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U-Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4-3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite-trondhjemite-granodiorite (TTG) samples yielded homogeneous 2.72-2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7-2.8 Ga had 2.9-3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9-3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low-HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72-2.98 Ga. [ABSTRACT FROM AUTHOR]

We have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling. U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of 207Pb/206Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum. In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of 207Pb/206Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of 207Pb/206Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes. [ABSTRACT FROM AUTHOR]

The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP–HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750–800 °C and 5–6 kbar, followed by near-isothermal decompression down to 3–4 kbar. The analysed monazites yielded 207Pb/206Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the ≥1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89–1.86 Ga fits well with the high-grade metamorphic event at 1.89–1.88 Ga observed elsewhere in southern and central Finland. The 1.87–1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83–1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80–1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones. • U-Pb monazite age data from southeastern Finland show ages of 1953 to 1737 Ma. • Age peaks at 1.89–1.86 Ga and 1.83–1.82 Ga record separate metamorphic events. • Major maximum at 1.80–1.77 Ga is related to the latest stage of exhumation. [ABSTRACT FROM AUTHOR]

U–Pb age data from Archean lower crustal granulites in the Varpaisja¨rvi area, central Finland, indicate several metamorphic events that took place in Mesoarchean–Neoarchean and Paleoproterozoic time. Zircons separated from the granulites fall into two distinct age groups. In the older group, the protolith age is 3.2 Ga and many zircons indicate metamorphism at ca. 3.1 Ga. In contrast, the oldest zircons in the younger group are ca. 2.73 Ga. Both terranes were migmatized at ca. 2.70 Ga with the emplacement and crystallization of igneous enderbites. The enderbites and the leucosomes of migmatites in both granulite terranes have 2.7 Ga zircons. High grade metamorphic conditions prevailed from 2.70 to 2.63 Ga which is the age range of most zircons in the eastern terrane. Also the enderbites show metamorphic zircon growth at ca. 2.64 Ga. Two distinct age groups are interpreted to be the result of juxtaposition of two terranes of different ages. This indicates that collisional and accretionary processes operated during Neoarchean time. The area east from the eastern granulite terrane underwent almost pervasive deformation and amphibolite facies metamorphism during the Svecofennian orogeny at ca. 1.9 Ga. However, the U–Pb ages of zircons in the retrograde area are similar to those of the eastern terrane, thus indicating that these rocks were metamorphosed under granulite facies conditions (2.70–2.63 Ga). Although the ca. 1.9 Ga Svecofennian metamorphism is not evident in the ion microprobe results, some Paleoproterozoic Pb-loss is indicated by TIMS U–Pb analyses of zircons. In addition, monazite age of 1.89 Ga from the retrograde area reflects Svecofennian metamorphism. [Copyright &y& Elsevier]

The Tuntsa Suite is a polymetamorphic Archean complex mainly consisting of metasedimentary gneisses. At least two strong metamorphic events can be distinguished in the area. The first took place at high temperatures in the Neoarchean at around 2.70–2.64 Ga, indicated by migmatisation and U-Pb ages of metamorphic zircon. During the Paleoproterozoic, metasedimentary gneisses were penetratively deformed and recrystallized under medium pressures producing staurolite, kyanite and garnet-bearing mineral assemblages. The suggested Paleoproterozoic PT path was clockwise where the temperature and pressure first increased to 540–550 °C and 6 kbar, crystallizing high Ca/low Mg garnet cores. The mineral compositions show that commonly garnet core was not in chemical equilibrium with staurolite but crystallized earlier, although garnet-staurolite-kyanite assemblages are common. The temperature and pressure increased to c. 650 °C and 8 kbars where staurolite and kyanite coexist. This was followed by decompression down to c. 550–600 °C and 3–4 kbars, shown by andalusite crystallization and cordierite formed in the breakdown of staurolite and biotite + kyanite. The observed garnet zoning where Mg increases and Ca decreases from the core to the rim was developed with both increasing and decreasing pressure, depending on the effective bulk composition. The U-Pb and Sm-Nd age determinations for monazite and garnet show that the Paleoproterozoic metamorphic cycle took place at 1.84–1.79 Ga, related with thrusting of the Lapland granulites onto the adjacent terranes and subsequent exhumation. [ABSTRACT FROM AUTHOR]

An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by combining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijärvi metamorphic region of southern Finland reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns characterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijärvi area formed by bulk north-south crustal shortening and associated vertical stretching. [ABSTRACT FROM AUTHOR]

Zircons from six samples collected from igneous and metamorphic rocks were dated using the NORDSIM ion microprobe, in order to investigate the tectonic evolution of the Palaeoproterozoic Svecofennian Orogen in southwestern Finland. These rocks represent pre-collisional, collisional and post-collisional stages of the orogeny. The ion microprobe results reveal two age groups of granodioritic–tonalitic rocks. The intrusions have different tectonic settings: the Orijärvi granodiorite represents pre-collisional 1.91–1.88 Ga island-arc-related magmatism and yielded an age of 1898±9 Ma, whereas the collision-related Masku tonalite was dated at 1854±18 Ma. The latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to ≈1.87 Ga. This is interpreted to reflect the collision between the Southern Svecofennian Arc Complex with the Central Svecofennian Arc complex and the formation of a suture zone between them during D2 deformation. Granulite facies metamorphism in the Turku area was dated at 1824±5 Ma using zircons from leucosome in the Lemu metapelite. This age constrains D3 folding related to post-collisional crustal shortening in this area. Crustal melting continued until ≈1.81 Ga, as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons. New metamorphic zircon growth took place in older granitoids at granulite facies, but not at amphibolite facies. Detrital zircons with ages between 2.91 and 1.97 Ga were found in the mesosome of the Lemu metapelite and 2.64–1.93 Ga inherited cores were found in the 1.87 Ga Masku tonalite. [Copyright &y& Elsevier]

Abstract The Paleoproterozoic Central Lapland Granitoid Complex (CLGC) forms the northern core of the Fennoscandian Shield. We present here new U-Pb age data and review the existing ones from plutonic and metasedimentary rocks of the CLGC, and integrate the results with the structural field observations to decipher the tectonic history of this region. Granitoids vary in age from 2.43 Ga to 1.78 Ga, whereas metamorphic zircon overgrowths, both in granites and metasedimentary rocks, vary between 1.90 Ga and 1.76 Ga with several age peaks indicating prominent metamorphic pulses. The almost continuous growth of metamorphic zircon domains favors that the central part of the CLGC was near solidus for 100–120 Ma. A succession of five deformations and associated foliation-producing events, which were near-orthogonal to each other, have been identified. East-vergent thrusting and stacking (D1) commenced at ca. 1.92 Ga, and led to the onset of Barrovian-type metamorphism followed by a radical tectonic shift that yielded E-W trending D2 structures. Exhumation at 1.88–1.87 Ga caused decompression melting of thermally evolved thickened crust and led to the emplacement of large volumes of granites and triggering of Buchan-type metamorphic conditions. NE-SW collision (D3) at the southwestern margin of Fennoscandia heterogeneously overprinted the S3 fabric on D2 structures. Orogen parallel (NNW-SSE) buckling at 1.87–1.85 Ga formed oroclines in the hot linear arc segments. Renewed NNW-SSE shortening (D4) at 1.84–1.81 Ga produced long wavelength folds and caused the formation of domal structure in the CLGC. The final 1.78–1.76 Ga far-field collision (D5) in the west overprinted all the pre-existing structures. This complex multi- directional tectonic scenario provoked the intermittent exhumation phases between different contraction events of the CLGC. Highlights • Central part of the Lapland Granitoid Complex (CLGC) was near solidus for 100–120 Ma. • Five major deformation events (D1-D5) occurred between 1.92 Ga and 1.76 Ga. • Barrovian- to Buchan-type metamorphism at 1.88–1.87 Ga. • Migmatitic core was transformed to long wavelength folds during D4. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

New field mapping, lithostratigraphic, whole-rock geochemical, U–Pb zircon and radiogenic isotope data on the Nampula Subdomain of southernmost Malawi show good correlation with those published from adjacent NE and NW Mozambique. The oldest rocks, dated between ∼1180 and 1070 Ma form a complexly interlayered sequence of supracrustal layered paragneisses and migmatites (Nampula Group), intimately interlayered with intermediate-basic (Mocuba Suite) and felsic (Mamala Suite) gneisses, likely meta-volcanic and meta-plutonic in origin. The Mocuba Suite has typical calc-alkaline, I-Type granitoid, TTG compositions, with generally positive ϵNd values, NdT dm ages around 1.2 Ga and low initial 87Sr/86Sr ratios. These characteristics indicate generation in a juvenile volcanic arc setting at this time. Minor exceptions include a few samples which show limited input of Palaeoproterozoic crust. The Nampula arc is viewed as one of a number that developed in a closing oceanic basin which lay between the Archaean-Palaeoproterozoic Congo-Bangweulu-Ubendian-Tanzania cratonic blocks in the north and the Zimbabwe Craton to the SW, and which can be traced "southwards" in Gondwana to continue in the NE part of the ∼1 Ga Maud belt in East Antarctica. The Nampula arc-complex was intruded at ∼1040 by syn-to late tectonic sheets of megacrystic granite/augen gneisses of the Culicui Suite and, at ∼1000 Ma, by a number of bodies of late-to post-tectonic, massif-type anorthosite (Tengani Suite), which are unique to the Malawian segment of the Nampula Subdomain. The anorthosites are considered to have intruded at a post-tectonic stage under a late Mesoproterozoic extensional regime. The rocks underwent deformation and high grade metamorphism in both the Mesoproterozoic (Irumide) and Neoproterozoic-Cambrian (Pan-African) orogenies. P-T data suggest the latter attained peak granulite facies conditions of ∼800 °C at a maximum of 13–18 kbar, comparable with Neoproterozoic granulites in adjacent areas. The data presented here do not rule out either Irumide (∼1 Ga) juxtaposition of the Congo and Kalahari cratons (in Rodinia), or Pan-African (∼0.5 Ga) collision (Gondwana). Instead of this "either-or" situation, a recent hypothesis suggests accretion/collision occurred during both orogenies, separated by a period of drift apart, by the "strange attractor" phenomenon. Our data do not rule exclude that contention; rather, they tend to support it. • Nampula Subdomain in Malawi equated with neighbouring Mozambique • Zircon dates from 8 Mesoproterozoic orthogneisses range from ∼1180 to 1000 Ma • Rocks have mostly "juvenile" Sm–Nd isotope signatures • WR geochemical suggest arc-like signatures for most igneous rocks • Neoproterozoic maximum metamorphic P-T conditions estimated around 800 °C at up to 13–18 kbar [ABSTRACT FROM AUTHOR]

The Svecofennian domain comprises a c. 800 km by 800 km area in Finland and Sweden, mainly composed of Palaeoproterozoic granitoids, schists and gneisses. In the central part of the domain in western Finland, the Vaasa complex hosts a sequence of high-grade metamorphic rocks and granites formed by crustal anatexis during the culmination of the ∼1.9 Ga Svecofennian orogeny. The Vaasa complex consists of a c. 8000 km 2 granitoid batholith surrounded by diatexitic and metatexitic migmatites as well as metasedimentary rocks. In order to enhance the perception of the crystallisation history, provenance, and maximum sedimentation age of the protoliths involved, nine granitoid-xenolith pairs and a palaeosome-neosome pair were collected from the main body of the Vaasa batholith and surrounding rocks, and zircon from them was U–Pb dated using the LA-MC-ICPMS method. The U–Pb zircon crystallisation ages of the granitoids are 1.88–1.87 Ga, which is in accordance with the few previous results from the Vaasa region, and confirms a relatively swift crystallisation of the granite core throughout the Finnish part of the complex. The samples from the central part of the batholith have younger zircon ages than those from batholith margin. The inherited zircon age distribution pattern of the granitoids resembles that of the metasedimentary xenoliths. Our new comprehensive data, combined with whole-rock geochemistry from previous studies, render an in situ origin of the Vaasa granite probable. The maximum sedimentation age of the supracrustal precursors of the Vaasa complex is 1.92 Ga. Some of the metasedimentary xenoliths from the Vaasa complex host zircon that is coeval with their host granitoids and register incipient melting. Based on these ∼1.88–1.86 Ga zircon grains, the maximum sedimentation age would be deemed too young. Therefore, our data calls for caution when determining maximum sedimentation ages of high-grade metasedimentary rocks. [ABSTRACT FROM AUTHOR]

Abstract: This paper discusses early Neoarchean (2.8–2.7Ga) plate tectonics by integrating knowledge from new geochemical observations and numerical models. Based on a geochemical dataset of 295 granitoid samples from the Karelian and Kola cratons of the Fennoscandian Shield, we divide Neoarchean juvenile (extracted from oceanic crust or mantle) granitoids into three groups: (1) low-HREE (heavy rare earth elements) TTGs (tonalite–trondhjemite–granodiorite) (high SiO2, low Mg, low-HREE, higher Sr, lower Yb n and higher Nb/Ta), (2) high-HREE TTGs (slightly lower range of SiO2, larger range of MgO contents and higher Cr and Ni contents, high-HREE, lower Sr, higher Yb n and lower Nb/Ta), and (3) high Ba–Sr sanukitoids (medium-HREE, high-Mg and high K–Ba–Sr). The main difference between the low- and high-HREE groups lies in their pressure-sensitive element contents, which indicates high-pressure melting conditions for the low-HREE group and low-pressure conditions for the high-HREE group. A possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau/protocrust. Melting in the lower part of thick basaltic oceanic crust could produce TTGs of the low-HREE type, whereas low-pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high-HREE TTGs. The third group of Archean high Ba–Sr sanukitoids was formed after the TTGs. Their low SiO2 and high Mg–K–Ba–Sr contents suggest an origin by melting in an enriched (metasomatized) mantle source, probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau/TTG protocontinent. Such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the Archean, which locally increased temperatures within the mantle wedge. More frequent breakoff resulted, because subducting plates were weaker (due to rheologically thinner lithosphere and a thicker basaltic crust), and tensile stresses within the subducting plate were larger (due to a thick crust that transforms from buoyant basalt to dense eclogite). [Copyright &y& Elsevier]