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Detrital zircon (DZ) U–Pb geochronology has improved the way geologists approach questions of sediment provenance and stratigraphic age. However, there is debate about what constitutes an appropriate sample size (i.e., the number of dates in a DZ sample,n), which depends on project objectives, sample complexity, and, critically, analytical budget. Additionally, there is ongoing concern about bias introduced by zircon grain size. We tested a recently developed rapid (3 s/analysis) data acquisition method by multicollector laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) that incorporates an automated selection routine and calculates two‐dimensional grain geometry from polished sample surfaces. Eleven samples were analysed from below and above the Late Cretaceous (Campanian) basal Castlegate unconformity of the Book Cliffs, Utah, in a down‐depositional‐dip transect including Price, Horse, Tusher, and Thompson canyons. 12,448 new concordant dates were generated during two measurement sessions. Results are consistent with recent studies suggesting there is no major provenance change and little time (1–2 Myr) represented across the unconformity. Grain size and sample size both exert a strong control on sample dissimilarity. Age distributions constructed from subsamples of large grains are systematically less similar to whole samples; age distributions composed of small grains are overall more similar to whole samples. As such, North American sediment sources that produce large grains such as the Grenville and Yavapi‐Mazatzal belts can bias age distributions if only large grains are analysed. A sample size ofn = 100 is inadequate for characterizing age distributions as complex as those of the Book Cliffs, whereas a sample size ofn = 300 provides good characterization. Sample size ofn ≈ 1000 or more is unnecessary unless project objectives include scanning for subordinate age groups, such as when identifying the youngest grains for calculating a maximum depositional age (MDA). Dates used in MDA calculations acquired with rapid acquisition are best re‐analysed with longer LA‐ICP‐MS acquisition methods or isotope dilution thermal ionization mass spectrometry for increased accuracy and precision. We include new MATLAB code and open‐source software programs,DZpickandDZmda, for automated spot picking and calculating MDAs.

 

Abstract The plate-boundary conditions of the Mesozoic North American Cordillera remain poorly constrained, but most studies support large (>800 km) southward motion of the Insular and Intermontane superterranes during Jurassic–Cretaceous time. An implicit feature in these models of large coastwise displacements is the presence of one or more continentalscale sinistral strike-slip faults that could have dismembered and displaced terrane fragments southward along the western margin of North America prior to the onset of mid-Cretaceous shortening and dextral strike-slip faulting. In this study, we documented a system of sinistral intra-arc shear zones within the Insular superterrane that may have accommodated large southward motion. Employment of a new large-n igneous zircon U-Pb method more than doubled the precision of measurements obtained by laser ablation–inductively coupled plasma–mass spectrometry (from ~1% to 0.5%) and allowed us to demonstrate the close temporal-spatial relationship between magmatism and deformation by dating comagmatic crosscutting phases. Crystallization ages of pre-, syn-, and postkinematic intrusions show that the intra-arc shear zones record an Early Cretaceous phase of sinistral oblique convergence that terminated between 107 and 101 Ma. Shear zone cessation coincided with: (1) collapse of the Gravina basin, (2) development of a single voluminous arc that stitched the Insular and Intermontane superterranes together, and (3) initiation of eastwest contractional deformation throughout the Coast Mountains. We interpret these concurrent tectono-magmatic events to mark a shift in plate kinematics from a sinistral-oblique system involving separate terranes and intervening ocean basins to a strongly convergent two-plate margin involving a single oceanic plate and the newly assembled western margin of North America. 

Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.

 

Abstract The southern Coast Mountain batholith was episodically active from Jurassic to Eocene time and experienced four distinct high magmatic flux events during that period. Similar episodicity has been recognized in arcs worldwide, yet the mechanism(s) driving such punctuated magmatic behavior are debated. This study uses zircon Hf and O isotopes, with whole-rock and mineral geochemistry, to track spatiotemporal changes in southern Coast Mountains batholith melt sources and to evaluate models of flare-up behavior and crust formation in Cordilleran arc systems. Zircon Hf isotope analysis yielded consistently primitive values, with all zircon grains recording initial εHf between +6 and +16. The majority (97%) of zircons analyzed yielded δ18O values between 4.2‰ and 6.5‰, and only five grains recorded values of up to 8.3‰. These isotopic results are interpreted to reflect magmatism dominated by mantle melting during all time periods and across all areas of the southern batholith, which argues against the periodic input of more melt-fertile crustal materials as the driver of episodic arc magmatism. They also indicate that limited crustal recycling is needed to produce the large volumes of continental crust generated in the batholith. Although the isotopic character of intrusions is relatively invariant through time, magmas emplaced during flare-ups record higher Sr/Y and La/Yb(N) and lower zircon Ti and Yb concentrations, which is consistent with melting in thickened crust with garnet present as a fractionating phase. Flare-ups are also temporally associated with periods when the southern Coast Mountains batholith both widens and advances inboard. We suggest that the landward shift of the arc into more fertile lithospheric mantle domains triggers voluminous magmatism and is accompanied by magmatic and/or tectonic thickening. Overall, these results demonstrate that the magmatic growth of Cordilleran arcs can be spatially and temporally complex without requiring variability in the contributions of crust and/or mantle to the batholith. 

Detrital zircon (DZ) U‐Pb laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has revolutionised the way geologists approach many Earth science questions. Although recent research has focused on rapid sample throughput, acquisition rates are limited to 100–300 analyses h⁻¹. We present a method to acquire zircon U‐Pb dates at rates of 120, 300, 600 and 1200 analyses h⁻¹(30, 12, 6 and 3 s per analysis) by multi‐collector LA‐ICP‐MS. We demonstrate the efficacy of this method by analysing twelve zircon reference materials with dates from ~ 3465 to ~ 28 Ma. Mean offset from high‐precision dates increases with faster rates from 0.9% to 1.1%; mean random 1suncertainty increases from 0.6% to 1.3%. We tested this new method on a sandstone sample previously characterised by large‐nDZ geochronology. Quantitative comparison shows increased correspondence among age distributions comprising > 300 dates. This new method holds promise for DZ geochronology because (a) it requires no major changes to hardware, but rather modifications to software; (b) it yields robust age distributions well‐suited for quantitative analysis and maximum depositional age calculations; (c) there is only a minor sacrifice of accuracy and measurement uncertainty; and (d) there is less burden to researchers in terms of time investment and analytical cost.

 

The timing of crustal thickening in the northern Central Andean Plateau (CAP), at 13–20°S, and its relationship to surface uplift is debated. Zircon qualitatively records crustal thickness as its trace element chemistry is controlled by the growth of cogenetic minerals and relative uptake of light and heavy Rare Earth Elements. Jurassic to Neogene zircons from volcanic rocks, sandstones, and river sediments reveal shifts in trace element ratios suggesting major crustal thickening at 80–55 Ma and 35–0 Ma, coincident with high‐flux magmatism. An intervening magmatic lull due to shallow subduction obscures the magmatic record from 55 to 35 Ma during which thickening continued via crustal shortening. Protracted thickening since the Late Cretaceous correlates with early elevation gain of the CAP western margin, but contrasts with Miocene establishment of near modern elevation in the northern CAP and the onset of hyperaridity along the Pacific coast, highlighting their complex spatial and temporal relationship.