic time. A northern Songpan-Ganzi system was sourced by erosion of the Qinling-Dabie orogen and the North China block throughout its deposition. These separate deposystems were later tectonically amalgamated to form one complex and then uplifted as the eastern Tibet Plateau. ?? 2006 Geological Society of America.

Geology↗
USGS2006

The Ajo Mining District, Pima County, Arizona: Evidence for middle Cenozoic detachment faulting, plutonism, volcanism, and hydrothermal alteration

Introduction: The Ajo porphyry copper deposit and surrounding Upper Cretaceous rocks have been separated from their plutonic source and rotated by detachment faulting. Overlying middle Cenozoic sedimentary and volcanic rocks have been tilted and show evidence for two periods of rotation. Following these rotations, a granitic stock (23.7?0.2 Ma) intruded basement rocks west of the Ajo deposit. This stock was uplifted 2.5 km to expose deep-seated Na-Ca alteration.

Arizona↗
USGS2005

Integrated provenance analysis of a complex orogenic terrane: Mesozoic uplift of the Bogda Shan and Inception of the Turpan-Hami Basin, NW China

We employ petrographic and advanced geochemical techniques to better document the evolution of the Turpan–Hami basin based on the unique geologic histories of the arc terranes that served as potential sources of Turpan–Hami deposits. First, a provenance study of Permian through Cretaceous sandstone of the Turpan–Hami basin reveals temporal and spatial changes in dominant source terranes that provided detritus to the basin. Volcanic-lithic-rich Upper Permian sandstone (mean Qm 19 F 18 Lt 63 ; Qp 7 Lvm 89 Lsm 4 ; Qm 48 P 39 K 13 ) followed by more quartzose compositions in Triassic sandstone (mean Qm 41 F 19 Lt 40 ; Qp 20 Lvm 75 Lsm 5 ; Qm 68 P 21 K 11 ) indicate progressive unroofing of the extinct northern and central Tian Shan arc terranes to the south of Turpan–Hami. A sharp change to sedimentary-lithic-rich Lower Jurassic sandstone (mean Qm 47 F 16 Lt 37 ; Qp 16 Lvm 42 Lsm 42 ; Qm 75 P 12 K 13 ) overlain by a return to volcanic–lithic-rich Middle Jurassic sandstone (mean Qm 39 F 21 Lt 40 ; Qp 14 Lvm 51 Lsm 35 ; Qm 65 P 21 K 14 ) points to the initial uplift and unroofing of the largely andesitic Bogda Shan to the north, which first shed its sedimentary cover as it emerged to become the partition between the Turpan–Hami and southern Junggar basins. Second, geochronological, trace-element, and Sm-Nd isotopic variations among granitoids in the late Paleozoic Tian Shan orogenic belt provide a further test of Mesozoic uplift of the Bogda Shan. On the basis of previous models of crustal compositions throughout the South, Central, and North Tian Shan, Bogda Shan, and East and West Junggar terranes, we infer that isotopically enriched granitic cobbles (average εNd i = −0.50, n = 6) contained in Lower Triassic deposits in the north-central Turpan–Hami basin were derived from the continental crustal Central Tian Shan terrane, south of Turpan–Hami, and not from the more oceanic North Tian Shan, Bogda Shan, and East and West Junggar terranes, north of the Turpan–Hami basin. We therefore infer that the ancestral Bogda Shan had not been uplifted by the Early Triassic, and that prior to this time, a unified Junggar–Turpan–Hami basin existed during Late Permian deposition of extensive lacustrine deposits

Turpan-Hami Basin↗
USGS2005

Ion microprobe measurement of strontium isotopes in calcium carbonate with application to salmon otoliths

The ion microprobe has the capability to generate high resolution, high precision isotopic measurements, but analysis of the isotopic composition of strontium, as measured by the 87 Sr/ 86 Sr ratio, has been hindered by isobaric interferences. Here we report the first high precision measurements of 87 Sr/ 86 Sr by ion microprobe in calcium carbonate samples with moderate Sr concentrations. We use the high mass resolving power (7000 to 9000 M.R.P.) of the SHRIMP-RG ion microprobe in combination with its high transmission to reduce the number of interfering species while maintaining sufficiently high count rates for precise isotopic measurements. The isobaric interferences are characterized by peak modeling and repeated analyses of standards. We demonstrate that by sample-standard bracketing, 87 Sr/ 86 Sr ratios can be measured in inorganic and biogenic carbonates with Sr concentrations between 400 and 1500 ppm with ∼2‰ external precision (2σ) for a single analysis, and subpermil external precision with repeated analyses. Explicit correction for isobaric interferences (peak-stripping) is found to be less accurate and precise than sample-standard bracketing. Spatial resolution is ∼25 μm laterally and 2 μm deep for a single analysis, consuming on the order of 2 ng of material. The method is tested on otoliths from salmon to demonstrate its accuracy and utility. In these growth-banded aragonitic structures, one-week temporal resolution can be achieved. The analytical method should be applicable to other calcium carbonate samples with similar Sr concentrations.

Geochimica et Cosmochimica Acta↗
USGS2005

U-Pb zircon ages and Pb isotope geochemistry of gold deposits in the Carolina slate belt of South Carolina

Volcanic rocks of the Persimmon Fork Formation host the largest known gold mines of the Carolina slate belt. U-Pb (SHRIMP) zircon ages have been obtained from rocks closely associated with pyrite-enargite-gold deposits at Brewer (quartz-topaz rhyolite breccia from the argillic alteration zone in the Brewer pit and felsic ash-flow tuff from the quartz sericite alteration zone), from the disseminated and semimassive pyrite-gold deposits at Haile (crystal lithic rhyolitic ash-flow tuffs from the Champion pit), and from the Ridgeway deposit (felsic ash-flow tuff from the stratigraphic host of the North pit gold deposit). Generally, the zircons are fine grained, fractured, and contain crystal imperfections (corrosion, inclusions, and pits). 206 Pb/ 238 U zircon spot ages for all deposits span a wide range, mostly from 400 to 760 Ma. Inclusions and cores indicative of inherited domains in the zircons were not found, and only a few analyses range from 1.1 to 1.8 Ga. A distinct xenocrystic zircon population was not identified. The 206 Pb/ 238 U weighted age averages of zircon indicate the following crystallization dates for the volcanic and volcaniclastic rocks closely associated with the gold deposits: 550 &plusmn; 3 Ma for Brewer, 553 &plusmn; 2 Ma for Haile, and 556 &plusmn; 2 Ma for the Ridgeway deposit. These zircon crystallization ages represent close estimates of the age of the original gold mineralizing events. Younger zircon spot ages can be attributed to the effects of Paleozoic regional metamorphism. Pb isotope compositions of sulfide minerals (galena, pyrite, enargite, sphalerite, chalcopyrite, and molybdenite) and silicate minerals (K-feldspar, and sericite) in the gold deposits help to constrain the sources of fluids and metals during the mineralizing events. The deposits are pyrite rich, containing multiple generations of pyrite, including early-crystallized pyrite that is closely associated with the original gold mineralizing event, as well as recrystallized pyrite formed in response to Paleozoic metamorphism. Pb isotope compositions of pyrite span a wide range, including the most radiogenic values for the sulfides. Galena and K-feldspar are not abundant but where present they are typically the least radiogenic minerals. Galena has a limited range of Pb isotope compositions that are representative of the gold deposits as a group ( 206 Pb/ 204 Pb = 18.020&ndash;18.326, 207 Pb/ 204 Pb = 15.550&ndash;15.639, 208 Pb/ 204 Pb = 37.605&ndash;38.286). Values of 207 Pb/ 204 Pb straddle the average crustal Pb growth curve, consistent with contributions involving the mantle and continental crust. Whole-rock Pb isotope compositions of volcanic and volcaniclastic rocks of the Persimmon Fork Formation nearly match the range for sulfides in the gold deposits. Subtle regional contrasts in Pb isotope compositions exist among the deposits. Sulfide minerals from Barite Hill (e.g., galena 206 Pb/ 204 Pb < 18.077) in southern South Carolina are generally less radiogenic than sulfides from Ridgeway ( 206 Pb/ 204 Pb > 18.169), Haile ( 206 Pb/ 204 Pb > 18.233), and Brewer ( 206 Pb/ 204 Pb > 18.311) in northern South Carolina. Because Pb isotope compositions of basement rocks from Grenville massifs in the southern Appalachians and sulfide minerals from the gold deposits do not match, a direct genetic connection cannot be established. Diversity in values of 206 Pb/ 204 Pb and the relatively high values of 207 Pb/ 204 Pb suggest that the deposits evolved adjacent to or closely related to continental blocks, perhaps linked to a back-arc tectonic setting. Among potential younger analogues of the slate belt gold deposits are the sulfide deposits of the Okinawa trough in the western Pacific. Mantle-derived isotopic contributions were more important at Barite Hill in southern South Carolina, the least radiogenic among the deposits where oceanic crust had developed, than at Brewer, Haile, and Ridgeway in northern South Carolina where rifting thinned the continental crust.

Georgia, North Carolina, South Carolina, Virginia↗
USGS2004

Proterozoic tectonostratigraphy and paleogeography of central Madagascar derived from detrital zircon U-Pb age populations

Detrital zircon U‐Pb ages determined by SHRIMP distinguish two clastic sequences among Proterozoic metasedimentary rocks from central Madagascar. The Itremo Group is older: zircon data, stromatolite characteristics, and carbon isotope data all point to a depositional age around 1500–1700 Ma. The Molo Group is younger, deposited between ∼620 Ma (the age of the youngest zircon) and ∼560 Ma (the age of metamorphic overgrowths on detrital cores). Geochronologic provenance analysis of the Itremo Group points to sources in East Africa as well as local sources in central and southern Madagascar but provides no evidence for a detrital contribution from northern and eastern Madagascar nor from southern India. Detrital zircon and sedimentologic similarities between rocks of the Itremo Group and the Zambian Muva Supergroup suggest a lithostratigraphic correlation between the two. The Molo Group has a strong 1000–1100 Ma detrital signature that also indicates an east African provenance and suggests a Neoproterozoic geographic connection with Sri Lanka but shows no indication of input from the Dharwar craton and eastern Madagascar. Central Madagascar was probably juxtaposed with the Tanzanian craton in the Paleo‐ and Mesoproterozoic, whereas northern and eastern Madagascar were connected to India. Internal assembly of Madagascar postdates Neoproterozoic Molo Group sedimentation and is likely to have occurred at about 560 Ma.

Journal of Geology↗
USGS2004

Geochemistry of quartzofeldspathic gneisses and metamorphic mafic rocks of the Indian Creek and Pony–Middle Mountain Metamorphic Suites, Tobacco Root Mountains, Montana

Quartzofeldspathic gneisses and metamorphic mafic rocks are the dominant lithologies in the Indian Creek and Pony–Middle Mountain Metamorphic Suites of the Tobacco Root Mountains. Field relations, geochemical discriminant analysis, and isotopic systematics indicate that these rocks derive from a bimodal volcanic suite ca. 3.3 Ga. The quartzofeldspathic gneisses contain sodic rocks of the tonalite-trond-hjemite-granodiorite suite as well as potassic varieties. This suite of rocks most likely contains some lithologies derived from sedimentary or volcaniclastic sources, and there is evidence that alkali metasomatism occurred prior to or during subsequent major tectonothermal events. The entire suite of gneisses and metamorphic mafic rocks has geochemical characteristics that are indicative of an active continental arc setting, with deposition most likely in an extensional, backarc setting similar to the Mesozoic through Tertiary rocks of the eastern Sierra Nevada Mountains or Mojave Desert. The formation of these rocks represents an early, distinct stage of crustal evolution that preceded the (unconformable?) deposition of one or more platform-type sedimentary sequences (e.g., marbles, pelitic schists, quartzites, banded iron formations). All primary lithologic contacts and textures or structures indicative of possible protoliths have been largely obliterated due to transposition during Archean and Paleoproterozoic (ca. 2.4 and ca. 1.8 Ga) deformation and metamorphism.

Montana↗
USGS2004

Age and evolution of the Precambrian crust of the Tobacco Root Mountains, Montana

U-Pb analyses of zircons from gneisses, anatectic leucosome, metasedimentary rocks, and a younger (metamorphosed) mafic dike from the Tobacco Root Mountains of southwestern Montana document a Precambrian history that extends from at least 3.90–1.77 Ga. The oldest U-Pb age reported here (3.8 Ga) is from a detrital zircon from a quartzite within the Spuhler Peak Metamorphic Suite, although younger ages of clearly detrital grains suggest the protolith was deposited subsequent to 3.2 Ga. Alternatively, a Pb-Pb age of ca. 2.45 Ga from a single subhedral zircon from this quartzite suggests the quartzite, and perhaps other Spuhler Peak Metamorphic Suite lithologies, may have formed in the Proterozoic. An Archean age, however, seems most compatible with the Archean Sm-Nd model ages of mafic and metasedimentary components of the Spuhler Peak Metamorphic Suite and the age distribution of zircons from the quartzite, which is very similar to the age distribution present in Archean quartzites in the region. The Spuhler Peak Metamorphic Suite lies in tectonic contact with volumetrically dominant, Archean, quartzofeldspathic gneisses and intercalated metasedimentary rocks. The protoliths of these gneisses were apparently emplaced 3.2–3.4 Ga, and are interpreted to be the basement upon which the intercalated (meta)sedimentary rocks were deposited. U-Pb analyses of zircons from anatectic leucosome near the boundary between the gneisses and the Spuhler Peak Metamorphic Suite, however, yield a significant population of 1.77 Ga grains, which are interpreted to have crystallized from the leucosome. All other grains are Archean (to 3.48 Ga) and interpreted to derive from the metasedimentary source of the leucosome. In addition, U-Pb analyses of zircons extracted from a granulite facies mafic dike that cuts across Archean gneissic banding indicate the dike was intruded at 2.06 Ga, but reached granulite facies at 1.76 Ga. Structural, petrologic, and geochronologic data suggest all lithologies experienced granulite facies metamorphism at ca. 1.77 Ga and that the Spuhler Peak Metamorphic Suite was tectonically emplaced after 2.06 Ga, but before 1.77 Ga. This Paleoproterozoic tectonic activity is most likely a result of burial during terrane collision (e.g., the juxtaposition of the Wyoming and Hearne provinces) and/or to postcollisional mafic underplating.

Montana↗
USGS2004

Ad Duwayhi, Saudi Arabia: Geology and geochronology of a neoproterozoic intrusion-related gold system in the Arabian shield

The Ad Duwayhi gold deposit, located in the east-central part of the Arabian shield, is the newest gold discovery in Saudi Arabia. Exploration to date has identified a gold resource of greater than 1 million ounces (oz) with significant potential for expansion. Gold mineralization was closely associated, in time and space, with emplacement of a late- to postorogenic, intracrustal, northwest-oriented granite body (659 &plusmn; 7 Ma) and comagmatic square quartz porphyry (646 &plusmn; 11 Ma), a hypabyssal and perhaps younger phase of the granite. Mineralization was largely confined to northeast-striking, southeast-dipping fault zones. Hydrothermal alteration consisted of early biotitic alteration overprinted by sericitic alteration. Sericitic alteration was coincident with gold mineralization and produced a quartz-sericite-carbonate-pyrite-rutile mineral assemblage, found both as vein fill and wall-rock alteration products. Mineralization styles, in the following general paragenetic sequence, include (1) quartz-molybdenite veins in and near the granite stock, (2) low-grade gold-bearing quartz vein breccia in and along the margins of the granite stock, (3) gold-bearing stockwork and sheeted quartz veins, and (4) massive to banded gold-rich tabular quartz veins. The gold-bearing stockwork, sheeted, and tabular veins are generally spatially associated with square quartz porphyry dikes and more distal to the granite stock. Mineralized zones at Ad Duwayhi are characterized by low sulfide and base metal content and gold/silver ratios of approximately 6/1. Gold shows no significant correlation with other metals, except lead, and moderate correlation with silver. Re-Os dating of molybdenite from a quartz-molybdenite vein and a tabular quartz vein with cogenetic gold produced robust ages of 655.6 &plusmn; 2.7 and 649.9 &plusmn; 2.3 Ma, respectively, documenting that gold mineralization and crystallization of granite and square quartz porphyry were, within uncertainty, coeval events. This age correlation combined with granite textural features, the presence of unidirectional solidification textures in granite and square quartz porphyry, and the nature and time-space distribution of mineralization styles, all indicate that mineralization evolved in and near the interface between a crystallizing magma and the surrounding rocks and, thus, is consistent with an intrusion-related genesis. In light of our findings at Ad Duwayhi, a reassessment of similar intrusion-hosted deposits in the Arabian shield is warranted, and areas of late- to postorogenic plutonism, particularly in the Afif composite terrane, should be considered prospective for intrusion-related gold systems.

Economic Geology↗
USGS2003

Monthly Strontium/Calcium oscillations in symbiotic coral aragonite: Biological effects limiting the precision of the paleotemperature proxy

In thermodynamic equilibrium with sea water the Sr/Ca ratio of aragonite varies predictably with temperature and the Sr/Ca ratio in coral have thus become a frequently used proxy for past Sea Surface Temperature (SST). However, biological effects can offset the Sr/Ca ratio from its equilibrium value. We report high spatial resolution ion microprobe analyses of well defined skeletal elements in the reef‐building coral Porites lutea that reveal distinct monthly oscillations in the Sr/Ca ratio, with an amplitude in excess of ten percent. The extreme Sr/Ca variations, which we propose result from metabolic changes synchronous with the lunar cycle, introduce variability in Sr/Ca measurements based on conventional sampling techniques well beyond the analytical precision. These variations can limit the accuracy of Sr/Ca paleothermometry by conventional sampling techniques to about 2°C. Our results may help explain the notorious difficulties involved in obtaining an accurate and consistent calibration of the Sr/Ca vs. SST relationship.

Geophysical Research Letters↗
USGS2001

SHRIMP-RG U-Pb zircon geochronology of mesoproterozoic metamorphism and plutonism in the southwesternmost United States

Mesoproterozoic intrusive and granulite‐grade metamorphic rocks in southern California have been inferred to be exotic to North America on the basis of perceived chronologic incompatibility with autochthonous cratonal rocks. Ion microprobe geochronology indicates that zircons in granulite‐grade gneisses, dated at 1.4 Ga using conventional methods, are composed of 1.68–1.80‐Ga cores and 1.19‐Ga rims. These Early Proterozoic gneisses were metamorphosed at extremely high temperatures and moderate pressures during emplacement of the 1.19‐Ga San Gabriel anorthosite complex. The lack of a 1.4‐Ga metamorphic event suggests that Proterozoic rocks in this region, rather than being exotic to North America, may in fact be a midcrustal window into Mesoproterozoic crustal evolutionary processes in southwestern North America.

California↗
USGS2000

Geochronology of the proterozoic basement of southwesternmost North America, and the origin and evolution of the Mojave crustal province

The Proterozoic Baldwin gneiss in the central Transverse Ranges of southern California, a part of the Mojave crustal province, is composed of quartzofeldspathic gneiss and schist, augen and granitic gneiss, trondhjemite gneiss, and minor quartzite, amphibolite, metagabbro, and metapyroxenite. Sensitive high resolution ion microprobe (SHRIMP) data indicate that augen and granitic gneisses comprise a magmatic arc intrusive suite emplaced between 1783 ± 12 and 1675 ± 19 Ma, adjacent to or through thinned Archean crust. High U/Th rims on zircons in most samples suggest an early metamorphic event at ∼1741 Ma, but peak amphibolite facies metamorphism and penetrative, west vergent deformation occurred after 1675 Ma. The Baldwin gneiss is part of a regional allochthon emplaced by west vergent deformation over a Proterozoic shelf-slope sequence (Joshua Tree terrane). We hypothesize that emplacement of this regional allochthon occurred during a late Early or Middle Proterozoic arc-continent collision along the western margin of Laurentia.

Tectonics↗