Ultrahigh‐pressure (UHP) rocks in North‐East Greenland lie within a larger region of high‐pressure Laurentian crust formed in the overthickened upper plate of the collision with Baltica. ...Coesite‐bearing zircon dates UHP metamorphism to 365–350 Ma, which formed at the end of the Caledonian collision as a result of intracontinental subduction facilitated by strike‐slip faults that broke the lithosphere. Rutile is the stable Ti‐bearing phase at UHP, while titanite forms on the retrograde path. Trace elements and U‐Pb in titanite were analyzed for six UHP gneisses. Zr‐in‐titanite temperatures range from 764 to 803°C and lie on the isobaric part of the pressure‐temperature path at 1.2 GPa, which fits Ti‐phase stability determined by thermodynamic modeling. Large (>600 μm), zoned titanite preserves three distinct trace element patterns that are due to metamorphism, melting and garnet breakdown. Weighted mean 206Pb/238U ages range from 347 ± 5 Ma to 320 ± 11 Ma, but age variation as a function of trace element domain for individual samples is not resolvable within uncertainty. Titanite records a prolonged period of exhumation that is also seen in the zircon record, where phengite decompression melting started at ca. 347 Ma, leucosome emplacement accompanied retrograde metamorphism from 350 to 330 Ma; and titanite grew during isobaric cooling from 345 to 320 Ma when the UHP rocks stalled at lower crustal levels. The same transforms that originally break the lithosphere play a significant role in channeling the UHP rocks back to the lower crust via buoyancy driven exhumation, after which time titanite formed.
Key Points
A slice of Laurentian crust was subducted to ultrahigh‐pressure (UHP) conditions in a strike‐slip regime after Caledonian crustal thickening
Titanite grows on an isobaric cooling path from 345 to 320 Ma, after exhumation to the lower crust, consistent with previous zircon dating
Exhumation of an UHP slice to the lower crust was driven by buoyancy forces also in a translational setting
New in situ laser ablation-inductively coupled plasma-mass spectrometry and sensitive high-resolution ion microprobe U–Pb geochronology of zircons from the Idaho batholith and spatially overlapping ...Challis intrusions reveals a series of discrete magmatic belts of different ages and compositions. Following the accretion of the Blue Mountains province to North America along the Salmon River suture zone, two compositionally diverse belts of metaluminous plutons formed both adjacent to the suture and well inboard of it. These were constructed from ~100 to 85 Ma and were followed by a voluminous pulse of peraluminous magmatism, forming the bulk of the Atlanta lobe and largest fraction of the batholith between ~80 and 67 Ma. Around 70 Ma, a later and more spatially restricted suite of metaluminous plutons formed around the Bitterroot lobe of the batholith. This was followed by another pulse of voluminous peraluminous magmatism in the Bitterroot lobe, lasting from ~66 to 54 Ma. The changes from low volume metaluminous to high volume peraluminous magmatism may reflect a combination of changes in the angle and segmentation of the subducting Farallon plate and over thickening of the continental lithosphere. All of these features were then cut by plutons and dikes associated with the Challis volcanic field, lasting from ~51 to 43 Ma. Inherited components are pervasive in zircons from most phases of the batholith. While Precambrian components are very common, zircons also often contain cores or mantles that are 5–20 million years older than their rims. This suggests that the early phases of the batholith were repeatedly cannibalized by subsequent magmas. This also implies that the older suites may have been originally more aerially extensive than their currently exposed forms.
U-Pb dating of zircon by LA-ICP-MS Chang, Zhaoshan; Vervoort, Jeffery D.; McClelland, William C. ...
Geochemistry, geophysics, geosystems : G3,
20/May , Volume:
7, Issue:
5
Journal Article
Peer reviewed
Open access
In this study we used LA‐ICP‐MS (laser ablation–inductively coupled plasma–mass spectrometry) to determine U‐Pb ages of 5 zircon samples of known age (∼1800 Ma to ∼50 Ma) in order to determine the ...reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double‐focusing ICP‐MS coupled with a New Wave Research UP‐213 laser system. The laser ablation pit sizes ranged from 30 to 40 μm in diameter. Laser‐induced time‐dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA‐ICP‐MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP‐MS, can be used for the U‐Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.
Abstract
Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on ...northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.
The Gravina belt consists of Upper Jurassic through Lower Cretaceous marine clastic strata and mafic‐intermediate volcanic rocks that occur along the western flank of the Coast Mountains in southeast ...Alaska and coastal British Columbia. This report presents U‐Pb ages and Hf isotope determinations of detrital zircons that have been recovered from samples collected from various stratigraphic levels and from along the length of the belt. The results support previous interpretations that strata in the western portion of the Gravina belt accumulated along the inboard margin of the Alexander‐Wrangellia terrane and in a back‐arc position with respect to the western Coast Mountains batholith. Our results are also consistent with previous suggestions that eastern strata accumulated along the western margin of the inboard Stikine, Yukon‐Tanana, and Taku terranes and in a fore‐arc position with respect to the eastern Coast Mountains batholith. The history of juxtaposition of western and eastern assemblages is obscured by subsequent plutonism, deformation, and metamorphism within the Coast Mountains orogen, but may have occurred along an Early Cretaceous sinistral transform system. Our results are inconsistent with models in which an east‐facing subduction zone existed along the inboard margin of the Alexander‐Wrangellia terrane during Late Jurassic‐Early Cretaceous time.
Key Points
Strata of the Gravina belt formed in two different sedimentary basins
Western strata are related to the outboard Alexander and Wrangellia terranes
Eastern strata formed outboard of the Yukon‐Tanana, Taku, and Stikine terranes
High-pressure low-temperature rocks from Svalbard are an excellent target for studying metamorphic reactions in Phanerozoic subduction zones. This study reveals the presence of monazite in an ...eclogite and a blueschist from the Vestgötabreen Complex, southwestern Svalbard. In order to investigate the monazite-forming reaction, we obtained pressure–temperature estimates coupled with U–Pb and Lu–Hf dating. Combined geothermobarometry allows to constrain three evolutionary stages of garnet growth in the eclogite: nucleation (1.6 ± 0.3 GPa at 460 ± 60 °C), peak-pressure (2.3 ± 0.3 GPa at 507 ± 60 °C), and peak-temperature (2.1 ± 0.3 GPa at 553 ± 60 °C). A zircon age of 482 ± 10 Ma is interpreted to belong to the prograde part of the pressure–temperature path. Monazite forms inclusions within garnet rims, or it is surrounded by allanite and apatite, altogether forming pseudomorphs of a tabular shape in the matrix. Textures, geothermobarometry and geochronology support the conclusion the monazite formed under high-pressure conditions at 471 ± 6 Ma. We propose that the monazite crystallization in the eclogite happened due to a decomposition of accessory phases during the decompression after peak-pressure of the metamorphic cycle. Monazite in the blueschist occurs as inclusions in garnet cores and gives an indicative age of 486 ± 6 Ma, which is interpreted to reflect the prograde growth of the garnet. Lu–Hf garnet dating resolves an age of peak-pressure metamorphism in the blueschist at 471.1 ± 4 Ma under conditions of 2.0 ± 0.03 GPa and 500 ± 30 °C. The Vestgötabreen Complex provides evidence for an early Ordovician modern-style subduction system in the proximity of the Baltica margin. Hence, this study also supports the tectonic models that favour a mixed Baltican and Laurentian provenance of south-western Svalbard.
Neoproterozoic to early Paleozoic strata exposed along the northeastern margin of North America (Franklinian Basin) record a prolonged history of rifting and passive margin development. An episode of ...Ediacaran–Cambrian extension is potentially recorded in volcanic and sedimentary rocks of the Yelverton Formation, exposed on northern Ellesmere Island, Nunavut, Canada. Here, we present new whole-rock trace element and isotope geochemistry and hornblende
40
Ar/
39
Ar geochronology from intrusive and extrusive rocks of the Yelverton Formation, along with isotope geochemistry from carbonate rocks underlying the volcanics and detrital zircon U–Pb and Lu–Hf isotopic data from the Yelverton Formation and overlying Grant Land Formation. The carbonate strata yielded an average
87
Sr/
86
Sr value of 0.7076 ( n = 6), constraining the overlying volcanics to the late Ediacaran–early Cambrian (ca. 570–530 Ma). Flows and dikes/sills show three distinct compositions: (1) a depleted, low La/Yb
PM
and Th/Nb group, (2) an enriched, higher La/Yb
PM
and low Th/Nb group, and (3) a low to moderate La/Yb
PM
and high Th/Nb group. One of the high Th/Nb intrusions produced saddle-shaped
40
Ar/
39
Ar hornblende spectra with ca. 482 ± 7 Ma age minima and Silurian–Devonian inverse isochron ages, highlighting a hitherto unknown suite of younger intrusions and volcanic rocks. The trace element geochemistry and
143
Nd/
144
Nd
(t)
ratios of enriched and depleted volcanic rocks of the indisputable Yelverton Formation are consistent with decompression melting during rifting along a continental margin. We suggest that late Ediacaran–early Cambrian rifting recorded in the Yelverton Formation resulted in the separation of peri-Laurentian terranes, such as the North Slope subterrane and eventual development of the passive margin of northern Laurentia.
Patients with acute HIV-1 infection (AHI) have elevated infectivity, but cannot be diagnosed using antibody-based testing. Approaches to screen patients for AHI are urgently needed to enable ...counselling and treatment to reduce onward transmission.
We pooled data from four African studies of high-risk adults that evaluated symptoms and signs compatible with acute retroviral syndrome and tested for HIV-1 at each visit. AHI was defined as detectable plasma viral load or p24 antigen in an HIV-1-antibody-negative patient who subsequently seroconverted. Using generalized estimating equation, we identified symptoms, signs, and demographic factors predictive of AHI, adjusting for study site. We assigned a predictor score to each statistically significant predictor based on its beta coefficient, summing predictor scores to calculate a risk score for each participant. We evaluated the performance of this algorithm overall and at each site.
We compared 122 AHI visits with 45 961 visits by uninfected patients. Younger age (18-29 years), fever, fatigue, body pains, diarrhoea, sore throat, and genital ulcer disease were independent predictors of AHI. The overall area under the receiver operating characteristics curve (AUC) for the algorithm was 0.78, with site-specific AUCs ranging from 0.61 to 0.89. A risk score of at least 2 would indicate AHI testing for 5-50% of participants, substantially decreasing the number needing testing.
Our targeted risk score algorithm based on seven characteristics reduced the number of patients needing AHI testing and had good performance overall. We recommend this risk score algorithm for use by HIV programs in sub-Saharan Africa with capacity to test high-risk patients for AHI.
The tectonic assembly of the Northern Cordillera is currently disputed and directly impacts Paleozoic‐to‐recent paleogeographic and plate tectonic reconstructions of North America. In this study, we ...present new U‐Pb zircon geochronology from the allochthonous Yukon‐Tanana terrane and the parautochthonous Cassiar terrane of the Northern Cordillera from south‐central Yukon, Canada. Our data provide new constraints for the assembly of the Northern Cordillera in this region. Metasedimentary samples from the Ingenika Group (Cassiar terrane) and the Snowcap and Finlayson assemblages (Yukon‐Tanana terrane) yielded detrital zircon age spectra that are comparable to known northwest Laurentia age spectra. One of our samples from the Snowcap assemblage yields a detrital zircon age spectrum that is anomalous for northwest Laurentia, but comparable to Early Paleozoic strata deposited in the Nevada‐Idaho‐Utah region. Zircon rim growth and Pb‐loss recorded by detrital zircon in the Snowcap assemblage and Ingenika Group samples record metamorphism at 370 ± 4 Ma and between 171 ± 5 and 135 ± 3 Ma. Late Devonian metamorphism and magmatism possibly corresponds to rifting of the Snowcap assemblage from the Laurentian margin. Middle Jurassic‐Early Cretaceous metamorphic zircon rim ages from the Cassiar terrane record metamorphism during collisions between the Intermontane superterrane (including the Yukon‐Tanana terrane) and Laurentia (Early to Middle Jurassic), and between the Insular superterrane and Laurentia (Middle to Late Jurassic). Our study suggests that collision between the Yukon‐Tanana terrane and the Laurentian margin began no earlier than ∼205 Ma.
Plain Language Summary
The Northern Cordillera (northwestern North America) represents an amalgamation of continental, volcanic, and oceanic crustal blocks (terranes), which collided with North America (Laurentia) over the last 380 million years. Constraining the timing and sequence of these collisional events is crucial for our understanding of the tectonic and paleogeographic evolution of North America and the natural resources and hazards that it provides. In Yukon, Canada, the most widely accepted models propose that the earliest Cordilleran collisional event occurred between the Yukon‐Tanana terrane and Laurentia, ∼270 to 250 million years ago. However, in this study, we present new data which suggest that collision between the Yukon‐Tanana terrane and North America took place sometime between 205 and 170 million years ago. Our findings also suggest that the Yukon‐Tanana terrane represents a collage of smaller crustal blocks, one of which originated from western North America. Importantly, the present‐day structurally overlapping relationships of Cordilleran terranes means that our new interpretation of the Yukon‐Tanana‐Laurentia collision directly influences the interpretation of all younger and older collisional events in Northern Cordillera. Our study adds to a growing body of evidence that argues that the current and most widely accepted models and terrane definitions for the Northern Cordillera require revision.
Key Points
Cassiar terrane in south‐central Yukon records Late Devonian and Middle Jurassic‐Early Cretaceous metamorphism of the Laurentian margin
Middle Jurassic‐Early Cretaceous metamorphism records collision between Yukon‐Tanana terrane and Laurentia, which began no earlier than ∼205 Ma
Yukon‐Tanana terrane contains metasedimentary rocks sourced from Idaho‐Nevada‐Utah region
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