The Late Cretaceous diamondiferous pyroclastic kimberlites from the Fort à la Corne (FALC) field, Saskatchewan craton, Canada: Petrology, geochemistry and genesis

Highlights

• First comprehensive petrological and isotopic study on FALC pyroclastic kimberlites

• Petrology and geochemistry indistinguishable from that of hypabyssal kimberlites

• Composition similar to global estimate of primary kimberlite melt composition

• Timing of source enrichment related to break-up of Rodinia supercontinent

• Substantial SCLM component in their genesis

Abstract

The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to − 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6–0.7 Ga) depleted mantle (T_DM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.

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).