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Investigation of the Särna Alkaline Complex in Dalarna, Sweden

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Investigation of the Särna Alkaline Complex in Dalarna, Sweden UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Investigation of the Särna alkaline complex in Dalarna, Sweden John Eliasson ISSN 1400-3821 B1019 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 1.0 Introduction ....................................................................................................................................... 3 1.1 Alkaline systems ............................................................................................................................ 3 1.2 Regional geology ........................................................................................................................... 5 1.3 Särna alkaline complex (SAC) ........................................................................................................ 6 1.4 Cancrinite ...................................................................................................................................... 7 2.0 Methodology ..................................................................................................................................... 8 2.1 Fieldwork ....................................................................................................................................... 8 2.2 Scanning electron microscope (SEM) ............................................................................................ 9 2.3 LA-ICP-MS ...................................................................................................................................... 9 2.4 Raman spectroscopy ................................................................................................................... 10 3.0 Results ............................................................................................................................................. 11 3.1 Mineral and sample description .................................................................................................. 11 3.1.1 Cancrinite (nepheline) syenites: fine grained samples ........................................................ 11 3.1.2 Cancrinite (nepheline) syenites: coarser samples ................................................................ 16 3.1.3 Other samples: the southeastern outcrop ........................................................................... 18 3.1.4 AS13-02: Tinguaite ............................................................................................................... 24 3.2 Major and minor chemistry ......................................................................................................... 25 3.2.1 Whole-rock analysis.............................................................................................................. 25 3.2.2 Clinopyroxenes ..................................................................................................................... 29 3.2.3 Titanite and Cancrinite ......................................................................................................... 32 3.3 87Rb–87Sr dating ........................................................................................................................... 37 3.3.1 Cancrinite (nepheline) syenite: K-feldspar, titanite and cancrinite grains ........................... 37 3.3.2 Tinguaite: Biotite grains with a forced initial ....................................................................... 38 3.3.3 Weighted mean age: Särnaite and Tinguaite ....................................................................... 39 4.0 Discussion ........................................................................................................................................ 40 Emplacement time of the SAC ........................................................................................................... 40 Cancrinite ........................................................................................................................................... 41 On the controls of oxygen fugacity .................................................................................................... 41 Extreme MREE depletion ................................................................................................................... 43 Evolution of the system ..................................................................................................................... 44 (1) Fractional crystallization of CPX (large Ca-Ti Tschermaks component) from a parent melt ... 44 (2) Potential silicate-carbonate immiscibility ................................................................................ 47 5.0 Conclusions ...................................................................................................................................... 50 6.0 Acknowledgements ......................................................................................................................... 50 7.0 References ....................................................................................................................................... 51 2 1.0 Introduction Rocks with alkaline affinity tend to display an exotic minerology, which often contain high concentrations of rare earth elements (REE), thereby making them of economic importance. Additionally, alkaline complexes are rare. Only a handful of complexes have been found in Sweden and even fewer has been thoroughly investigated, thereby elevating the academic importance of Särna alkaline complex. These factors combined is why more studies are needed to get an understanding of how they form and develop. Alkaline to peralkaline igneous rocks are typically found within intracontinental extensional settings, for instance this is well illustrated in the alkaline Illímaussaq intrusion in the Gardar province in South Greenland. However, they have also been observed within Oceanic Islands (Sørensen, 1992). Given the stable nature of the Baltic shield the scarcity of alkaline complexes in Sweden is understandable, but this is also a reason to study them. Why would they be found in a tectonically stable environment? Sørensen (1992) further describes, to some extent, the pre-requisites needed to form rocks with alkaline affinities. Among others, the need of low degree of partial melting and crystallization in a fluid- or gas-rich magmatic system is crucial. The low degree of partial melting results in a silica deficiency and can be the start of potential alkalinity. This study aims to further constrain the time of emplacement of the Särna alkaline complex (SAC) and further correlate the time of emplacement to coeval tectonic events on a regional scale in the Fennoscandian Shield. Furthermore, it sets out to contribute with additional insights into the magmatic evolution of the system, such as: • What parent melt composition was needed to form the SAC? • What fluid phases appear to have been present during the formation of the SAC? • How is the oxygen fugacity in the melt affecting the mineralogy? By attempting to answer questions such as these, this thesis aims to clarify some aspects of how and when the SAC formed. Additionally, by placing the SAC in a larger context it might provide new insights into how alkaline complexes can form and evolve. 1.1 Alkaline systems Rocks with a large ratio of alkalis (sodium and potassium) compared to silica or alumina are typically considered to be alkaline. This can for instance be syenites, which are found in the left corner of a QAPF diagram (Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid) diagram). Syenites are compositionally very similar to granites but inherently have a quartz deficiency. This does not, however, make it alkaline. It does simplify the process of becoming alkaline because it inherently has one of the components necessary. Due to this specific composition, large alkali to silica and alumina ratio, alkaline rocks tend to adhere to an unusual mineralogy. Nepheline, for instance, is a commonly found mineral in alkaline rocks and can to some extent be seen an indicator mineral. Nepheline has a chemical formula of Na3K(Al4,Si4O16), or simplified (Na,K)AlSiO4 (Hålenius et al., 2018). It is quite evident that this stoichiometric structure is more suited for a system with lower silica concentrations than for instance albite (NaAlSi3O8). The sodium±potassium to alumina to silica ratios are 1:1:1 and 1:1:3 respectively, further illustrating this. Furthermore, pyroxenes such as aegirine are quite common in alkaline rocks, often crystallizing as elongated needles. It has a chemical formula of 3+ NaFe Si2O6 and is typically only formed in systems that have an abundance of sodium and where feldspars have used most of the alumina. The reason why high concentrations of sodium are needed for aegirine to form is that it can readily incorporate quite substantial amounts of Na while still being 3 silica and alumina conservative. Similar pyroxenes, like jadeite (NaAlSi2O6), requires higher concentrations of Al2O3 to be left in the melt to form. Therefore, the ratio of alkalis to silica and alumina is essential to create an alkaline rock. These uncommon minerals, such as nepheline and aegirine or augite are often rock forming together with K-feldspar and albite. They produce rocks such as syenites, nepheline syenite (syenite with a large
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