SIMS U-Pb, Sm-Nd isotope and geochemical study of an arkosite-amphibolite suite, Peräpohja Schist Belt: evidence for ca. 1.98 Ga A-type felsic magmatism in northern Finland

In the northern and north-eastern part of the Peräpohja Schist Belt, northern Finland, an extensive supracrustal rock unit has been identified which is composed of alternating amphibolitic and arkositic components. The amphibolites form layers whose thickness varies from one millimeter to some tens of meters, being most often a few tens of centimeters. They represent mafic tuff beds deposited concurrently with more abundant arkositic rocks. Most of the arkosites have a modal and major and trace element compositionsimilar to that of A2-type granites. For example, they exhibit high LREE/HREE, negative Eu anomalies, and flat HREE and are moderately enriched in Nb, Zr, and Y. The genesis of the arkosites is enigmatic as they show features supporting either a volcaniclastic or an epiclastic origin. In the latter case, they were derived via erosion of a source dominated by A2-type granitic rocks. Previous conventional ID-TIMS and new SIMS U-Pb dating of zircons from two arkosite samples and one mica schist sample, all three picked from the northern part of the schist belt, indicate that these rocks contain a single population of zircons with an age of ca. 1975 Ma suggesting that they are among the youngest supracrustal rocks in the schist belt. In contrast, one mica schist sample from the western part of the belt revealed only the presence of Archean zircons. The samples do not differ markedly in terms of their Nd isotopecomposition as they all have a moderately negative εNd(1900 Ma). Regardless of the genesis of the arkosites, their isotopic and geochemical data suggest a previously unknown occurrence of extensive A-type felsic magmatism at ca. 1.98 Ga, contemporaneously withsome continental flood basalts. However, concrete evidence for this felsic A-type magmatism in the form of ca. 1.98 Ga felsic plutonic rocks is virtually absent in the presently exposed Fennoscandian Shield

Intracratonic Palaeoproterozoic granitoids in northern Finland: prolonged and episodic crustal melting events revealed by Nd isotopes and U-Pb ages on zircon

Zircons from twelve Palaeoproterozoic granitoids from the Central Lapland Granitoid Complex (CLGC) and from the Hetta Complex (HC) in northern Finland were dated using the NORDSIM ion microprobe. In addition, U-Pb age determinations on zircons from two samples were made using TIMS. Results reveal a wide range of U-Pb ages from 2.13Ga to 1.76 Ga. The oldest samples include the porphyritic, weakly deformed Nilipää granite, which has an ion probe concordia age of 2126±5 Ma, compatible with the previously published TIMS age of 2136±5 Ma. The Tohmo granodiorite provided a slightly younger ion microprobe age of 2105±4 Ma. The age of these two granitoids overlaps the depositional age of Karelian cratonic metasediments, suggesting that they represent an unusual tectonic setting for granitoid magmatism in the Fennoscandian Shield. The Ruoppapalo granodiorite, which is also porphyritic and weakly deformed, yielded an ion probe zircon concordia age of 1905±5 Ma, in accord with the conventional TIMS result. The strongly deformed Molkoköngäs granite has an ion probe concordia age of 1855±13 Ma, which is within error limits the same as the TIMS age, 1843±23 Ma. The Pernu monzogranite has a concordia age of 1813±6 Ma. All other granitoids in the CLGC, including those of appinitic affinity, as well as a leucosome from the Kappera migmatite in the Hetta Complex, are dated at 1.79 – 1.76 Ga, although many granitoids have older inherited zircons. The abundance of deformed granitoids of this age show that intensive ductile deformation, metamorphism and melting occurred at around 1.79 – 1.76 Ga in the northern Fennoscandian Shield, coeval with post-collisional magmatism in the southern part of the Shield. Most granitoids in the Central Lapland Granitoid Complex have strongly negative εNd values (1.8 Ga), ranging from -8 to -5. It is evident that they have a major Archaean component in their source, indicating derivation from Archaean crust during crustal reworking in an intracratonic tectonic setting

A1-tyypin graniittien ja niihin liittyvien intermediääristen kivien geokemiallinen ja termodynaaminen mallinnus: esimerkki Fennoskandian kilven keskiosista

The origin of ferroan A-type granites in anorogenic tectonic settings remains a long-standing petrological puzzle. The proposed models range from extreme fractional crystallization of mantle-derived magmas to partial melting of crustal rocks, or involve combination of both. In this study, we apply whole-rock chemical and Sm-Nd isotopic compositions and thermodynamically constrained modeling (Magma Chamber Simulator, MCS) to decipher the genesis of a suite of A1-type peralkaline to peraluminous granites and associated intermediate rocks (monzodiorite-monzonite, syenite) from the southwestern margin of the Archean Karelia craton, central Finland, Fennoscandian Shield. These plutonic rocks were emplaced at ca. 2.05 Ga during an early stage of the break-up of the Karelia craton along its western margin and show trace element affinities to ocean island basalt-type magmas. The intermediate rocks show positive epsilon Nd(2050 Ma) values (+1.3 to +2.6), which are only slightly lower than the estimated contemporaneous depleted mantle value (+3.4), but much higher than average epsilon Nd(2050 Ma) of Archean TTGs (-10) in the surrounding bedrock, indicating that these rocks were essentially derived from a mantle source. The epsilon Nd(2050 Ma) values of the peralkaline and peraluminous granite samples overlap (-0.9 to +0.6 and -3.2 to +0.9, respectively) and are somewhat lower than those in the intermediate rocks, suggesting that the mafic magmas parental to granite must have assimilated some amount of older Archean continental crust during their fractionation, which is consistent with the continental crust-like trace element signatures of the granite members. The MCS modeling indicates that fractional crystallization of mantle-derived magmas can explain the major element characteristics of the intermediate rocks. The generation of the granites requires further fractional crystallization of these magmas coupled with assimilation of Archean crust. These processes took place in the middle to upper crust (-2-4 kbar, -7-15 km) and involved crystallization of large amounts of clinopyroxene, plagioclase and olivine. Our results highlight the importance of coupled FC-AFC processes in the petrogenesis of A-type magmas and support the general perception that magmas of A-type ferroan granites become more peraluminous by assimilation of crust. They further suggest that variable fractionation paths of the magmas upon the onset of assimilation may explain the broad variety of A-type felsic and intermediate igneous rocks that is often observed emplaced closely in time and space within the same igneous complex.Peer reviewe

Petrography, geochemistry, and geochronology of the Sc-enriched Kiviniemi ferrodiorite intrusion, eastern Finland

The Kiviniemi mafic intrusion, near the eastern margin of the Paleoproterozoic Central Finland Granitoid Complex, is both spatially and temporally associated with post-kinematic Fe-Ti-P-enriched Svecofennian orogenic mafic magmatism. The main rock types in this small (~ 15 ha) intrusion are garnet-bearing fayalite ferrodiorite, leucoferrodiorite, ferromonzodiorite, and pyroxene diorite. The garnet-bearing fayalite ferrodiorite and leucoferrodiorite contain 50-281 ppm Sc, 275-5600 ppm Zr, and 58-189 ppm Y (n = 42), delineating a mineralized deposit some 2.5 ha in extent. Overall, these rocks show an evolved (iron-enriched) tholeiitic character; low values of Ni (<20-40 ppm), Cr (<20 ppm), and Cu (<20-80 ppm); and high contents of Zn (213-700 ppm). The rock-forming minerals in the ferrodioritic rocks are (ferro)hedenbergite, plagioclase (~ An(40)), ferropargasite and ferroedenite, almandine garnet, and fayalite (Fo(1-4)). Accessory minerals include zircon, ilmenite, fluorapatite, biotite, pyrite, pyrrhotite, potassium feldspar, grunerite, and clinoferrosilite. Some relict cumulate textures have been preserved, but primary magmatic features have largely been overprinted by strong recrystallization and corona formation. The main carriers of Sc are amphibole, clinopyroxene, and apatite. The remarkably strong enrichment of Sc in ferromagnesian silicates and apatite, rather than in specific Sc-minerals, implies magmatic enrichment. Post-kinematic mafic intrusions in central Finland constitute a bimodal association with co-existing granitoid counterparts. The Kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts, indicative of contemporaneity of the two magmas. This conclusion is in accord with the coincident U-Pb zircon ages for the ferrodiorite, at 1857 +/- 2 Ma (multigrain ID-TIMS) and the megacrystic granite, at 1860 +/- 7 Ma (single-crystal LA-MC-ICP-MS). The initial epsilon Nd value of the ferrodiorite and the granite are + 0.1 and - 2.5, respectively. These Nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some Archaean crustal material into the granite in the course of magmatic evolution. The resource estimation calculated for Kiviniemi intrusion by using 40 g/t Sc cut off value is 13.4 Mt of rock with an average grade of 162.7 g/t scandium, 1726 g/t zirconium, and 81 g/t yttrium.Peer reviewe

Paleoproterotsooisten arclogiittien jäljillä - Muutos 1.88 Ga kalkkialkalisesta magmatismista 1.86 Ga korkean Nb:n ja adakiitti-tyyppiseen magmatismiin Fennoskandian kilven keskiosissa

Arclogites, i.e., lower crustal gamet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modem continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90-1.87 Ga subduction-related talc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F. LILE, and HFSE (especially Nb: 18.9-44 ppm), show positive initial epsilon(Nd) value, and near-chondritic but variable initial zircon epsilon(Hf) values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive epsilon(Hf), and positive but variable zircon epsilon(Hf) values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive CNd value, and chondritic to negative zircon epsilon(Nd) values. The three groups yield zircon U-Pb ages of similar to 1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at similar to 1.88 Ga. The ages and compositions are dearly different from the adjacent 1.90-1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90-1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga. (C) 2020 Elsevier B.V. All rights reserved.Peer reviewe

The age and origin of the Vaasa migmatite complex revisited

The origin of the Vaasa migmatite complex was studied by using whole-rock Sm-Nd and zircon Lu-Hf and U-Pb data in conjunction with whole-rock major and trace element geochemistry. The concordia ages of five Vaasa area granitoid samples are 1.88-1.86 Ga, constraining the age of peak migmatization. The ages of inherited zircon cores in the samples show two clear age populations at 2.02-1.92 Ga and c. 2.7 Ga, which correspond to ages yielded by a mica schist sample from the adjacent Evijarvi belt, as well as with published values for the Evijarvi belt zircon. The initial epsilon(Nd) values of the Vaasa complex samples are relatively unradiogenic (from -3.0 to -2.0). Such values are comparable to a value (-3.6) calculated for the Evijarvi mica schist, as well as to literature values (from 3.0 to -0.5) for the Evijarvi belt. The average initial zircon epsilon(Hf) values of four of the granitoids range from -10 to -5 and are in agreement with the Nd-isotopic results, whereas the northmost sample has a significantly higher value (+1). The deviation is suggested to result from disequilibrium melting of zircon. The isotopic and geochronological data indicate that the Vaasa complex granitoids formed by partial melting of the Evijarvi belt metasedimentary rocks.Peer reviewe

Tracing arclogites in the Paleoproterozoic Era - A shift from 1.88 Ga calc-alkaline to 1.86 Ga high-Nb and adakite-like magmatism in central Fennoscandian Shield

Arclogites, i.e., lower crustal gamet-pyroxenite cumulates, are suggested to play an important role in controlling magma differentiation in modem continental arcs. Until now, arclogite-related magmatism has only been described from the Phanerozoic Era. The Svecofennian orogen in the central Fennoscandian Shield hosts a rare association of 1.86 Ga igneous rocks geochemically distinct from the surrounding and much more abundant 1.90-1.87 Ga subduction-related talc-alkaline magmatism. The 1.86 Ga magmatic rocks are divided into three groups: 1) high-Nb gabbros (HNB) which are enriched in Fe2O3T, TiO2, P2O5, F. LILE, and HFSE (especially Nb: 18.9-44 ppm), show positive initial εNd value, and near-chondritic but variable initial zircon εHf values; 2) high-Mg gabbros (HMG) which are characterised by high MgO, CaO, Cr and Ni contents, slight enrichment in LILE, positive εHf, and positive but variable zircon εHf values; 3) adakite-like rocks showing high Al2O3 and Na2O contents, slight enrichment in LILE, relative depletion in some HFSE, positive CNd value, and chondritic to negative zircon ε​​​​​​​Nd values. The three groups yield zircon U-Pb ages of similar to 1.86 Ga and exhibit undeformed textures in contrast to the surrounding supracrustal rocks metamorphosed at similar to 1.88 Ga. The ages and compositions are dearly different from the adjacent 1.90-1.87 Ga arc-related igneous rocks suggesting a distinct origin. Despite similar ages and close spatial relationship, separate sources are required for each of the different 1.86 Ga rock groups. Trace element modelling of partial melting suggests that arclogites, with compositions similar to pyroxenite xenoliths found in the kimberlite pipes of eastern Finland, are the source for the HNB rocks. In contrast, subduction-modified mantle peridotite is the source for the HMG rocks, and a mafic lower crustal source is suggested for the adakite-like rocks. The following geodynamic model is suggested: (rutile-bearing) arclogite formation at 1.90-1.87 Ga followed by arclogite delamination and partial melting during extension of the thickened Svecofennian crust at 1.86 Ga. </p

Magmatic ore deposits in mafic–ultramafic intrusions of the Giles Event, Western Australia

More than 20 layered intrusions were emplaced at c. 1075 Ma across > 100 000 km2 in the Mesoproterozoic Musgrave Province of central Australia as part of the c. 1090–1040 Ma Giles Event of the Warakurna Large Igneous Province (LIP). Some of the intrusions, including Wingellina Hills, Pirntirri Mulari, The Wart, Ewarara, Kalka, Claude Hills, and Gosse Pile contain thick ultramafic segments comprising wehrlite, harzburgite, and websterite. Other intrusions, notably Hinckley Range, Michael Hills, and Murray Range, are essentially of olivine-gabbronoritic composition. Intrusions with substantial troctolitic portions comprise Morgan Range and Cavenagh Range, as well as the Bell Rock, Blackstone, and Jameson–Finlayson ranges which are tectonically dismembered blocks of an originally single intrusion, here named Mantamaru, with a strike length of > 170 km and a width of > 20 km, constituting one of the world's largest layered intrusions. Over a time span of N200 my, the Musgrave Province was affected by near continuous high-temperature reworking under a primarily extensional regime. This began with the 1220–1150 Ma intracratonic Musgrave Orogeny, characterized by ponding of basalt at the base of the lithosphere, melting of lower crust, voluminous granite magmatism, and widespread and near-continuous, mid-crustal ultra-high-temperature (UHT) metamorphism. Direct ascent of basic magmas into the upper crust was inhibited by the ductile nature of the lower crust and the development of substantial crystal-rich magma storage chambers. In the period between c. 1150 and 1090 Ma magmatism ceased, possibly because the lower crust had become too refractory, but mid-crustal reworking was continuously recorded in the crystallization of zircon in anatectic melts. Renewed magmatism in the form of the Giles Event of the Warakurna LIP began at around 1090 Ma and was characterized by voluminous basic and felsic volcanic and intrusive rocks grouped into the Warakurna Supersuite. Of particular interest in the context of the present study are the Giles layered intrusions which were emplaced into localized extensional zones. Rifting, emplacement of the layered intrusions, and significant uplift all occurred between 1078 and 1075 Ma, but mantle-derived magmatism lasted for N50 m.y., with no time progressive geographical trend, suggesting that magmatism was unrelated to a deep mantle plume, but instead controlled by plate architecture. The Giles layered intrusions and their immediate host rocks are considered to be prospective for (i) platinum-group element (PGE) reefs in the ultramafic–mafic transition zones of the intrusions, and in magnetite layers of their upper portions, (ii) Cu–Ni sulfide deposits hosted within magma feeder conduits of late basaltic pulses, (iii) vanadium in the lowermost magnetite layers of the most fractionated intrusions, (iv) apatite in unexposed magnetite layers towards the evolved top of some layered intrusions, (v) ilmenite as granular disseminated grains within the upper portions of the intrusions, (vi) iron in tectonically thickened magnetite layers or magnetite pipes of the upper portions of intrusions, (vii) gold and copper in the roof rocks and contact aureoles of the large intrusions, and (viii) lateritic nickel in weathered portions of olivine-rich ultramafic intrusions