Browsing by Subject "Svecofennian orogeny"

Tiivistelmä/Referat – Abstract The Geological Survey of Finland (GTK) conducted extensive bedrock mapping and sampling along the southern border of the Central Finland Granitoid Complex (CFGC) between 2016–2017. The 40000 km2 complex was formed during the early stages of the 1.92–1.77 Ga Svecofennian orogeny and is dominated by felsic plutonic rocks. The main crust-forming phase at 1.91–1.87 Ga generated the felsic syn- and postkinematic suites during active subduction. Jyväskylä suite corresponds to the synkinematic rocks (1.89–1.88 Ga) and Saarijärvi and Rautalampi suites to the postkinematic (1.88–1.87 Ga). Previous studies have divided the postkinematic rocks into Types 1, 2, 3a and 3b, of which Types 1–3a represent the Saarijärvi suite and Type 3b the Rautalampi suite. The postkinematic suites are bimodal because of the local mafic association. Volcanic rocks associated to a continental margin setting are especially found along the southern-southwestern margin of the CFGC. Abundant metasediments with a mainly turbiditic origin surround the complex. The purpose of this thesis is to classify the granitoids and dioritoids of the Jämsä region into either the syn- or postkinematic types, discuss their petrogenesis and compare the data to the adjacent volcanic rocks. New petrographical, geochemical and geochronological data are used to set the plutonic rocks into the regional geological framework. Six distinct types are recognized from the study area: Mettisuo, Akkasuo, Linjamaa, Haavistonmäki, Kalaoja and Hangasjärvi. The first three were found to be synkinematic (Jyväskylä) and the last three postkinematic (Saarijärvi) with respect to the orogenic stages. The synkinematic Mettisuo (1890 ± 4 Ma) and Akkasuo-type (1880 ± 6 Ma) equigranular to inequigranular granitoids are metaluminous to peraluminous with Akkasuo being slightly more mafic. They have higher CaO and Sr and lower K2O and FeOt/MgO than the postkinematic types. The Linjamaa-type inequigranular dioritoids represent the Vaajakoski quartz diorite lithodeme (Jyväskylä suite) owing to their more juvenile character. However, there are discrepancies within the Linjamaa division. Partial melting of biotite- and hornblende-rich high-K calc-alkalic rocks by basaltic underplating produced the synkinematic types, leaving behind a granulitic residue. The Haavistonmäki-type (1876 ± 7 Ma) alkali feldspar-porphyritic granites are postkinematic Type 1 with a sedimentary component from the Pirkanmaa migmatite belt. They are peraluminous with low Na2O and FeOt/MgO and high FeOt and Al2O3. The Kalaoja-type inequigranular to porphyritic granitoids are transitional between postkinematic Types 2 and 3. The lack of fluorite and pyroxenes complicate the classification. They are metaluminous to peraluminous with relatively low FeOt, MgO and CaO and higher FeOt/MgO and K2O. The Hangasjärvi-type (1885 ± 5 Ma) granitoids are silica-rich, resembling postkinematic Type 2. They differ from the Kalaoja rocks by their equigranular to inequigranular character, but the same mineralogical restrictions apply in the classification. A mantle-derived basaltic magma assimilated variable amounts of the lower crust, producing the postkinematic magmas. A depleted granulitic residue likely does not produce sufficient amounts of K2O and LILE contents for the postkinematic magmas. The postkinematic types are bimodal because of the locally associated mafic rocks. The volcanic rocks of the study area belong to the same continental subduction environment, are locally coeval and share a partially similar origin as the Jyväskylä suite and the mafic members of the Saarijärvi suite.