Elsevier

Lithos

Volumes 268–271, January 2017, Pages 114-130
Lithos

Age, origin, and thermal evolution of the ultra-fresh ~ 1.9 Ga Winnipegosis Komatiites, Manitoba, Canada

https://doi.org/10.1016/j.lithos.2016.10.033Get rights and content

Highlights

  • Pristine Palaeoproterozoic komatiites are described, their cooling and crystallisation history is reconstructed

  • Parental melts were nominally dry, with depleted compositions, and MgO contents ≈ 24 wt%

  • Liquidus temperature of ~ 1501 °C requires anomalously hot mantle

  • Formation from a mantle plume that was deflected by strong gradients in lithospheric thickness towards pre-existing rifts

  • New U–Pb zircon age of 1870.3 ± 7.1 Ma for the Winnipegosis Komatiite Belt

Abstract

The Proterozoic spans the longest portion of earth's history, yet in contrast to the Archaean, the record of komatiites and related high MgO igneous rocks from this Eon is sparse. This paper describes the pristine Palaeoproterozoic Winnipegosis Komatiites, from Manitoba, Canada, which form part of the Circum-Superior Belt large igneous province. We present a comprehensive petrographical investigation, mineral and bulk rock geochemistry, and Al-in-olivine thermometry for the Winnipegosis Komatiites, along with new U-Pb SHRIMP dating of zircons from a mafic unit, which yield an age of 1870.3 ± 7.1 Ma for the Winnipegosis Komatiite Belt. The komatiites are Al-undepleted and dominated by massive olivine porphyritic flows with a median thickness of 6 m. Differentiated flows containing layers of olivine spinifex are present, but rare. Trace element data indicate the komatiites were derived from depleted mantle, and subsequently contaminated with 2–3% continental crust. Temperatures from Al-in-olivine thermometry are consistent with a nominally dry melt, and combined with olivine-melt Mg–Fe partitioning, suggest a parental melt with ~ 24 wt% MgO and a liquidus (olivine) temperature of ~ 1501 °C, approximately 100 °C cooler than their hottest Archaean counterparts. At ~ 1424 °C chromite joined olivine as a crystallising phase. Olivine and chromite phenocrysts were re-mixed with residual melt shortly before or during komatiite eruption, which occurred by the time the magma had cooled to ~ 1321 °C. Combined geochemical and geological evidence requires that the Winnipegosis Komatiites erupted onto rifting continental crust. Their high liquidus temperatures require anomalously hot mantle. Considering the Winnipegosis Komatiites in the context of the broader Circum-Superior Belt, we suggest that these magmas formed from a mantle plume that was deflected towards the margins of the Superior craton by strong gradients in lithospheric thickness. This interpretation of the mode of formation of the Circum-Superior Belt casts doubt on ambient mantle potential temperatures as high as 1600 °C during the Proterozoic.

Introduction

Komatiites are generally believed to represent high-temperature, large-degree melts of the mantle (Arndt et al., 2008, Nisbet et al., 1993). As such, they have long been used as probes of both the chemical and thermal evolution of the mantle through time (Bickle et al., 1976, Campbell and Griffiths, 2014, Maier et al., 2009). However, the temporal record of mantle temperature and chemistry provided by komatiites is irregular; the vast majority of komatiites erupted during the Archaean (Arndt et al., 2008), with only two occurrences known from the Phanerozoic (Echeverria, 1980, Hanski et al., 2004). Despite comprising almost half of Earth history, only a handful of komatiites and related high MgO igneous rocks have been reported from the Proterozoic Eon, including: Ti-rich komatiites of the Karasjok and Central Lapland Greenstone Belts (Barnes and Often, 1990, Hanski et al., 2001); basaltic komatiites of the Vetreny Belt, (Puchtel et al., 1997); and a number of basaltic komatiites and compositionally similar intrusions from the Circum-Superior Belt (Arndt, 1982, Arndt et al., 1987, Hynes and Francis, 1982, Minifie et al., 2013).

In light of the paucity of Proterozoic komatiites, this paper describes the exceptionally fresh ~ 1.87 Ga Winnipegosis Komatiites from Manitoba, Canada. The minimally altered nature of the Winnipegosis Komatiite sequence provides an excellent opportunity both to study komatiite formation during the Proterozoic, and to compare the chemical and thermal state of Archaean and Proterozoic mantle. We present petrographical and geochemical observations with a view to understanding the mode of formation and thermal evolution of the Winnipegosis Komatiites. We demonstrate that the Winnipegosis lavas are komatiites by any definition, showing spinifex textures and derivation from a liquid with > 18 wt% MgO (Kerr and Arndt, 2001). The Winnipegosis Komatiites show many geochemical similarities to Archaean Al-undepleted komatiites, and formed from a nominally dry, depleted mantle source. Their high liquidus temperatures are ~ 100 °C lower than their hottest Archaean counterparts, but still require thermally anomalous mantle for their formation. We discuss possible modes of formation of the Winnipegosis Komatiites in light of these findings and the regional geological context.

Section snippets

Geological setting

The Winnipegosis Komatiite Belt (WKB; Hulbert et al., 1994) is located in the Superior Boundary Zone, in Manitoba, Canada, adjacent to the subsurface extension of the Thompson Nickel Belt (Fig. 1). The Superior Boundary Zone lies along the northwestern margin of the Archaean Superior Craton, and forms a narrow eastern foreland to the ~ 1.8 Ga Trans-Hudson Orogen (e.g. White et al., 2002). The Trans-Hudson Orogen was formed during the closure of the Manikewan Ocean (Stauffer, 1984), and resulted

Petrography and flow descriptions

Thin sections from approximately 200 samples from a 250 m section of komatiite intersected by borehole RP91-1A (hereafter RP1A), and approximately 70 samples from a 230 m section of komatiite intersected by borehole RP94–12 (hereafter RP12) were investigated petrographically. Previous core logging and petrographical examination of RP1A (Hulbert et al., 1994) identified at least 28 komatiite flows ranging between ~ 3 m and ~ 36 m in thickness, with a median thickness of 6 m. Core logging of RP12 (

Sample selection for geochemistry

A large body of core samples, rock powders, and thin sections, from > 250 samples used during the initial characterisation of the rocks by both Cominco and the Geological Survey of Canada (GSC), was obtained from the GSC archives in Ottawa. We utilised a large database (hereafter the ‘GSC database’) of unpublished geochemical data collected during the 1990s, including whole rock major element data measured by XRF; whole rock Fe2O3:FeO, H2Ot, and CO2 determined by wet chemical methods; and

XRF major and minor elements

Variations of elements measured by XRF are shown in Fig. 4. Some incompatible elements, such as Al2O3 and TiO2, show extremely tight correlations which extrapolate through measured olivine compositions. Others show more scatter and regression lines extend to higher (e.g. CaO) or lower (e.g. Na2O) MgO than Winnipegosis olivine. Samples from borehole RP12 lie along the same regression trends as the RP1A samples, but are generally more scattered.

The elements SiO2, FeO, and MnO show less

Olivine control and screening for the effects of alteration

Strong correlations between MgO and many incompatible elements that intersect olivine compositions are indicative of olivine control; consistent with the petrographical observation that olivine was the dominant crystallising mineral. A regression of Cr2O3 against MgO for the RP1A samples intersects a mixing line between average olivine and chromite compositions at 1.1 ± 0.1% chromite (95% confidence interval), suggesting the spread of data can be explained by the addition/subtraction of a mixture

Winnipegosis Komatiite Belt as part of the Circum-Superior Belt

The new 1870.3 ± 7.1 Ma age of the Winnipegosis Komatiite Belt is identical, within uncertainty, to the youngest mafic and ultramafic bodies previously dated in the Circum-Superior Belt, both in the adjacent Thompson Nickel Belt and more than 2000 km away in the New Quebec Orogen (Heaman et al., 2009). The Winnipegosis Komatiites also share many petrological features with other igneous rocks of the CSB: mafic/ultramafic rocks are abundant; magmatic olivine with Mg# ≥ 0.92, indicative of high

Conclusions

The Winnipegosis Komatiites formed at 1870.3 ± 7.1 Ma as part of the Circum-Superior Belt LIP. They occur as both massive and differentiated flows containing olivine spinifex layers. Their parental melt contained ~ 24 wt% MgO, which, along with the presence of spinifex layers and a median MgO of 22.4 wt% over > 250 m thickness of flows, confirms that these lavas are Proterozoic komatiites.

The parental melts crystallised only olivine from 1501 °C to approximately 1424 °C, and then a mixture of olivine + 

Acknowledgements

Pedro Waterton performed this work under the tenure of an NSERC Vanier Scholarship at the University of Alberta. Analytical work was funded by the Canada Excellence Research Chairs Program and the Geological Survey of Canada. We thank Stan Mertzman and Karen Mertzman for the XRF analyses, and Thomas Stachel and Gerhard Brey for supplying and discussing olivine standards. Constructive discussions with Yannick Bussweiler, Richard Stern, Tom Chacko, Mark Hamilton, Richard Ernst and Larry Heaman

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    Present address: NiPtx Inc. 262 Cranleigh Place S.E., Calgary, Alberta T3M 0N5, Canada.

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