2.a. A Mesozoic Labrador Sea sub-province

Modified from Steenfelt et al. (Reference Steenfelt, Hollis and Secher2006), Fig. 1(a) shows that a greater abundance of lamprophyre/kimberlite intrusions (orange dots) and associated carbonatite centres (yellow pentagons) outcrop across southern West Greenland, compared to along its conjugate Labrador margin (Sinclair et al. Reference Sinclair, Barr, Culshaw and Ketchum2002; Wilton et al. Reference Wilton, Taylor, Sylvester and Penney2002; Tappe et al. Reference Tappe, Foley, Jenner, Heaman, Kjarsgaard, Romer, Stracke, Joyce and Hoefs2006), even if the latter includes the Aillik Bay complex, as the type locality for aillikites. This bias may relate to Greenland being underlain by a thicker and more fertile/carbonated Archaean core, whereas Labrador is truncated by a Paleoproterozoic Core Zone and incorporates a greater abundance of Mesoproterozoic batholiths. The bias could also relate to asymmetric Jurassic-Cretaceous rifting (Tappe et al. Reference Tappe, Foley, Jenner, Heaman, Kjarsgaard, Romer, Stracke, Joyce and Hoefs2006), eventually leading to a Paleogene opening of the Labrador Sea, along which progressively more feldspar-bearing, alkali to eventually even tholeiitic magmas were emplaced (Tappe et al. 2007; Larsen et al. Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009).

Figure 1.

(a) Mesozoic reconstruction, showing lamprophyre, kimberlite (both as orange dots) and carbonatite (yellow pentagon) locations across an Archaean North Atlantic Craton of southern W Greenland and E Labrador (modified from Steenfelt et al. Reference Steenfelt, Hollis and Secher2006), where Neoproterozoic carbonatite centres are subdued. Ages are from Larsen et al. (Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009). As argued in the paper, lamprophyre dykes converge onto an inferred FHI carbonatite centre, like Aillik dykes (Tappe et al. Reference Tappe, Foley, Jenner, Heaman, Kjarsgaard, Romer, Stracke, Joyce and Hoefs2006). Craton is bound to the north and south by Paleoproterozoic orogens in paler shades of grey and Mesoproterozoic units to the east in even paler shades. Paleotectonic domains and Archaean terranes separated by solid black lines, whereas dotted lines separate systematic southern amphibolite to northern granulite facies shifts across Archaean terrane blocks, as defined by Windley & Garde (Reference Windley and Garde2009). (b) Their crustal cross section, onto which are added Ketilidian and Nagssugtoquidian thrusts from Garde et al. (Reference Garde, Hamilton, Chadwick, Grocott and McCaffrey2002) and van Gool et al. (Reference Van Gool, Connelly, Marker and Mengel2002), respectively. GI = Grønnedal-Ika, FHI = Frederiks Håb Isblink, T = Tikiusaaq, Q = Qaqarsuuk, S = Sarfartoq.

Focusing on the Mesozoic, much research addresses petrogenetic aspects (e.g., Tappe et al., 2011, Reference Tappe, Romer, Stracke, Steenfelt, Smart, Muehlenbachs and Torsvik2017a) – tentatively attributed to a subducted Pacific plate (Kjarsgaard et al. Reference Kjarsgaard, Heaman, Sarkar and Pearson2017; Tappe et al. Reference Tappe, Pearson, Kjarsgaard, Nowell and Dowall2013), regional rifting (Tappe et al. 2014, 2017b) and a Great Meteor hot spot track (Heaman & Kjarsgaard, Reference Heaman and Kjarsgaard2000) – as well as its entrained diamonds (e.g., Kjarsgaard, Reference Kjarsgaard and Goodfellow2007). It is debatable, however, whether a mantle plume head of a maximum ∼1000 km radius (White & McKenzie, Reference White and McKenzie1995) triggered the Mesozoic Labrador Sea lamprophyre–kimberlite–carbonatite sub-province, because (1) an extrapolation of Lawver and Müller’s (Reference Lawver and Müller1994) proto-Icelandic hot spot track at ∼150 Ma roughly positions that plume centre north of Greenland, requiring ∼1700 km of lateral mantle plume flow to the nearest Mesozoic Tupertalik complex (Fig. 1), and (2) a ∼150 Ma Great Meteor plume centre near a more kimberlite-dominated Kirkland Lake – Timiskaming area (Heaman & Kjarsgaard, Reference Heaman and Kjarsgaard2000) would have been located at an even greater radial distance of ∼1800 km from the west coast of Greenland. Since edge-driven convection, proposed for younger kimberlites along the western edge of the Canadian Shield (Kjarsgaard et al. Reference Kjarsgaard, Heaman, Sarkar and Pearson2017), unlikely played a role this far east of a subducting Farallon plate, it remains to be shown if lithospheric rifting was sufficient – on its own – to induce Mesozoic lamprophyre–kimberlite magmatism across the Labrador Sea sub-province.

2.b. Dyke and sheet clusters around common magmatic centres

Previous mapping of the Labrador Sea sub-province has identified clusters of calcite kimberlites and lamprophyres that occasionally surround coeval carbonatites (e.g., Steenfelt et al. Reference Steenfelt, Hollis and Secher2006). Apart from radiating dykes converging onto some offshore Aillik Bay centre (cf., Fig. 2 in Tappe et al. Reference Tappe, Foley, Jenner, Heaman, Kjarsgaard, Romer, Stracke, Joyce and Hoefs2006), however, other complexes have not been related to any plumbing system, let alone to associated carbonatites, where FHI’s radiating dyke swarm and centrally inclined cone sheets towards an inferred carbonatite in Fig. 1(a) are all contributions from this study, as presented in more detail in Fig. 2(e). That geological map further includes an aeromagnetic map, superimposed onto the FHI glacier and colour-coded according to the legend’s total magnetic intensity (TMI) scale, that exhibits a distinct positive anomaly (up to ∼400 nT) at the southern edge of the glacier, towards which all studied dykes (in red) and inclined sheets (in green) converge.

Figure 2.

Field relationships. (a-d) AEROMAG TMI (1996) extracts (copied from Naalakkersuisut’s ‘Greenland Portal’) of four known carbonatite centres along the southern west coast of Greenland, compared to (e) glaciated areas inside 1:100 000 geological map of outcropping area (Kokfelt et al., Reference Kokfelt, Weng and Willerslev2019). Twenty-two published and 25 new dyke, sheet and sill sampling localities are shown, using colours and symbols as in Fig. 8. Moving average rose diagrams for dyke (red) strike measurements from the central (C) and southern (S) study area, converge – together with inclined sheet (green) dip directions – onto a positive aeromagnetic anomaly (up to 400 nT). (f-g) Google Earth images of study areas C and S, onto which this paper’s sampled dykes and inclined sheets are located and where each sample number terminates a 5197-prefix. (h) Conceptional cross section through how a mushroom-shaped igneous centre (yellow) gives rise to a local stress field that allows the injection of both bladed radiating dykes from its cylindrical stem, as well as inclined cone sheets from the crown of a more oblate head.

Such conspicuous convergence – as traced from 100:000 scale geological maps (Fig. 2e) and mapped within study areas S, C & N in 2010 (Fig. 2e-g) – conforms to how Chadwick & Dieterich (Reference Chadwick and Dieterich1991) model a similar combined radiating dyke and circumferential cone sheet swarm on a Galapagos Island. In their model, a mushroom-shaped central magma source (Fig. 2h) generates a combined local stress field, in which an over-pressurized cylindrical stem triggers the injection of radiating dykes, while concentric and centrally inclined sheets are injected from its oblate crown. It remains to be seen if FHI’s radiating dykes are limited to the northern side of its proposed centre or other associated dykes can also be mapped and sampled to the south of that centre. For the latter, the nearest Paamiut dykes are deemed too distally located (Fig. 1a), too ultramafic (cf., later Fig. 8a) and too old (∼166 Ma) to have been injected from the same FHI centre.

Purely based on similarities with the aeromagnetic anomalies at four known carbonatite complexes along the southern west coast of Greenland (Fig. 1), shown at similar scales in Fig. 2(a-d) for comparisons, it is tempting to propose that the FHI anomaly/centre is likewise carbonatitic, at least as a working hypothesis for this paper. Even if the FHI anomaly does not exhibit any anomalously low nT-rim – like the one around the Neoproterozoic Sarfartoq carbonatite (Fig. 3a), likely reflecting how its demagnetized fenitized host rocks contrast its magnetite-rich carbonatite centre – the other verified carbonatite examples argue that such low-magnetic rims are not prerequisite. Thus, even if unavailable gravimetric or radiometric data would strengthen interpretations (Thomas et al. Reference Thomas, Ford and Keating2016), available evidence allows the FHI anomaly to be a carbonatite centre that – due to its combined glacial cover and proximity to the sea – may have been missed by both geological and stream sediment mappers (cf., Steenfelt et al. Reference Steenfelt, Jensen, Nielsen and Sand2009, for the latter).

Figure 3.

Field photos: (a) a westerly view across study area S, intruded by both radiating dykes and shallow W-dipping sheets, both of which converge onto an aeromagnetic anomaly that is located at the edge of the FHI glacier, ∼6.5 km behind this outcrop. Each sample number terminates a 5197-prefix. (b) Weakly biotite-phyric 519705 LMD-dyke. (c) Inclined 519704 PN-sheet. (d) A 2.4 m-thick damtjernite dyke, located ∼28 km from the aeromagnetic anomaly in Fig. 2(e), from which a more aphyric LMD margin (519750) and more porphyritic HMD core (519751) was sampled. (e) A 0.2 m-thick and 64˚SW-dipping dyke from the central area (not sampled), with numerous margin-parallel ocelli strands that plunge down-dip.

If the FHI anomaly turns out to be a carbonatitic centre, this would follow upon another recent discovery of an almost coeval and neighbouring Tikiusaaq carbonatite (Steenfelt et al. Reference Steenfelt, Hollis and Secher2006; Fig. 2c). As illustrated in Fig. 1(a), it would also result in a conspicuous regular zig-zag ‘chain’ of carbonatitic centres across the Labrador Sea sub-province, with Mesozoic centres – emplaced during Pangea break-up – located 170–176 km apart across the core of the North Atlantic Craton, while more peripheral Neoproterozoic centres – emplaced during the assembly of Pangea – occur slightly closer (130–137 km) to each other and even inside Paleoproterozoic orogens that verge onto the craton from north and south.

Another note may be made on how all known carbonatite centres across the North Atlantic Craton, with their surrounding clusters of lamprophyres and kimberlites, appear restricted to the cores of lower-order Archaean craton blocks, as defined by Windley & Garde (Reference Windley and Garde2009), rather than along their boundaries (cf., Fig. 1a-b). While this could be used as an argument against the presence of a carbonatitic FHI centre, its existence could equally well question the boundary between a more northerly located Bjørnesund and southerly located Kvanefjord block, hidden by the particularly large FHI glacier. If so, Bjørnesund’s southern amphibolite grade zone, which Windley & Garde (Reference Windley and Garde2009) use as an argument for a domino-block-like craton block boundary below the FHI glacier, may represent a northward extension of their retrogressed nappe covering parts of the Kvanefjord block (magenta unit in Fig. 1b).

3.a. Petrographic classifications

During previous studies of the FHI complex, Hansen (Reference Hansen1980) mainly distinguished between nephelinites and carbonate-bearing melilitites, where nephelinites were further sub-divided into being olivine-bearing and progressively more evolved than that. She also identified melilitites, where the central portion of one composite melilitite dyke (sample 118101) is even carbonatitic. In a systematic review of intrusions across southern West Greenland (Fig. 1a), Larsen et al. (Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009) adopted a more petrographic classification scheme by Le Maitre (Reference Le Maitre2002), relabelling Hansen’s (Reference Hansen1980) kaersutite-, Ti-rich augite-, olivine- and biotite-phyric nephelinites as monchiquites and her melilitites as alnöites. The latter is in accordance with Tappe et al. (2005), who argue for an addition of three ultramafic lamprophyre types into Le Maitre’s (Reference Le Maitre2002) classification scheme, namely (1) melilite-bearing alnöites, (2) less magnesian and more nepheline and/or alkali feldspar-bearing damtjernites and (3) more magnesian, carbonate-rich and garnet-bearing aillikites.

Since it will be shown that two dykes with porphyritic cores and almost aphyric margins neither contain melilite, leucite or garnet, but rather interstitial sodic foids, we reclassify the monchiquites by Larsen et al. (Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009) as damtjernites, following Tappe et al. (Reference Tappe, Foley, Jenner, Heaman, Kjarsgaard, Romer, Stracke, Joyce and Hoefs2006) and Pandey et al. (Reference Pandey, Rao, Dhote, Pandit, Choudhary, Sahoo and Lehmann2018). A further distinction between typically more porphyritic olivine damtjernites and often almost aphyric damtjernites will be supported by bulk rock geochemistry (Section 4.1) and here forthwith referred to as either high- or low-Mg damtjernites (abbreviated as HMD and LMD, respectively). It will also be shown how more evolved monchiquites by Larsen et al. (Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009) host too much foids to classify as lamprophyres and thereby conform better to Hansen’s nephelinites and may even be regarded as phonolitic (i.e., here forthwith referred to as phonolitic nephelinites (PN), or simply nephelinites for short). Finally, for the lack of a better name, we adopt alnöite from Larsen et al. (Reference Larsen, Heaman, Creaser, Duncan, Frei and Hutchinson2009), even if these rocks may contain insufficient melilite and where we wish to stress their more carbonaceous nature (i.e., here forthwith referred to as carbonaceous alnöites (CA), or simply alnöites for short).

In the following sub-sections, microscope petrography and scanning electron microscope (SEM) mineral chemistry (Supplement A) focus on one sample from each of the four main rock types (HMD, LMD, alnöite and nephelinite). Due to space restrictions, an additional sample of each rock type is presented as Supplementary Material B, together with autolithic nodules (Fig. B4) from within a proximal LMD dyke (519714) in study area S (Figs. 2g & 3a). Thus, representing damtjernites, a better exposed and 2.4 m-thick distal dyke from study area N (Fig. 3d) was sampled from both its almost aphyric LMD margin (519750A) and more porphyritic and olivine-bearing HMD core (519750B), with another even more porphyritic core sample (519751) collected a farther 156 m NNW and along that same dyke (Fig. 2e). Representing a more evolved alnöite–nephelinite suite, identified geochemically in Section 4, this main paper includes the most carbonaceous, or least silicic, of two aphyric and radiating alnöite dykes (510754) from study area N (Fig. 2e), as well as a radiating nephelinite dyke (519715), from inside the most proximal study area S (Fig. 2g).

3.b. Damtjernites

In Fig. B1, a pair of thin section slides of the porphyritic HMD core and almost aphyric LMD margin of the thickest dyke in this study (519768C & -M, respectively) quantify how locally touching euhedral olivine and a greater abundance of faintly beige coloured augite phenocrysts, together with rarer magnetite micro-phenocrysts, abound inside this dyke’s HMD core (-68C in Table 1). These phenocrysts define a panidiomorphic texture, indicative of a cumulative origin. Even if shared assemblages in both margin and core samples are consistent with flow segregation, the marginal sample’s 2.9% of (micro)phenocrysts record a greater proportion of magnetite and less olivine, compared to the more porphyritic dyke core, yet almost equal augite proportions (cf., −68M and −68C in Table 1). Phenocrysts exhibit little internal zonation, except for thin, darker and more titaniferous augite rims, where centrally positioned SEM spot analyses (Supplement A) were made on individual phenocrysts within the dyke’s porphyritic HMD core. These results (Fig. B9) show that 20 olivine and 26 augite phenocrysts inside this dyke core (519768C) are the most magnesian among all analyses in this study (Fo_88-80 and Di_90-71, respectively), which together with 20 more magnesian and aluminous magnetites are consistent with this dyke being more primitive than another damtjernite dyke with a porphyritic core (519750-1 in Fig. 3d), described next.

Table 1.

Phenocryst assemblages within damtjernites

All proportions are out of 100%, excluding matrix and ocelli.

The three thin section scans in Fig. 4(a-c), together with additional traced copies in Fig. B2(a-c), quantify both phenocryst abundances and grain sizes (cf., Fig. B2d-f for the latter). These results (Table 1) show that this dyke’s pair of porphyritic HMD core samples (collected 156 m apart along the same dyke) host roughly similar phenocryst assemblages, especially, after combining slightly paler brown kaersutites with darker brown and often more euhedral biotite phenocrysts. As supported by SEM spot analyses from the most porphyritic core sample 519751 (cf., small, colour-coded circles in Fig. 4c), with largest and freshest phenocrysts, this dyke core not only includes anhydrous olivines, augites and magnetites, like in the previous more primitive dyke, but also hydrous kaersutites and biotites, consistent with a more evolved composition. This is substantiated by less magnesian olivines (18 analyses of Fo_84-78) and augites (43 analyses of Di_84-71), while 11 kaersutite and 22 biotite spot analyses have Mg numbers ranging between 72-66 and 54-52, respectively (Fig. B9). In addition to magnetites also being less magnesian and aluminous than in sample 519768C, sample 519751 – by chance – also hosts a single and anomalously large ilmenite, likely as a xenocryst.

Figure 4.

Damtjernite petrography. (a-c) Three thin section scans, where 519750A & -B (a & b, respectively) were collected next to each other and 519751 (c) was collected 156 m NNW along the same dyke. Colour-coded circles in (c) locate SEM spot analyses (Fig. B9). Red and yellow frames in (c) locate microphotos (d-e), matrix phase maps (f-g) and ocelli in Fig. 5(a-d). (d) Partly resorbed olivine (ol) phenocrysts, where one has a ‘corona’ of epitaxially overgrown euhedral biotites (bi). (e) Other phenocrysts, including augite (au) and magnetite (mt), together with circular and internally heterogeneous ocelli (oc). (f-g) 3.1 mm²-large matrix phase maps (cf., Fig. B3) for (f) more euhedral matrix crystals, forming a framework inside which (g) interstitial phases crystallized. Colour-coded text acts as a legend, quantifying modal proportions.

Even if phenocrysts are not conducive towards interpretations of crystallization sequences, the presence of only anhydrous olivines, augites and magnetites in the more primitive HMD sample 519768C argues for these being the earliest crystallizing phases. The addition of both kaersutite and biotite phenocrysts in the more evolved HMD samples 519750-1 indicates that early crystallizing anhydrous phases were followed by more hydrous phases, reflecting a P_H2O increase during early differentiation. In addition, conspicuously rounded olivine (and to a lesser extent also kaersutite) phenocrysts are resorbed (e.g., lower right of Fig. 4d), and this resorption occurred while some olivines were epitaxially overgrown by biotite, as evidenced by how an olivine phenocryst in Fig. 4(d) is less resorbed beneath protective biotite. In contrast, equally rounded ocelli are rather surrounded by what appears to be tangentially adhered biotites (Fig. 4e), suggesting that ocelli nucleated and rapidly expanded relatively late, after biotites had started crystallizing.

SEM element maps (Fig. B3) across a 3.1 mm²-large portion of a less easily defined matrix confirm that both olivine and kaersutite had stopped crystallizing, while augite, magnetite and biotite continued crystallizing together with accessory apatite needles, perovskite, minute rutile and even sulphides (Fig. 4f and -51_m in Table 2). While these more euhedral, early crystallizing phases constitute roughly half of this matrix area, the remaining half (Fig. 4g) is comprised of mostly analcime, some nepheline, minor calcite and accessory melilite, as late crystallizing interstitial phases (Table 2). Based on textural relationships between these more anhedral phases, it appears that analcime crystallized last, while it is more difficult to disclose crystallization sequences between the other three interstitial phases. There is no indication of the analcime having replaced another equally interstitial phase.

Table 2.

Bulk modal mineral proportions (incl. ocelli)

Including (p) phenocrysts, (m) matrix and ocelli, *22.5% Na-Ca-Si, and †8.1% dolomite.

Sample 519751’s conspicuously circular and transparent ocelli (Figs. 4e & B2c) differ from the sample’s rounded olivines (e.g., Fig. 4d), by comprising more heterogeneous aggregates of mainly transparent minerals in among pale green and acicular aegerine crystals. While such ocelli – after first being regarded as amygdales – were subsequently suspected to be immiscible carbonatite droplets, only two out of 64 SEM spot analyses on this thin section’s ocelli (Fig. 4c) are calcitic and – in decreasing number of spot analyses within brackets – rather made up of silicic analcime (27), nepheline (14), augite-aegerine (8), leucite (2) and titanite (1). The first 27 spot analyses provide our most quantitative constraints on analcime (Na,H₂O)[AlSi₂O₆], with half of its cations being silica, a quarter being aluminium, little less than a quarter being sodium, traces of calcium and totals between 84-89% allowing for additional water, not analysed for (cf., ‘29 analcimes’ in Supplementary Data D).

More informative SEM backscatter and elemental maps (Figs. B3 & 5) further reveal that four ocelli are modally made up of 42–90% analcime, 0–43% nepheline and 0–2% sanidine, together with 4–6% augite-aegerine needles, 0–8% calcite and 0–5% melilite, as well as accessory apatite, magnetite and pyrite (Table 2); thereby replicating the interstitial phases found in the matrix of the same thin section (Fig. 4g). We find no indication of the analcime having crystallized from hydrothermal fluids, such as radiating zeolite or a concentrically zoned amygdale. Even if some augite-aegirine needles may represent partially incorporated microcrysts or even protrusions from a surrounding matrix, all phases inside these four ocelli (i.e., Fig. 5) – representing 0.96% of the thin section, compared to all ocelli constituting 6.3% – are included in Table 2’s estimate of sample 519751’s modal mineral proportions. The estimate excludes biotite; however, since these crystals appear to have adhered onto the outer surface of such melt droplets (Fig. B3), rather than having crystallized from it and draping the inside of droplet walls.

Figure 5.

Phase maps of four ocelli, traced from SEM electronic backscatter and elemental maps in Fig. B3. Labelled circles locate SEM spot analyses for analcime (al) nepheline (ne), calcite (c), magnetite (mt) and pyrite (py). ‘depleted’ refers to a zone between nepheline and analcime with lower Na and higher Ca, inside which most calcite and melilite also resides. Otherwise, as in Fig. 4.

Textural relationships inside the studied ocelli improve constraints on the crystallization sequence among late-stage phases, with more euhedral augite-aegerine and magnetite → (crystallizing before) subhedral nepheline and sanidine → anhedral melilite and calcite → surrounding analcime. The melilite-rich ocellus section in Fig. 5(a) provides the best evidence of melilite crystallizing epitaxially along the walls of this ocelli and – in the absence of nepheline and sanidine – possibly even together with augite-aegerine. Rather than forming a second stage of immiscible droplets inside these ocelli, however, calcite more often appears to also have crystallized against ocelli walls or pre-existing nepheline (e.g., in the most calcitic ocellus section in Fig. 5b). This is substantiated by the most intriguing ocellus section in Fig. 5(c), which not only includes all mentioned phases but also an unconstrained envelope that is more calcic and less sodic than analcime and incorporates a high concentration of both melilite and calcite. This enigmatic (hybrid?) zone drapes nephelines and sanidines and is in turn draped by analcime, suggesting that while more calcic phases grew onto pre-existing nephelines and sanidines, hydrous analcime was the last to crystallize inside this ocellus. Again, there is no indication of the analcime having replaced another phase and must thereby have crystallized as a primary magmatic – albeit late-stage and fluid-rich – phase.

3.c. Damtjernite-hosted autoliths

Two coarse-grained nodules, from inside a proximal dyke (519714; Fig. B4), mainly comprise of more euhedral and likely cumulus augites (no olivine) and subhedral magnetites, surrounded by intercumulus kaersutite (no biotite). This assemblage resembles that of phenocrysts inside evolved damtjernites but where these nodules more clearly show how kaersutite crystallized after augite and magnetite. The nodules also contain additional larger abundances of euhedral and thereby early crystallizing apatites, as well as isotropic fluorites, which are not observed as phenocrysts within the two studied damtjernite dykes. SEM spot analyses further reveal how these nodules contain cumulus augites (Di_50-47), kaersutites (Mg# ∼58-34) and magnetites, which are all only slightly more evolved than analysed damtjernitic phenocrysts (Fig. B9). These nodules arguably provide the most direct information about cumulates within a central magma chamber and could potentially have accumulated from magmas like their damtjernitic host dyke.

3.d. Carbonaceous alnöites

Two selected samples serve to illustrate the petrography of typically more aphyric, thinner and relatively scarce alnöite intrusions (Fig. 2e). Because of their fine-grained nature and presence of rarely more than micro-phenocrysts, it is difficult to identify minerals under a microscope. Reconnaissance SEM spot analyses only reveal that both samples include augite, apatite, magnetite and some biotite, typically as slightly larger crystals. Relatively low-resolution SEM elemental maps for a more altered 519749, with partly amphibolitized augite micro-phenocrysts, did not allow all phases to be fully traced. Nevertheless, the resulting phase map in Fig. B5(b) still reveals that this thin section’s dominating paler patches are richer in feldspathoids, apatites and magnetites, while slightly darker zones in between these patches are much more carbonaceous (including both calcite and ankerite), devoid of aluminium and hosting rutile rather than magnetite. It is tempting to interpret these different zones as an incipient separation into more feldspathoidal and carbonaceous magma parts, which could have segregated after augite started crystallizing, judging from how augite crystals appear more evenly dispersed across both zones.

A slightly coarser-grained and fresher 519754 (Figs. B6 & 6) does not exhibit as much zoning as 519749 but a more evenly aphyric to slightly augite-microphyric texture. A complete SEM elemental mapping of a 9.9 mm²-large area of this thin section shows that 519754 is made up of early crystallizing augite, biotite, magnetite, apatite and perovskite (Fig. 6b). Even if this mineral assemblage resembles that of damtjernites (Fig. 4f), the alnöite matrix in Fig. 6(b) hosts less augite and a greater proportion of apatite and biotite compared with the matrix of the porphyritic HMD (Fig. 4f) and, most importantly, a smaller proportion of early crystallizing phases (44.3%) compared to either the HMD’s matrix (50.8% for -51_m in Table 2) or the entire thin section, including phenocrysts (68.4% for -51_p in Table 2).

Figure 6.

The most carbonaceous of two alnöite thin sections, 519754. (a) Thin section scan with more obvious apatite and augite micro-phenocrysts and even smaller magnetites, surrounded by biotites. Colour-coded circles locate SEM spot analyses (Fig. B9). (b) A 9.9 mm²-large phase map of matrix portion located in (a), based on SEM electronic backscatter and elemental maps in Fig. B5. Otherwise, as in Fig. 4.

Like within the damtjernites, interstitial phases of sample 519754 are also made up of mostly analcime, but also an equal proportion of unidentified more calcic and less sodic Ca+Na+Si phase that – much like an ocelli’s ‘hybrid’ zone in Fig. 5(c) – typically is located in between the analcime and apparently earlier crystallizing calcite (Fig. 6b). All these late crystallizing interstitial phases constitute a significantly larger proportion within this alnöite, compared to the HMD (55.6% vs 31.7% of interstitial phases, respectively, in Table 3).

Table 3.

Interstitial analcime:carbonate proportions

* 40% analcime and 41% ‘hybrid’ Ca+Na+Si phase.

3.e. Phonolitic nephelinites

Samples from an inward dipping sheet (519711) and a radiating dyke (519715), serve to illustrate the petrography of typically more aphyric and thinner nephelinite intrusions, emplaced closest to the magmatic centre (Fig. 2g). The inclined sheet (519711) exhibits an overall pale-spotted texture (Fig. B7a) that incomplete SEM mapping (Fig. B7b) reveals to represent (K, Si)-rich spots, surrounded by (Na, Ca)-rich darker zones. Among more euhedral crystals that are large enough to be identified and mapped in Fig. B7(b), augite and apatite needles, as well as a remarkably large abundance of euhedral and equigranular perovskites, appear to be dispersed relatively evenly throughout both matrix zones, suggesting that these crystallized before the (K, Si)- and (Na, Ca)-rich portions segregated from each other. Preferentially SEM spot analysed sanidine micro-phenocrysts on the other hand reside exclusively inside (K, Si)-blebs, while magnetite, pyrite and some euhedral analcime crystals appear restricted inside intervening (Na, Ca)-zones, together with euhedral nephelines. Thus, it appears that 519711’s sanidines and nephelines started crystallizing after earlier augites, magnetites, apatites and perovskites and thereby left behind an interstitial melt inside the (Na, Ca)-rich darker zones that subsequently segregated into more analcime-rich and carbonaceous parts (cf., Fig. B7b).

The crystallization sequence for the above nephelinite sheet sample 519711 is more convincingly supported by nephelinite dyke sample 519715 (Fig. 7), which is sufficiently coarse to allow all its phases to be identified from SEM elemental maps (Fig. B8). While this thin section is not as distinctly spotted as 519711, it still contains ∼5.7% of more irregular greyish patches, in among paler areas, separated by darker transition zones (Fig. 7a). The 6.6 mm²-large SEM element maps in Fig. 7(c-d) reveals that one such grey patch is dominated by carbonates, while its paler surroundings incorporate more analcime. Both interstitial phases constitute 61.6% of this map and reside inside an open network of early crystallizing phases, dominated by sanidines throughout but without any augites residing inside the carbonated patch (Fig. 7c). Magnetite and rutile are the most abundant accessory phases, as opposed to mainly pyrite in 519711, with rutile being more common among carbonates. Little – if any – nepheline occur together with mainly analcime, while the carbonaceous patch comprises of more dolomite than calcite.

Figure 7.

Sample 519715 from a proximal nephelinite dyke. (a) Scanned thin section with few micro-phenocrysts of mainly opaque magnetites, a solitary apatite and mostly analcime-bearing paler patches, including one distinct ocellus, set in a fine-grained matrix with 6.3% greyish patches. Colour-coded circles locate SEM spot analyses (Fig. B9). Phase maps, based on SEM elemental maps in Fig. B8, for (b) ocellus, as well as (c) early and (d) late crystallizing phases within a 6.6 mm²-large matrix area, dominated by a central grey patch. Otherwise, as in Fig. 4.

The thin section of sample 519715 (Fig. 7a) also contains a thin vein and isolated paler spots, including a larger ocellus that is linked to that vein. A segment of this vein, mapped in Fig. 7(d), comprises 89% analcime and 11% calcite, resembling proportions calculated for the entire thin section (90% vs 10%, respectively), as detailed in Section C2 and listed in Table 3. In comparison, the ocellus (Fig. 7b) is almost entirely made up of analcime (99%), with only ∼1% calcite (Table 3), after excluding early crystallizing acicular sanidines that likely protrude from the surrounding matrix and accessory pyrites and magnetites that line the walls of the ocellus. Together with analcime-dominated ocelli in our damtjernite sample (Fig. 6), this suggests that analcime rest melts more readily form ocelli than carbonaceous rest melts.

3.f. Summary of key petrographic results

Through various textural relationships, it may be generalized that the crystallization sequence is olivine → augite → magnetite → kaersutite → biotite (during resorption of olivine and kaersutite) → apatite/fluorite/perovskite/rutile/pyrite (more difficult to discriminate between these five phases) → sanidine → nepheline → melilite → calcic-, magnesian- and/or ferrous carbonates → analcime. Through phenocryst and SEM phase maps, as well as discriminations between predominantly analcime-bearing ocelli and veins versus more carbonaceous zones, it is possible to quantify phase proportions for all four rock types (Tables 1–2). The ubiquitous presence of analcime, together with carbonates, across all rock types suggests both a cogenetic relationship and that all phases are igneous, rather than secondary. This view is supported by how proportions of early crystallizing phases diminish, while proportions of interstitial late phases increase, and how ratios between analcime and carbonates, as dominating interstitial phases, differ between CA and PN (Table 3), for example, induced by fractional crystallization and late-stage (± immiscible) segregations within interstitial rest melts.

4.a. Geochemical classification

Geochemically, FHI samples plot along the more aluminous lamprophyre base of Bergman’s (Reference Bergman1987) ternary classification diagram, separate from more magnesian kimberlites, aillikites and associated (often magnesian) carbonatites, as well as more potassic lamproites (Fig. 8a). FHI samples further plot across the silica-poorer foidite side of a Total Alkali vs Silica (TAS) diagram (Fig. 8b), after preferably not eliminating loss on ignitions (LOIs) and normalizing major elements to 100%, because these rocks may have substantial igneous CO₂ components. Fig. 8(b) also shows how FHI’s damtjernites plot where Rock’s (Reference Rock1987) compositional ranges by ultramafic and alkali lamprophyres overlap (UML and AL, respectively, in Fig. 8b). In contrast, more alkali CA to PN mostly plot along a rough CA-PN array above these damtjernites, which extends from the carbonatite end of Rock’s (Reference Rock1987) UML field, across the TAS diagram’s foidite field and into the more alkali and silica-rich extent of Rock’s (Reference Rock1987) AL field, to almost plot as phonolites.

4.b. Damtjernites

Twelve selected variation diagrams in Figs. 9–10, with MgO as a common differentiation index, illustrate how HMDs and LMDs define two distinct geochemical groups, with more or less than ∼8.5 wt% MgO, respectively. Among these plots, Sc (Fig. 10e) is the only element that exhibits a single coherent, albeit scattered, linear trend, which even extends into the alnöite–nephelinite suite. The other eight plots exhibit more or less elbowed HMD and LMD trends, where elbows for SiO₂, CaO, P₂O₅, MnO and rare earth elements (REEs) (Figs. 9a, e & 10c, f) are convex upward and Fe₂O₃, TiO₂ and Co (Fig. 10a-b, d) are convex downward. Despite some being poorly defined, the commonly discordant trends for HMDs and LMDs can still be reversely modelled separately, as detailed in Supplement C and briefly summarized in the following text for a maximum of three co-fractionating phases.

Figure 8.

Classification diagrams by (a) Bergman (Reference Bergman1987) and (b) Le Maitre (Reference Le Maitre2002), where this paper’s samples group into either high- or low-Mg damtjernites (HMD and LMD, respectively) and an associated suite of carbonaceous alnöites (CA) to phonolitic nephelinites (PN). Subdued sample symbols in (b) exclude volatiles and are normalized to 100%. According to Rock (Reference Rock1987), blue UML-field = ultramafic lamprophyres and red AL-field = alkali lamprophyres. Carb = Carbonatite.

Figure 9.

Six major oxide variation diagrams with MgO as differentiation indices. (a-c) ‘Alkali feldspathic’ oxides with negative sloping CA-PN trends. Insert plots K₂O against Na₂O. (d-f) ‘Carbonatitic’ oxides and loss of ignition (LOI) with opposite positively sloping CA-PN trends. Yellow-filled circles represent concentrations in the four different ocelli cross sections in Fig. 5, calculated from mapped phase proportions and their stoichiometric compositions. Symbols and annotations as in Fig. 8.

Figure 10.

Three major oxide (a-c) and three trace element (d-f) variation diagrams with MgO as differentiation indices. (g-h) Reverse modelling results for HMD and LMD trends, derived from Figs. C2 & C3, respectively, where every intersection collectively delineates a colour-coded area that quantifies the range of possible fractionating/accumulating phenocryst assemblages, listed next to these. Symbols and annotations as in Fig. 8.

Our reversed geochemical modelling uses SEM spot analysed HMD phenocryst compositions (Fig. B9), firstly, for earliest crystallizing olivines, augites and magnetites, which were all seen to accumulate inside the more primitive and panidiomorphic dyke core sample 519768C (Fig. B1f). These phases define irregular phase triangles in Fig. C2, through which manually fitted HMD and LMD trend lines consistently pass. The ways that trends intersect these irregular phase triangles are transposed onto a more quantifiable ternary diagram (Fig. 10g), where six HMD trends are consistent with the fractionation/accumulation of between 70–88% augite, 5–24% olivine and 0–22% magnetite, while six intersecting LMD trends require more magnetite (29–45%), together with roughly similar proportions of olivine (4–28%) and less augite (33–68%), likely having fractionated from these more aphyric samples. These results may be simplified as HMD trends being controlled by mostly augite, including some olivine; whereas, LMD trends are controlled by additional magnetite. A similar dominance of augite fractionation also controlled the diversification of lamprophyric magmas within the Kola Peninsular (Nosova et al. 2021a).

Since biotites appear to replace resorbed olivines within the more evolved dyke core sample 519751 (cf., Fig. 4d), the same trends are reversely re-modelled through augite–magnetite–biotite phase triangles (Fig. C2) but result in roughly similar proportions (Fig. 10h). Thus, suggesting that augite is the most important phase for both HMDs and LMDs, magnetite becomes important for LMDs, while it matters less what type of additional mafic phase (whether early crystallizing olivine, late crystallizing biotite, intermediate kaersutite or any mixture of these) fractionated, or accumulated, together with these. The question just becomes how the above-modelled assemblages are either fractionated or accumulated in order to form both HMDs and LMDs? In Section 5(a), the following three fractionation models will be discussed: (1) elbowed HMD and LMD trends simply representing liquid lines of descent from a more magnesian parent; (2) in situ dyke flow segregation, where magnetite is mysteriously decoupled from accumulating into dyke cores; and (3) dykes tapping a fractionated magma chamber top, occasionally followed by the entrainment of an unconsolidated mafic cumulate mush (without denser magnetites).

In addition to having discordantly elbowed trends, HMD and LMD trends defined by Al₂O₃ (Fig. 9b) and total alkalis (Fig. 9c) are rather offset from, than connected to, each other, with HMD samples having overall more elevated concentrations than LMD samples. Since analcime-rich ocelli are particularly sodic and aluminous (yellow-filled circles in Figs. 9–10 & C1), it is reasonable to suspect that these offsets reflect an accumulation of such ocelli into HMD samples. Since three dykes have both porphyritic HMD cores and almost aphyric LMD margins, it may be further speculated that ocelli flow segregated from dyke margins and more readily accumulated into HMD dyke cores. Such an interpretation is supported by a higher proportion of ocelli within the most porphyritic core sample 519751 (6.3%), compared to a less porphyritic core sample 519750C (0.8%) from the same dyke (Table B1 and Fig. B2).

4.c. T-trends by more evolved rocks

More evolved alnöites and nephelinites define trends that occasionally form an extension of LMD trends (e.g., Fig. 10a-e), as expected if these simply formed through continued biotite, augite and magnetite fractionation from an LMD parent. In the remaining 8 of 13 selected variation diagrams in Figs. 8(b) and 9–10, however, alnöite–nephelinite trends define discordant T-junctions to LMD trend extrapolations, much like how one would expect liquid immiscibility to split towards carbonatitic and feldspathoidal end members. Two types of T-junctions are recognized, where (1) nephelinites become more enriched in SiO₂, Al₂O₃ and total alkalis (Fig. 9a-c) and (2) alnöites become more enriched in CaO, MgO, P₂O₅ and REEs (PN-CA in Figs. 9d-e & 10f). These oxides and elements are either the building blocks of, or highly compatible with, interstitial analcime or carbonates, in total constituting 62 and 56 modal-%, respectively, of nephelinite sample 519715 and alnöite sample 519754 (Table 2). Together with how ratios vary between analcite and carbonates (Table 3) and their bleb-like mingling amongst each other (Figs. B5a, 7a & B7a), it is therefore tempting to relate these alnöite–nephelinite trends to segregations of such interstitial analcime and carbonatite rest melts.

One observation that could question such late-stage analcime–carbonatite segregation is that extrapolations of LMD trends by SiO₂ and Al₂O₃ (Fig. 9a-b) do not consistently intersect where more evolved T-trends separate into either nephelinites or alnöites but intersect more nephelinitic compositions. As will be discussed in Section 5(c), this likely reflects superimposed SiO₂ and Al₂O₃ loss as magmas transitioned from an evolved LMD parent and into the alnöite–nephelinite segregation, for example, through sanidine fractionation. This sanidine fractionation is consistent with how nephelinites are more sodic than other rock types, as shown by the Na₂O versus K₂O insert in Fig. 9(c). Finally, one may also note that it is difficult to resolve which of the two segregated end members is more evolved than the other, where alnöites are more magnesian, and thereby arguably more primitive, yet on the other hand also – as illustrated by REEs (Fig. 10f) – more enriched in incompatible elements than nephelinites, typical for more evolved magmas.

4.d. Incompatible element signatures

Both REEs and additional incompatible elements (left and right columns of Fig. 11, respectively) display remarkably parallel patterns, which for our four main rock types are displaced, relative to each other, so that (1) HMDs (Fig. 11a-b) have overall lowest concentrations, as expected for more primitive rocks, including cumulates that further expelled intercumulus melts, while (2) LMDs (Fig. 11c-d) have higher concentration, as expected after fractionation of phenocrysts with relatively low concentrations. These LMD patterns are, nevertheless, lower than even more evolved nephelinites and alnöites, where (3) CAs (Fig. 11e-f) have the overall highest incompatible element concentrations (Fig. 11h), while (4) nephelinite patterns have concentrations that are intermediate to alnöites and LMDs. PN further include a subgroup with particularly low Middle Rare Earth Element (MREE) concentrations (convex-down REE patterns in Fig. 11g) that partly overlap LMD patterns and may have experienced apatite fractionation. Apart from this low-MREE nephelinite sub-group, all samples from all four rock types within the FHI complex – as mentioned – share conspicuously similar incompatible element patterns (i.e., signatures), supporting a cogenetic relationship.

Figure 11.

Chondrite-normalized rare earth element (REE) and OIB-normalized incompatible element patterns (Sun & McDonough, Reference Sun and McDonough1989). (a-b) LMDs (green), compared to selected Tikiusaaq kimberlites (yellow) from Tappe et al. (Reference Tappe, Romer, Stracke, Steenfelt, Smart, Muehlenbachs and Torsvik2017a). (c-d) HMDs (purple). (e-f) Carbonaceous alnöites (cyan). (g-h) Phonolitic nephelinites (red), where a subgroup (magenta) has relatively low (concave-up) MREE patterns.

Incompatible element patterns are for most parts more enriched than Ocean Island Basalts (OIB), thought to be derived from an enriched mantle plume. Patterns are not particularly more enriched in large ion lithophile elements (LILE), compared to high field strength elements (HFSE) and do not possess negative Nb-Ta anomalies, consistent with parents derived from a mantle source that has never been metasomatized within a supra-subduction zone. Instead, patterns are noted for their consistently negative K-spikes, which also become more pronounced upon differentiation and must thereby partly relate to some phlogopite/biotite fractionation from damtjernitic magmas, as well as sanidine from nephelinites. Likewise, progressively more pronounced negative Ti-anomalies for more evolved samples support notions from Fig. 10(a-b) and petrographical observations that indicate a relatively early onset of magnetite fractionation, shortly after olivine and augite. Furthermore, the proposed apatite fractionation from low-MREE nephelinites (pink in Fig. 11h) is supported by their more negative P-spikes. Since this P and MREE depletion appears coupled with less negative K-anomalies, compared to other nephelinites, low-MREE nephelinites likely experienced less fractionation or even accumulation of sanidines.

Despite some fractionation of K-, P- and Ti-rich phases, the most parental damtjernite patterns still exhibit negative K-anomalies, as well as less distinct P-anomalies, which could have been inherited from the mantle source. Such an interpretation is substantiated by how roughly coeval kimberlites from a nearby Mesozoic Tikiussaq complex (cf., yellow background in Fig. 11b) – with the exception of having much lower Heavy Rare Earth Element (HREE) concentration, likely related to deeper segregations from their more garnet-rich mantle source – share conspicuously similar patterns with FHI’s LMDs and are regarded as primary magmas (Tappe et al. Reference Tappe, Romer, Stracke, Steenfelt, Smart, Muehlenbachs and Torsvik2017a). Consequently, this not only argues for LMDs having close to primary compositions but also records a similar mantle source type for both complexes, even if Tiklussaq’s kimberlitic source was deeper than that of FHI’s damtjernites.