DATA ARTICLE | SHORT Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 1 of 7 New insights from field observations of the Younger giant dyke complex and mafic lamprophyres of the Gardar Province on Tuttutooq island, South Greenland Lot Koopmans*1 , Robert A. Webster1 , Rory Changleng1 , Lucy Mathieson1 , Alasdair J. Murphy1 , Adrian A. Finch1 , William McCarthy1 1School of Earth and Environmental Sciences, University of St Andrews, St Andrews, United Kingdom Abstract The Gardar Province of south Greenland is defined by the products of alkaline igneous magmatism during the Mesoproterozoic. The most laterally exten- sive Gardar intrusions are a series of giant dyke complexes best exposed on the Tuttutooq archipelago. We present new field observations and a geolog- ical map of north-east Tuttutooq island that provide fresh insights into the temporal evolution of the Younger giant dyke complex and two associated ultramafic lamprophyres. Our data demonstrate that distinctive crystallisa- tion regimes occurred in different sectors of the dyke complex, leading to the formation of marginal gabbros and ovoid pod-like domains displaying lam- ination, modal layering and/or more evolved differentiates. We infer that at least two pulses of magma contributed to the formation of the Younger giant dyke complex. In addition, the relative ages of two ultramafic lamprophyre diatremes are constrained and attributed to two distinct phases of rifting in the Gardar Province. Introduction Determining how magmas intrude and crystallise in the crust using struc- tural data and textural features provides fundamental insights into the pro- cesses inherent in the construction of igneous intrusions. Despite decades of research, the mechanisms that control layer formation, magma transport and storage, and in-situ magma evolution in magma chambers remain elusive (e.g. Cashman & Giordano 2014). The Younger giant dyke complex (YGDC) is an elongate igneous intrusion that extends for over 145 km across south- ern Greenland (Upton 2013). The YGDC contains regions in which igneous layering, foliation and/or differentiated compositions are observed (Upton 1962). It therefore provides a unique opportunity to study these enigmatic magmatic processes. This study presents a detailed field analysis of eastern Tuttutooq island, focusing on the sections of the YGDC exposed on the island and two ultramafic lamprophyre intrusions. We use these data to elucidate the structural and crystallisation history of the YGDC magma and associated lamprophyres, and thereby gain insights into the processes that controlled the formation of these spectacular intrusions. *Correspondence: lk55@st-andrews.ac.uk Received: 05 Feb 2021 Accepted: 20 Apr 2021 Published: 16 June 2021 Keywords: Gardar Province, rift magmatism, Younger giant dyke complex, magma chamber, lamprophyre Abbreviations: BFD: Big feldspar dykes OGDC: Older giant dyke complex TIN: Tuttutooq-Ilimmaasaq-Narsarsuaq YGDC: Younger giant dyke complex GEUS Bulletin is an open access, peer- reviewed journal published by the Geological Survey of Denmark and Greenland (GEUS). This article is distributed under a CC-BY 4.0 licence, permitting free redistribution, and reproduction for any purpose, even commercial, provided proper citation of the original work. Author(s) retain copyright. Edited by: Kristian Svennevig (GEUS, Denmark) Reviewed by: Lotte Melchior Larsen (GEUS, Denmark), and one anonymous reviewer Funding: See page 7 Competing interests: None declared Additional files: See page 7 https://doi.org/10.34194/geusb.v47.6526 https://orcid.org/0000-0002-4018-2588 https://orcid.org/0000-0002-9149-6106 https://orcid.org/0000-0001-7660-1927 https://orcid.org/0000-0002-8840-7926 https://orcid.org/0000-0002-0920-1912 https://orcid.org/0000-0002-3689-1517 https://orcid.org/0000-0002-7214-1449 mailto:lk55@st-andrews.ac.uk Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 2 of 7 www.geusbul let in.org Geological background The Tuttutooq–Ilimmaasaq–Narsarsuaq (TIN) Zone is the southern of two ENE-orientated rift zones that comprise the Gardar Province. Rifting occurred during the break-up of the Columbia supercontinent, and magmatism was confined to two distinct periods: early (1320–1250 Ma) and late (1200–1140 Ma) Gardar (Upton 2013). Today, the TIN Zone is composed of a granitic basement (Juliane- håb batholith), a fault-bound sedimentary and extrusive volcanic sequence (Eriksfjord Formation), and a range of intrusive bodies, of which those exposed on Tuttutooq are presented in Table 1. Apart from minor occurrences of brown dykes, Gardar magmatism on Tuttutooq began with the emplacement of the Older giant dyke complex (OGDC) – a composite giant dyke with syenogabbroic margins and a central zone grading from augite syenite in the west, to peralkaline sodalite-nepheline syenite in the east. This predates a suite of subvertical dykes, obtaining widths of up to 800 m, known as the YGDC. The YGDC is dom- inantly troctolitic but encloses several large ovoid pods of layered and/or more evolved rocks aligned parallel to the strike of the dyke (Upton 1962; Upton et al. 2003). Following the giant dykes, the main Gardar dyke swarm was emplaced throughout the TIN Zone (Upton 2013). Relatively small occurrences of ultramafic lamprophyre, occasionally associated with breccia pipes and carbon- atites, are thought to have been emplaced throughout rifting. Two of these are preserved on Tuttutooq (Upton et al. 2006). The latest known igneous events were the emplacement of large central complexes along the rift axis c.1160–1140 Ma, including the Tugtutôq Central complex and Ilímaussaq complex (Waight et al. 2002). After rifting ceased, the Gardar Province remained remarkably well preserved. Block faulting, erosion and uplift have set present day exposure at an estimated depth of 3–4 km below the contemporary land surface (Upton 2013). It is thought that Tuttutooq sits struc- turally below the nearby Ilímaussaq complex (Upton 2013), and hence the island provides an opportunity to study deeper rift plumbing systems below economi- cally critical deposits. Results A new geological map produced during the current work is presented in Fig. 1. Mapping was conducted at the 1:5000 scale, and a full resolution copy can be found in the supplementary files. In the following section, we provide a summary of field relationships within the YGDC and two ultramafic lamprophyre diatremes in the study area. Younger giant dyke complex YGDC facies description The basis for subdivisions of lithological units used in this study is provided later. YGDC marginal gabbro. The YGDC marginal gabbro is a fine- to medium-grained gabbro. Dendritic plagioclase (<3 cm) and leucocratic melt segregate lenses are observed; the long axis of both are perpendicular to the contacts between host rock and YGDC at several localities (Figs. 2b, d). Rounded host rock xenoliths up to 15 cm are also observed sporadically within this facies (Fig. 2a). YGDC central troctolite. A coarse-grained troctolite comprised of predominantly olivine and plagioclase primocrysts. Minor oxides and interstitial clinopyroxenes are also present. Four variations of the YGDC central troctolite exist: (1) a texturally homogeneous facies typically with <1% anorthosite xenoliths; (2) an anorthosite xenolith facies with 50–80% anorthosite xenoliths (0.1–150 m) surrounded by a troctolite matrix; (3) a glomerocrystic facies comprising a framework of 2–3 cm ‘snowflake’-like plagioclase aggregates; (4) a layered facies, which has modal layering and/or a foliation defined by a plagioclase crystal framework. YGDC syenogabbro. Alkali feldspar-bearing gabbro with primocrysts of clinopyroxene. Assorutit syenogabbro has characteristic dendritic clinopyroxene (<30 cm). Krydssø syenogabbro grades from modally layered through foliated, to texturally homogeneous (L3, Fig. 1). Table 1 Gardar intrusions on Tuttutooq Name Rock Unit Trend Age (Ma) ± (Ma) Reference Brown dykes (BD0) Dolerite WNW–ESE 1279 1.3 Upton 2013 Older Giant Dyke Nepheline syenite E–W 1184 5 Heaman (unpublished data) Younger Giant Dyke Troctolite ENE–WSW 1163 2 Heaman (unpublished data) Gardar dyke swarm Various, including big feldspar dykes (BFDs) ENE–WSW 1163–1160 - Upton 2013 (Relative age) Tuttutooq ultramafic lamprophyre Mela-aillikite N/A <1163 - Upton et al. 2006 (Relative age) Tuttutooq Central Complex Quartz Syenites, Granites N/A 1143 36 Blaxland et al. 1978 See Upton 2013 for original references and isotopic systems in detail. https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 3 of 7 www.geusbul let in.org 400 200 20 0 200 300 20 0 200 20 0 100 100 10 0 20 0 200 10 0 20020 0 10 00 0 10 0 100 20 0 100 100 200 20 0 200 20 0 100 200 100 100 10 0 300 200 20 0 30 0 20 0 300 20 0 200 200 300 300 200 200 30 0 300 200 30 0 300 20 0 200 200 10 0 200 20 0 20 0 200 0 0 0 100 0 100 0 100 10 0 100 100 100 10 0 100 0 10 0 0 10 0 100 200 200 100 100 100 100 100 100 100 51 3524 30 86 18 20 38 57 5042 79 24 16 25 8 46°7'0"W46°8'0"W46°9'0"W46°10'0"W46°11'0"W46°12'0"W46°13'0"W46°14'0"W46°15'0"W46°16'0"W 60 °5 4' 0" N 60 °5 3' 0" N 60 °5 2' 0" N 0 1 2 km L4 - Assorutit L2 - Sissarluttooq Layering and foliation Contacts Sharp Fault Gradational Structural Data Horizontal layering Julianehåb granite YGDC marginal gabbro Late dyke swarm Geological Units Late Gardar Units YGDC syenogabbro (homogeneous) YGDC alkali feldspar syenite Anorthosite xenolith troctolite YGDC central troctolite (homogenous) Glomerocrystic troctolite Evolved veins Assorutit syenogabbro (dendritic px) Krydssø syenogabbro (foliated) Krydssø syenogabbro (layered) Layered troctolite Younger Giant Dyke Complex Nepheline syenite Syenogabbro Older Giant Dyke Complex Early Gardar Units Brown dykes Lamprophyre localities Basement Assorutit Sissarluttooq Nasaasarli Narsaq Sund TunuaraqIteq Marraat Afersuaq L3 - Krydssø L1 - Marraat Not Mapped Kryd ss Ø Store PilesØ Fig. 2a Fig. 3g Fig. 3aFig. 3e Fig. 3d Fig. 2d Fig. 2b Fig. 2c Fig. 3b Fig. 3c Fig. 1 Composite map of the studied area, with location identified by a red arrow in the inset map. Lamprophyre locality maps are presented in Fig. 4. Localities 1–4 (circled in red) refer to select evolved or layered ‘pods’ and are discussed in the text. a Granite xenolith Melt segregate lenses b c) Marginal facies Central facies c Dendritic plagioclase Central facies Marginal facies d Fig. 2 Selected photographs of key exposure within the YGDC. a: Granite xenolith incorporated in the YGDC marginal gabbro. b: Melt segregate lenses in the marginal gabbro perpendicular to contact at Sissarluttooq. c: Dendritic plagioclases within the marginal gabbro sharply truncated against the central facies troctolite. d: Wavy contact between marginal gabbro and central troctolite. The north arrow is 15 cm, and the hammer handle is 10 cm. Location of each image can be found in Fig. 1. YGDC alkali feldspar syenite. Alkali feldspar-dominated syenite with clinopyroxene, quartz (<5%), amphibole, olivine, and calcite. Evolved veins (commonly <1 m wide) are similar but contain more quartz (up to 20%) and are occasionally pegmatitic. YGDC facies relationships Two sections (henceforth limbs) of the YGDC are present in the study area. Both have subvertical, sharp contacts with the basement granite. They have wider (up to 800 m) and narrower sections (minimum 150 m), and trend ENE–WSW. https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 4 of 7 www.geusbul let in.org The outermost 2–100 m of each limb consists of the YGDC marginal gabbro, which shares a lobate or diffuse contact with the homogeneous facies troctolite (Figs. 2c, d). The gross morphologies of the limbs are therefore equivalent. Several petrographically distinct ovoid pods occur in the centre of the widest parts of the YGDC (Fig. 1). Contacts between the homogeneous facies troctolite and the pods are not exposed. Where pods are con- centrically zoned, internal contacts are gradational over c. 3 m. Four individual pods are discussed in detail later. Marraat pod (L1) The Marraat pod is 300 m wide, is composed of layered troctolite and in places sheathed with glomerocrystic troctolite (L1, Fig. 1). Contacts between the two facies are complex, and occasional ‘glomerocrysts’ are found within layers. Within the layered troctolite, alternat- ing picritic and thicker troctolitic layers dip towards the centre of the pod, defining a synformal structure. Layers are laterally continuous and typically thicker at the axis (up to 30 cm) than the edges (1 cm). Contacts between layers are generally gradational over 1  cm; however, some sharp boundaries are observed to the south of the pod where the layers steepen to near vertical. Feldspar crystals define a weak layer-parallel foliation within the troctolite and a non-pervasive, lay- er-parallel foliation in the picrite that is most apparent at the southern margin. Pegmatitic autoliths of the YGDC central troctolite (up to 30 cm) occur in the south- ern section of the layered pod. Sissarluttooq pod (L2) The Sissarluttooq pod is dominated by a layered trocto- lite that is best exposed along the eastern coast of Tut- tutooq, where the YGDC attains a width of 800 m (L2, Fig. 1). A well-developed foliation is persistent through- out the pod defined by a framework of plagioclase. Foli- ations dip concentrically inwards and shallow from 50° at the edge towards a horizontal central point, defining a canoe-shaped morphology. Crystal size is uniform throughout the pod. Modal layering is observed at only one locality where decimetre-scale picritic layers alter- nate with foliated troctolite across a 5 m interval. Elongate mafic enclaves composed of olivine-mag- netite cumulates (60–40%) with interstitial plagioclase are observed both aligned with (up to 6 m long) and cross-cutting the foliation (0.15–1 m long). Magnetite crystals are often aggregated and have poikilitic tex- tures enclosing rounded olivine and/or plagioclase crys- tals. Enclave margins are cuspate against the troctolite, are not chilled (Fig. 3), and entrain ellipsoidal autoliths or individual phenocrysts from the neighbouring-foliated troctolite. A thin (4–50 m) sheet of homogeneous facies troc- tolite dissects the YGDC through the Sissarluttooq pod (Fig. 3a). It has a fine-grained chilled margin and termi- nates bluntly outside of the pod (Fig. 3a). a b c d e f g Homogenous finger Foliated troctolite Chilled margin North Foliated troctolite enclaves enclave Foliated troctolite Anorthosite Evolved vein Kryddsø syenogabbro North Layered troctolite Fig. 3 a: Contact between a ‘homogenous finger’ and foliated troctolite at Sissarluttooq. b: Modal layering in the Krydssø pod. c: Overview of layered synform in the Marraat pod. d: Foliated troctolite and parallel mafic enclaves in the Sissarluttooq pod. e: Large mafic enclave entraining portions of foliated troctolite in the Sissarluttooq pod. f: Bilateral veins (e.g. white arrow) within mafic lamprophyres. g: Evolved vein cross-cutting anorthosite at Assorutit. The north arrow is 15 cm, and the thumb is 2 cm. Location of each image can be found in Fig. 1. https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 5 of 7 www.geusbul let in.org Krydssø pod (L3) Exposed c. 1 km SSW of Assorutit (L3, Fig. 1), this pod is composed of Krydssø syenogabbro. Thick modal layer- ing (5–15 cm) of laterally continuous feldspar-rich (<80% mode) and feldspar-poor (>40% mode) layers is exhib- ited in the western third of the pod. To the east, layering grades into a foliation defined by euhedral aligned 2–10 mm pyroxene (c. 20% mode), before diminishing into texturally homogeneous Krydssø syenogabbro. Layering and foliation both dip from c. 40° at the margins towards the centre of the pod and define a central synformal axis plunging 10° towards the north-east. Two large evolved veins (up to 60 m wide) have been identified near the Krydssø pod. Assorutit pod (L4) The Assorutit pod (L4, Fig. 1) grades concentrically from the homogeneous facies troctolite through 50–100 m of Assorutit syenogabbro to a core of Alkali feldspar syenite. Towards the tip of Assorutit peninsula, an anor- thosite xenolith troctolite is juxtaposed against the pod by a WNW–ESE fault marked by a 2–5 m wide zone of brecciated troctolite. Pegmatitic (0.5–10 cm) evolved veins cross-cut and radiate outwards from the YGDC alkali feldspar syenite facies and can be observed on either side of the fault. The veins also cross-cut the large anorthosite xenoliths (Fig. 3g). Ultramafic lamprophyre Two ultramafic lamprophyres are associated with the northern limb of the YGDC (Fig. 4). Eastern lamprophyre and breccia The eastern lamprophyre and an associated breccia dia- treme outcrop at the apex of a narrowing in the YGDC, both are circular in map view with diameters of c. 100 m (Fig. 4). The lamprophyre’s mineralogy consists of oliv- ine, pyroxene, phlogopite and garnet, and occasional phlogopite pegmatitic segregations. Cross-cutting, sin- uous and bilateral veins (Fig. 3) are observed radiating from the centre. The breccia pipe comprises clasts with rounded edges and a matrix that grades from a felsic to mafic composition. Clasts include quartzite, a fine- grained mafic lithology and coarse granite. The lamprophyre cross-cuts the breccia pipe causing further brecciation but no contact between either unit with the YGDC is exposed. Dykes from the main Gardar dyke swarm have variable relationships with the units. Trachytic dykes cross-cut both the lamprophyre and brec- cia. However, a doleritic dyke with large felspar xenocrysts (i.e. a big feldspar dyke; BFD) is traceable for less than a metre into the breccia before it becomes indistinguish- able from the surrounding breccia matrix (Fig. 4). The matrix at this point is mafic in composition. Cross-cutting relationships that nearby indicate the doleritic BFDs were some of the earliest members of the main Gardar dyke swarm, emplaced prior to the trachytic dykes. Western lamprophyre The western lamprophyre is found wholly within the  YGDC near the northern contact with the OGDC. Exposure is limited, but the unit appears to have an Breccia (mafic host) Trachyte dyke Ultramafic Lamprophyre Breccia (felsic host) YGDC evolved veins Big feldspar dyke, doleritic Julianehåb granite OGDC syenogabbro OGDC nepheline syenite YGDC central troctolite Contacts Observed Inferred Gradational 46°11'25"W46°11'30"W46°11'35"W46°11'40"W 60 °5 4' 25 "N 46°14'50"W46°14'55"W46°15'0"W46°15'5"W 60 °5 3' 40 "N 60 °5 3' 35 "N 50 m 50 m Western Eastern Fig. 4 Focus maps of the western and eastern lamprophyre bodies (see Fig. 1 for locations). https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 6 of 7 www.geusbul let in.org irregular shape. The mineralogy can be divided into two groups: (1) oikocrysts consisting of 2–10 mm pla- gioclase, amphibole and biotite in the south and <2 mm biotite crystals in the north and (2) uniformly sized mafic chadacrysts that are enclosed by group 1. The northern contact with the YGDC is sharp and curviplanar, whereas the southern contact displays a complex melange of troctolite and lamprophyre. In one outcrop, orbicular minerals (potentially pseudo-leucite) are present. Discussion Origin of layering The layered pods of the YGDC have previously been inter- preted as regions where convective overturn was vigor- ous enough to produce dynamic layering (Upton 1962, 2013). However, we find the structural, mineralogical and textural differences between the Marraat and Sissarlut- tooq pods sufficient to warrant the presence of two dif- ferent layer-forming processes. The synformal structure with layers thickening towards the centre at Marraat may be explained by crystal-laden density currents originating from gravitational instabilities at the walls, implying con- vection may not be required. A similar process has been suggested to occur in parts of the Skaergaard intrusion (Irvine 1980), as well as other regions of the YGDC (Upton 2013). This process is inconsistent with the uniform crys- tal size and concentric structure at Sissarluttooq; instead, we hypothesise layering occurs here through gravita- tional settling below a convective cell. Petrogenesis of YGDC magmas Assorutit and Krydssø magmas The presence of evolved rocks at the Krydssø and Assoru- tit pods has been attributed to either in-situ fractionation of the troctolitic melt (Upton 1962) or the emplacement of distinct later injections of magma (Upton & Thomas 1980). Field results presented here are not exclusive to either process. However, the layering mechanisms dis- cussed earlier could fractionate a melt by mechanically separating crystals (i.e. dynamic fractional crystallisa- tion). This process could occur during layer formation in the Krydssø and Sissarluttooq pods, driving the fraction- ation of the YGDC to more evolved compositions. Furthermore, the presence of cross-cutting evolved veins within the anorthosite xenolith troctolite, known to be a part of the YGDC roof zone (Upton 2013), indicates that silicic melt migrated within the YGDC along vein networks. This presents a process by which the central complexes of the TIN Zone could be fed by evolved melts fractionated at depth. The silica-saturated nature of the Assorutit pod, in otherwise undersaturated lithologies, has been postulated to be the result of crustal assimilation (Upton 2013). Gran- ite xenoliths found in this study provide direct evidence that this process was occurring. Mafic enclaves We interpret the mafic enclaves at Sissarluttooq to be cogenetic with the YGDC, and not later melt injections, due to the lack of chilled margins. Such enclaves may be the result of the YGDC melt crossing into the Fe-immiscibility field during fractional crystallisation. The resulting dense, Fe-rich melts sank through the crystal mush below, whilst the lighter melts ponded at the top of the intrusion, form- ing the anorthosite. A similar process has been suggested to occur in the Isortôq Giant Dykes (Rosa et al. 2020). Homogeneous sheet The presence of central facies troctolite that cross-cuts the Sissarluttooq pod (L2) indicates that minor troctolitic injections occurred over a prolonged time period. Both injections of YGDC melt are thought to have the same source due to their mineralogical and textural similarities. Timing of lamprophyre magmatism The gradational change of a felsic to mafic matrix in the breccia diatreme at the eastern lamprophyre pipe is interpreted to record mixing between a primary felsic material and a mafic magma. We suggest that the felsic material formed through the melting of Julianehåb gran- ite and the mafic host was sourced from the BFD and possibly the lamprophyre. This requires the emplace- ment of these bodies before the breccia’s matrix had fully crystallised, which provides a tight relative age on the emplacement of the lamprophyre – contemporane- ous with the emplacement of the BFDs at the start of the main Gardar dyke swarm. The equigranular ultramafic mineralogy present in the western lamprophyre implies a consistent crys- tallisation regime. In contrast, the variable size of the enclosing poikilitic mineralogy suggests that the miner- als crystallised within a high thermal gradient. The tex- tural differences between the north and south contacts of this lamprophyre support the presence of a high thermal gradient, which may be explained by the prox- imity to the external contact of the YGDC. In addition, the complex southern contact of the lamprophyre pro- vides no evidence of a liquid–liquid contact between the lamprophyre and the YGDC. We suggest this body is a xenolith of an ultramafic lamprophyre emplaced just before the YGDC, producing an equigranular mafic mineralogy. Density differences have led it to sink through the YGDC post-emplacement, generating the poikilitic mineralogy due to the interaction with the host magma. Conclusion Our field results lead to several conclusions: (1) density currents and/or convection developed the Marraat and Sissarluttooq pods; (2) the YGDC melts were available https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org Koopmans et al. 2021: GEUS Bulletin 47. 6526. https://doi.org/10.34194/geusb.v47.6526 7 of 7 www.geusbul let in.org for a prolonged time period; (3) magmatic fractionation drove the formation of the Assorutit and Krydssø pods as a result of crystal settling under dynamic conditions; (4) the presence of evolved veins radiating from the Assorutit pod demonstrates that there was mobilisa- tion of magmatic differentiates within the YGDC; (5) the two ultramafic lamprophyres on Tuttutooq are of two distinct ages in the late Gardar (>1163 and 1163–1160 Ma). Targeted geochemical, anisotropy of magnetic sus- ceptibility and spatial analyses will enhance our under- standing of the YGDC petrogenesis presented here and further study the associated lamprophyre intrusions. Acknowledgements The maps and field data presented here are the result of a 6-week expedition to Tuttutooq in 2019. We acknowledge expedition support from Terra Nova, Expedition Foods, Balfour Beatty, Veitchi group and Blacks Outdoor Retail Ltd. We also thank Prof. Brian Upton for his insightful comments and discussions. Place names are given in modern Greenlandic orthography – older spellings present in the literature are Tugtutôq (Tuttutooq), Sigsardlugtôq (Sissarluttooq), Kryds Sø (Krydssø), Marrait (Marraat) and Ilímaussaq (Ilimmaasaq). The older spelling has been retained for geological features named before the introduction of modern Greenlandic orthography. Additional information Funding statement LK, RW, RC, LM and AM received funding from the Mining Institute of Scotland, Institute of Materials, Minerals and Mining, the Edinburgh Geological Society, the Augustine Courtauld Trust and the Scott Polar Research Institute. LK received funding from the Society of Economic Geology Hickok-Radford Fund. Author contributions LK, RW, RC, LM and AM: fieldwork, sample collection, drafting and editing of the manuscript. WM and AF: research supervision and revision of the manuscript. Competing interests The authors declare no competing interests. Additional files A high-resolution geological map and a methods description can be found at https://doi.org/10.22008/FK2/NAX2IT References Blaxland, A.B., Van Breemen, O., Emeleus, C.H. & Anderson, J.G. 1978: Age and origin of the major syenite centers in the Gardar province of south Greenland: Rb-Sr studies. 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Chemical Geology 186(1–2), 99–116. https://doi.org/10.1016/S0009-2541(01)00420-X https://doi.org/10.34194/geusb.v47.6526 http://www.geusbulletin.org https://doi.org/10.22008/FK2/NAX2IT https://doi.org/10.1130/0016-7606(1978)89 https://doi.org/10.1016/j.jvolgeores.2014.09.007 https://doi.org/10.1016/j.jvolgeores.2014.09.007 https://doi.org/10.34194/geusb.v44.4626 https://doi.org/10.34194/geusb.v29.4692 https://doi.org/10.34194/geusb.v29.4692 https://doi.org/10.1016/j.lithos.2006.03.046 https://doi.org/10.1016/j.lithos.2006.03.046 https://doi.org/10.1016/S0024-4937(03)00030-6 https://doi.org/10.1016/S0024-4937(03)00030-6 https://doi.org/10.1093/petrology/21.1.167 https://doi.org/10.1093/petrology/21.1.167 https://doi.org/10.1016/S0009-2541(01)00420-X New insights from field observations of the Younger giant dyke complex and mafic lamprophyres of the Abstract Introduction Geological background Results Younger giant dyke complex YGDC facies description YGDC facies relationships Marraat pod (L1) Sissarluttooq pod (L2) Krydssø pod (L3) Assorutit pod (L4) Ultramafic lamprophyre Eastern lamprophyre and breccia Western lamprophyre Discussion Origin of layering Petrogenesis of YGDC magmas Assorutit and Krydssø magmas Mafic enclaves Homogeneous sheet Timing of lamprophyre magmatism Conclusion Acknowledgements Additional information References Figures Fig. 1 Composite map of the studied area, with location identified by a red arrow in the inset map. Fig. 2 Selected photographs of key exposure within the YGDC. a: Granite xenolith incorporated in the Fig. 3 a: Contact between a ‘homogenous finger’ and foliated troctolite at Sissarluttooq. b: Modal l Fig. 4 Focus maps of the western and eastern lamprophyre bodies (see Fig. 1 for locations). Table Table 1 Gardar intrusions on Tuttutooq