Larry M. Heaman

Global mafic magmatism at 2.45 Ga: Remnants of an ancient large igneous province?

Radiometric dating, including numerous high-precision U-Pb baddeleyite and zircon ages, of Paleoproterozoic flood basalts, dike swarms, and layered mafic intrusions worldwide, indicates that a substantial volume of mafic magma was produced on Earth ca. 2.45 Ga. New U-Pb data presented here for the 2473 Ma Matachewan and 2446 Ma Hearst diabase dike swarms in North America establish a critical temporal link with ancient flood-basalt volcanism in Karelia and constrain the timing of a very ancient magnetic field reversal on Earth. The potential volume, magnitude, and extent of this magmatism may approach many of the better-known Mesozoic flood-basalt provinces and represent the oldest recognized large igneous province. The initiation and subsequent proliferation of flood-basalt and associated magmatism near the Archean-Proterozoic boundary may reflect a fundamental change in heat flux at the core-mantle boundary that heralded the breakup of a Late Archean supercontinent.

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.

Timing of eastern North American kimberlite magmatism: continental extension of the Great Meteor hotspot track?

Abstract Twenty-nine new high precision U–Pb perovskite ages for kimberlite and other CO2-rich ultrabasic rocks, primarily from five fields or clusters in eastern North America (Rankin Inlet, Attawapiskat, Kirkland Lake, Timiskaming and Finger Lakes), indicate that at least five periods of Mesozoic kimberlite magmatism can be distinguished. These new data document four periods of kimberlite magmatism previously unrecognized in eastern North America: at 196, 180–176, 148–146 and 142–134 Ma. In addition, the detailed emplacement history of Jurassic kimberlites in the Kirkland Lake field indicates magmatism spanned a period of 13 Myr from 165 to 152 Ma with approximately half of the kimberlites in this field emplaced in less than 2 Myr between 156.9–155.3 Ma. These U–Pb results demonstrate for the first time that there is a NW–SE Triassic to Cretaceous age progression in kimberlite magmatism, which is consistent with the relative plate motions of North America during this interval. This age progression in kimberlite magmatism extends for more than 2000 km from Rankin Inlet through to the Attawapiskat, Kirkland Lake and Timiskaming fields and is interpreted, in part, to be the continental expression of magmatism linked to the Great Meteor mantle plume hotspot track. If correct, then the timing and location of this magmatism provides a more rigid constraint for both the exact position of the hotspot and the relative direction and rate of North American plate motion during the Mesozoic opening of the North Atlantic Ocean.

U–Pb zircon dating by laser ablation-MC-ICP-MS using a new multiple ion counting Faraday collector array

This study reports U–Pb geochronological data for zircon obtained by laser ablation-multi-collector-ICP-MS using a new collector block design that includes three ion counters and twelve Faraday buckets. The collector configuration allows for simultaneous detection of ion signals from mass 238U to 203Tl, an important factor for the achievement of highly precise and reproducible Pb–Pb and Pb–U ratios. The main advantage of the multiple ion counting system is the capability to readily measure low Pb ion signals ( 50) of zircon grains.

Gunbarrel mafic magmatic event: A key 780 Ma time marker for Rodinia plate reconstructions

Precise U-Pb baddeleyite dating of mafic igneous rocks provides evidence for a widespread and synchronous magmatic event that extended for >2400 km along the western margin of the Neoproterozoic Laurentian craton. U-Pb baddeleyite analyses for eight intrusions from seven localities ranging from the northern Canadian Shield to northwestern Wyoming–southwestern Montana are statistically indistinguishable and yield a composite U-Pb concordia age for this event of 780.3 ± 1.4 Ma (95% confidence level). This 780 Ma event is herein termed the Gunbarrel magmatic event. The mafic magmatism of the Gunbarrel event represents the largest mafic dike swarm yet identified along the Neoproterozoic margin of Laurentia. The origin of the mafic magmatism is not clear, but may be related to mantle-plume activity or upwelling asthenosphere leading to crustal extension accompanying initial breakup of the supercontinent Rodinia and development of the proto– Pacific Ocean. The mafic magmatism of the Gunbarrel magmatic event at 780 Ma predates the voluminous magmatism of the 723 Ma Franklin igneous event of the northwestern Canadian Shield by ∼60 m.y. The precise dating of the extensive Neoproterozoic Gunbarrel and Franklin magmatic events provides unique time markers that can ultimately be used for robust testing of Neoproterozoic continental reconstructions.

Magmatism of the mid-Proterozoic Gardar Province, South Greenland: chronology, petrogenesis and geological setting

Abstract A lithospherically weak zone embracing the southern margin of the Greenland Archaean craton and the Ketilidian Julianehab batholith underwent repeated rifting during the interval 1350–1140 Ma, accompanying breakup of Palaeopangaea. The Gardar Province comprises the area affected by the faulting and asssociated alkaline magmatism. While an estimated 2–5 km of Proterozoic cover has been eroded, rift-fill successions have been preserved in early fault-bounded basins. The orientation of dyke swarms changed from WNW–ESE to nearly NE–SW during Gardar times. The principal swarms are inferred to occupy zones of lithospheric thinning and graben development. Central-type intrusive complexes, largely of syenites and nepheline syenites, reached shallow levels. The principal magmatic evolution was from transitional olivine basalt through to phonolites. Accompanying silica-oversaturated magma generation involved greater degrees of crustal assimilation. Anorthositic xenoliths in the Gardar intrusions imply the presence of an extensive anorthositic complex at depth, regarded as an integral part of the North American Proterozoic massif anorthosite association. The most primitive Gardar basalts are themselves relatively evolved, probably as a result of olivine±pyroxene fractionation during crustal underplating. The Gardar basic rocks are troctolitic with elevated Al 2 O 3 /CaO ratios: their incompatible element patterns suggest a significant input from lithospheric sources. The Ca-deficient nature of the Gardar basalts is attributed to an origin involving lithospheric mantle depleted by previous melting events. Trace element and isotopic signatures suggest considerable heterogeneity in the mantle sources which are ascribed to differential metasomatism of clinopyroxene-poor peridotite sources by small-fraction asthenospheric melts. The ultramafic lamprophyre/carbonatite association that recurred throughout the period is inferred to have originated from melting of metasomites deep within the lithospheric mantle. Affinities between the alkaline intrusions over an interval of >100 Ma characterize the Gardar as a coherent magmatic province and support the contention that the magmas are largely of lithospheric origin. The energy required to generate the very large requisite volumes of primitive magmas may have been supplied by successive mantle plumes. The Gardar magmatism pre- and post-dates the ∼1.27 Ga Mackenzie Igneous Events of North America but wholly pre-dates the ∼1.1 Ga Keweenawan magmatism associated with the Midcontinent Rift.

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.

Provenance of the Meguma terrane, Nova Scotia: rifted margin of early Paleozoic Gondwana

Detrital zircon ages from the lower part of the Late Proterozoic(?) to Middle Cambrian Goldenville Group in the Meguma terrane of Nova Scotia suggest derivation from local sources in the Avalonian and Pan-African orogens on the margins of Early Cambrian Gondwana. Samples from near the top of the group show a broader distribution, including ages back to Archean. The eNd data show a corresponding trend, from slightly positive in the lower Goldenville Group to highly negative in the upper Goldenville Group and overlying Upper Cambrian to Lower Ordovician Halifax Group. The trends are consistent with deposition of the lower part of the Meguma succession in a rift, in which uplifted rift-flanks were the main source of the early basin fill, whereas subsequent thermal subsidence of rift margins allowed for more widespread sediment sourcing in younger units. The rift was possibly located between Gondwana and Avalonia, and may have been the locus for separation of Avalonia from Gondwana to form part of the Rheic O...

Direct dating of Archean microbial ichnofossils

Well-preserved Archean pillow lavas from the ca. 3.35 Ga Euro Basalt of the Pilbara Craton, Western Australia, contain micron-sized tubular structures mineralized by titanite (CaTiSiO 4 ) with residual organic carbon preserved along their margins. Direct U-Pb dating of titanite in the tubular structures demonstrates an Archean age. These tubular microstruc- tures are identical to microbial ichnofossils in modern basalts, ophiolites, and greenstone belts, and are interpreted as a biogenic signature in these ancient rocks. Microbial colonization of basaltic glass thus appears to have been part of a deep subsurface biosphere established early in Earths history.

Circa 2.3‐Ga Magmatism of the Arrowsmith Orogeny, Uranium City Region, Western Churchill Craton, Canada

The western margin of the Churchill craton records a complex history of Paleoproterozoic tectonism. The most prominent of these tectonic events is recorded within the 2.0–1.9‐Ga Taltson‐Thelon magmatic zone. The widespread magmatism and high‐grade metamorphism of this zone, especially in Alberta and Saskatchewan, obscures an older tectonic belt, the ca. 2.4–2.3‐Ga Arrowsmith Orogeny. In the Uranium City region of northwestern Saskatchewan, a suite of Paleoproterozoic granites represent a magmatic product of this orogen. U‐Pb zircon crystallization ages were identified for the Macintosh Bay monzogranite ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape