Geological Survey of Denmark and Greenland Bulletin 33, 2015, 45-48 45 Magma mixing, mingling and hybridisation at different crustal levels: snapshots from 1.9 billion years of magmatism in south-eastern Greenland Thomas F. Kokfelt, Samuel M. Weatherley, Jakob K. Keiding and Trygvi B. Árting During fi eld work in 2014, we investigated a suite of igne- ous intrusions in south-eastern Greenland between 65° and 67°N. Many of the intrusions show widespread evidence for juxtaposition of diff erent magmas in the liquid state and sub- sequent mixing, mingling and hybridisation. Here we pre- sent fi eld evidence for these processes from three areas that diff er in age and geological setting. We discuss the signifi - cance of mingling, mixing and hybridisation features in the fi eld area, motivated by their abundance in the area, the mor- phological variation between intrusions that were emplaced at diff erent crustal levels, the implications for magma genesis in collisional and rift settings, and the implications for the interior dynamics of igneous bodies. Magmatism in south-eastern Greenland Th e magmatic history of south-eastern Greenland broadly falls into two episodes: (1) the Proterozoic Nagssugtoqidian orogeny and preceding subduction, and (2) the Palaeogene opening of the North Atlantic. Th e Nagssugtoqidian Orogen formed by a NE–SW collision between the Rae and the North Atlantic Cratons that started at c. 1880 Ma and ended with a post-orogenic collapse at c. 1740–1690 Ma (see Kolb 2014 and references therein). Several intrusions and intrusive complexes are associated with the orogeny (Fig. 1). Th ese include the c. 1885 Ma Ammassalik Intrusive Complex, interpreted as orig- inating in an island arc, several post-tectonic granites, and the intermediate to felsic Ikaasartivaq intrusive complex with an uncertain intrusion age of 1680 Ma (Kalsbeek et al. 1993). Th ree separate intrusions of diorite and tonalite are found north of Ikaasartivaq strait. Th e ages of these Palaeoprotero- zoic intrusions, their geotectonic setting and relationship to the other intrusive bodies are poorly constrained. Intrusive complexes from the Palaeogene extend from 66° to 75°N. Th ey were emplaced between c. 55 and 25 Ma (Lar- sen et al. 2014 and references therein) and encompass mafi c to felsic compositions of alkaline to tholeiitic affi nity in ad- dition to carbonate-related lithologies (Nielsen 2002). Th e intrusive complexes and associated coeval lavas and coast- parallel dyke swarms are all part of the Palaeogene North Atlantic Igneous Province (NAIP) that formed as a result of oceanic rift ing under the infl uence of the ancestral Icelandic mantle plume (Brooks 2011). In this contribution, we present evidence for magma min- gling, mixing and hybridisation in south-eastern Greenland, focusing in particular on the Ammassalik Intrusive Com- plex, the Ikaasartivaq intrusive complex and the Kialineq Intrusive Centre. Magma mixing, mingling and hybridisation Magma mixing is a recognised process operating in most magmatic systems worldwide and may be a trigger for gen- erating volcanic eruptions (Sparks & Sigurdsson 1977). Th e term ‘mixing’ refers to a process where two magmas blend to form a new magma of intermediate composition. Th e ex- tent to which the magmas blend or mix critically depends on diff erences in the rheological properties and the relative proportions of the juxtaposed magmas (Sparks & Marshall 1986). Important parameters that determine the rheology of a given magma include temperature, composition and crys- tal load. For large property contrasts, the mixing process will © 2015 GEUS. Geological Survey of Denmark and Greenland Bulletin 33, 45–48. Open access: www.geus.dk/publications/bull Kialineq Intrusive Centre Tasiilaq intrusion Ikaasartivaq intrusive complex Ammasalik Intrusive Complex Tasiilaq 67°N 34°W Laube Gletscher Laube Gletscher syenite intrusion Ikaasartivaq strait Palaeogene basalt 50 km Greenland Pre-Cretaceous rocks Cretaceous/Palaeogene sedimentary rocks Intrusion desribed here Other intrusions Fig. 1. Geological map of south-eastern Greenland showing some of the prominent intrusions and intrusive centres in the region. The intrusions described here are the c. 1885 Ma Tasiilaq intrusion of the Ammassalik Intrusive Complex, the c. 1680 Ma Ikaasartivaq intrusive complex and the c. 35 Ma Kialineq Intrusive Centre. +10 –8 4646 typically be incomplete and the original magmas remain as identifi able units; the result is a mingled magma. Hybridisa- tion refers to the general mixing process that may produce a range of variably heterogeneous intermediate magmas. Classical examples of magma mixing and mingling include eroded plutonic systems within the NAIP including Iceland (Brooks 2011). A common feature in many of these exhumed magma reservoirs is the occurrence of ‘net-veined’ complexes consisting of felsic, mafi c and hybrid (intermediate) rocks. Tasiilaq intrusion of the Ammassalik Intrusive Complex – Th e Tasiilaq intrusion, dated to 1886 ± 2 Ma (Hansen & Kalsbeek 1989), is the central part of the Palaeoproterozoic Ammassalik Intrusive Complex that forms a c. 85 × 20 km WNW–ESE-trending array of three large mafi c intrusions (Fig. 1). Th e intrusion mainly comprises leuconorite that cross-cuts melagabbro, and late anorthosite and hypersthene veins that cross-cut the leuconorite and melagabbro. Th e wall rock is quartzofeldspathic garnet-rich gneiss, which has granulite facies assemblages close to the contact, and has been mobilised and mixed with the intrusive rock forming a marginal zone of hybrid rocks. Based on thermobarymetry and fl uid inclusion data, Andersen et al. (1989) concluded that the intrusion crystallised at c. 1000–1100°C and 6–8 kbar (c. 18–24 km depth), i.e. in the middle to deep crust. In the fi eld, melagabbro oft en forms 20–100 m wide trains within larger domains of leuconorite, defi ning the same gen- eral WNW–ESE-orientation as the overall structures of the Ammassalik Intrusive Complex. In some cases leuconorite has intruded melagabbro forming sharp angular contacts (Fig. 2A); in other outcrops, the two lithologies form lenticu- lar intermingled bodies with smeared out, wavy contacts and with no chilled margins (Fig. 2B). Ikaasartivaq intrusive complex – Th e Ikaasartivaq intru- sive complex is composed of mafi c and felsic intrusions, which straddles the Ikaasartivaq fj ord c. 25 km north of Ta- siilaq (Fig. 1). Th e complex measures c. 32 × 25 km and com- prises rocks of granitic, dioritic and gabbroic composition. It was fi rst mapped by Wright et al. (1973), who described it as a late- or post-orogenic calc-alkaline suite related to the up- lift of the area. Th ese authors proposed that the complex was emplaced into relatively shallow crustal levels based on ob- servations of sharp contacts between intrusive units and neg- ligible evidence for contact or retrogressive metamorphism. Mixing and hybridisation features within the intrusive complex are ubiquitous and show a range of styles and mor- phologies. In the main body of the complex, mingling features between diorite and granite are observed at millimetre- to metre-scales in the form of blobs and globules and occasion- ally as angular net-veined areas. Although mingling features are quite pervasively developed throughout the complex, they tend to be more prominent and abundant in the western part of the intrusion. Here they oft en occur as rather extensive pil- Fig. 2. Examples of composite magmatic systems intruded at different crustal levels. A, B: The Tasiilaq Intrusion representing deep to middle crustal levels. A: Leuconorite intruding melagabbro in a semi-molten state. B: Mingling structures with lensoidal wavy contacts between leuconorite (no) and melagab- bro (gb). C, D: The Ikaasartivaq intrusive complex intruded at intermediate to upper crustal levels. C: A c. 5 m wide composite mafic-felsic dyke intruded into gneiss basement. D: Contact of dyke with gneiss (gn) wall rock, marginal zone of hybridised magma (hyb), and interior part of dark, distorded diorite pillows (di) in pale granite (gr) matrix; note crenulated, chilled margins in diorite. E, F: The Kialineq Intrusive Centre that represents a shallow intrusion (<5 km). E: A 4–5 m wide composite sheet of syenite with up to metre-sized mafic pillows in diorite. F: Composite dyke in the syenite body (sy) at Laube Gletscher showing evidence of mingling and hybridisation on a small scale. di di A B C D E F gb no hyb gr gn sy hyb 47 low complexes that texturally resemble the mafi c-felsic mixed rocks in the Kialineq Intrusive Centre (see below). Th e com- plex also hosts a range of composite dykes and sills, some of which are texturally similar to the classical net-veined com- plexes in Iceland. Mingling and mixing features within these smaller bodies include caulifl ower textures, irregular margins between mafi c globules and felsic host melt (Fig. 2C, D), and decimetre- to several metre-scale mafi c pillows within felsic material. Th e margins of intrusive features and edges of indi- vidual mafi c globules commonly show signs of hybridisation as indicated by intermediate coloured rocks (Fig. 2D). Kialineq Intrusive Centre – Th e Kialineq Intrusive Centre is located c. 200 km north of Tasiilaq (Fig. 1) in an extremely rugged terrain. It is dominated by gabbro-diorite-syenite- granite plutons and is cut by numerous coast-parallel mafi c dykes. Many of the plutons are closely associated with exten- sive mafi c-felsic mixed magma complexes (Brooks 2011 and references therein). Only a few radiometric ages are available, but they appear to show a bimodal distribution with mafi c magmatism at 56–49 Ma, represented by the mafi c Imilik intrusion, followed by a hiatus and subsequent activity at 37– 35 Ma of plutons with more evolved compositions (Larsen et al. 2014). Although the chronology is poorly constrained, it agrees with the fi eld observations and suggests a general sequence of emplacement of gabbro, mafi c dykes, mafi c-felsic complexes, syenite and granite. Mafi c-felsic complexes, oft en referred to as net-veined com- plexes, are widespread and ubiquitous in the Kialineq district, and have also been reported elsewhere from the NAIP (Niel- sen 2002). Th e vertical extent of the mafi c-felsic complexes is rarely well exposed in the Kialineq Intrusive Centre, but fi eld evidence suggests that these complexes can be at least 400 m thick. Although the fi eld relationships are complex and vary substantially with locality, they show that the mafi c- felsic complexes are dominated by dioritic pillows surrounded by comagmatic syenites (Fig. 2E). Th e pillows range in size from c. 0.1 to 2 m in diameter and consist of fi ne- to medium- grained diorite, typically coarsening from rim to centre with some pillows having chilled rims against the syenites. Th is implies a signifi cant thermal gradient existed between the two liquids. More typical sensu stricto net-veined textures of angular mafi c blocks separated by rather homogeneous sye- nite matrix also occur. Such breccia-type textures testify that the basic material in some cases was partly solidifi ed when the syenite was emplaced. In contrast, the pillow complexes sug- gest synmagmatic interaction and mingling that demonstrate the co-existence of mafi c and felsic liquids. Hybridisation was observed at the Laube Gletscher (Fig. 2F), but is generally rare in the area. Interestingly, vesicles and centimetre-sized miarolitic cavities are common features in Kialineq intrusive rocks, showing that exsolution of gaseous phases occurred. Th ese exsolution features, along with the occurrences of ring dykes and bell-jar plutons, indicate a shallow emplacement depth in a subvolcanic (cauldron) environment. Discussion, summary and outlook We have described three examples of mafi c-felsic complexes in south-eastern Greenland that show distinct mingling features irrespective of diverse geological settings, level of emplacement in the crust and compositional range. Here we briefl y discuss the main diff erences between the three areas in order to constrain the key responsible processes that gener- ated the observed features. Th e Tasiilaq intrusion represents a deep crustal end-mem- ber of the study area. Compared to the other two examples, the mingled lithologies of the Tasiilaq intrusion (leuconorite and melagabbro) contrast less strongly in composition, and so large diff erences in solidus temperatures are unlikely. Further support for this hypothesis is off ered by the lack of chilled margins at lithological contacts, although this could in part also refl ect the slow cooling of the intrusion. Th e textural re- lationships between the two main lithologies are consistent with leuconorite being intruded into melagabbro at a point where the latter was partly solidifi ed. Th e mingling-like fea- tures are oft en smeared out defi ning a general WNW–ESE orientation, suggesting a syn-magmatic infl uence from the regional stress fi eld of the orogen. Overall, the Tasiilaq intru- sion seems to record a markedly diff erent and more tectonised image of mingling relations between magmas of minor com- positional contrast than shown by the other two examples. Th e mingling features found in the Ikaasartivaq intrusive complex and the Kialineq Intrusive Centre show many over- all similarities, but also some notable diff erences. Both areas represent fairly high crustal levels as opposed to the Tasiilaq intrusion and contain an apparent bimodality of magma compositions (oft en referred to as the ‘Daly Gap’), includ- ing diorite and syenite/granite as end-members. Th e com- mon observation in both systems are chilled mafi c pillows or blobs in contact with felsic melts, indicating that a consid- erable temperature contrast existed between the respective mingling liquids. Th is excludes a model in which the min- gling components are formed by silicate liquid immiscibil- ity processes that otherwise are believed to be important in many magmatic systems. Only limited evidence for hybridisation is seen in the Kia- lineq Intrusive Centre as compared to the common occur- rence in the Ikaasartivaq intrusive complex (Fig. 2C vs. Fig. 2E). Th is could refl ect diff erences in a range of parameters, 4848 especially in the crustal level. Since temperature gradients between magma and host rock are greater at shallow crustal levels, the time window for mixing and hybridisation in the Kialineq system is expected to be shorter. Th is may account for better preservation of end-member compositions in the Kialineq Intrusive Centre than the other two examples studied. In all three areas studied, the scales at which mafi c and felsic components mingle to form blobs of diff erent sizes probably refl ect multiple factors, such as contrasts in viscos- ity, temperature and density of the mingling magmas, the relative abundance of mafi c to felsic magmas. It is beyond the scope of this paper to discuss these parameters in detail, but we suggest that blob development indicates an environ- ment where diff erences in magma temperatures are small. Unsurprisingly, these features are found in the largest intru- sions. Caulifl ower textures with grain size reduction indicate environments with stronger thermal gradients and these are typically found in the shallower environments or in smaller, sill or sheet-like bodies. In summary, igneous rocks in south-eastern Greenland provide abundant evidence for bimodal magmatism, and magma mingling, mixing and hybridisation in diff erent tectonic settings and crustal levels. Key diff erences between the three areas relate partly to various extents of mingling vs. hybridisation and partly to textural and structural character- istics of the interface between mafi c and felsic components (e.g. blob vs. pillow size, extent of grain-size reduction, degree of surface crenulation). Th e governing factors are several, in- cluding (1) depth of emplacement which determines thermal conditions and cooling rates, (2) diff erences in magma-in- tensive parameters for the sets of dual end-members (tem- perature, density, viscosity, chemical composition, crystal contents), (3) the relative proportions of the mafi c and silicic end-members, and (4) size and geometry of the intrusive bod- ies (small dykes or sheets vs. large magma chamber systems). Future work aims to qualify the reasons for the diff eren- ces in more detail, to assess how the region fi ts into a broader understanding of magmatism and petrogenesis in diff erent tectonic settings and to investigate the coupled chemical and physical dynamics of juxtaposed magmas. One aspect will be to understand how the mingling features relate to the geological setting and intrusion shape, and investigate how these rocks compare to net-veined complexes in rift set- tings and andesites in arc and collisional settings. Another aspect regards the origin of the apparent bimodality and un- derstanding the role of magma mixing as a process masking the compositional gap. Detailed petrological, geochemical and isotopic studies will be carried out to characterise end- member components in order to address the petrogenetic link between the mafi c and felsic components. Th is will help to distinguish between models of fractional crystallisation versus partial melting of the Archaean crust and to explain the apparent bimodality. Acknowledgements Th e work is part of the SEGMENT project that aims to evaluate the eco- nomic potential of the larger Tasiilaq area and to gather geological infor- mation in general and is fi nanced by the Ministry of Industry and Min- eral Resources in Greenland and the Geological Survey of Denmark and Greenland. Special thanks to Christian Tegner, Chip Lesher and Th omas Ulrich for collaborative fi eldwork in the Kialineq area. 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