Timing of Palaeoproterozoic intra-orogenic sedimentation in the central Fennoscandian Shield; evidence from detrital zircon in metasandstone

Self Cite

Detrital zircon studies suggest that the few quartzite occurrences in southern Finland are younger than 1.87 Ga and express sedimentation after 1.89-1.87 Ga accretional deformation and metamorphism in the Svecofennian orogenic belt. Detailed field work in the high-grade metamorphic Pyhäntaka area allowed to distinguish an overturned formation within metagraywackes (cordierite paragneisses) and psammites. The Pyhän-taka formation has a maximum thickness of 1000 meters and consists of quartzite overlain by meta-arkose, metatuff, and metabasalt on top. An uncorformity, expressed by a weathering surface, separates the quartzite from underlying metagraywacke. The metavolcanic rocks within, stratigraphically underlying and overlying the Pyhäntaka formation are mostly basalts and basaltic andesites, but a felsic volcanic rock and dacitic fragments in volcaniclastic rocks imply bimodal affinity. The quartzite was deposited during a stable intra-orogenic period probably after accretion but before 1.83-1.80 Ga collisional deformation and metamorphism in the Svecofennian orogen. Rifting during the intraorogenic period and accumulation of variable material in the rift from nearby sources by fluvial processes is a viable scenario for deposition and preservation of the Pyhäntaka formation. Geochemical diagrams of the metavolcanic rocks show a scatter that is best explained by source heterogeneity and crustal contamination. Despite their (likely) postaccretion setting the basaltic rocks show arc-type characteristics due to subductionmodified lithospheric mantle sources. Because of recycling, also the paragneisses in the Pyhäntaka area are geochemically similar in spite that they represent different tectonic settings. The use of elemental geochemistry alone appears to be insufficient for discriminating tectonic settings of basalts or graywackes in the Svecofennian of southern Finland where accretion and post-accretion settings were largely obliterated by late collision.

Section: Introductionsupporting

confidence: 55%

The Pyhäntaka formation, Southern Finland: A sequence of metasandstones and metavolcanic rocks upon an intra-orogenic unconformity

Nironen¹

2011

Self Cite

“…Detrital zircon age patterns of the xenoliths in the Vaasa batholith (Kotilainen et al, 2016) are similar to the patterns of previously dated detrital zircon from Svecofennian metasedimentary rocks (cf. Huhma et al, 1991;Claesson et al, 1993;Lahtinen et al, 2002Lahtinen et al, , 2009Rutland et al, 2004;Bergman et al, 2008). Previous U-Pb monazite analyses of the Vaasa batholith have yielded ages between 1872 and 1858 Ma (Alviola et al, 2001;Sipilä et al, 2008).…”

Section: Geologic Settingmentioning

confidence: 98%

New monazite U-Pb age constraints on the evolution of the Paleoproterozoic Vaasa granitoid batholith, western Finland

Kotilainen¹,

Mänttäri²,

Kurhila³

et al. 2016

The Vaasa batholith, western Finland, is a large, peraluminous granitoid pluton that crystallized at 1.88-1.87 Ga during the culmination of the Svecofennian orogeny. The batholith has gradual contacts, through metatexites and diatexites, with the enveloping metasedimentary rocks of the Bothnian Belt. We present ID-TIMS U-Pb age data on monazite from granitoids and xenoliths of the Vaasa batholith and combine these with published U-Pb zircon ages in order to shed further light on the evolution of the Vaasa batholith. The apparent monazite ages for seven of the examined samples are 1870-1863 Ma, and 1855±3 Ma for one further sample from the southern part of the batholith. Combined with pre-existing data, the monazite ages of the granitoids are 9 to 18 Ma (face values) or 3 to 9 Ma (external errors considered) younger than the U-Pb zircon crystallization ages from respective samples. Our new data suggest slow cooling for the Vaasa batholith -the closure/saturation temperature of the monazite U-Pb system was probably reached in ~10 m.y. after the crystallization of magmatic zircon in the examined rocks.

“…Magmatism and metamorphism during the Svecofennian orogeny in Southern Svecofennia may be divided into the early Svecofennian (1.90-1.87 Ga) and the late Svecofennian (1.84-1.79 Ga) periods. The early Svecofennian period was followed by an intra-orogenic phase of minor magmatism (Bergman et al, 2008;Väisänen et al, 2012). The late Svecofennian granitic magmatism and associated high-temperature, low-pressure type metamorphism overprinted the early Svecofennian magmatism, metamorphism and deformation in a wide belt called the Late Svecofennian GraniteMigmatite zone (Ehlers et al, 1993) that covers southernmost Finland.…”

Section: Geological Settingmentioning

confidence: 99%

The Salittu Formation in southwestern Finland, part I: Structure, age and stratigraphy

Nironen¹,

Mänttäri²,

Väisänen³

2016

Self Cite

Because of the relatively low metamorphic grade, low strain and well-preserved early structures in volcanic and volcaniclastic rocks, the Orijärvi triangle (in the center of the larger Orijärvi area) is one of the few locations in the Svecofennian orogen of southern Finland where lithostratigraphy has been determined. The geochemistry of the picriticbasaltic metavolcanic rocks of the Salittu Formation, located in the northeastern part of the Orijärvi area, has been characterized but otherwise the bedrock and structures have been barely touched. After remapping we give an interpretation of structural evolution, provide new age data to constrain duration of volcanism at Salittu, and refine the stratigraphy in the Orijärvi area.The original stratigraphy is visible at Salittu: metabasalt overlies migmatitic gneisses, and metapicrite is on top. The rocks were folded during early Svecofennian D 1 -D 2 deformations, and the large synformal structures developed as D 2 -D 5 interference structures formed during late Svecofennian D 5 deformation. The structural pattern at Salittu is much the same as in the Orijärvi triangle.The new age data, combined with earlier published data, constrains the Salittu volcanism at ca. 1875 Ma. The stratigraphy in the Orijärvi area consists of the early (1.90-1.89 Ga) volcanic Orijärvi Formation, overlain by the sedimentary Vetio Formation, the volcanic Kisko Formation, the volcanic-sedimentary Ahdisto Formation, the volcanic Toija Formation, and on top the Salittu Formation, all emplaced at 1.88-1.87 Ga. We propose a model in which the Orijärvi Formation represents magmatism at the margin of a microcontinent, and the overlying package represents sedimentation and magmatism above a subduction zone during an initial stage of microcontinental accretion. D 1 deformation occurred in an advanced stage of accretion, after emplacement of the volcanic rocks of the Salittu Formation.