The Late Cretaceous diamondiferous pyroclastic kimberlites from the Fort à la Corne (FALC) field, Saskatchewan craton, Canada: Petrology, geochemistry and genesis
Graphical abstract
Introduction
Kimberlites are small-volume, volatile-rich, potassic, and ultrabasic rocks recorded from almost all cratons and range in age from the Palaeoproterozoic to the Holocene. Owing to their (i) deeper depths of origin than any known terrestrial rock types, (ii) entrainment, on ascent, of a variety of crustal and mantle xenoliths from wall-rocks, (iii) unusual geochemistry with high enrichment of both compatible and incompatible trace elements which can shed light on the nature and composition of the sub-continental lithospheric (SCLM) mantle and deeper mantle processes, (iv) often diamondiferous nature, and (v) links to supercontinent cycles, Large Igneous Provinces, mantle plumes and mass extinctions, kimberlites continue to attract intense research from a variety of perspectives (e.g., Phipps Morgan et al., 2004, Gurney et al., 2005, Becker and Le Roex, 2006, Coe et al., 2008, Francis and Patterson, 2009, Jelsma et al., 2009, Chalapathi Rao and Lehmann, 2011a; Donnelly et al., 2011; Yaxley et al., 2013; Tappe et al., 2013, Tappe et al., 2014, Griffin et al., 2014, Kamenetsky et al., 2014, Bailey and Lupulescu, 2015, Heaman et al., 2015, Ault et al., 2015). Besides, some of the petrogenetic aspects concerning kimberlites such as the composition of the parental magmas (Price et al., 2000), the role of lithospheric assimilation during the magmatic ascent (Patterson et al., 2009), status within the supercontinent cycles (Tappe et al. 2016) and plate tectonic paradigm (Stern et al., 2016) also remain contentious issues of contemporary interest. Based on their volcanic/sub-volcanic nature and also taking into consideration grain size and the nature of infilling components at least four distinct types of kimberlite pipes have been recognised: (i) hypabyssal kimberlites, (ii) tuffisitic kimberlites, (iii) re-sedimented volcaniclastic kimberlites, and (iv) pyroclastic kimberlites (see Dawson, 1994, Hetman, 2008, Scott-Smith and Smith, 2009, Mitchell et al., 2009, Brown et al., 2012, Das et al., 2013). Owing to their ultramafic nature, kimberlites are readily prone to alteration and surface deposits are seldom preserved, thereby resulting in the predominance of hypabyssal varieties compared to the other kimberlite types. As a result, much of our existing knowledge and understanding about the petrology, geochemistry and genesis of kimberlites is strongly influenced by studies on hypabyssal kimberlites.
Pyroclastic kimberlites, within crater as well as extra-crater, are relatively the least preserved of all kimberlite types but are known from (i) Mwadui, Tanzania (Stiefenhofer and Farrow, 2004), (ii) Orapa A/K/1 and Jwaneng, Botswana (Field et al., 1997, Brown et al., 2008), (iii) Tokapal, Central India (Mainkar et al., 2004), (iv) Igwisi Hills, Tanzania (Willcox et al., 2008), (v) Mbuji-Mayi, Democratic Republic of Congo (Demaiffe et al., 1991), and (vi) Ontario, NW Territories, North Alberta and Fort à la Corne, Canada (Eccles et al., 2004, Zonneveld et al., 2004, Nowicki et al., 2008, Grunsky and Kjarsgaard, 2008, Van Straaten et al., 2009). However, a great majority of the studies yet carried out on them addresses various aspects of their geology, volcanology and emplacement mechanism, discriminating various volcanic facies within the larger pipes, diamond prospectivity and geochronology. Studies involving mineral chemistry and geochemistry of the pyroclastic kimberlites and addressing petrogenetic issues are very few owing either to the poor preserval of these rocks or due to confidential/commercial reasons. It should be mentioned here that attempts to utilise geochemical data of pyroclastic samples to infer petrogenetic processes are available in the literature for the kimberlites from Tanzania (Willcox et al., 2015) and northern Alberta, Canada (Eccles et al., 2004). Some bulk geochemical data of magmaclastic rocks from the FALC field has also been earlier documented by Lehnert-Thiel et al. (1992, Table 2) which revealed their similarity to kimberlites in general (see also Scott-Smith, 2008).
The Late Cretaceous diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan craton, Canada, is one of the largest (~ 200 ha) kimberlite fields on Earth comprising essentially subaerial and shallow marine pyroclastic kimberlites (Scott-Smith, 2008). Not much information is available in the public domain on the mineral chemistry and geochemistry of these kimberlites despite their discovery more than two decades ago. To the best of our knowledge, the only major work yet published on these lines is a published Ph.D. thesis of Leahy (1996) on borehole samples OFS-93-002, 003,004, 009, 010 and 012 which included mineral chemistry of the grains and whole-rock geochemical analyses by XRF; many of the trace elements (including REE) and Nd isotopic ratios were not reported. In this paper we document mineralogical and geochemical data of 41 samples, including Nd isotopic compositions on 8 samples, drawn from five widely separated boreholes of the FALC field. The focus of this study on the FALC kimberlites is to (i) identify and document petrological and geochemical variations within and across various kimberlite types, (ii) compare the data to other pyroclastic and hypabyssal kimberlites from southern Africa, India and Canada which constitute an integral component of the paleocontinents of Laurentia, Rodinia and Gondwanaland, and (iii) constrain their genesis. Throughout this paper we use the term kimberlite with reference to the Group I kimberlites, and the term orangeite with reference to the Group II kimberlites from their ‘type area’ of the Kapvaal craton of southern Africa (see Mitchell, 1995).
Section snippets
Geological setting of the Sask craton and the FALC kimberlite field
The Saskatchewan craton (or Sask craton) is a pericratonic tectonic segment in the Canadian shield, the bulk of which is made up of the Slave, Superior and Rae cratons (Snyder and Grütter, 2010). Seismic reflection studies have identified the configuration of the Sask craton (Lucas et al., 1993, Hajnal et al., 2005) and zircon ages of xenoliths showed its crustal age to be 3100–2450 Ma (Bickford et al., 2005). The presence of macrodiamonds in FALC kimberlites demonstrates the presence of
Age of kimberlites
Radiometric (U-Pb perovskite; Rb-Sr phlogopite) and biostratigraphic age constraints available for the FALC kimberlites reveal episodic eruptions over a time span of about 7 Ma (99–106 Ma) in the Albian (Leckie et al., 1997, Zonneveld et al., 2004, Berryman et al., 2004, Kjarsgaard et al., 2009a). During kimberlite emplacement, the environment changed from an overall terrestrial to dominantly marine conditions due to oscillatory marine transgressions and regressions and the kimberlites are, thus,
Analytical methods
Mineral chemistry of the kimberlite liquidus minerals and xenocrysts was determined by a CAMECA-SX100 electron microprobe (EPMA) at Mineral Resources, Technical University of Clausthal, Germany. Wavelength-dispersive spectrometry was deployed for mineral analysis. An accelerating voltage of 15 kV, a beam current of 50 nA and a beam diameter of 1 μm was used. TAP, PET and LLIF crystals and a PAP online correction program were employed. Several in-house natural and synthetic standards were used for
Petrography
Both conventional optical microscopy as well as back scattered electron (BSE) imaging were deployed for petrographic study of the drillcore samples and their salient aspects are depicted in Fig. 2, Fig. 3. A petrographic description of each of the studied samples corresponding to different depths from within each drill-core is summarised in Table 1 and their overall textural and mineralogical aspects are discussed here. All kimberlites of this study are loosely packed, clast-supported and
Mineral chemistry
Representative mineral chemistry data of various liquidus as well as xenocryst phases in the samples is presented in Supplementary Table A1, Supplementary Table A2, Supplementary Table A3, Supplementary Table A4, Supplementary Table A5, Supplementary Table A6, Supplementary Table A7 and discussed individually below. Mineral chemistry data of FALC samples reported by Leahy (1996) has also been considered along with our results.
Bulk-rock geochemistry
Petrographic features such as serptinised olivines, re-sedimentation textures and carbonatisation imply that the studied rocks have experienced deuteric alteration and sea water interaction. Furthermore, the presence of granitic and carbonate xenoliths (Table 1) implies possible crustal contamination in some of the samples. It is well known that kimberlites (including hypabyssal varieties) are probe to crustal entrainment, and post-magmatic (deuteric) alteration that may limit the utility of
Neodymium isotope composition
Eight bulk-rock samples from the diamondiferous #147 kimberlite (147-3A, 3C, 3H and 3K) and the non- or very-low-diamondiferous #168 (168-1C and 1G) and #181 (181-2A and 2E) kimberlites, were measured for their Nd isotope composition and the data is presented in Table 2 together with the bulk-chemical data. Measurement of Sr isotopes was not attempted because of the alteration of the samples by seawater interaction. The measured 143Nd/144Nd values range from 0.512536 to 0.512612 whereas the
Comparison with world-wide kimberlites and orangeites
An important outcome of this mineralogical and geochemical study on the pyroclastic FALC kimberlites is the strong resemblance of their mineral composition (olivine, spinel, mica, and perovskite), bulk-rock geochemistry and radiogenic Nd isotopic composition to that of world-wide hypabyssal-facies kimberlites from the paleocontinents of Laurentia, Rodinia as well as Gondwanaland (e.g., Chalapathi Rao et al., 2004, Becker and Le Roex, 2006, Coe et al., 2008, Tappe et al., 2014). This
Petrogenesis
Whilst there is an overall consensus for the role of sub-continental lithospheric mantle (SCLM) as the source region of orangeites (e.g., Fraser and Hawkesworth, 1992, Tainton and McKenzie, 1994, Mitchell, 2006, Coe et al., 2008, Chalapathi Rao et al., 2011a, Giuliani et al., 2015), the depth of melting and the source region of kimberlites are unconstrained and remain controversial. Based on bulk-rock geochemistry, high-pressure experimental studies, and entrained diamond inclusions, a number
A geodynamic model for the FALC kimberlites
Kimberlites are considered as time capsules in a global plate-tectonic framework and their emplacement ages are considered to be of geodynamic significance (Jelsma et al., 2009). Many models exist for the trigger of kimberlite eruptions and include (i) hotspots and mantle plumes (e.g., Crough et al., 1980, Heaman and Kjarsgaard, 2000, Torsvik et al., 2010, Chalapathi Rao and Lehmann, 2011), (ii) subduction of oceanic lithosphere and partial melting of its overlying continental lithospheric
Conclusions
- 1.
A detailed petrological and geochemical study on five kimberlite bodies of the Late Cretaceous diamondiferous FALC kimberlites, Sask craton, reveals that they are loosely packed, clast-supported and variably sorted pyroclastic kimberlites characterised by the presence of juvenile lapilli and single crystals dominated by olivine. Their xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite compositions overlap with those reported from world-wide kimberlites. Our
Acknowledgements
We thank Uranerz Exploration and Mining, Saskatoon, and Brent Jellicoe for access to drill core during the German-Canadian technical co-operation project KAN GEO 99 (1998–2001). Dan McKenzie, Cambridge, has kindly extended his help with the inversion modelling carried out in this study. Klaus Herrmann, Clausthal, helped with electron microprobe work, and Peter Dulski, Potsdam, provided the REE analytical data. NVCR acknowledges financial support from the Alexander von Humboldt Foundation,
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