Thomas E. Krogh

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.

High precision U-Pb ages for granulite metamorphism and deformation in the Archean Kapuskasing structural zone, Ontario: implications for structure and development of the lower crust

Abstract Precise U-Pb isotopic ages for two subhorizontal granulite gneisses in the Kapuskasing structural zone indicate that formation and emplacement occurred more than 50 m.y. after cessation of volcanism in the steeply folded nearby greenstone belts by successive stages of ductile underplating across a zone of crust-mantle delamination. Volcanism and batholith formation had ceased by 2700 and 2680 Ma, respectively, while granulite formation occurred between 2660 and 2640 Ma and ductile deformation continued until 2585 Ma. Emplacement of the oldest granulite caused the development of shear planes and melting in the older overlying tonalite, while emplacement of the younger granulite caused rupturing and rehydration of the overlying older granulite. Widespread tectonic underplating is implied since all Superior Province granulites have similar young ages. Anomalous Ar ages that post-date accretion by about 150 m.y., and scrambled Pb-Pb and Rb-Sr whole-rock systems, also suggest that late low-temperature alteration accompanied this event. Temporal relationships and the presence of post-volcanic (2670 Ma) conglomerate cobbles at granulite grade support a model of ductile underplating of a post-accretion, thermally softened back arc-fore arc complex, with components welded onto an overlying volcanic-plutonic complex at successively deeper levels. Multistage deformation in active fault zones and hydrothermal emplacement of major gold deposits in greenstone belts are coeval with ductile underplating with ages between 2670 and 2585 Ma. Anomalously abundant gold production from Archean rocks may indicate that shallow underplating, granulitization and devolatization was a widespread phenomenon at this time. Post-accretion ductile underplating must be considered as a primary cause of seismic reflectors in Archean terrains.

Historical Development of Zircon Geochronology

An exact knowledge of rock formation ages is perhaps the single most important tool needed for assembling the geologic record into a coherent history. Moreover, the age of Earth and the time scale of pre-human events are central to a civilization’s sense of origin and purpose. Therefore, the quest for precise and reliable geochronometers has had a scientific and a cultural importance that few other enterprises can match. Since the beginning of the last century it has been recognized that long-lived radioactive decay systems provide the only valid means of quantifying geologic time. The uranium-lead decay system has always played a central role for several reasons. Minerals that contain very high U concentrations, although rare, are well known and easily obtained. The half lives of the natural U isotopes 238U and 235U are long enough to span all of Earth’s history but short enough that both parent and radiogenic daughter elements could be measured in such minerals even with the methods of a century ago. After the discovery that the U decay system is paired \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[^{238}U\ {\rightarrow}\ ^{206}Pb\ +\ 8\ {\alpha}\ +\ 6\ {\beta}^{{-}}\ (T_{1/2}\ =\ 4468\ Myr)\] \end{document} \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[^{235}U\ {\rightarrow}\ ^{207}Pb\ +\ 7\ {\alpha}\ +\ 4\ {\beta}^{{-}}\ (T_{1/2}\ =\ 704\ Myr)\] \end{document} it was realized that two age determinations could be made on the same sample using the same two elements. If the system has been closed to mobility of parent or daughter these two ages should agree, thus furnishing an internal test on the accuracy of the age. Further, the chemical coupling of the decays allows the age of the radiogenic daughter to be determined solely from its isotopic composition without knowing the parent-daughter ratio, a more difficult and less reliable parameter to measure. The wide utilization of the U-Pb geochronometer would not have been possible without the mineral zircon (ZrSiO4). Zircon normally contains U in concentrations well above its host rock average but discriminates strongly against the daughter element Pb during crystallization. …

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.

Last gasp of the Grenville Orogeny; thermochronology of the Grenville Front tectonic zone near Killarney, Ontario

We present U-Pb (titanite, zircon) and

Timing and thermal influence of late orogenic extension in the lower crust: a UPb geochronological study from the southwest Grenville orogen, Canada

^{40}Ar/^{39}Ar

Chicxulub crater source for shocked zircon crystals from the Cretaceous-Tertiary boundary layer, Saskatchewan: Evidence from new U-Pb data

(hornblende, mica, K-feldspar) data from a transect across the western part of the Grenville Front Tectonic Zone (GFTZ) near Killarney, Ontario. High-grade metamorphic assemblages (~1450 Ma) in this part of the GFTZ pre-date the Grenvillian orogeny and were primarily exhumed, with little or no metamorphic overprinting, by Grenvillian deformation. The titanite and zircon data form a discordant array with an upper intercept of

1.45 Ga granulites in the southwestern Grenville Province; geologic setting, P-T conditions, and U-Pb geochronology

1454 \pm 8 Ma

Zircon and rutile UPb geochronology of the Niquelândia layered mafic and ultramafic intrusion, Brazil: constraints for the timing of magmatism and high grade metamorphism

and a lower intercept of