Ion microprobe UPb zircon geochronology and isotopic evidence for a trans-crustal suture in the Lapland–Kola Orogen, northern Fennoscandian Shield

The ~1.8 Ga Nattanen-type granites in the Finnish Lapland and western Kola Peninsula are found as several relatively small, high-level, discordant plutons that are easily discernible as weak maxima on aeromagnetic maps. We present U-Pb mineral isotope data on the Finnish plutons. The concordia ages are in the 1.79−1.77 Ga range and there is little evidence for inheritance. Initial radiogenic isotope compositions (our common-Pb data combined with previously published whole-rock Nd and Hf data) imply a major, yet varying, Archean source component. Elemental geochemical data on five Finnish intrusions (the Nattanen stock, the Tepasto and Pomovaara complexes, the Riestovaara and Vainospää batholiths), as well as, associated dyke rocks allow the Nattanen-type granites to be classified as oxidized A-type granites. Their petrogenesis may be related to partial melting of the lower crust by mafic underplating (extensional setting) or as a result of thermal relaxation in thickened crustal setting.

Section: Geologic Settingmentioning

confidence: 99%

The Paleoproterozoic Nattanen-type granites in northern Finland and vicinity - a postcollisional oxidized A-type suite

Heilimo¹,

Halla²,

Lauri³

et al. 2009

“…Its petrology has been a subject of many studies during the last 50 years (Eskola, 1952;Meriläinen, 1976;Hörman et al, 1980;Raith & Raase, 1986;Barbey & Raith, 1990;Korja et al, 1996;Leibinger, 1996;Korja et al, 1996;Leibinger, 1996;Leibinger, 1996;Perchuck et al, 2000) and has been recently revised ) and has been recently revised by Tuisku et al (2006) who demonstrated a regional scale, clockwise PT evolution. Isotopic dating has confirmed that the LGB is Palaeoproterozoic in age (Meriläinen, 1976;Bernard-Griffiths et al, 1984;Huhma & Meriläinen, 1991;Bibikova et al, 1993;Sorjonen-Ward et al, 1994;Tuisku & Huhma, 1998Daly et al, 2001) and geochemical data suggest that the igneous rocks of the belt were formed in arc setting (Barbey et al, 1986;Bibikova et al, 1993). Tectonically the LGB is thought to represent a crustal scale, SW-directed thrust sequence (Marker, 1988;Korja et al, 1996).…”

Section: Introductionmentioning

confidence: 96%

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part II: Isotopic dating

Tuisku¹,

Huhma²

2006

The migmatitic metapelites of the Lapland granulite belt (LGB) in the NE part of the Fennoscandian Shield represent an arc-related greywacke basin metamorphosed in the granulite facies. Detrital zircons from migmatitic metapelites are derived from 1.94 etrital zircons from migmatitic metapelites are derived from 1.94 zircons from migmatitic metapelites are derived from 1.94 derived from 1.94 − 2.9 Ga old acid source rocks (U-Pb SIMS ages). The clustering of detrital zircon ages between 1.97 and 2.2 Ga is problematic, because abundant felsic crust of this age is absent in the shield. The metasediments are characterized by Sm-Nd model ages of ca. 2.3 Ga. A younger, 1905 − 1880 Ma population of homogeneous zircons was formed during regional metamorphism. The peak high-grade metamorphism took place at ~1900 Ma and the latest chronological record from subsequent decompression and cooling phase is from ca. 1870 Ma. The norite-enderbite series of the LGB represents arc-magmas, which were intruded into the metasediments at ~1920 − 1905 Ma ago according to zircon U-Pb ages and were probably an important heat source for metamorphism. Older, zoned zircon grains in a quartz norite vein, initial ε Nd values of 0 to +1 and the continuous spectrum of LILE enrichment in the enderbite-series probably reflect assimilation of metasediments by magmas. Monazite U-Pb ages of migmatitic metasediments in the range 1906 − 1910 ± 3 Ma overlap the late stage of enderbite intrusion and growth of early metamorphic zircons. Garnetwhole rock Sm-Nd ages from leucosomes in the range 1880 − 1886 ± 7 Ma are concurrent with the growth of the youngest metamorphic zircons and probably indicate the crystallization of leucosomes or the influence of a fluid liberated from them. Isotopic and petrologic data reveal that the evolution of Lapland Granulite belt from the erosion of source rocks to the generation of a sedimentary basin, its burial, metamorphism and exhumation took place within ca. 60 Ma.

“…According to the available evidence, most of the anorthositic, enderbitic and metapelitic rocks have a Palaeoproterozoic age range from about 2.0 to 1.9 Ga (Meriläinen, 1976;Bernard-Griffi ths et al, 1984;Huhma & Meriläinen, 1991;Sorjonen-Ward et al, 1994;Daly et al, 2001).…”

Section: Geological Settingmentioning

confidence: 99%

“…Also, the LGB was earlier thought to form part of the Archaean basement complex of the shield (i.e. Simonen, 1980) but later structural (Marker, 1988;Korja et al, 1996) and geochronological data showed that it is in most part a Palaeoproterozoic high-grade mobile belt surrounded by Archaean complexes (Meriläinen, 1976;Bernard-Griffi ths et al, 1984;Huhma & Meriläinen, 1991;Sorjonen-Ward et al, 1994;Tuisku & Huhma, 1998Daly et al, 2001).…”

Section: Introductionmentioning

confidence: 99%

Evolution of migmatitic granulite complexes: Implications from Lapland Granulite Belt, Part I: Metamorphic geology

Tuisku¹,

Mikkola²,

Huhma³

2006

The Palaeoproterozoic Lapland granulite belt was juxtaposed between Archaean and Proterozoic terrains in the NE part of the Fennoscandian Shield concurrently with the accretion of Svecofennian arc complexes at ~1.9 Ga. The belt consists mainly of aluminous migmatitic metagreywackes. Abundant noritic to enderbitic magmas were intruded concordantly into the metasediments and were probably an important heat source for metamorphism, which took place during the crystallization of the magmas. This is supported by structural and contact relations of metasediments and igneous rocks, and by the lack progressive metamorphic reaction textures in the igneous rock series. The peak of metamorphism took place above the dehydration melting temperature of the biotite-sillimanite-plagioclase-quartz assemblage at 750−850°C and 5−8.5 kbar which lead to formation of a restitic palaeosome and peraluminous granitic melt in metapelites. Subsequently, the rocks were decompressed and cooled below the wet melting temperature of pelitic rocks (650°C) under the stability fi eld of andalusite coexisting with potassium feldspar (2−3 kbar). Cooling was accompanied by the crystallization of the neosomes, often carrying aluminium-rich phases. Postmetamorphic duplexing of the LGB is clearly seen in the distribution of calculated PT conditions.