New detrital zircon U-Pb ages and Lu-Hf isotopic data from metasedimentary rocks along the western boundary of the composite Avalon terrane in the southeastern New England Appalachians

ABSTRACT West Avalonia is a composite terrane that rifted from the supercontinent Gondwana in the Ordovician and accreted to Laurentia during the latest Silurian to Devonian Acadian orogeny. The nature and extent of West Avalonia are well constrained in Nova Scotia, New Brunswick, and Newfoundland, Canada, by U-Pb detrital zircon data and/ or isotope geochemistry of (meta)sedimentary and igneous rocks. The southeastern New England Avalon terrane in eastern Massachusetts, Connecticut, and Rhode Island has generally been interpreted as an along-strike continuance of West Avalonia in Canada, but the ages and origins of metasedimentary units along the western boundary of the Avalon terrane in Massachusetts and Connecticut remain poorly constrained. In this study, new detrital zircon U-Pb and Lu-Hf laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data from three samples of metasedimentary units along the western boundary of the southeastern New England Avalon terrane in Connecticut and Massachusetts were compared with existing data to test whether these metasedimentary units can be correlated along strike. The data were also compared with existing detrital zircon U-Pb and εHf data in New England and Canada in order to constrain the extent and provenance of West Avalonia. The maximum depositional age of two of the three detrital zircon samples analyzed in this study, based on the youngest single grain in each sample (600 ± 28 Ma, n = 1; 617 ± 28 Ma, n = 1) and consistency with existing analyses elsewhere in the southeastern New England Avalon terrane, is Ediacaran, while that of the third sample is Tonian (959 ± 40 Ma, n = 4). Detrital zircon analyses of all three samples from this study showed similar substantial Mesoproterozoic and lesser Paleoproterozoic and Archean populations. Other existing detrital zircon U-Pb data from quartzites in the southeastern New England Avalon terrane show similar Tonian populations with or without Ediacaran grains or populations. Most published detrital zircon U-Pb data from (meta)sedimentary rocks in West Avalonia in Canada yielded Ediacaran youngest detrital zircon age populations, except for a quartzite unit within the Gamble Brook Formation in the Cobequid Highlands of Nova Scotia, which showed a Tonian maximum depositional age, and otherwise a nearly identical detrital zircon signature with rocks from the southeastern New England Avalon terrane. All samples compiled from the southeastern New England Avalon terrane and West Avalonia in Canada show main age populations between ca. 2.0 Ga and ca. 1.0 Ga, with major peaks at ca. 1.95, ca. 1.50, ca. 1.20, and ca. 1.00 Ga, and minor ca. 3.1–3.0 Ga and ca. 2.8–2.6 Ga populations. The εHf(t) values from the three samples yielded similar results to those from West Avalonia in Canada, suggesting that both regions were derived from the same cratonic sources. The εHf(t) values of all West Avalonian samples overlap with both Amazonia and Baltica, suggesting that there is a mixed signature between cratonic sources, possibly as a result of previous collision and transfer of basement fragments between these cratons during the formation of supercontinent Rodinia, or during subsequent arc collisions.

Zircon of Triassic Age in the Stuttgart Formation (Schilfsandstein)—Witness of Tephra Fallout in the Central European Basin and New Constraints on the Mid-Carnian Episode

The Carnian Stuttgart-Formation (Schilfsandstein) of the Central European Basin contains relics of Triassic volcanic detritus in form of euhedral zircon grains and authigenic analcime. Multiple LA-ICP-MS spot analyses of single zircon crystals from an outcrop near Heilbronn (SW Germany) yielded weighted average 206Pb/238U ages between 250 and 230 Ma, providing evidence for tephra fallout in the southern part of the Central European Basin related to Olenekian, Anisian–Ladinian and Carnian volcanic activity. The tephra was probably transported by monsoonal circulations from volcanic centres of the NW Tethys to the Central European Basin. The four youngest zircon crystals gave a weighted average 206Pb/238U age of 231.1 ± 1.6 Ma (10 analyses), which is interpreted to date syn-depositional tephra fallout into the fluvial Lower Schilfsandstein Member of the Stuttgart Formation. This new maximum depositional age provides the first evidence that deposition of the Stuttgart Formation, which represents the type-example of the mid-Carnian episode, a global episode of enhanced flux of siliciclastic detritus and related environmental perturbations, occurred during the Tuvalian 2 substage at ca. 231 Ma, about 3 million years later than suggested by previous correlations. Zircon grains with weighted average 206Pb/238U ages of 236.0 ± 1.2 Ma (n = 17) and 238.6 ± 1.5 Ma (n = 6) and 206Pb/238U ages between 241 ± 6 and 250 ± 3 Ma point to the presence of tephra in early Carnian to Olenekian strata of the Keuper to Buntsandstein Groups. Traces of these reworked tephra were incorporated into the Stuttgart Formation due to fluvial erosion in the southern Central European Basin and at its margins.

Open-system behaviour of detrital zircon during weathering: an example from the Palaeoproterozoic Pretoria Group, South Africa

Detrital zircon in six surface samples of sandstone and contact metamorphic quartzite of the Magaliesberg and Rayton formations of the Pretoria Group (depositional age c. 2.20–2.06 Ga) show a major age fraction at 2.35–2.20 Ga, and minor early Palaeoproterozoic – Neoarchaean fractions. Trace-element concentrations vary widely, with Ti, Y and light rare earth elements (LREEs) spanning over three orders of magnitude. REE distribution patterns range from typical zircon patterns (LREE depletion, heavy REE enrichment, well-developed positive Ce and negative Eu anomalies) to patterns that are flat to concave downwards, with indistinct Ce and Eu anomalies. The change in REE pattern correlates with increases in alteration-sensitive parameters such as Ti concentration and (Dy/Sm) + (Dy/Nd), U–Pb discordance and content of common lead, and with a gradual washing-out of oscillatory zoning in cathodoluminescence images. U and Th concentrations also increase, but Th/U behaves erratically. Discordant zircon scatters along lead-loss lines to zero-age lower intercepts, suggesting that the isotopic and chemical variations are the results of disturbance long after deposition. The rocks sampled have been in a surface-near position (at least) since Late Cretaceous time, and exposed to deep weathering under intermittently hot and humid conditions. In this environment, even elements commonly considered as relatively insoluble could be mobilized locally, and taken up by radiation-damaged zircon. Such secondary alteration effects on U–Pb and trace elements can be expected in zircon in any ancient sedimentary rock that has been exposed to tropical–subtropical weathering, which needs to be considered when interpreting detrital zircon data.

Luminescence age calculation through Bayesian convolution of equivalent dose and dose-rate distributions: the D_e_D_r model

In nature, any mineral grain (quartz or feldspar) receives a dose-rate (Dr) specific to its environment. The dose-rate distributions, therefore, reflect the micro-dosimetric context of grains of similar size. If all the grains have been well bleached at deposition, this distribution corresponds, within uncertainties, to the distribution of equivalent doses (De). Their combination (convolution of the De and Dr distributions in the De_Dr model proposed here) allows the calculation of the true depositional age. If grains whose De values are not representative of this age (hereafter called "outliers") are present in the De distribution, the model allows them to be identified before the age is calculated. As the De_Dr approach relies only on the Dr distribution, the model avoids any assumption representing the De distribution, which is usually difficult to justify. Herein, we outline the mathematical concepts of the De_Dr approach (more details are given in Galharret et al., accepted) and the exploitation of this Bayesian modelling based on an R code available in the R package 'Luminescence'. We also present a series of tests using simulated Dr and De distributions with and without outliers and show that the De_Dr approach can be an alternative to available models for interpreting De distributions.

Revised Maximum Depositional Age for the Ediacaran Browns Hole Formation: Implications for Western Laurentia Neoproterozoic Stratigraphy

Constraining the depositional age of Neoproterozoic stratigraphy in the North American Cordilleran margin informs global connections of major climatic and tectonic events in deep time. Making these correlations is challenging due to a paucity of existing geochronological data and adequate material for absolute age control in key stratigraphic sequences. The late Ediacaran Browns Hole Formation in the Brigham Group of northern Utah, USA, provides a key chronological benchmark on Neoproterozoic stratigraphy. This unit locally comprises <140 m of volcaniclastic rocks with interbedded mafic-volcanic flows that lie within a 3500 m thick package of strata preserving the Cryogenian, Ediacaran, and the lowermost Cambrian history of this area. Prior efforts to constrain the age of the Browns Hole Formation yielded uncertain and conflicting results. Here, we report new laser-ablation-inductively-coupled-mass-spectrometry U-Pb geochronologic data from detrital apatite grains to refine the maximum depositional age of the volcanic member of the Browns Hole Formation to 613±12 Ma (2σ). Apatite crystals are euhedral and pristine and define a single date population, indicating they are likely proximally sourced. These data place new constraints on the timing and tempo of deposition of underlying and overlying units. Owing to unresolved interpretations for the age of underlying Cryogenian stratigraphy, our new date brackets two potential Brigham Group accumulation rate scenarios for ~1400 m of preserved strata: ~38 mm/kyr over ~37 Myr or ~64 mm/kyr over ~22 Myr. These results suggest that the origins of regional unconformities at the base of the Inkom Formation, previously attributed to either the Marinoan or Gaskiers global glaciation events, should be revisited. Our paired sedimentological and geochronology data inform the timing of rift-related magmatism and sedimentation near the western margin of Laurentia.

The Pre-Grenvillian assembly of the southeastern Laurentian margin through the U–Pb–Hf detrital zircon record of Mesoproterozoic supracrustal sequences (Central Grenville Province, Quebec, Canada)

The detrital zircon perspective on the pre-collisional crustal evolution of the Grenville Province remains poorly explored. In this study, we conducted in situ laser ablation U–Pb–Hf isotopic microanalysis on detrital zircon grains from three pre-orogenic (>1 Ga) supracrustal sequences that crop out in the Central Grenville Province (Lac Saint-Jean region, QC, CA). Detrital zircon grains from vestiges of these sequences record three dominant age peaks at c. 1.46 Ga, 1.62 Ga, 1.85 Ga, and a subordinate peak at 2.7 Ga. The 1.46 Ga and 1.62 Ga age peaks are recorded in detrital zircon grains from a quartzite associated with a metavolcanic sequence (i.e. Montauban Group) with a maximum depositional age of c. 1.44 Ga. In contrast, the c.1.85 Ga age peak is observed from recycled zircon grains in metasediments with maximum depositional ages between 1.2 and 1.3 Ga. The suprachondritic Hf isotope composition in detrital zircon grains of the 1.46 Ga and 1.62 Ga age populations records juvenile crustal growth during peri-Laurentian accretionary orogenesis related to the Pinwarian (1.4–1.5 Ga) and Mazatzalian–Labradorian (1.6–1.7 Ga) events. The detrital zircon grains associated with Penokean–Makkovikian (1.8–1.9 Ga) source rocks record reworking of c. 2.7 Ga continental crust derived from a near-chondritic mantle reservoir. Overall, crust-forming and basement reworking events associated with accretionary orogenesis in southeastern Laurentia are retained in the detrital zircon load of Precambrian basins even after the terminal Grenvillian collision and assembly of Rodinia.

Testing local and extraregional sediment sources for the Late Cretaceous northern Nanaimo basin, British Columbia, using 40Ar/39Ar detrital K-feldspar thermochronology

Detrital K-feldspar 40Ar/39Ar thermochronology was conducted on clastic sedimentary rock samples collected from northern exposures of the Upper Cretaceous Nanaimo Group on Vancouver Island and adjacent Gulf Islands of British Columbia to constrain the denudation history of the local Coast Mountains batholith source region and determine the origin of extraregional sediment supplied to the basin. Strata of the northern Nanaimo Group deposited between 86 and 83 Ma (Comox and Extension formations) exhibit a 130–85 Ma age distribution of detrital K-feldspar 40Ar/39Ar ages that lack age maxima. These are interpreted to have been sourced from the southwestern Coast Mountains batholith. Younger strata deposited between 83 and 72 Ma (Cedar District and De Courcy formations) yield a broader age range (150–85 Ma) with an age maximum near the depositional age. These results indicate focused denudation of deeper-seated rocks east of the Harrison Lake fault. The youngest units deposited after 72 Ma (Geoffrey, Spray, and Gabriola formations) primarily yield younger than 75 Ma detrital K-feldspar ages with pronounced age maxima near the depositional age. This sediment was sourced extraregionally relative to the Coast Mountains batholith. We sought to constrain the origin of the extra-regional sediment by measuring the thermal histories of 74 samples of basement rocks from throughout the Pacific Northwest, and by compiling a database of over 2400 biotite 40Ar/39Ar and K/Ar cooling ages from predominantly Cretaceous batholiths along the western North American margin. This analysis focused upon two previously proposed source regions: the Idaho batholith and the Mojave-Salina margin of southern California. The Nanaimo detrital K-feldspar 40Ar/39Ar age distributions favor the peraluminous Late Cretaceous Idaho batholith and its Proterozoic Belt-Purcell Supergroup sedimentary wall rock as the more likely source of the extraregional sediment and disfavor the Baja–British Columbia hypothesis for 2000–4000-km-scale translation of rocks along the margin during the Late Cretaceous.

Zircon (U-Th)/He thermochronology of Grand Canyon resolves 1250 Ma unroofing at the Great Unconformity and <20 Ma canyon carving

Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the &lt;250 °C thermal history of Precambrian basement over the past &gt;1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to &lt;200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ~3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ~80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity.