Wouter Bleeker

Boninite series: low Ti-tholeiite associations from the 2.7 Ga Abitibi greenstone belt

Abstract Boninite series volcanic flows, interfingered with komatiites and tholeiitic basalts, occur at several localities in the ∼2.7 Ga Abitibi greenstone belt. Flows from Whitney Township, Ontario, have a compositional range of SiO2 44–60 wt%, MgO 24–7.4, Mg# 83–69, and Ni 930–200 ppm. Low TiO2 (0.14–0.31 wt%) but high Al2O3 (13–25 wt%) contents yield variably high Al2O3/TiO2 ratios of 48–100. These boninite series volcanics are characterized by fractionated HREE where Gd/Ybn 0.3–0.7; positive normalized Zr(Hf)/MREE anomalies, and Zr/Hf > 36; generally negative normalized Nb anomalies; and LREE depletion to enrichment (La/Smn 0.72–1.4). Flows with similar compositional affinities occur in the neighbouring Kidd Volcanic Complex and Tisdale volcanic group. Alteration, and/or contamination by continental crust can be ruled out as the cause of the distinctive and coherent compositions. If the areally extensive komatiite–tholeiite association represents an ocean plateau derived from a mantle plume and the boninite series formed in a convergent margin, then the interfingering of komatiite and boninite series flows may represent interaction of a plume with a subduction zone.

Lithosphere development in the Slave craton: a linked crustal and mantle perspective

The late tectonic evolution of the Slave craton involves extensive magmatism, deformation, and high temperature-low pressure (HT-LP) metamorphism. We argue that the nature of these tectonic events is difficult to reconcile with early, pre-2.7 Ga development and preservation of a thick tectosphere, and suggest that crust–mantle coupling and stabilization occurred only late in the orogenic development of the craton. The extent and repetitiveness of the tectonic reworking documented within the Mesoarchean basement complex of the western Slave, together with the development of large-volume, extensional mafic magmatism at 2.7 Ga within the basement complex argue against preservation of a widespread, thick, cool Mesoarchean tectosphere beneath the western Slave craton prior to Neoarchean tectonism. Broad-scale geological and geophysical features of the Slave craton, including orientation of an early F1 fold belt, distribution of ca. 2.63–2.62 Ga plutonic rocks, and the distribution of geochemical, petrological and geophysical domains within the mantle lithosphere collectively highlight the importance of an NE–SW structural grain to the craton. These trends are oblique to the earlier, ca. 2.7 Ga north–south trending boundary between Mesoarchean and Neoarchean crustal domains, and are interpreted to represent a younger structural feature imposed during northwest or southeast-vergent tectonism at ca. 2.64–2.61 Ga. Extensive plutonism, in part mantle-derived, crustal melting and associated HT-LP metamorphism argue for widespread mantle heat input to the crust, a feature most consistent with thin (<100 km) lithosphere at that time. We propose that the mantle lithosphere developed by tectonic imbrication of one or more slabs subducted beneath the craton at the time of development of the D1 structural grain, producing the early 2.63–2.62 Ga arc-like plutonic rocks. Subsequent collision (external to the present craton boundaries) possibly accompanied by partial delamination of some of the underthrust lithosphere, produced widespread deformation (D2) and granite plutonism throughout the province at 2.6–2.58 Ga. An implication of this model is that diamond formation in the Slave should be Neoarchean in age. D 2003 Elsevier B.V. All rights reserved.

Archaean tectonics: a review, with illustrations from the Slave craton

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Episodic, mafic crust formation from 4.5 to 2.8 Ga: New evidence from detrital zircons, Slave craton, Canada

The ϵHf and δ18O values in detrital zircons from the Slave craton, Canada, indicate three episodes of crust formation between ca. 4.5 and 2.8 Ga, namely at ca. 4.4–4.5 Ga, ca. 3.8 Ga, and ca. 3.4 Ga. Most of the juvenile crust appears to have been mafic in composition, and there is no clear evidence for initial granitic protocrust in the Hadean of the Slave craton. The range of initial ϵHf values in zircons increases from 3.9 to 2.8 Ga, indicating that both extraction of new material from mantle and reworking of the older crust are important for the secular evolution of the continental crust. A preliminary review of available Hf data in zircons suggests that the three episodes of crust generation may have been of global importance. The mafic crust formed in the Archean and the Hadean was then reworked for at least ~0.5–1.5 b.y., as indicated by data from the Slave craton, Gondwana, and the Limpopo Belt of Africa.

Timing and tempo of the Great Oxidation Event

Significance We present U-Pb ages for the extensive Ongeluk large igneous province, a large-scale magmatic event that took place near the equator in the Paleoproterozoic Transvaal basin of southern Africa at ca. 2,426 Ma. This magmatism also dates the oldest Paleoproterozoic global glaciation and the onset of significant atmospheric oxygenation. This result forces a significant reinterpretation of the iconic Transvaal basin stratigraphy and implies that the oxygenation involved several oscillations in oxygen levels across 10−5 present atmospheric levels before the irreversible oxygenation of the atmosphere. Data also indicate that the Paleoproterozoic glaciations and oxygenation were ushered in by assembly of a large continental mass, extensive magmatism, and continental migration to near-equatorial latitudes, mirroring a similar chain of events in the Neoproterozoic. The first significant buildup in atmospheric oxygen, the Great Oxidation Event (GOE), began in the early Paleoproterozoic in association with global glaciations and continued until the end of the Lomagundi carbon isotope excursion ca. 2,060 Ma. The exact timing of and relationships among these events are debated because of poor age constraints and contradictory stratigraphic correlations. Here, we show that the first Paleoproterozoic global glaciation and the onset of the GOE occurred between ca. 2,460 and 2,426 Ma, ∼100 My earlier than previously estimated, based on an age of 2,426 ± 3 Ma for Ongeluk Formation magmatism from the Kaapvaal Craton of southern Africa. This age helps define a key paleomagnetic pole that positions the Kaapvaal Craton at equatorial latitudes of 11° ± 6° at this time. Furthermore, the rise of atmospheric oxygen was not monotonic, but was instead characterized by oscillations, which together with climatic instabilities may have continued over the next ∼200 My until ≤2,250–2,240 Ma. Ongeluk Formation volcanism at ca. 2,426 Ma was part of a large igneous province (LIP) and represents a waning stage in the emplacement of several temporally discrete LIPs across a large low-latitude continental landmass. These LIPs played critical, albeit complex, roles in the rise of oxygen and in both initiating and terminating global glaciations. This series of events invites comparison with the Neoproterozoic oxygen increase and Sturtian Snowball Earth glaciation, which accompanied emplacement of LIPs across supercontinent Rodinia, also positioned at low latitude.

Detrital zircon geochronology and grain-size analysis of a ∼2800 Ma Mesoarchean proto-cratonic cover succession, Slave Province, Canada

Abstract Detrital zircon populations of five quartzite samples of the ∼2800 Ma Central Slave Cover Group, Slave Province, have been investigated to explore regional lithostratigraphic correlation/diachroneity, maximum depositional ages, ages of underlying basement, and general provenance issues. A total of 346 grains were analyzed by sensitive high resolution ion microprobe for U–Pb ages and by digital image analysis for grain size. Ages of individual grains range from 3918 to 2808 Ma and define several important modes, notably at ∼3400 Ma, 3150 Ma, 2950 Ma, and 2826 Ma. Although the samples show significant differences, suggesting localized rather than well-mixed sediment provenances (supported by grain-size statistics), some modes are shared among the samples in a pattern that supports regional correlation. Nevertheless, the ages of the youngest detrital modes, in conjunction with independent age constraints from a parallel study, indicate either diachroneity in the cessation of quartz arenite deposition across the realm of the Central Slave Cover Group or a difference in basement domains.

Emplacement and deformation of granites during transpression: magnetic fabrics of the Archean Sparrow pluton, Slave Province, Canada

Abstract The Sparrow pluton is part of the Prosperous Suite of two-mica granites that crop out within amphibolite grade meta-greywackes of the southern Yellowknife Domain, in the Slave Province of the Canadian Shield. The magnetic susceptibility ( K ) and the anisotropy of magnetic susceptibility (AMS) were used to systematically map the structural patterns in the pluton and to establish the relationship between plutonism and Late Archean tectonics in the region. Paramagnetic Fe-phyllosilicates (biotite, chlorite) and very fine-grained magnetite contribute to K and to the AMS. The magnetic foliation and the magnetic lineation are predominantly controlled by the biotite fabrics and their orientations are consistent with the regional D 2 strain field. The horizontal magnetic lineations in the Sparrow pluton suggest a horizontal stretching component associated with the regional D 2 event. The zonation defined by K values is compatible with a fold pattern trending parallel to the regional F 2 folds and S 2 foliation and to the magnetic fabric trends in the pluton. The intensities and symmetries of the AMS also define map patterns that are consistent with D 2 deformation. Microstructural study indicates the pluton recorded D 2 strain as it crystallized and cooled from the solidus, demonstrating syn- D 2 emplacement. The results indicate the pervasive structural patterns in the Sparrow pluton are an integral part of the regional strain field, and that they are kinematically consistent with a transpressive D 2 strain regime. Mapping the fabric patterns within syntectonic plutons provides a useful approach to the kinematic analysis of synemplacement deformation events in multiply deformed metamorphic terranes.

Needs and opportunities in mineral evolution research

Abstract Progress in understanding mineral evolution, Earth’s changing near-surface mineralogy through time, depends on the availability of detailed information on mineral localities of known ages and geologic settings. A comprehensive database including this information, employing the mindat.org web site as a platform, is now being implemented. This resource will incorporate software to correlate a range of mineral occurrences and properties vs. time, and it will thus facilitate studies of the changing diversity, distribution, associations, and characteristics of individual minerals as well as mineral groups. The Mineral Evolution Database thus holds the prospect of revealing mineralogical records of important geophysical, geochemical, and biological events in Earth history.

Suture-zone geometry along an irregular Paleoproterozoic margin: The Superior boundary zone, Manitoba, Canada

Lithoprobe acquired a 190-km-long deep seismic reflection profile across the Superior boundary zone at the eastern margin of the Paleoproterozoic Trans-Hudson orogen. The profile is located ∼250 km south of a major promontory along the ancient margin of the Superior craton and complements earlier profiles closer to the promontory. The new data image the remnants of a craton-verging thrust belt, younger than 1864 Ma, within the Superior boundary zone extending to a maximum depth of 15 km above a shallowly west dipping basal decollement. Across the surface suture zone, a dip reversal to east-dipping reflections occurs within the adjacent Reindeer zone, where a crustal-scale imbricate stack is imaged. Reindeer zone lower crust and upper mantle extend eastward beneath the Superior craton margin for 40–100 km. Preservation of a foreland thrust belt within the Superior boundary zone is explained as a result of reduced collisional convergence and subsequent exhumation within a reentrant flanking the Thompson promontory.

Towards a ‘natural’ time scale for the Precambrian – A proposal

A proposal is put forward to redefine the geological time scale for the Precambrian. Flaws of the present, chronometrically defined, time scale are discussed and illustrated. It is concluded that we need to go back to the rock record to define a “natural” time scale, in which major divisions (eons, eras, etc.) are defined in terms of first-order events and transitions in the observable stratigraphic record. For the earliest part of Earth history, we need a time scale, including a formalized Hadean eon, that is fully consistent with rapidly evolving insights from planetary science.