Browsing by Subject "Geochronology"

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  • Helama, Samuli; Arppe, Laura; Timonen, Mauri; Mielikäinen, Kari; Oinonen, Markku (2018)
    Tree-ring stable isotope chronologies provide very high-resolution palaeoclimatic data, and the number of records is increasing rapidly worldwide. To extend the chronologies back in time, before the period covered by the old living trees, the use of subfossil wood samples is required. Typically, the longest continuous subfossil chronologies consist of regionally collected tree-ring materials, rather than tree rings from a single site, and are likely more sensitive to data heterogeneity. Yet, the characteristics of such datasets remain hitherto unexplored. Here we produce a continuous, decadally resolved chronology of C-13/C-12 ratio (delta C-13) from Finnish Lapland over the past 7.5 ka (5500 BCE to 2010 CE) for which there is replication of at least five Pinus sylvestris trees. Less negative delta C-13 values were observed as trees age and for western sites (higher in elevation and further from the cold oceanic air flow). The age -related trends in living tree delta C-13 data were expressed mainly over the first fifty years mimicking the "juvenile effect" whereas the subfossil data showed trend over the trees' lifespan. These findings demonstrated the need to detrend the individual delta C-13 series before averaging them into the mean chronology. The corresponding biases were removed from the isotopic data using the methods frequently applied for tree-ring width and density proxies, the Regional Curve Standardization (RCS) combined with signal-free approach. While the RCS procedures commonly preserve the long-term variations in the resulting chronology, not all types of them did so as demonstrated for chronologies produced using separate RCS models for the delta C-13 series with relatively high and low isotopic level (offset from the grand mean). It was shown that these delta C-13 levels (i.e. the relative isotopic enrichment) result both from the low-frequency climate signal and biogeographical aspects (the site longitude/altitudes). The non-climatic biases were removed from the delta C-13 series by using separate RCS models for the subsets (western and eastern) of isotopic series. Similar to previous investigations using annually resolved delta C-13 data from Lapland, our chronology had strong negative correlations to variations in cloud cover. Here, a bootstrapping experiment was used to verify this dendroclimatic association. The resulting palaeo-cloud reconstruction portrayed decadal to multi-millennial variations with centennial anomalies coinciding with the mid and late Holocene events of climate transitions, highlighting the value of subfossil isotope chronologies from tree rings in synthesising climate dynamics from several proxy sources over the present interglacial.
  • Pietilä, Maija (Helsingin yliopisto, 2020)
    Geological Survey of Finland conducted bedrock mapping in the eastern parts of Central Finland Granitoid Complex (CFGC) and the area next to the Archean craton in the 1990s. The area consists mainly of Paleo-proterozoic paragneisses, with minor volcanic rocks present. The granitoids belonging to the Central Finland Granitoid Complex make up part of the bedrock in the area. The granitoids of CFGC are divided into a 1.89-1.88 Ga syn-kinematic group, and a crosscutting, 1.88-1.87 Ga post-kinematic group. In this Master’s thesis, three post-kinematic granitoid intrusions of Löytölamminvuori, Sorsakoski and Karvalevä are studied, covering their lithological, petrographical and geochemical features. The intrusions are non-foliated, porphyritic granites and quartz-monzonites, with a minor mafic phase of mostly dioritic composition in the Karvalevä intrusion. The main mafic silicates in the granite phase are biotite and hornblende, in the quartz-monzonite and mafic phases also clino- and orthopyroxene are present. Resembling the other post-kinematic plutons of the CFGC, the studied intrusions are geochemically high in Al2O3, FeO and K2O, and low in MgO, CaO and Sr. One U-Pb age of 1876+6 Ma has been measured for the Löytölamminvuori intrusion, which places the intrusion at the same time frame as the other post-kinematic plutons. Geochemically the intrusions show A-type affinity and close similarities to the post-kinematic pluton Types 2 and 3, fitting best with the Type 3a, which is transitional between the two. The magmas forming Löytölamminvuori, Sorsakoski and Karvalevä were derived from partial melting of mantle derived basalts, which underwent crustal contamination by partial melts from the lower crust. Slight deviation in composition from the strictly A-type magma and the volcanic arc affinity can be explained by the crustal component. The mafic phases show more primitive geochemistry, and thus present the mantle-derived source component with less crustal assimilation in the source. The intrusions show signs of bimodal mafic-felsic magmatism, the dioritic phases in Karvalevä intrusion and one syn-plutonic dyke in Sorsakoski intrusion representing the mafic component. The mafic magmatism was cogenetic with the felsic phases, but not comagmatic, the diorites intruding simultaneously but slightly after the felsic phases. The mafic phases show a continuum in chemical composition to the granites and quartz-monzonites, but with a slight compositional gap.
  • Tepsell, Johanna; Rämö, O. Tapani; Heinonen, Aku; Lahaye, Yann; Haapala, Pieti; Halkoaho, Tapio; Heinonen, Jussi S.; Höytiä, Henri; Konnunaho, Jukka; Järvinen, Ville (2020)
    Archean terranes are generally poorly exposed and structurally complicated, making models of geological evolution difficult to establish. Because of pervasive metamorphism, strong deformation, and poorly exposed bedrock, the stratigraphy, geochronology, and geological evolution of the northeastern part of the Archean Western Karelia Subprovince, Finnish Lapland, is still deficient. Three new U-Pb age determinations were made using the LA-ICP-MS method on zircon in search of more profound understanding of the geochronology of the area. Data from the Pultoselka granite yield an age of 2795 +/- 5 Ma, providing a minimum age of 2.8 Ga for the Tulppio suite. The age is coeval with widespread 2.74-2.70 Ga granitoids of the Karelia Province, likely representing the same magmatic event. The data from the Kontioselka pegmatite define a maximum age for the pegmatite and a minimum age for mafic volcanic rocks in Kontioselka at 1781 +/- 27 Ma. The age coincides with major deformation and metamorphism in the Karelia Province around 1.76 Ga. Zircon from the Janesselka mafic-ultramafic complex yields a minimum age of 2425 +/- 17 Ma, complying rather well with the 2.5-2.4 Ga mafic layered intrusions of the northern Fennoscandian Shield. Our new data show that the Archean of the northeastern part of the Western Karelia Subprovince was affected by Proterozoic thermal events more extensively than thus far perceived.
  • Chopin, Francis; Corsini, Michel; Schulmann, Karel; El Houicha, Mohamed; Ghienne, Jean-Francois; Edel, Jean-Bernard (2014)
    Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310–295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.