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Geochemistry of Volcanic Rocks from the Southeast of the Central Finland Granitoid Complex

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Geochemistry of Volcanic Rocks from the Southeast of the Central Finland Granitoid Complex Department of Geography and Geology, University of Turku Krista Mönkäre GEOCHEMISTRY OF VOLCANIC ROCKS FROM THE SOUTHEAST OF THE CENTRAL FINLAND GRANITOID COMPLEX Master’s Thesis in Geology Turku 2016 UNIVERSITY OF TURKU Faculty of Mathematics and Natural Sciences Department of Geography and Geology MÖNKÄRE, KRISTA: Geochemistry of volcanic rocks from the southeast of the Central Finland Granitoid Complex Master’s Thesis, 76 p., 1 appendix Geology April 2016 The approximately 60 by 80 km wide study area is situated in the southeastern corner of the Central Finland Granitoid Complex. The study area consists mainly of different plutonic rocks, whereas the supracrustal and subvolcanic rocks form a discontinuous and scattered NE-SW oriented belt in the middle of the area. The aim of the Master’s thesis is to characterize the volcanic rocks in the area and interpret whether they resemble other Svecofennian volcanic rocks in southern Finland. The Master’s thesis study is part of the Central Finland mineral potential estimation –project conducted by the Geological Survey of Finland. Due to scattered occurrence of the volcanic rocks and an extensive Quaternary coverage in the study area, geochemical approach was chosen. The material consists of bedrock observations, whole-rock geochemical analyses, thin sections and aeromagnetic maps. Altogether, 184 whole-rock analyses were used in the study. On the basis of field relationships and lithological differences, the volcanic rocks were divided into five different belts (Makkola, Kauppila, Halttula, Leivonmäki and Korospohja) which were compared with each other. Apart from a few exceptions, all the volcanic rocks have the same geochemical features and all of the belts are characterized by a wide range of volcanic rocks from basaltic to rhyolitic in composition. Intermediate, calc-alkaline compositions are the most common in all of the belts. All the volcanic rocks are enriched in LILEs and LREEs relative to HFSEs and are depleted in Ti and Nb. The trace element geochemistry of the volcanic rocks is more strongly correlated with the SiO2 content than the location of the sample as the felsic rocks have more evolved compositions than the mafic rocks. The Makkola and Korospohja belts represent slightly more felsic and evolved compositions on average, whereas the Kauppila, Halttula and Leivonmäki belts have more mafic compositions. Based on the available data it seems that all of the volcanic belts are part of the same system as no significant compositional differences between them exist. The study revealed that both the major and trace element geochemistry of the volcanic rocks indicate a mature, Andean-type continental arc setting. Some of the volcanic rocks also exhibit adakite-like geochemical features. The geochemical features of the volcanic rocks resemble in many ways the volcanic rocks from the Tampere and Häme belts. Similarities with the Tampere belt are more evident because of its more mature characteristics compared to the Häme belt. Key words: geochemistry, volcanic arc, volcanic rock, Proterozoic TURUN YLIOPISTO Matemaattis-luonnontieteellinen tiedekunta Maantieteen ja geologian laitos MÖNKÄRE, KRISTA: Keski-Suomen granitoidikompleksin kaakkoisosan vulkaniittien geokemia Pro gradu -tutkielma, 76 s., 1 liite Geologia Huhtikuu 2016 Tutkimusalue sijaitsee Keski-Suomen granitoidikompleksin kaakkoisosassa ja on kooltaan noin 60 x 80 km. Pääosa tutkimusalueesta koostuu erilaisista syväkivistä, kun taas suprakrustiset ja subvulkaaniset kivet muodostavat alueen keskelle epäyhtenäisen koilis-lounas-suuntaisen vyöhykkeen. Pro gradu -työn tarkoituksena on tutkia tarkemmin alueen vulkaniitteja ja vertailla niitä Etelä- ja Keski-Suomessa sijaitsevien liuskealueiden vulkaniittien kanssa. Pro gradu-tutkielma on suoritettu osana geologian tutkimuskeskuksen Keski-Suomen mineraalipotentiaalin arviointi-projektia. Tutkimuksen aineisto koostuu kallioperähavainnoista, kokokivikemian analyyseista, ohuthieistä sekä aeromagneettisista kartoista. Tässä tutkimuksessa käytettiin yhteensä 184 kokokivianalyysiä. Tutkimusmenetelmäksi valittiin kivien kokokivigeokemian tutkiminen, sillä vulkaniitit esiintyvät tutkimusalueella hyvin hajanaisina vyöhykkeinä ja tutkimusaluetta päällystää laaja kvartääriaikainen peite. Vulkaniitit jaettiin kenttähavaintojen ja litologisten erojen avulla viiteen eri alueseen (Makkola, Kauppila, Halttula, Leivonmäki ja Korospohja), joita vertailtiin keskenään. Lukuun ottamatta muutamia poikkeuksia, vulkaniittien geokemialliset piirteet ovat kaikilla alueilla samanlaiset. Kaikille alueille on ominaista vulkaniittien laaja koostumusvaihtelu basalteista ryoliitteihin, mutta intermediäärinen ja kalkki-alkalinen koostumus on yleisin. Makkola ja Korospohja edustavat koostumukseltaan hieman felsisempiä ja kehittyneempiä alueita, kun taas Kauppila, Halttula ja Leivonmäki ovat pääkoostumukseltaan mafisempia. Kaikki vulkaniitit ovat rikastuneita LIL-alkuaineista suhteessa HFSE-alkuaineisiin, ja vulkaniiteilla on alhaiset Ti- ja Nb-pitoisuudet. Hivenalkuainekoostumuksen vaihtelun huomattiin liittyvän enemmän SiO2-pitoisuuden vaihteluun kuin vulkaniittien sijaintiin. Tutkimus osoitti, että vulkaniittien pääalkuaine- ja hivenalkuainegeokemia viittaa mantereiseen, Andit-tyypin vulkaanisen kaaren ympäristöön. Lisäksi joillakin vulkaniiteilla havaittiin adakiittisia piirteitä. Vulkaniiteilla todettiin lisäksi olevan samanlaisia piirteitä Tampereen ja Hämeen liuskealueiden vulkaniittien kanssa, mikä viittaa samanlaiseen tektoniseen synty-ympäristöön. Yhteneväisyys Tampereen liuskealueen kanssa on ilmeisempi vulkaniittien kehittyneemmän koostumuksen takia. Asiasanat: geokemia, vulkaaninen kaari, vulkaniitti, proterotsooinen Table of contents 1. INTRODUCTION ....................................................................................................... 1 1.1 PURPOSE OF THE STUDY ............................................................................................ 3 2. OVERVIEW OF VOLCANIC ARCS ....................................................................... 3 2.1 GEOLOGICAL STRUCTURE ......................................................................................... 4 2.2 PETROGENESIS AND MAGMATISM ............................................................................. 6 2.2.1 Sources of magma ............................................................................................. 8 2.2.2 Role of fluids ..................................................................................................... 9 2.2.3 Evolution of magma .......................................................................................... 9 2.2.4 Magmatypes .................................................................................................... 10 2.2.5 Volcanic outputs ............................................................................................. 11 2.3 GEOCHEMICAL FEATURES ....................................................................................... 12 3. MATERIAL AND METHODS ................................................................................ 14 3.1 SCREENING OF THE GEOCHEMICAL DATA ................................................................ 15 3.2 FIELD WORK ........................................................................................................... 16 3.3 PETROGRAPHY........................................................................................................ 16 3.4 GEOCHEMICAL ANALYSES ...................................................................................... 17 4. GEOLOGY OF THE STUDY AREA ..................................................................... 17 4.1 REGIONAL GEOLOGY .............................................................................................. 17 4.1.1 Plutonic rocks ................................................................................................. 18 4.1.2 Subvolcanic and supracrustal rocks ............................................................... 19 4.2 LOCAL GEOLOGY AND SUBDIVISION OF THE STUDY AREA ....................................... 20 4.2.1 Makkola .......................................................................................................... 22 4.2.2 Kauppila.......................................................................................................... 22 4.2.3 Halttula ........................................................................................................... 23 4.2.4 Leivonmäki ...................................................................................................... 23 4.2.5 Korospohja...................................................................................................... 23 5. PETROGRAPHY OF THE VOLCANIC ROCKS ................................................ 25 5.1 SUBVOLCANIC ROCKS ............................................................................................. 25 5.1.1 Plagioclase porphyrites .................................................................................. 25 5.1.2 Plagioclase-uralite porphyrites ...................................................................... 25 5.1.3 Uralite porphyrites ......................................................................................... 25 5.2 SUPRACRUSTAL ROCKS ........................................................................................... 26 5.2.1 Mafic tuffs ......................................................................................................
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