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

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