James Whitehead

Late Eocene impact ejecta: geochemical and isotopic connections with the Popigai impact structure

Late Eocene microtektites and crystal-bearing microkrystites extracted from DSDP and ODP cores from the Atlantic, Pacific, and Indian oceans have been analyzed to address their provenance. A new analysis of Nd and Sr isotopic compositions confirms previous work and the assignment of the uppermost microtektite layer to the North American tektites, which are associated with the 35.5 Ma, 85 km diameter Chesapeake impact structure of Virginia, USA. Extensive major element and Nd and Sr isotopic analyses of the microkrystites from the lowermost layer were obtained. The melanocratic microkrystites from Sites 216 and 462 in the Indian and Pacific oceans possess major element chemistries, Sr and Nd isotopic signatures and Sm–Nd, T_(CHUR), model ages similar to those of tagamite melt rocks in the Popigai impact structure. They also possess Rb–Sr, T_(UR), model ages that are younger than the tagamite T_(CHUR) ages by up to ∼1 Ga, which require a process, as yet undefined, of Rb/Sr enrichment. These melanocratic microkrystites are consistent with a provenance from the 35.7 Ma, 100 km diameter Popigai impact structure of Siberia, Russia, while ruling out other contemporaneous structures as a source. Melanocratic microkrystites from other sites and leucocratic microkrystites from all sites possess a wide range of isotopic compositions (ϵ(^(143)Nd) values of −16 to −27.7 and ϵ(^(87)Sr) values of 4.1–354.0), making the association with Popigai tagamites less clear. These microkrystites may have been derived by the melting of target rocks of mixed composition, which were ejected without homogenization. Dark glass and felsic inclusions extracted from Popigai tagamites possess ϵ(^(143)Nd) and ϵ(^(87)Sr) values of −26.7 to −27.8 and 374.7 and 432.4, respectively, and T_(CHUR) and T_(UR) model ages of 1640–1870 Ma and 240–1830 Ma, respectively, which require the preservation of initially present heterogeneity in the source materials. The leucocratic microkrystites possess diverse isotopic compositions that may reflect the melting of supra-basement sedimentary rocks from Popigai, or early basement melts that were ejected prior to homogenization of the Popigai tagamites. The ejection of melt rocks with chemistries consistent with a basement provenance, rather than the surface ∼1 km of sedimentary cover rocks, atypically indicates a non-surficial source to some of the ejecta. Microkrystites from two adjacent biozones possess statistically indistinguishable major element compositions, suggesting they have a single source. The occurrence of microkrystites derived from a single impact event, but in different biozones, can be explained by: (1) diachronous biozone boundaries; (2) post-accumulation sedimentary reworking; or (3) erroneous biozonation.

U-Pb geochronology and origin of granitoid rocks in the Thetford Mines ophiolite, Canadian Appalachians

We integrate new and existing age data from ophiolite and subophiolite rocks in the Thetford Mines region, Quebec Appalachians, to determine the timing of: (1) oceanic crust formation; (2) oceanic decoupling; (3) ophiolite emplacement onto the continent, and; (4) postemplacement Taconic orogenesis. Few ophiolites possess high-precision temporal data for all these events. This allows us to develop models to explain the origin of emplacement-related granitoids that have intruded the ophiolite. U-Pb zircon ages have been determined for four samples from the Thetford Mines ophiolite. Oceanic plagiogranite from the Lac de l9Est region of the Thetford massif yields an age of 480 ± 2 Ma. This age is identical to that of the adjacent Mont Ham massif thrust sheet (478 +3 / −2 Ma), suggesting a close temporal and spatial association of these two units in the source oceanic domain. Two peridotite-hosted granitoid samples yield crystallization ages of 470 +5 / −3 Ma and 469 ± 4 Ma. A Precambrian inherited zircon component is also present in one sample. These granitoids are strongly peraluminous, contain abundant inherited zircon, exhibit variable but high 87 Sr/ 86 Sr initial ratios, and low 208 Pb/ 206 Pb ratios in igneous zircon, which suggest that they were derived by partial melting of Laurentian margin sedimentary rocks. The ages indicate that the ophiolite was thrust over the Laurentian margin within 1–17 m.y. of oceanic crustal crystallization and that these obduction-related granites were generated during a period of Shear heating calculations indicate that shearing can generate the peridotite-hosted granitoid melts and their observed contact-zone mineralogies under specific conditions, although margin sedimentary rock melting in response to west-dipping subduction-related arc magmatism cannot be excluded.

Origin of “toasted” quartz in terrestrial impact structures

Quartz present in the target rocks at impact sites and as clasts within impact melt rocks and some pseudotachylytes commonly exhibits a brown, nonpleochroic grainy appearance in plane- and cross-polarized light, known as ‘‘toasted’’ quartz. This variety of quartz also possesses a higher albedo in hand sample than colorless examples. Despite the intense coloration, no compositional origin for the browning is evident. A high proportion of exceptionally small (typically submicron scale) fluid inclusions, which are typically too small to be observed with the petrographic microscope, is considered to be the cause. These fluid inclusions are predominantly located along decorated planar deformation features, and they enhance scattering of transmitted light and reflect larger quantities of incident light. Statistically lower SiO 2 and total oxide contents present in toasted quartz, relative to untoasted quartz, are consistent with increased surface electron scattering during microprobe analysis by fluid inclusions intersecting the surface.

The sub-ophiolitic metamorphic rocks of the Québec Appalachians

Abstract Amphibolitic metamorphic rocks are associated with the Thetford, Asbestos and Orford ophiolites as well as the Mont Albert and Pennington Sheet peridotites of the Quebec Appalachians. An augmented compilation of the existing data on their field relations, mineralogy, geochronology, structural features and geothermobarometry is presented in order to help reconstruct the timing and processes of marginal basin closure and ophiolite emplacement during the Ordovician. A new, refined 40 Ar 39 Ar incremental release spectrum and isochron age is presented for the Thetford Mines ophiolite dynamothermal sole. The new 477 ± 5 Ma age resolves the existing disparity between the crustal (plagiogranite) and sole ages. The sole was formed shortly after crustal formation, suggesting that the ophiolite was decoupled at or near a spreading centre. A hitherto undescribed ultramafic-mafic amphibolitic sole beneath the Asbestos ophiolite was decoupled and rotated during the continental emplacement of the overlying ophiolite. Dating of the sole at Asbestos was hampered by presence of low K2O amphiboles, but an Acadian (377 Ma) age was obtained from orthoclase. The Orford sole was dismembered and incorporated within a serpentinitic melange that contains other ophiolitic lithologies. Sheared amphibolites from alongside the Pennington Sheet in the Flintkote Mine are reinterpreted as a dynamothermal sole, rather than a metasomatically generated amphibole-bearing metasediment.

40AR/39AR AGE CONSTRAINTS ON TACONIAN AND ACADIAN EVENTS IN THE QUEBEC APPALACHIANS

40Ar/39Ar dating of hornblende, muscovite, and orthoclase from southeastern Quebec provides new constraints on the timing, grade, and regional extent of Taconian and Acadian metamorphism in the Canadian Appalachians. Muscovite ages reveal that the Taconic orogeny occurred at ca. 463 Ma throughout the region. Lack of reset amphibole indicates that the metamorphic grade did not exceed greenschist facies. Acadian metamorphism, with a peak of <350 °C, is recorded by an orthoclase cooling age of 377 ± 4 Ma. This later metamorphism partially reset finer-grained muscovite toward the Vermont border. The calculated 300–340 °C closure temperature of orthoclase indicates that this age currently provides the closest estimate of the timing of peak Acadian metamorphism north of New England. Despite the lower grade of Taconian and Acadian metamorphism in Quebec compared to New England, peak temperatures appear to have been achieved synchronously. In addition, investigation of the timing of pre-Taconic decoupling of the Quebec oceanic fragments suggests a protracted obduction history. Obduction may have displaced slivers of both the subducting and overthrusting oceanic plates.

Statistical Evaluation of Compositional Differences Between Upper Eocene Impact Ejecta Layers

Melt droplets formed by the impact of a large meteorite impact event(s) have been found in upper Eocene marine sediments from the Atlantic, Pacific, and Indian oceans. The number of discreet impacts that occurred to form these melt droplets can be gleaned by compositionally analysing the droplets and establishing the number of distinct groups. Previous studies have inferred two, three, or more source impact events, although we believe the statistical techniques used to distinguish the groupings are open to criticism. Multivariate and univariate analysis (including discriminant analysis) of the major-element composition of an increased data set of impact melt ejecta droplets have been performed. The results demonstrate that the uppermost ejecta layer is geochemically distinct from other late Eocene melt ejecta. Our statistical analysis suggests two underlying, purportedly stratigraphically separate ejecta layers, possess minimal differences that cannot be distinguished clearly from one another by discriminant analysis, which adds to the plausibility that they have a common source. Finally, our results reveal apparent disparities between the new major-element data from this study and data compiled from existing sources.