Spinel-bearing symplectites in Palaeoproterozoic ultramafic rocks from two different geological settings in Finland: Thermobarometric and tectonic implications

Sm-Nd data were determined for eight mafic-ultramafic intrusions from the Svecofennian (1.88 Ga) Kotalahti Nickel Belt, Finland. The intrusions represent both mineralized and barren types and are located at varying distances from the Archaean/Proterozoic boundary. The samples for the 23 Sm-Nd isotope analyses were taken mostly from the ultramafic differentiates. Results show a range in initial ε Nd values at 1880 Ma from -2.4 to +2.0. No relaNo relationship can be found between the degree of Ni mineralization and initial ε Nd values, while a correlation with the geological domain and country rocks is evident. The Majasaari and Törmälä intrusions, which have positive ε Nd values, were emplaced within the Svecofennian domain in proximity to 1.92 Ga tonalitic gneisses, which have previously yielded initial ε Nd values of ca. +3. In contrast, the Luusniemi intrusion, which has an ε Nd value of -2.4 is situated close to exposed Archaean crust. Excluding two analyses from the Rytky intruArchaean crust. Excluding two analyses from the Rytky intru-. Excluding two analyses from the Rytky intrusion, all data from the Koirus N, Koirus S, Kotalahti, Rytky and Kylmälahti intrusions, within error limits, fall in the range -0.7 ± 0.3. The results support the concept of contamination by Archaean material in proximity to the currently exposed craton margin. The composition of the proposed parental magma for the intrusions is close to EMORB, with initial ε Nd values near +4.

Section: Majasaarimentioning

confidence: 99%

Sm-Nd data for mafic-ultramafic intrusions in the Svecofennian (1.88 Ga) Kotalahti Nickel Belt, Finland - Implications for crustal contamination at the Archaean/Proterozoic boundary

Makkonen¹,

Huhma²

2007

“…Tectonically the LGB is thought to represent a crustal scale, SW-directed thrust sequence (Marker, 1988;Korja et al, 1996). Tuisku and Makkonen (1999) found eclogite facies rocks in the SW marginal zone of the LGB suggesting that this could represent part of the NE-subducted slab. Hoffman (1989), Bridgwater et al (1990) and Hall et al (1995) have proposed a palaeogeographic correlation of the LGB palaeogeographic correlation of the LGB correlation of the LGB with the Nagssugtoqidian orogen of Greenland and the Nagssugtoqidian orogen of Greenland and ~1.9 Ga orogenic belts of Baffin Island and Labrador in the Canadian Shield.…”

Section: Introductionmentioning

confidence: 99%

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.

“…pressure was at least 7 kbar and the temperature 650−700 °C. From the same zone, Tuisku and Makkonen (1999) found metamorphosed ma- fi c rocks with the assemblage olivine-orthopyroxeneclinopyroxene-pargasite-spinel giving metamorphic conditions of ca. 11−12 kbar and 670 °C.…”

Section: Key Metamorphic Equilibria and Featuresmentioning

confidence: 98%

“…This, together with the intensive transposition during the thrusting of the LGB, suggests that the areas to the SW were included in the subducting slab and formed a foreland area (Tuisku & Makkonen, 1999). Yet the areas to the SW contain Archaean gneiss complexes (3.1−2.6 Ga) and Palaeoproterozoic, greenstonedominated supracrustal sequences (ca.…”

Section: S T R I K E S O F S -S U R F a C E S I N S W M A R G I N A Lmentioning

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.