Robert H. Rainbird

Detrital Zircon Analysis of the Sedimentary Record

The composition of “heavy,” or accessory, detrital minerals in sediments and sedimentary rocks has been a topic of quantitative study for at least the last seventy years, beginning with the first issue of the Journal of Sedimentary Petrology in May 1931 (Tyler 1931, Pentland 1931). Zircon has since played a prominent and complex role in interpreting the composition and history of modern and ancient sediments. Because zircon is highly refractory at Earth’s surface, it occurs in virtually all sedimentary deposits and so provides a critical link in understanding the source history of a deposit. Twenhofel (1941), in a pioneering paper on the frontiers of sedimentary mineralogy and petrology, noted that the simple presence of detrital zircon would be of little value in determining its source: “Zircons from a half dozen sources with as many different properties may be present in a sediment and merely be identified as zircon. Parent rocks cannot be positively identified on such data . The variety or varieties must be identified and their optical properties determined.” From very early on, then, it was recognized that detrital zircon would be a powerful tool in understanding provenance, and thus, sedimentary dispersal systems. Interpretive goals matured considerably in the subsequent decades, especially with major advances in microscopy, mineral chemistry, isotope tracer geochemistry, and geochronology, each addressing different aspects of provenance, sedimentation, and Earth history. The hundreds of published studies utilizing detrital zircon in the last 20 years indicate the increasing success in assessing provenance, paleogeography, and tectonic reconstructions. Selected studies are highlighted in this review to illustrate ways in which detrital zircon can be used for interpreting the stratigraphic record, and thus, the past surface conditions of Earth. In it we will outline the quantitative techniques involved in the sampling protocol and interpretation of data and then …

Nature and timing of Franklin igneous events, Canada : implications for a Late Proterozoic mantle plume and the break-up of Laurentia

Abstract U Pb baddeleyite/zircon ages have been determined for gabbro sills and diabase dykes emplaced during Franklin igneous events in northern Canada. Results for seven widely separated sills from Victoria Island, Coronation Gulf and Bathurst Inlet, and for a single diabase dyke from southeastern Baffin Island, indicate that most of the Franklin igneous activity was synchronous and occurred at 723 + 4/−2 Ma. A U Pb baddeleyite/ zircon age of 718 ± 2 Ma for one of the Victoria Island sills suggests that the magmatism continued for at least 5 m.y. after the initial outburst. The short-duration and large volume of the intrusive tholeiitic magmatism and its association with the Natkusiak flood basalts suggest a link with a mantle plume-generated hotspot. The onset of magmatism may have coincided with the arrival of the plume at the base of the lithosphere. Stratigraphic relationships on Victoria Island indicate that differential uplift occurred just prior to the initial outbreak of igneous activity, which is interpreted as a response to doming above the upwelling plume. The exposed record provides no indication of significant pre-volcanic extension and rifting, although some NE-SW extension occurred after the initial eruptive phase. The 779 Ma mafic magmatism in the Mackenzie Mountains, the 723 Ma Franklin events and the 615 Ma Long Range dykes in Labrador are three precisely dated mafic events in North America that may be contemporaneous with various phases of Late Proterozoic break-up of Laurentia. These ages establish key chronological constraints for global correlation of Late Proterozoic epicontinental basins and for Precambrian plate reconstructions.

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

Low oxygen limited the rise of animals Oxygen levels in Earths early atmosphere had an important influence on the evolution of complex life. Planavsky et al. analyzed the isotopic signature of chromium in sedimentary rocks from across the globe—a proxy for past oxygen levels. Oxygen levels in the mid-Proterozoic (1.6 billion to 900 million years ago) were very low: less than 0.1% of the modern atmosphere. These low levels were probably below the minimum oxygen requirements for the earliest animals, delaying their emergence and diversification. Science, this issue p. 635 Oxygen levels in Earth’s early atmosphere were often less than 1% of modern levels. The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

U-Pb geochronology of Riphean sandstone and gabbro from southeast Siberia and its bearing on the Laurentia-Siberia connection

Thirty-one detrital zircons from the mid–late Riphean Mayamkan Formation sandstone (Uy Group) of the Sette–Daban fold belt, southeast Siberia yielded SHRIMP 207Pb/206Pb ages ranging between 1500 and 1050 Ma. Other grains yielded ages between 2.7 and 1.8 Ga. There is no known source region for the Mesoproterozoic zircons in Siberia; however, this range of ages closely matches those of detrital zircons from Neoproterozoic sandstones from northwest Canada, which are considered to have been derived from the Grenville Province of southeast Laurentia (all directions cited are with reference to present-day coordinates). These data suggest a formerly close connection between southeast Siberia and northwest Laurentia prior to their separation in the Neoproterozoic. However, two gabbro sills which intrude the Riphean sedimentary succession of the Sette–Daban fold belt are dated here at 1005±4 Ma and 974±7 Ma (U–Pb baddeleyite), an unknown age in northern Laurentia and unlike the widespread and well characterized 723 Ma Franklin and 1267 Ma Mackenzie mafic magmatic events. These somewhat incongruous results cast uncertainty on existing continental reconstructions, which link Siberia to Laurentia from about 1900 to 700 Ma. Our data can be reconciled with existing data by proposing an alternative continental configuration based on former continuity of the following tectonic entities: Archean Tungus Province (Siberia) with Archean Slave Province (Laurentia); Paleoproterozoic Angara fold belt (Siberia) with Paleoproterozoic Wopmay orogen and Great Bear magmatic zone (Laurentia); and Paleoproterozoic Akitkan fold belt (Siberia) with Paleoproterozoic Thelon–Taltson magmatic zone (Laurentia). Our reconstruction also considers the proposed northern extension of the Grenville orogen to be a potential source for Mesoproterozoic detrital zircons from the Mayamkan Formation. Such an orientation also is required to explain the apparent absence of Franklin and/or Mackenzie mafic magmatic rocks and the lack of distinctive Neoproterozoic lithofacies in the Sette–Daban fold belt. An additional conclusion of our study is that the lowermost Uy Group can be no younger than ca. 1010 Ma because it is intruded by a diabase sill dated at 1005±4 Ma. Previous work indicated that the Uy Group and underlying Lakhanda Group are of late Riphean age (1000–650 Ma). The youngest detrital zircon from the Mayamkan Formation provides a maximum U–Pb age of 1070±40 Ma for the upper Uy Group.

The early Neoproterozoic sedimentary Succession B of northwestern Laurentia: Correlations and paleogeographic significance

The Mesoproterozoic–Neoproterozoic stratigraphic record of ancestral North America (Laurentia) comprises three, unconformity-bounded sedimentary successions that are termed, from oldest to youngest, A, B, and C. Recent and ongoing detailed stratigraphic studies of Succession B, along with improved geochronology, allow extension and refinement of existing correlation schemes for northwestern Canada and Alaska. Succession B strata include the Shaler Supergroup of the Amundsen Basin, Mackenzie Mountains supergroup of the Mackenzie Mountains fold belt, Pinguicula group of the Wernecke Mountains inlier, Fifteenmile group of the Ogilvie Mountains inliers, and the lower Tindir Group of Tatonduk inlier. The Katakturuk Dolomite, in the northeast Brooks Range of Alaska, is included with Succession B on the basis of platformal character, geochronology, and inferred paleogeographic affinity. The framework for regional lithostratigraphic correlation of Succession B is built on recognition of four distinctive lithostratigraphic assemblages: two thick stromatolitic platformal carbonate assemblages separated by two largely subaerial siliciclastic assemblages. The correlation is supported by geochronology of detrital zircons from the upper quartzarenite assemblage, which indicates a maximum age of ca. 1000 Ma for the lower part of Succession B. These rocks are interpreted to be remnants of a northwesterly trending (present coordinates) early Neoproterozoic basin-margin promontory (Amundsen-Ogilvie-Mackenzie platform) that developed within an intracratonic basin on the northwest margin of Laurentia. The Neoproterozoic stratigraphic record of northwestern North America bears striking similarity to contemporaneous stratigraphy on other continents, particularly in the Amadeus Basin and Adelaide fold belt of central and southern Australia. Reconstructions of the Neoproterozoic supercontinent juxtapose the eastern margin of ancestral Australia against the western margin of Laurentia during the time these strata were being deposited. The Amundsen-Ogilvie-Mackenzie platform consequently may represent a segment (of the margin) of a large intracratonic basin that rifted apart with the breakup of the supercontinent during the latest Proterozoic. This hypothesis provides a template for future sequence stratigraphic, chemostratigraphic, biostratigraphic, paleomagnetic, and geochronologic comparisons and has implications for predictive economic geology in both areas. Capitalization of group and supergroup indicates formalization according to the International Stratigraphic Code. Noncapitalization indicates that the names have not been formalized.

Detrital zircon geochronology and provenance of the Torridonian, NW Scotland

Between 30 and 50 single detrital zircons from each of four specimens of the Stoer Group and two specimens from the unconformably overlying Torridon Group were analysed on the GSC SHRIMP II ion probe. 207Pb/206Pb ages of zircons from the Stoer Group range between 3.00 and 1.74 Ga with 95% concentrated between 2.93 and 2.48 Ga. The Bay of Stoer Formation has a small mode c. 2.55 Ga, matching overgrowth ages on some older grains. A few grains between 1.92 and 1.74 Ga occur in each of the Stoer Group specimens. These data are consistent with conventional provenance information and sedimentology, which indicate that the bulk of the Stoer Group probably was derived from local basement of the Lewisian Gneiss Complex and that deposition occurred adjacent to basin-margin growth faults. The two Torridon Group samples have similar detrital zircon age profiles with distinctive modes at 1.80 Ga, 1.66 Ga and 1.10 Ga, interpreted to represent sources of Ketilidian, Labradorian and Grenvillian affinity, respectively. A less well defined cluster c. 2.85–2.55 Ga reflects reworking of the underlying Stoer Group and possibly direct contribution from the Lewisian Gneiss Complex. The youngest concordant detrital zircon yields a maximum age of 1060±18 Ma for the Torridon Group (Applecross Formation). Our data, together with palaeocurrents from the Applecross Formation, suggest that the Torridon Group could have been deposited by a late to post-Grenvillian intermontane or foreland trunk river system flowing northeasterly, parallel to the Grenvillian orogenic belt.

Sedimentary evolution of the Riphean–Vendian basin of southeastern Siberia

Abstract The Riphean to Vendian ( ≈1600–540 Ma) sedimentary succession of the southeastern margin of the Siberian platform is 12–14 km thick and consists of terrigenous-carbonate successions termed, from oldest to youngest, the Uchur, Aimchan, Kerpyl, Lakhanda, Uy and Yudoma Groups. Group boundaries typically are regional unconformities; local angular unconformities occur at the base of the Aimchan and Yudoma Groups. Deposition mostly occurred in terrestrial to shallow marine sedimentary environments; only the Uy Group contains evidence of deep-water sedimentation. Paleocurrent and facies trends show that provenance of the Uchur, Aimchan and most of Kerpyl Groups was from the Siberian craton to the west. This corresponds with the mineralogical and chemical composition of sandstones, which suggests continental block to recycled orogen provenance with predominance of granites in the source area. Sandstones from the Uy and Yudoma Groups were derived from both western (Siberian) and eastern (non-Siberian) sources. The Uy Group contains graywacke that implies local recycled orogen to arc orogen provenance. Trace and rare earth element geochemistry suggests provenance from post-Archean source rocks and this is supported by U–Pb detrital zircon geochronology which indicates that only 3 of the 96 grains analyzed are of Archean age. Detrital zircons ≈2050 Ma predominate at the base of the Uchur Group. At the base of the Kerpyl Group ≈2060–1880 Ma zircons predominate with youngest grains ≈1300 Ma. The latter represents an unknown source, as rocks younger ≈1700 Ma are not reported from the basement of the Siberian platform. Zircons in the uppermost part of the Uy Group range in age from 1500 to 1050 Ma suggesting a non-Siberian provenance, perhaps from the Grenville orogen of Laurentia. Conventional U–Pb analysis of a few detrital zircon grains from the Yudoma Group sandstones yielded ages ≈2200–2000. Sedimentological and stratigraphic studies indicate that the Riphean–Vendian sedimentary basin of southeastern Siberia initiated by rifting that subsequently failed, allowing the development of a long-lived intracratonic sedimentary basin. Mafic magmatism and depositional features of the Uy Group suggest that there was renewed rifting ≈1000 Ma, when the basin evolved into an aulacogen. Rifted arms spread to form the Verkhoyansk ocean, the margins of which were approximately parallel to the modern margin of Siberian platform and Okhotsk massif.

Behavior of major and trace elements (including REE) during Paleoproterozoic pedogenesis and diagenetic alteration of an Archean granite near Ville Marie, Québec, Canada

Abstract This paper presents a complete set of chemical data from a pre-2.0-Ga weathering profile that formed on the Archean granite near Ville Marie, Quebec. Examination of data from two sections of the paleosol indicates that Na, Ca, P, and Sr were lost from the paleosol, whereas Ti Nb, Th, Zr, Hf, and Ta remained immobile during chemical weathering and all subsequent alteration. Rare earth mobility during genesis of the Ville Marie paleosol closely follows the pattern of early stages of chemical weathering in modern profiles. Average concentration of REE in both profiles is 20–40% lower than what is found in the fresh granite, suggesting mobilization during pedogenesis. HREEs seem to be less mobilized than the LREEs. (La/Sm)N ratios gradually decrease toward the top of the paleosol but the values remain very close to that of the protolith. (Gd/Yb)N values remain unchanged in the weathering profile. (La/Yb)N values in the weathering profile are quite variable and generally lower than that of the protolith. Eu/Eu∗ and Ce/Ce∗ values are essentially the same as those of the protolith, but two samples from the uppermost part of the profile have developed positive cerium anomaly of ≈0.3 relative to the protolith.

Diverse organic-walled fossils, including “possible dinoflagellates,” from the early Neoproterozoic of arctic Canada

A shallow-water shale unit from the early Neoproterozoic Wynniatt Formation, arctic Canada, preserves an unusually high diversity of organic-walled fossils, including abundant cyanobacteria, several multicellular protists and/or problematica, and more than 30 distinct acritarch species. Recognition of 13 new acritarchs, based on novel ornamentation, excystment structures, and/or wall structure, substantially increases their known diversity for this interval and points to a severe undersampling of the Proterozoic fossil record. Three of these new acritarchs exhibit features characteristic of dinoflagellate cysts and are reasonable candidates for early representatives of the clade, particularly in light of recent molecular phylogenetic analyses and biomarker data. The high diversity of acritarchs in the Wynniatt Formation also bolsters the potential for biostratigraphic resolution in the Neoproterozoic.

U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada

U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analysis of detrital zircon in 11 sandstone samples from the late Paleo-proterozoic Dubawnt Supergroup of the Baker Lake and Thelon Basins are presented here. Alluvial fan and braided stream facies of the ca. 1.84–1.78 Ga Baker Sequence yield age spectra with significant groupings at 2.70 and 2.60 Ga, reflecting provenance chiefly from local sources in the northwestern Hearne domain of the western Churchill Province. Age spectra are consistent with sed-imentological and stratigraphic information, which indicate provenance from the proximal uplifted shoulders of an intracontinental rift that developed in response to inboard effects of the Hudsonian orogeny (collision of Superior craton into western Churchill Province). The unconformably overlying 1.76–1.74 Ga Whart Sequence exhibits a more varied provenance and has an abundance of 2.0–1.9 Ga detrital zircon. These ages match those of synorogenic plutons in the Taltson-Thelon orogen (collision of Slave craton into the western Churchill Province). Provenance from the Taltson-Thelon orogen is supported by easterly paleocurrents and Rb-Sr cooling ages from mafic dikes that indicate reactivated collision of the Slave craton and related uplift and denudation coeval with deposition of the Whart Sequence. The depositional age of the Whart Sequence is constrained by intercalated rhyolite flows with conventional 207 Pb/ 206 Pb crystallization ages of 1757.6 ± 3.4 Ma and 1753.0 ± 1.7 Ma (2σ). The thermal effects of this collision also are recorded in isotopic data from basement rocks throughout the western Churchill Province and in syndepositional to postdepositional faults that parallel the main boundary faults of the Slave indentor. Late reactivation and associated metamorphism, faulting, and basin development could be the result of the inboard effects of subduction and related collision along the southern margin of Laurentia. The detrital zircon geochronology of the Barrens Sequence supports recycling of local supracrustal sequences and weathering of underlying granitic gneisses. Zircons of Hudsonian affinity (ca. 1.9–1.8 Ga) are nearly absent, which argues against a distant eastern source region for the Barrens Sequence. The Dubawnt Supergroup detrital zircon age data set shows concentrations ca. 2.75–2.6 Ga, 2.1–1.9 Ga, and 1.85–1.75 Ga, which are concordant with the known ages of major juvenile crust–forming events leading up to final amalgamation of Late Archean and Paleoproterozoic supercontinents. Detritus of this age was shed cratonward and captured by interior basins, such as the Baker Lake and Thelon Basins, during major collisional events that reworked the margins of the western Churchill Province (e.g., Hudsonian and Thelon-Taltson orogenies).