Testing a New Approach on Koster Dikes Dolerites with Incipient Amphibolite-Facies Overprint

Home , Dike swarm, Mackenzie dike swarm

UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre

Reconstructing melt composition

from clinopyroxene phenocrysts:

testing a new approach on Koster

dikes dolerites with incipient

amphibolite-facies overprint

Cesare Albasio Lodi-Cusani

ISSN 1400-3821 B1033 Master of Science (120 credits) thesis Göteborg 2018

Mailing address Address Telephone Geovetarcentrum Geovetarcentrum Geovetarcentrum 031- 786 19 56 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN

Table of Contents

ABSTRACT ...... 3

1. INTRODUCTION ...... 4 1.1 Aim of the study ...... 5

2. BACKGROUND INFORMATION ...... 6 2.1 Studying mafic dike swarms ...... 6 2.2 Background on Columbia Supercontinent ...... 8 2.3 Introduction on Koster Dikes geological background ...... 8 2.4 Rock composition according to Hageskov (19879 ...... 10 2.5 Intra dike compositional variation ...... 12 2.6 Age of the dikes and the host rock ...... 13 2.7 Issues concerning the old dataset...... 13

3. METHOD ...... 14 3.1 Sampling ...... 14 3.2 Reconstructing the Equilibrium melt from Cpx with ICP-MS data ...... 14 3.3 Factors affecting the Dcpx/melt ...... 16 3.4 Analytical methods ...... 16 3.5 Whole rock with modal mineral abundances to have a comparison ...... 17 3.6 Assessing the mobile elements and the impact of metamorphism ...... 18 3.7 Reconstructing the equilibrium melt from CPX and determining the viability of the technique . 18

4. OBSERVATIONS AND RESULTS OF APPLYING THE METHOD...... 19 4.1 New observations on petrology and mineralogy ...... 19 4.2 Geochemical analyses ...... 22 4.2.1 ICP-MS signal ...... 23 4.3 Whole rock estimate ...... 24 4.4 Reconstructed equilibrium melt ...... 24 4.5 The reconstructed melt trace elements patterns...... 27 4.5.1 Cpx spidergrams ...... 29 4.5.2 Sr in the reconstructed melt from both Cpx and Plag ...... 29

5. DISCUSSION...... 30 5.1 Coherence with previous findings ...... 30 5.2 Magmatic inside pristine Cpx ...... 31 5.3 Spidergrams ...... 33 5.4 Potential of the technique, petrogenesis and tectonic setting ...... 35

6. CONCLUSIONS ...... 37

7. REFERENCES ...... 38

1 8. APPENDIX ...... 44 Appendix A Koster Archipelago map ...... 44 Appendix B. Geochemical analyses ...... 45 Appendix.C Element analysis quality assessment...... 46

2 Reconstructing melt composition from clinopyroxene phenocrysts: testing a new approach on Koster dikes dolerites with incipient amphibolite-facies overprint

Cesare Albasio Lodi-Cusani, 2018 Göteborgs Universitet, Guldhedsgatan 5A, Göteborg, Sweden 60hp Master Thesis

Abstract Resolving the problem posed by the presence of post-magmatic influences on whole rock composition is a relevant issue; whole rock data concerning trace elements cannot be employed in some metamorphic settings, to perform petrogenetic and tectonic modelling, through the usage of, REE, multi-element spider diagrams and discrimination diagrams. Clinopyroxenes (Cpx) in mafic rocks have been proposed to record pristine magmatic conditions, despite metamorphism or hydrothermal overprint occurring at some point in the rock history. The trace elements concentrations of clinopyroxene phenocrysts in five samples from the Koster dikes (W Sweden) have been analyzed with a LA-ICP-MS. Equilibrium melts for each sample have been calculated, by using experimentally obtained partition coefficients (Dcpx/melt) for clinopyroxene in basic lavas. These equilibrium melts plot similarly to the older data series by Hageskov (1987), concerning the immobile elements suggesting the technique works. Moreover, the equilibrium melt follows trends that are linear and coherent with magmatic differentiation, in particular concerning mobile elements, for instance Sr. Given, that some classification diagrams employ mobile elements, this study would render viable those graphs, in situations whereby it is not usually possible, as metamorphism and alteration change the chemical composition of rocks, in particular mobile elements. The data from the Koster dikes reconstructed melt show that the magma chemistry is MORB-like plotting also as WPB, with a possible involvement of a plume (magma shows LREE enrichment). The Koster dikes could be associated with the dike complexes already thought to be related in NE Canada and the break-up of the supercontinent Columbia; even though further studies are needed on these supposedly related complexes by using the same analytical technique described in this thesis. Keywords: Geochemistry, Koster, Mafic dikes, Trace elements, REE

3 1. Introduction One of the main problems of whole rock analyses of mafic rocks is that data might not mirror the original composition of the magma; rather it is affected by post magmatic processes like metamorphism, hydrothermal infiltration and weathering. Particularly regarding mobile elements, such as Sr and also partially mobile elements like La. Eliminating the post-magmatic influence is an issue that has been raised by several authors (Gill et al., 1981; Polat and Hofmann, 2003; Polat et al., 2002). Following the lead of Nisbet and Pearce (1977) clinopyroxenes withhold clues about the tectonic origin of ancient basaltic rocks, even though the study focused only on major elements. Other authors ranging from (Maruyama, 1976; Komiya et al., 2004) pointed out that clinopyroxene (Cpx) records the magmatic history of a mafic rock. However, metamorphism can change the composition of the rims in clinopyroxenes, as shown in Komiya et al. (2004), showing that metamorphism not only affects the whole rock trace elements composition, but clearly the mineral trace elements composition in the rims as well, even though the rims remain composed of clinopyroxene. Accordingly, it is necessary to give particular attention in selecting the spots, which should be located close to the core of the clinopyroxene. A dike complex like the Koster dikes (W Sweden, near Norway) make a suitable object of study since these dikes: -have been already studied previously (geochemistry, tectonics and paleomagnetism) -have variable amphibolite-facies overprint -the outcrops are well exposed and close to Gothenburg

Therefore, analyzing the unaltered core of clinopyroxene in an overprinted sample, with accurate analytical techniques like LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometer)- should unlock the original composition of the magmatic rock, exactly when it cooled down and crystallized. Indeed, it would be important to demonstrate that it is possible through LA-ICP-MS analyses of Cpx cores to find the purely magmatic trace elements patterns, after reconstructing the melt through the use of partition coefficients (Dmin/melt). The trace element ratios can be subsequently used to determine in a more accurate way the tectonic setting and the exact type of magma (i.e. MORB, OIB, etc.). Applying this technique to mafic dikes allows a wide range of possibilities, since the so-called geochemical fingerprint allows to exclude which dike swarms are not related. Moreover, the technique, provided it is valid, would allow to bypass whole rock analyses, to obtain readily usable trace element patterns.

4 1.1. Aim of the study The first aim of the study is analyzing clinopyroxene phenocrysts inside the amphibolite overprinted Koster Dikes dolerite with a LA-ICP-MS, in order to know if the reconstructed equilibrium melt is consistent with previous knowledge. Subsequently if trace element patterns in the reconstructed melt are coherent with magmatic trends, they will be used to discuss the petrogenesis and the tectonic setting of the Koster Dikes and their involvement in the break-up of the supercontinent Columbia (1.4 Ga ago). The third goal is to suggest what the results of the study imply on obtaining information about petrogenesis and tectonics from Cpx analyses, in settings influenced by metamorphism other than the Koster dikes

5 2. Background information 2.1. Studying mafic dike swarms Studying mafic dikes allows to perform several kinds of reconstructions, thanks to their tabular shape (that can be assumed to be 2-D in models) suitable for geophysical, geological and geochemical modelling and the presence of paramagnetic and magnetic minerals employable for paleomagnetic/paleogeographic reconstructions (Bleeker and Ernst, 2006; Ernst and Bleeker, 2010; Ernst and Buchan, 1997a). Dike swarms found nowadays in the world can be part of ancient radiating dike swarms, which have eventually been broken apart by plate tectonics; branches of the same swarm now lie thousands of kilometers apart on other landmasses. Hence, it is possible to obtain the distribution of past igneous provinces (Ernst et al., 2008) by stitching the dike complexes of similar ages together; also reconstructing the dike swarms allows to reconstruct the past supercontinents, before they broke up. Mafic dike swarms are known to be related to mantle plume activity (Ernst and Buchan, 1997b; Emst and Buchan, 2001; Guo et al., 2004; Peng et al., 2011) and/or extensional tectonics (Goldberg, 2010; Kröner et al., 2006; Yang et al., 2004). Mantle plumes are suspected to initiate the breaking apart of continents and supercontinents, such as Rodinia (Li et al., 1999). Dike swarms stretch for tens to thousands of kilometers, with width of individual dikes spanning from decimeter to decameter. They exist in several arrangements (fig.1); the focal point usually indicates the position of the hotspot. The origin of a dike swarm cannot be singled out to one phenomenon. Dike swarms are found throughout the world and have different ages, many are Archean meaning that their age is older than 2.5 Ga, whereas part of the complexes date to around 1.2 Ga (Mesoproterozoic). When two sets of dikes with different trends are present in the same swarm, it is usually considered that two separated events generated the two sub-swarms with different trends, even though there are cases when there are cross-cutting dikes that have been originated by a single magmatic episode; one known case among them is the 600 Ma dikes intruding in Archean gneisses: Sarfartoq, Sisimiut, Maniitsoq areas in western Greenland (Gill and Bridgwater, 1979; Mitchell et al., 1999). The geochemistry of a mafic dike swarm is important, as they can be the key to the petrogenesis of larger magmatic events, as their chemistry might be more primitive and less touched by crustal interaction (Peng, 2010); knowing the element and isotope ratio in the whole rock is very helpful to understand the evolution of the mantle lithosphere and the origin of the dike themselves (Liu et al., 2006). However, one of the most common problems is the influence on the rock composition of post-magmatic

6 Figure 1. Types of dike swarms: I, II and III radiate from a supposed hotspot, IV and V are called subparallel and VI arcuate; from Ernst and Buchan (2001) processes, thus having a composition that would be free of post-magmatic influence would allow to discriminate easily the petrogenesis and the setting, allowing the use of elements that are typical added/removed by metamorphism, hydrothermal fluids and weathering. An orogen that has been extensively studied and it is also related to the Sweconorwegian orogeny, in which the Koster-Kattsund dike swarm is found, is the Grenville province (Gower and Owen, 1984; Gower and Tucker, 1994), which lies in Atlantic Canada. Since the Grenville province is related, providing further data on the Koster dikes and their geochemical composition would give a more detailed insight on how these two provinces relate.

7 2.2. Background information on Columbia Supercontinent

Figure 2. Various hypothesized assemblages of Columbia supercontinent (Meert, 2012)

Columbia (also called Nuna) was a supercontinent that was assembled about 1.7 Ga ago; it was earlier than the more known Rodinia (formed 1.1 Ga). The break-up of Columbia is dated around 1.3-1.2 Ga (Hawkesworth et al., 2010; Hou, et al., 2008; Zhao et al., 2003). A possible assemblage of Columbia is shown in fig.2. Dike swarms that have been associated with extensional activity coincidental with Columbia breaking apart are for instance the Mackenzie Dikes in Canada (LeCheminant and Heaman, 1989), whose focal point lies on present day Victoria Island in the Canadian Arctic or the 1227 Ma dikes in Northern China Craton (Yang et al., 2011). Other dike swarms that match the ages of Columbia break-up are those located in Western Australia, in the Yilgarn Craton (Rogers and Santosh, 2002) that are a prosecution of the dikes in Eastern Antarctica, since Australia was attached to Antarctica, when Columbia was present. 2.3. Introduction on Koster dikes geological background The Koster dikes (fig. 3) are geologically part of the Østfold-Marstrand belt located in the Sweconorwegian shield, and more in detail part of the Koster-Kattsund dike swarm. It is one of the most prominent dike swarms located on the West Coast of Sweden. The Koster-Kattsund dikes origin is tied to intra-plate extensional stress, where tholeiitic magma intruded into in fractures formed due to the stress (Hageskov,

8 Figure 3. The Koster dykes geological context (The Kattsund-Koster dyke swarm and the Kongsberg-Bamble-Østfold segment of the Sveconorwegian Province (shown in the inset map as the KBØ segment). KBZ = Kristiansand-Bagn shear zone, DB = Dalsland boundary thrust. Hageskov (1988) 1987). The dike swarm has a width of 8 km and cuts across the crust. The composition of the dike rock is a tholeiitic basalt, and more accurately according the existing literature a N-MORB (Normal-Mid Oceanic Ridge Basalt) (Hageskov, 1987). The dikes in the southern part trend NNE-SSW with a dip around 67 towards W, these dikes are located in the area where the southern islands of the Koster archipelago lie. They are referred as anorogenic, which means that in this particular case that they have formed before the Sweconorwegian orogeny, dated conventionally with U-Pb around 1200-850 Ma; the Koster dikes have yielded an age around 1421±25 Ma, by using Rb- Sr dating (Hageskov and Pedersen, 1988). Most of the studies on Koster Dikes are from a single author: Bjørn Hageskov. He also discovered that the Koster Dikes originated from a different reservoir, even though the Kattsund dikes are part of the same complex. The rock type (concerning the least metamorphosed ones) is an acidic aphyric tholeiite, even though olivine and plagioclase porphyric dikes are known to occur (Hageskov,

9 1987). A metamorphic phase is dated around 1015 Ma with Rb-Sr (Hageskov and Pedersen, 1988). Koster dikes are located on the paleocontinent Baltica which was attached to Laurentia (Gower et al., 1990; Åhall and Gower, 1997) According to several authors (Daly et al. 1983; Gower and Tucker, 1994), the Gothian orogen is related with the Grenville Province, located on the Atlantic coast of Canada. On the Grenville province, as noticed by Hageskov (1988) mafic complexes and dikes with around the same whole rock Rb-Sr age of the Koster dikes are documented: the Michael Gabbro (Fahrig and Loveridge, 1981) Mealy Dikes 1380 Ma (Emslie, 1978; Emslie et al. 1984) and the Harp dikes (Emslie, 1978). The Koster dike swarm is divided in three sectors (Hageskov, 1984, 1985), named with roman numerals (Appendix A, fig. 1); I stands for the least metamorphosed, that means maximum 10% of recrystallization, and III for the most deformed and recrystallized. Even though no major change in igneous chemistry occurred in Sector I, narrow dikes (less than 2m wide) are shown to be enriched in H2O and Rb and depleted in Na2O and Sr (Hageskov, 1985). In Sector II metadolerites are found with a recrystallization ratio spanning from 15% to 100%, the original mineralogical make up, hence is not very much preserved. Sector three includes amphibolites, which are characterized by a lineated fabric, with a foliation that can be present; they are enriched in H2O though no other significant alteration of the chemistry occurred. Sector I, where dikes still have the original strike, is the most interesting to be studied, because little or no metamorphism occurred, making it a suitable source of samples, to carry out analyses on. It can also be highlighted that related dike swarms, with a Rb-Sr age around 1.4 Ga such as the Mealy Dikes in Atlantic Canada have been re-dated with U-Pb to 1.2 Ga (Gower, 1996; Gower and Krogh, 2002). Therefore, it is possible that the Koster dikes are about 200-150 Ma more recent than the age calculated with Rb-Sr. An unequivocal study to retrace the Koster dikes origin to a possible hotspot has never been carried out.

2.4. Rock composition according to Hageskov (1987) The rock which constitutes the Koster dikes is a microcrystalline diabase, occurring in several degrees of metamorphism (Hageskov, 1987); in the most metamorphosed ones, the original geochemistry (fig. 4) is preserved quite well, while the mineralogy is not.

The SiO2 content is around 48-50% in weight percent (wt. %). The main acidic minerals are plagioclase (around 40-45 wt. %) and orthoclase 4-6 wt. %.

10 Figure 4. The AFM diagram from Hageskov (1987) on which the Koster dikes plot along the Tholeiitic series. The line is the Skaergard liquid (Wager, 1960)

Table 1. Mean Koster dikes compositions. Oxides in weight %, elements in ppm. Hageskov (1987)

11 Quartz is found though in minimal percentages, never above 2 wt. %. A low amount of apatite usually less than 1 w% is present, with one exception of 3.5 wt. %. The mafics are: pyroxene ranging from 1 to-25% normative (Hyperstene), diopside normative 12-22% and minor olivine 3-7%, which is absent in some samples. Iron and Titanium minerals like Magnetite and Ilmenite together constitute about 6-9% normative in wt. %. The rock samples from Koster plot along the tholeiitic series in the AFM diagram (fig.4); in the Zr, Ti/100, Yx3 discrimination diagram (Pearce and Cann, 1973) they fall in between the Ocean Floor Basalts (OFB) and Calc-Alkali Basalt areas. The most primitive rock sample is LS7 which is located in Sector II, on Sydkoster.

2.5. Intra-dike compositional variation The compositional variation within the same dike is the most salient characteristic found so far in previous findings on Koster dikes; this was pointed out in the report (fig. 5) on the Koster-Kattsund dike swarm (Hageskov, 1987). A 10 m wide dike located on Ursholmen has been sampled along its width. Ten samples were taken from edge to edge every meter (the first one after 0.5 m). The Sr, Zr, U and Nb content is correlated quite weakly to water content. The only factor that seems to affect clearly the distribution of elements is only the location across the dike, where the sample was taken. K was not included in the original graph, and a graph with K/Ti (fig. 6), an index of crustal contamination shows a higher level close to the edges. An unequivocal explanation for the transversal compositional variation as a general occurrence in mafic

Figure 5. The compositional variation along a 10m dike profile on Ursholmen (Hageskov, 1987)

12 Figure 6. K/Ti variation along the horizontal section of a 10m dike. Point 2 is 0.5 m from the edge dikes is still missing. Both alteration and magmatic phenomena have been taken into account. This phenomenon is documented as well in other dike swarms; one major example is the 1.2 Ga Mackenzie dike swarm, mostly located in NW Canada (Baragar et al. 1996). The dike center samples plot distinctively, for instance Ti plots below the trend in “Al vs Ti” graph, compared with every other sample. Hence, the compositional difference between the edges and the center of the 10 m dike on Ursholmen cannot be ascribed to a single phenomenon, such as crustal assimilation. The processes, occurring altogether, behind the compositional variation suggest fractionation, different magma tapping, contamination with the felsic crust, and/or the interaction in the dike margins with the host rock as it was previously discussed in other cases with dolerite dikes (Ross, 1986; Wiebe, 1973).

2.6. Age of the dikes and the host rock The Rb-Sr dating technique employed in Hageskov and Pedersen (1988) yields an age of 1421±25 Ma for the Koster dikes. The author pointed out that the differential distributions of Rb and Sr across the dike might affect the whole dating process; moreover, as said earlier in this text, the author had doubts whether the different amount of trace elements in the bordering area close to the contact, were due to reaction with host rock, magmatic fractionation or just to alteration. The gneissic/granitic and amphibolitic Gothian host rock of the related Kattsund dikes is dated around 1200 Ma with Rb-Sr (Hageskov and Pedersen, 1981).

2.7. Issues concerning the old dataset The most relevant issue in Hageskov (1987) data is the lack of analyzed elements, compared to post-mid-1990s studies of this kind. All the Lanthanides except La are absent in Hageskov (1987) data, as well as many other trace elements that withhold information on the rock history like Pb. It is not possible to obtain a full multi-element

13 spidergram and a plot displaying the ratio to Chondrite of Lanthanides in whole rock analyses. No quantitative analyses of Cpx and Plagioclase are available in the previous study to compare with.

3. Method 3.1. Sampling Samples (9, 18, 34 and 37) have been collected from the center of the dikes, and one from the contact (named “cont”) with the host rock. Particular attention has been paid to determine, if the interior of the samples has been altered by weathering. The dikes sampling location has been recorded, with the help of Android™ app FieldMove Clino. The map with the location of samples is found in the Appendix A fig. “A.1”. 3.2. Reconstructing the Equilibrium melt from Cpx with ICP-MS data The reconstruction of the equilibrium melt by using ICP-MS data is an innovative technique. Having the trace elements composition of the melt, rather than of the whole rock allows to obtain the actual chemistry of the magma at the time it solidified, as some elements (such as Sr, Pb and Ba) are mobile (see sec. 3.4). Also, this technique, if proven to be effective, would be in general superior to using whole rock data, as some elements might be mobile in some settings, like La and the compatible metals, while in others are not. Generally, the first step is selecting the mineral phases that are almost certainly magmatic in an overprinted rock. One of the best options is Clinopyroxene (Cpx), as pointed out in the introduction, with attention in avoiding areas, which have likely reacted with other mineral phases, like rims, areas with inclusions or generally characterized by heterogeneities. Cpx phenocrysts should

Figure 7. SEM BSE picture displaying spots and the general appearance of selected crystals

14 Figure 8. The behavior of Dcpx/melt with varying amounts of Al (iv). The arrows indicate the line with the lowest and highest Al (iv) fraction. The behavior of the lines is roughly similar despite the variation in Al (iv). (Zack et al. 1997) define a magmatic texture (ophitic) and never porphyroblastic, which is index of recrystallization (Nehring et al. 2010). In large magmatic complexes such as Ulvö, earlier crystallized Cpx have a composition mirroring the magma, whereas the later (Claeson et al., 2007) ones have been affected by a differentiation process; hence their composition does not mirror the actual magma, that intruded first. The typical location selected on a Cpx phenocrysts, to be analyzed with LA-ICP-MS (fig. 7) looks in BSE- SEM slightly darker than the edges and show almost no heterogeneities, and has in this case an average CaO composition around 12%±3.5% (WDS-SEM data). The reconstruction of the equilibrium melt technique, which consists in obtaining the equilibrium melt that formed the magmatic phases from the analysis of a mineral phases composing has been already explored in some form, though in most cases it involved partial melting and never in detail concerning mafic rocks (Bernstein et al., 1998; Chauvel et al., 2001; Nehring et al. 2010; Storkey et al. 2005). Despite the differences in composition of the various Cpx crystals the partition coefficients are proportionally the same (fig. 8) In theory by knowing the partition coefficient Dmin/melt of a magmatic mineral phase and the amount of X element in the said mineral, it should be possible to obtain the amount of X element in the melt that was at equilibrium with the mineral, simply by dividing the amount of X element in the crystal by the partition coefficient for that elements.

15 Of course, this procedure is complicated by several factors like the variation in composition of a mineral phase and sometimes by pressure and temperature at which the magma crystallized. Obtaining the exact amounts for each element in the equilibrium melt is hence impossible, however it is explained further below how element ratios (useful to infer the magma type and processes behind its origin) are nearly constant. 3.3. Factors affecting the Dcpx/melt The partition coefficient between a mineral phase and a melt (often an artificial glass) can be determined experimentally. The main factor that regulates partition coefficients of Lanthanides, Y and in the case of clinopyroxene is the Al (iv) proportion, related to Ca-Tschermak content (Forsythe et al. 1994; Hill et al. 2000; Zack, et al. 1997); Also, according to Zack et al. (1997) as long as mafic rocks with silica around 50% are concerned, only the magnitude of Dcpx/melt changes when Al (iv) content in Cpx varies, without disruptions in behavior of the curve, as already displayed in fig.8. Restating the concept, Dcpx/melt absolute values may change due to Cpx composition, both inside the same crystal (i.e. core and rim), and between samples, despite the pattern remaining similar as summarized in Green et al. (1994). Even if REEs and other trace elements partition coefficients are controlled by Al(iv), Ti makes an exception and it is also affected by temperature besides composition (Johnson, 1998); REEs and Ti are positively correlated with Al (iv) content, other transition elements like Co are negatively correlated, and others like Sr are unaffected. Though Ca and Al content are not constant within Cpx, the trace element ratios in the reconstructed melt remain almost unaltered. Trace elements ratios are usually best represented with a Primitive Mantle normalized multi-element spidergram. Therefore, despite the errors in element absolute quantities, the element ratios are solid, since the partition coefficient ratios of REE, Y and Zr are almost constant. The physical explanation for the predictable curve shaped behavior of the REE partition coefficients (except Eu and Ce) is that the radius decreases with atomic number Z, whilst the charge of the ions is always 3+, making the larger ions like La less compatible in Cpx and the smaller like Lu more compatible. 3.4. Analytical methods An Agilent LA-ICP-MS was used to measure the trace elements in selected mineral phases. A SEM has been used both for picture taking and quantitative analyses of minerals, WDS calibrated with a Cobalt standard (error 3%). No whole rock data are available for the analyzed samples. The samples are in the format of polished slabs. No data on the oxidation of Fe were possible to be obtained with available instruments. SiO2 has been used as an internal standard for Cpx and Plag, SiO2 from WDS = 50%.

16 The ICP-MS data have been elaborated with GLITTER! Software (Access Macquarie), Nd (0.3 ppm), Sm (0.2), Eu (0.05 ppm), Gd (0.02 ppm) Tb (0.04 ppm), Dy (0.4 ppm), Ho (0.05 ppm), Er (0.01 ppm), Yb (0.02 ppm), Lu (0.02 ppm), Y (1.5 ppm). 3.5. Whole rock with modal mineral abundances to have a comparison Since no whole rock data are available for the samples analyzed with LA- ICP-MS an estimated whole rock (table 2) obtained with mineral abundances has been used. The purpose is to assess whether the amounts in the reconstructed equilibrium melt match with more robust data for some elements controlled by the major mineral phases. Due to the very small texture of the contact sample it was not possible to calculate reliably the mineral abundances. The current whole rock composition would hardly mirror the composition of the dolerite, the exact moment after it solidified, above all concerning the typically mobile elements, both trace like Sr and major like Fe. Magnetite mineralizations have been deposited by fluids, after the crystallization of the rock. Only elements solely controlled by magmatic phases can be considered. Assuming, that the pristine dolerite was mainly made up by pyroxene, plagioclase, spinels and sulphides, the whole rock has been approximated (with some variation between samples) as 48-52% Pl and 47-49% Cpx and 1-3% Ti-Mag/Ilm. Data for the composition for the Ti-mag have been averaged from Jensen (1993). Then by using the previous dataset from Hageskov (1987), major elements Al and Mg which are mostly controlled by Cpx + Plag, are plotted to verify if they lie close on the Al vs Mg trend defined by the whole old dataset. The technique can

Table 2. The reconstructed whole rock (all data in ppm). *Element plots well below the trend defined by the Hageskov (1987) series Whole Rock est. 9 34 18 37 Al 75640 67513 71089 72495 Mg 39878 39299 41966 40036 Mn 1355 1678 1275 1525 Ti* 5608 8762 7226 6561 Cr* 1040 3372 3882 2086 Ni 35 55 63 40 V* 337 542 510 415 Sr 179 131 145 163 be used for some elements controlled by major phases, however only Mn and Sr plots within the already present trends, whereas other elements that might have been used such as V, Ni, Cr cannot used as proxies for magmatic differentiation, as some other mineral is influencing their quantity in the whole rock. Cr is widely erratic from Laser spot to spot, and Ti, even with Ti-mag included plots, when plotted against Mg and Al, is more than an order of two lower than the trend. V as well plots at the lower end of 17 the trend. However, employing a graph that uses the immobile proxy Mg in the estimate in the whole rock (e.g. sample 37) against a trace element for instance Y calculated in the reconstructed melt could suffice to tell whether the data in the reconstructed melt fits the already defined trend by Hageskov (1987) data. However, this approach is limited to few elements usable as proxies for magmatic differentiation, since most trace elements in the whole rock are controlled by mineral phases, such as apatite and monazite that have not been analyzed with LA-ICP-MS due to time constraints.

3.6. Assessing the mobile elements and the impact of metamorphism Telling mobile elements from immobile is necessary to single out the element distributions in the whole rock, which have been changed by processes subsequent to rock crystallization. Typical mobile elements like Rb might be immobile in certain settings, while other might behave as mobile or as partly mobile. The lack of dispersion (Cann, 1970) in a scatter graph featuring two elements against each other, one immobile like Y and the other one whose status is unknown, suggests no mobility of the element, whereas dispersions suggests that the element is mobile. The dispersion in Koster dikes whole rock graphs, when present is probably linked to metamorphism, as in Sector I trace elements like Ni, Cr and V against Y tend to plot as a linear pattern, whereas in sector II and III they are more dispersed. Assessing the mobility of each element is useful to figure out whether spidergrams from whole rock data show an entirely magmatic signal and to know which elements are influenced by post-magmatic processes.

3.7. Reconstructing the equilibrium melt from Cpx and determining the viability of the technique The equilibrium melt calculation process starts from selecting the most reliable (the immobile elements in Cpx that are close to the amounts in whole rock, such as Y and Zr) element abundances in Cpx. Subsequently, a partition coefficient Dcpx/melt from an experimental set, like the well-known one by Hart and Dunn (1993) is used. Then, to obtain the quantity of a certain element (Zr for instance) in the equilibrium melt, it is only necessary to take the Zr amount in Cpx and then divide it by the partition coefficient Dcpx/melt found in the set. In this case, Dcpx/melt for most elements is from Hart and Dunn (1993), in Appendix B is specified the source of partition coefficients. To verify whether the ICP-MS analyses are viable to the purpose of calculating the equilibrium melt, a Ti vs. Zr discrimination diagram (Pearce & Cann, 1973) is drawn picturing all the spots from the older Hageskov whole rock dataset. Successively, the equilibrium melt calculated from each LA-ICP-MS analyzed sample is plotted altogether. A further step is to double check is using the Zr vs Zr/Y diagram (Pearce

18 and Norry, 1979). The newer reconstructed melt data should plot coherently with both the previous data, petrology and tectonic setting suggested by those aforementioned diagrams. The petrology is already, rock clearly determined: a Tholeiitic basalt plotting as a MORB as written in Hageskov (1987). For other elements, which are not featured in discrimination diagrams, another approach can be used to determine the viability. First for the x axis on the plot either the Al or the Mg estimate, from modal abundances, in the bulk rock is used; on the y axis a certain trace element in the equilibrium melt. For example La is plotted against the estimated Mg (through modal abundances); to be viable the equilibrium melt should plot within an order of two the trend defined by the previous whole rock analyses by Hageskov (1987). The equilibrium melt points have a 5% error bars, to allow a wide enough margin deriving from instrumental errors. In order to show the presence of a magmatic signal two different approaches can be employed: comparing the trend of a mobile (Sr) versus an immobile like Y in the Koster dikes equilibrium melt against an unaltered basalt to see if they match. The other approach is comparing the multi elements spidergrams from the Koster dikes melt and from unaltered basalts; especially concerning compatible elements that might also be mobile like Co and Zn.

4. Observations and results of applying the method 4.1. New Observations on petrology and mineralogy

Figure 9. SEM BSE featuring a garnet porphyroblast

19 The rock is mostly composed of subhedral millimetric Cpx (Augite with around 30% Wollastonite component), Plagioclase (An60) in the core of the “freshest” plagioclases; they occur in both euhedral and subhedral millimetric laths. Orthopiroxene (Opx) might occur on rims of Cpx and rarely as anhedral crystals. In Sector I samples metamorphism has occurred though magmatic phases are still present. The outer rim of Cpx has been turned into amphibole. Garnet (fig.9) is occasionally present in the form of subhedral to euhedral submillimentric crystals growing from Cpx. Simplectite is also found and is composed by Plagioclase, amphibole and rarer quartz. Ti-Magnetite has several habits subhedral and anhedral; it displays the exolution lamellae into Ilmenite. Magnetite also occur into veins. The metamorphic overprint varies, some samples feature more recrystallization and symplectites and the original magmatic texture is in part lost, whereas in others is still in part recognizable (fig. 10) The texture in the sample which includes the contact (fig.11) with the gneissic host rock has a completely different texture: it is trachitic, with submillimetric plagioclase laths roughly aligned and many Cpx crystals commonly surrounded by an Opx rim. Some Opx with an anhedral habit is sometimes found in the sample along the contact. The rest of the groundmass is Px + Pl + Titanomagnetite/Ilmenite. In the groundmass micrometric spinels and sparse zircons are found as well.

20 Figure 10. SEM BSE image from the sample bordering the contact

Figure 11. SEM BSE image from sample 9 displaying the mineral phases

21 4.2. Geochemical analyses Data are averaged from ICP-MS spots; only the spots with a stable ICP-MS signal have been taken into account. The analyses units are in ppm even for major elements (Mg for example), since it is more practical to be used in graphs, as all other elements, are given in ppm, instead of oxide percent. EDS-SEM has been used for major elements and LA-ICP-MS for trace elements. Some important elements that are important for petrogenesis like Cr, could not be included, as the ICP-MS is seldom stable. Others like U and Ba have been recorded above LOD (Limit of Detection), yet the ICP-MS signal is markedly unstable and displayed an excessively pronounced intra-sample scattering. Analyses are found in the Appendix, section B ”Geochemical Analyses”, figure B.2. In the table also are information on mobility of elements, the used partition coefficients, the source of these coefficients, primitive mantle coefficients together with their source. The criteria to determine which elements are mobile in the Koster dikes is explained in sec. 3.4. In Appendix C, fig. C.1 it is shown the criteria to tell which element in the ICP-MS are reliable, which are weak (present but at levels borderline LOD, thus featuring high scattering), and unusable (unstable ICP-MS signal and high intra-sample variation).

22 4.2.1. ICP-MS signal The signal used is in Counts per Seconds (Cps) and the scale is logarithmic. Only the samples where elements plateau have been employed, as they mirror the content in the crystal, not affected by heterogeneities, like zonations and inclusions that might be hit by the laser. Sr is a useful element to understand the petrogenesis and the magmatic processes happened during the formation of the rock, though it might be disturbed as the laser might pierce through the Cpx and hit a Sr richer plagioclase accidentally, therefore showing an unstable signal and distorting the actual amount in Cpx. In fig. 12

Figure 12. ICP-MS signal for some relevant elements. a) Spot with stable signal. b) Bad quality spot is an example of one of the best Cpx signals (from sample 34), paired with a worse quality spot characterized by an unsteady signal in particular concerning trace elements rather than majors like Al.

23 4.3. Whole rock estimate

Figure 13. The reconstructed whole rock Al and Mg quantities a) against the trend from the old data series. b) is Al vs Mn. Error bars = 3.5% The estimate bulk rock approximately plots within the old data series trend concerning elements like Al, Mg and Mn (fig. 13). Graphs used are Al vs Mg, Al vs Mn and Mg vs Mn. Cr is not usable because its ICP-MS signal is not coherent between spots in the same sample and rarely features a stable ICP-MS signal; Ti plots outside the trend together with Ni and V. Sr, though lacking a definite trend plots within the area occupied by most of the points. 4.4. Reconstructed equilibrium melt The reconstructed melt plots along the trend already traced by the previous Hageskov data (fig.14), this concerning the immobiles, Ti, Zr and Y. The melt plots as a MORB in fig. 14a and transitionally between MORB and WPB (Within Plate Basalt) in fig. 14b. Regarding other more mobile elements like La, the reconstructed melt spots plots within an order of two. Since there is no data on Fe3+/Fe2+ for the current samples, to calculate Mg#, the differentiation plots include immobiles influenced by

24 differentiation, like Y or Ti plotted versus other elements. The graphs below in fig. 15 represent the equilibrium melt for each sample, with arrows indicating the influence on the element abundance by a certain mineral phase, during the magmatic differentiation process. The slope of the arrows is indicative, when an element is not compatible the slope is higher than 45 degrees; a roughly linear trend traced by the reconstructed melt data can be noticed. The reconstructed melt plots also as MORB in “Ti/100, Zr, Sr/2” diagram (fig. 16), featuring Sr which is a mobile element. Spots from other MORBs have been added, as the area in the original graph is not necessarily strictly defined. In appendix C a table with the reliability of analyzed elements is available.

Figure 14. Diagrams after Pearce (1979). The samples plot within the MORB in the picture a, whilst it plots somewhere in the middle in picture b

25 a) b)

c) d)

Figure 15. Several elements in equilibrium melts plotted against Y. Arrows represent the influence of mineral phases. Lighter circle means more differentiated 26

Figure 16. The Ti/100, Zr, Sr/2 diagrams after Pearce (1979), that have been added with points by Jing et al. (2016).BABB=Back Arc Basin Basalt.CAB= Calc-Alkali Basalt. IAT= Island Arc Tholeiite. Red squares is Koster Dikes reconstructed melt

4.5. The reconstructed melt trace element patterns The primitive mantle normalized spidergrams (fig. 17) have some scattering concerning the elements, from Ba to Nb, due to the amount in CPX that are borderline LOD. There are however marked similarities between samples regarding the elements after Nb in the series. Some elements are ”weak”, namely that they are borderline LOD (Limit Of Detection) and/or they show significant intra-sample change, yet the signal is higher than the background noise. A plot including the the immobile elements suggested in Pearce (1996) is compared to the reconstructed melt. Concerning the REE the graph normalized to chondrite an enrichment in the lighter elements is visible together with a mild Eu negative anomaly. In fig. 18 it is provided an example for several samples.

Figure 17. Spidergrams representing the average reconstructed melt from each sample. The most differentiated and the most primitive samples (Sec. I and II) from Hageskov dataset have been added for comparison

a) WPB b) MORB

c) WPB d) MORB

Figure 18. REE plot representing the best spots from several samples normalized to the Chondrite (Sun & McDonough 1989). In the graph is written how they plot in the Zr vs Zr/Y graph, a and c WPB, b and d MORB 28 4.5.1. Cpx spidergrams

Figure 19. Chondrite normalized plot (Sun and McDonough 1989) of the Cpx best spots

All the spidergrams (fig.19) normalized to chondrite display an evident negative Eu anomaly. Their pattern is generally close to each other, except the La-Pr slope, which sometimes varies slightly.

4.5.2. Sr in the reconstructed melt from both Cpx and Plag The strontium levels in the reconstructed melt have been calculated from both Cpx and Plagioclase (Plag), to have more robust data. 2.7 has been used as D for Plagioclase, and it has been calculated with the macro by Sun et al. (2017). In tab. 3 are the data used to obtain the Dplag/melt. P and T have been selected for plausible conditions in this tectonic setting. In fig. 20 are the Sr levels calculated from both plagioclase and clinopyroxene; they plot within an order of two, showing that the equilibrium melt from Cpx is close to the actual amounts in the original magma Table 3: The composition of the plagioclase used to calculate Dplag/melt. Error = 3% Plagioclase composition w% 34; T = 1200 C; P = 0.6 GPa Na2O 5.45 Al2O3 28.15 SiO2 54.87 K2O 0.23 CaO 10.35 FeO 0.70

Figure 20. The obtained Sr in the equilibrium melt from both Cpx and Plag. Dplag/melt = 2.70 calculated with a macro by Sun et al. (2017)

5. Discussion 5.1. Coherence with previous findings In the ”Ti vs Zr” and ”Zr vs. Zr/Y” diagrams (fig. 15) by Pearce and Cann (1973), recommended to be used in case of overprint/alteration, almost all samples from Hageskov and the equilibrium melt plot in the same area; more specifically falling in between the MORB/WPB area, which is coherent with the observation that the dikes are made up of tholeiitic basalt intruding the crust composed mainly of a gneissic and locally amphibolitic host rock. In the “Ti/100, Y*3, Zr” diagram the reconstructed melts fall in the MORB field as already mentioned in previous findings by Hageskov; the author used the once then (1987) available classification diagrams (Le Roex et al., 1985; Pearce and Cann, 1973) to determine the type of the Koster dikes rock, without using the more recent spidergrams. By using classification methods which are more recent than the 1987 Hageskov study, in conjunction with LA-ICP-MS the rock plots on the REE chondrite normalized spider diagrams as a T- MORB, which means, as discussed in later studies concerning dike complexes in the region nearby the presence of a mantle plume (Åhall and Gower, 1997). This T-MORB-like magma is present in Koster archipelago at various degrees of differentiation and crustal assimilation, which is shown by indexes like K/ Ti ratio and the Pb spike on the spidergram normalized to the Primitive Mantle. There is no evidence suggesting a magma tapping from a different source in dikes with a NNW strike (sample 18).

30 5.2. Magmatic signal inside the pristine Cpx Given that the Sr in Cpx is very similar to the Sr in plagioclase as seen in the result section (fig.20), it means that Sr in plagioclase likely reflects the Sr present in the pristine rock. The trend in the reconstructed melt is similar to those of the pristine Isua (SW Greenland) MORBs and OIBs (Ocean Island Basalts) in fig.21; in particular concerning Sr, an element which is mobile. The upward trend of pictured trace elements is similar and it is very likely linked to magmatic differentiation. Given the similarity of behavior between the unaltered MORB and the Koster dikes reconstructed melt, it would mean that inside the Cpx the original magmatic element distribution is still preserved, despite the metamorphic overprint. A further piece of evidence is that the Koster dikes whole rock data plot in a chaotic way with a very slight negative trend line, which is completely opposite to the behavior shown by the Isua basalts from Komiya et al. (2004). This would imply that the Sr in the whole rock is no longer due to just magmatism, but also that other factors, including metamorphism and weathering altered the original amounts. As it mentioned in the introduction the rims of Cpx have been partially altered, hence analyzing them with a smaller LA-ICP-MS spot size such as 20µ or even less, at least concerning the trace elements that still have a strong enough signal at 40µ. The ICP-MS signal might drop to three to four times less when a spot which is two or more times smaller. Performing such analyses would definitely allow to tell that the core of the pyroxenes is definitely unaltered and how metamorphism changed the trace elements distributions.

31 a)

Figure 21. Unaltered MORB and OIB from Isua represented black circles Komiya et al. (2004). Y is a proxy for differentiation, the more Y the more differentiation b) The Koster dikes reconstructed melt c) Sr in the whole rock from Hageskov (1987) displays no trend 32 5.3. Spidergrams The multi-element spidergrams (fig.17) in the results section display an almost similar pattern among samples, which indicates that the source magma, despite the several differentiation degrees, it is the same in every observed case. The behavior of incompatibles is remarkably similar; Pb shows a spike, which is a well-known index of interaction with the crust (Davidson, 1987). Moreover, the Pb positive anomaly would be solely due to crustal interaction, as it is from the magmatic signal inside Cpx. Another interesting aspect is that the primary melt was not already heavily affected by plagioclase fractionation, since there is no noticeable Eu negative anomaly. In crustal rocks Eu has a negative anomaly, because the plagioclase that forms first deep down in the mantle is known to capture the Eu2+. However, Cpx chondrite normalized spidergrams in fig.19 show a negative Eu anomaly, suggesting that plagioclase had already fractionated in the magma source. Metamorphism has probably altered the trace element patterns in the whole rock, though some dikes from Hageskov data (1987) still display linear trends as the original melt. In particular among the most scattered elements in fig. 17 from the start of the series in the spidergram until Nb. Rb is usually below LOD and cannot be used in any reconstruction, though Ba in Cpx is most likely magmatic since it is present in amounts that are both above LOD and consistent with predictions by using the Dcpx/melt from the whole rock to estimate the Ba in Cpx. In Appendix C, fig C.2. it is possible to see that the elements like U, Th, Nb and Ba, which all have a significant degree of scattering in spidergrams are present in the Cpx of the Koster dikes at levels similar to the concentration obtained by applying the reverse process of using the partition coefficient to calculate element amounts in Cpx from whole rock data from other mafic complexes. The HREE, Y and Ti are known to be the least touched by crustal contamination; this would mean that these elements very likely mirror the original distribution ratio in the primary melt. The negative Sr anomaly is due to the temperature around 1200 °C and pressure conditions (estimated around 0.6 GPa), which in this case were likely to favor the incorporation of Sr in Plagioclase, as summarized by Blundy and Green (2000). Otherwise, it would not explain the very shallow Eu anomaly displayed on the spidergram, which is as well a consequence of plagioclase crystallization and separation from the main melt. Also, to rule out that the Sr negative anomaly on the spidergram is an artifact, the Sr value from the melt calculated from the plagioclase does not differ more than an order of two. The compatible elements all have a similar behavior, with some of them

33 Figure 22. The dikes from Austurhorn compared with both the Koster dikes whole rock and two reconstructed melt. Only two Koster samples have been plotted for practical purposes mobile in some settings like Co and Zn. Also, the similarity in behavior is found with recent and unaltered Icelandic basalts in Austurhorn (Furman et al., 1992) shown in fig. 22. The lower V in Koster Dikes can be explained that the samples from Iceland included are less differentiated. Since the pattern in the aforementioned mobile elements is similar to those fresh basalts, it would imply that the trace element patterns given by Cpx and its reconstructed equilibrium melt are purely magmatic. There is no substantial difference in Chondrite normalized REE spidergrams between fig. 18 and fig. 23 (which is the sample located close to the contact). This lack of difference in patterns is important, as the rock bordering the contact has been very rapidly chilled. As mentioned in sec. 3.2, gradual cooling (and crystallization) of a mafic rock would mean that the earlier forming Cpx record the magma composition tapped from the source, while the later would have a composition of the residual melt, with a trace element pattern reflecting this kind of magmatic differentiation process (Claeson et al., 2007). In rapidly chilled rocks this phenomenon cannot occur, and in the case of the Koster dikes it can be safely assumed that this effect in the rock located in the middle of dikes has a negligible impact.

Figure 23. The spots from sample near the contact with the host rock normalized to Chondrite (Sun and McDonough, 1989)

5.4. Potential of the technique, petrogenesis and tectonic setting A major issue that this analytical technique would address is allowing the usage of discrimination diagrams that employ mobile elements, in settings where they cannot be used. For instance (Ueda et al., 2000) showed that whole rock analyses to reconstruct the origin of a mafic rock do not allow to obtain accurate results in areas like Greenstone Belts. Being able to use discrimination diagrams similar to those by Pearce and through spidergram signature compared to fresh mafics, would unlock a larger potential, in areas where whole rock analyses provided only fuzzy and ambiguous results. The Lanthanides spider plot normalized to Chondrite shows an enrichment in REE. The pattern resembles an E-MORB (despite not being a basalt intruding oceanic crust), even in the case when the melt is E-MORB normalized. However, this is similar to other cases of continental tholeiites, which tend to have a geochemical signature intermediate between N-MORB and Plume- MORB, when they have the lowest contamination (Dupuy & Dostal, 1984). The spider diagram of Lanthanides normalized to Chondrite is very similar to those of Continental Flood Basalts (CFB). Concerning why in the Pearce (1977) “Ti vs Zr/Y” diagrams the Koster Dikes plot in between WPBs and MORBs it could be explained by this hypothesis: first in the rifting process the basalt would be of the WPB type, as it intrudes in continental host rock and originates lower in the mantle than a MORB; afterwards when the rift widens the magma source would be shallower, hence with a geochemistry similar to a MORB. There are no significant differences in LREE enrichment between the reconstructed melt spots that in some cases plot as MORBs or WPBs. Hence, it is possible that a plume could have played some role in initiating the rifting related with the Koster Dikes

35 and the supposedly related mafic complexes in Atlantic Canada. On the discrimination diagram ”Ti/100, Zr, Sr/2” (fig.15) after Pearce and Cann (1973), the reconstructed melt plots within the MORB field as well, hence reinforcing and being consistent with the data from the other plots. It can be noticed some geochemical similarity with the nearby younger Late Gothian Dikes (Hageskov, 1997). In particular, concerning the REE patterns and even though these complexes are separated by a chronological interval of tens of millions of years, yet they might share a similar reservoir. However, the multi-element spidergrams, which can be compared to the Koster dikes, for rocks composing the Late Gothian Dikes are constructed from whole rock analyses, and given that the whole rock might have an altered trace elements composition from the original one. Broadly speaking it would mean that discrimination diagrams (tailored to fresh rocks) that include elements which are easily mobilized like Ba are often of no use, if the rock is altered/weathered in the slightest. Mobile elements on spidergrams display little or no information on the magmatic history of a rock, rather they can only be used to tell the processes that occurred after the rock crystallization. It can be also proposed that the Primitive mantle spidergrams in the results section resemble those of Mackenzie dykes, the Coppermine River Basalts and the Bear River dikes (Schwab et al., 2004). Though, the fact that the analyses for the Canadian dykes are whole rock weakens the comparison. However, it can be highlighted that the dikes located in Greenland and Atlantic Canada plot as WPB (fig. 24); this could mean that the reservoir was roughly similar and somewhat related, given that the continent Baltica and Laurentia were once joined by the Grenville-Gothian orogen, when the Supercontinent Columbia was present. It is known that the MacKenzie dikes have been originated by a mantle plume (Baragar et al., 1996) as well as the Harp Dykes (Ernst and Buchan, 2002). Hence, it is possible that the Koster Dikes might related to the same plume activity, as the Koster Dikes seem to record some form plume activity as the former cases. The timing however is supposedly different, as demonstrated by the fact that the Koster dikes date some 180 Ma earlier than the MacKenzie dikes, despite their whole rock Rb-Sr age might be questioned, as happened with the re-dating of the Mealy Dikes with U-Pb. When the Koster Dikes were dated at 1420 Ma in 1988 and the Harp dykes were assumed to be roughly coeval with the Koster Dykes according to the Rb-Sr age. To have more insights on the dikes lying on the opposite side of the Atlantic it would be necessary to perform the same kind of analysis on the Cpx phenocrysts, in order to know if the dike swarms found on the other side of the Atlantic are geochemically similar, through the use of discrimination diagrams which include mobile elements and spidergrams. If the timing differs by some tens of millions of years, then the plume could have initiated the break-up first were along the present-day Sweden W coast and later in the contemporary Canadian Shield. Probably, the rifting started along present-

36 day W coast of Sweden progressively opening up and initiating diking where the rift was just incipient 1250 Ma in present day NE Canada.

Figure 24. Plot displaying how the dikes from Bayan Obo (North China Craton), dykes in Greenland and in Canada plot similarly (Yang et al. 2011)

6. Conclusions The Koster dikes, coherently with the previous analyses are composed by a tholeiitic basalt, to be specific with a MORB-like chemistry and more accurately characterized by a T-MORB geochemistry. Even though the rock is clearly a basalt that intruded continental crust. Despite the magmatic overprint, the Cpx seems to contain a signal which is likely magmatic since the recalculated equilibrium melt follows trends resembling those of fresh basalts. This in particular concerning the elements that are known to be mobile like Sr. Hence, this technique would allow to use diagrams that include mobile elements, that are usually reserved for ”fresh” rocks. The REE pattern is similar to the late Gothian mafic dikes, which is possible since the geographical proximity, despite the late Gothian mafic dikes being older than the Koster dikes. The T-MORB-like magma type might be related with plume activity, which initiated the break-up of the supercontinent Columbia, roughly 1.4 Ga ago. Using the same analytical technique as with the Koster dikes, on swarms that were once located relatively nearby on Columbia like the Bear River Dikes in Canada would allow to know more how they relate to each other.

37 7. References

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43 Appendix A. Koster Archipelago map

Figure A.1: Map displaying the location of samples (in blue) and the partition of the area in sectorns (I least overprinted, II intermediate, III totally amphibolitized). Modified from Hageskov and Pedersen (1988)

44 Appendix B. Geochemical analyses

Figure B.2: Geochemical analyses table comprehensive of the D/Cpx source, Primitive Mantle values source and mobility of elements. EDS in oxide percent, all other data in ppm. * Elements below LOD

45 Appendix C. Element analysis quality assessment

Figure C.1: The criteria to tell if an element reliability is OK: Clustering between samples and visible trends, stable ICP-MS signal and little intra-sample scattering. Weak elements: only above LOD to high intra-sample scattering a) An element which is OK: Y b) U is an element defined as weak, given it is above LOD, but there is wide dispersion. Elements in ppm.

Table C.1: Table with the elements used according to their quality. Weak elements are present above LOD but show too wide scattering. Unusable means that the ICP- MS signal is never stable and/or is borderline LOD.

Element reliability Quality: OK, Weak, Unusable REEs OK Y OK Ti OK Li OK Sc OK Co OK Ni OK Zn OK Zr OK Sr OK V OK Pb OK Ba Weak U Weak Th Weak Nb Unusable Rb Unusable Ta Unusable Cs Unusable Cr Unusable

47 Fig. C.2: divergence of weak elements between measured values in Koster dikes CPX and reverse estimated quantitities in CPX of other mafic rocks