Sandra L. Kamo

TEMORA 1: a new zircon standard for phanerozoic U-Pb geochronology

The role of the standard is critical to the derivation of reliable U–Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation–inductively coupled plasma–mass-spectrometry (ELA–ICP–MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U–Pb intercomparisons between different laboratories that do not use a common spike.

Rapid eruption of Siberian flood-volcanic rocks and evidence for coincidence with the Permian–Triassic boundary and mass extinction at 251 Ma

The Siberian flood-volcanic event is the most voluminous and explosive, continental, volcanic event known in the Phanerozoic record. U^Pb perovskite and zircon ages were obtained for lavas of the lowermost unit (251.7 9 0.4 Ma) and near-uppermost unit (251.1 9 0.3 Ma), respectively, of the volcanic sequence in the Maymecha^Kotuy area, Russia. Along with stratigraphic correlations and paleomagnetic evidence, these ages suggest that rapid extrusion of the entire V6500 m thick composite sequence occurred in less than 1million years. The time of extrusion coincides precisely with an age of 251.4 9 0.3 Ma previously obtained for the Permian^Triassic mass-extinction event, the most devastating biotic crisis known. Emplacement of the Noril’sk^Talnakh ore-bearing intrusions, notable for their prodigious Cu^Ni^PGE deposits, was synchronous with these two major geologic events at 251.2 9 0.3 Ma. The Guli volcanic-intrusive complex in the Maymecha^Kotuy area appears to represent the final mafic magmatism of the entire Siberian flood-volcanic event. Baddeleyite from a carbonatite that intrudes the complex gives an age of 250.2 9 0.3 Ma, and shows possible 231 Pa excess. The Bolgokhtokh granodiorite stock has a zircon age of 229.0 9 0.4 Ma, and represents the youngest known magmatism in the region.

The application of SHRIMP to Phanerozoic geochronology: a critical appraisal of four zircon standards

Derivation of Phanerozoic zircon 206Pb/238U ages by SHRIMP depends on calibration against an independently dated standard. The qualities of four different zircon standards (SL13, QGNG, AS3 and TEMORA 1) are assessed herein. Not all of these behave consistently on SHRIMP with respect to their ages as determined by IDTIMS. SL13, the most commonly used standard over the past decade and a half, is the most heterogeneous in Pb/U. In addition, when SL13 is used as the calibration standard, the varied ages resulting from that heterogeneity are generally younger than ages derived from the other three standards. AS3-calibrated ages are the oldest of the group. Only QGNG and TEMORA 1, when calibrated relative to each other, yield ages on SHRIMP that are consistent with their IDTIMS ages. Of these two, TEMORA 1 has the distinct advantage of producing consistent IDTIMS ages at high precision. Because of these factors and its availability, we recommend its use in geological studies where precise and accurate Pb/U zircon ages are imperative. Approximate conversion factors have been derived to improve quantitative inter-comparison between SHRIMP ages that have been calibrated against the different standards. These refinements significantly advance the role that SHRIMP can play in the numerical calibration of the Phanerozoic timescale.

A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre

Abstract U Pb isotope systematics of shock metamorphosed zircon grains from pseudotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an impact origin for this structure. Zircon grains from these rocks exhibit planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks from the Sudbury Structure. A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. U Pb isotopic results for individual, pre-impact zircon grains indicate crystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 2023 Ma and indicate both impact- and post-impact related Pb loss. The data and granular morphology of a zircon grain from the Granophyre indicates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar microstructures in zircon are ubiquitous, there are also some unshocked, low-U grains, and these record a ca. 3.1 Ga primary age. The older ca. 3.1–3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma. The resilience of zircon shock features to post-impact alteration and annealing, in combination with precise U Pb dating of individual shocked grains provide a valuable method for indentifying ancient, metamorphosed and tectonically modified impact structures.

Evolution of the southern Abitibi greenstone belt based on U–Pb geochronology: autochthonous volcanic construction followed by plutonism, regional deformation and sedimentation

Abstract New mapping and U–Pb zircon geochronological results in the southern Abitibi greenstone belt (SAGB) support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages, rather than the collage of allochthonous terranes proposed in recent publications. Based on lithological and geochronological criteria these supracrustal assemblages also encompass the Swayze, Shining Tree and Montcalm greenstone belts indicating that, despite separation by large granitic intrusions, they are in fact part of one extensive greenstone belt. In conjunction with the mapping, the geochronological results support coherent, upward-facing stratigraphic sections with about 20% of the samples containing inherited zircons with ages similar to those found in underlying assemblages. The seven oldest assemblages represent semi-continuous volcanism from 2750 to 2697 Ma. The volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc–alkaline mafic to felsic lavas. Intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section. Regional fault control on the distribution of the volcanic assemblages provides evidence for early dip–slip movement associated with volcanic extension dating back to at least 2725 Ma. The two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults. The earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 Ma, and the youngest one of subaerial conglomerates, fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 Ma. The sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons, regional folding and reactivated movement on the regional faults probably related to accretion of the Abitibi to the Superior Province craton. Autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic ± komatiitic assemblages and two dominantly calc–alkaline assemblages, as well as the extensive intermingling of these different magma clans in a number of the assemblages, appears to be unique to the Archean. It suggests complex, large-scale and long-lived interaction between mantle plumes and subduction zone magmas. The Nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with eNd=2.5±0.5 demonstrating a lack of contamination by ancient enriched components.

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite

High-grade rocks of the Wilmington Complex, northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia. The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built. In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning, shown by cathodoluminescence (CL), includes both concentric, oscillatory patterns, indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies, metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.

Birthdate for the lapetus Ocean? A precise U-Pb zircon and baddeleyite age for the Long Range dikes, southeast Labrador

Mafic dike swarms represent direct evidence of early tensional environments that may have eventually led to ocean formation. The first precise U-Pb zircon and baddeleyite age of 615 {plus minus} 2 Ma for a Long Range dike, southeastern Labrador, could mark the beginning of Iapetus Ocean formation. Baddeleyite (ZrO{sub 2}) is more widespread than previously recognized. It is ideal for use with the U-Pb system because it has low initial common lead and sufficient uranium levels, and it does not lose significant radiogenic lead with time. Baddeleyite therefore provides a unique opportunity for dating dikes associated with rift-related events with unprecedented accuracy.

A minimum UPb age for Siberian flood-basalt volcanism

Abstract Establishing an accurate and precise age for Siberian flood-basalt volcanism is of great importance in evaluating causes for the unequaled mass extinction of flora and fauna at the Permian-Triassic boundary. We report a new, minimum UPb age obtained from zircon and baddeleyite from the mineralized Norilsk I intrusion that cuts the lower third of this rapidly deposited, 3500-m-thick volcanic sequence near Norilsk. This 251.2 ± 0.3 (2σ) Ma age is within analytical error of the SHRIMP UPb age for zircon from the Permian-Triassic boundary at Meishan, South China [251.1 ± 3.6 Ma (2σ)], and confirms Siberian basaltic volcanism as a possible contributor to the mass extinction.

Fingerprinting the K/T impact site and determining the time of impact by UPb dating of single shocked zircons from distal ejecta

Abstract U Pb isotopic dating of single 1–3 μg zircons from K/T distal ejecta from a site in the Raton Basin, Colorado provides a powerful new tool with which to determine both the time of the impact event and the age of the basement at the impact site. Data for the least shocked zircons are slightly displaced from the 544 ± 5 Ma primary age for a component of the target site, white those for highly shocked and granular grains are strongly displaced towards the time of impact at 65.5 ± 3.0 Ma. Such shocked and granular zircons have never been reported from any source, including explosive volcanic rocks. Zircon is refractory and has one of the highest thermal blocking temperatures; hence, it can record both shock features and primary and secondary ages without modification by post-crystallization processes. Unlike shocked quartz, which can come from almost anywhere on the Earths crust, shocked zircons can be shown to come from a specific site because basement ages vary on the scale of meters to kilometers. With U Pb zircon dating, it is now possible to correlate ejecta layers derived from the same target site, test the single versus multiple impact hypothesis, and identify the target source of impact ejecta. The ages obtained in this study indicate that the Manson impact site, Iowa, which has basement rocks that are mid-Proterozoic in age, cannot be the source of K/T distal ejecta. The K/T distal ejecta probably originated from a single impact site because most grains have the same primary age.

UPb geochronological constraints on the geological evolution of the Pinware terrane and adjacent areas, Grenville Province, southeast Labrador, Canada

Abstract UPb geochronological data show that the Pinware terrane and adjacent areas in the Grenville Province in southeast Labrador experienced three orogenic events: Labradorian, Pinwarian and Grenvillian. Labradorian (1710-1600 Ma) rocks in the Pinware terrane, previously only known from a dated felsic volcanic enclave in a younger granite, are now recognized to be widespread and include both supracrustal units and granitoid intrusions. One quartzite was deposited between ∼1805 and 1500 Ma, the ages of the youngest detrital zircon and subsequent metamorphism, respectively, and includes 1878 and 2720 Ma detrital zircon suggesting derivation of source material from pre-Labradorian Laurentia. A volcaniclastic(?) unit is interpreted to have an age of 1637 ± 8 Ma. Two Labradorian quartz monzonite intrusions yielded ages of 1650+18−9 Ma and 1649 ± 7 Ma. Coupled with previous information, these data indicate coeval plutonism, volcanism and sedimentation taking place at the same time as formation of the Trans-Labrador batholith farther north. The Pinwarian Orogeny is now extended to include events between 1510 and 1450 Ma. Granitoid rocks of the Upper Paradise River pluton, a large AMCG suite in the Mealy Mountains terrane, has been dated at 1495 ± 7 and 1501 ± 9 Ma, making it the only suite of this type and age known in North America. Migmatitic quartz monzonite, dated at 1450+15−21 Ma, provides the first proof of high-grade metamorphism in the Pinware terrane, and, in conjunction with recent geochronological data from other parts of the eastern Grenville Province, justify upgrading the Pinwarian to orogenic status. Grenvillian metamorphism throughout the Pinware terrane occurred between 1050 and 985 Ma, as indicated mainly on the basis of zircon lower intercepts, but including some monazite data and constraints imposed by dated younger rocks. This range of ages contrasts with the time span for Grenvillian metamorphism in the Lake Melville terrane, previously dated to between 1080 and 1000 Ma, and with which a newly determined age of 1047 ± 2 Ma from a granodiorite dyke at the Mealy Mountains-Lake Melville terrane boundary conforms. In the Pinware terrane, near the end of Grenvillian orogenesis, widespread alkalic magmatic activity occurred between 990 and 980 Ma. Units investigated include an aegerine-bearing alkali-feldspar syenite emplaced at 991 ± 5 Ma, a clinopyroxene-fayalite alkali-feldspar syenite having a probable age of 985 Ma, and alkalic mafic dykes emplaced at 985 ± 6 Ma. Similar (although amygdaloidal) alkalic mafic dykes in the Lake Melville terrane may be slightly younger, having an age of 974 ± 6 Ma. The short-lived alkalic activity was followed by late-tectonic magmatism in the Pinware terrane at 983 ± 3 Ma, which heralded post-tectonic granitoid plutons between 974 and 956 Ma. Cooling and uplift in the Pinware terrane is documented by titanite ages between 972 ± 5 Ma and 939 ± 5 Ma. It is inferred that cooling occurred 30 million years sooner in the southeast Pinware terrane than near its northwest margin.