Pentti Hölttä

Tectonic setting of post-collisional magmatism in the Palaeoproterozoic Svecofennian Orogen, SW Finland

Abstract Five bimodal post-collisional intrusions in southwestern Finland have been investigated. Geochemically, the mafic rocks are shoshonitic monzodiorites, which are highly enriched in Fe, P, Ti, F, LREE and in incompatible trace elements. The felsic rocks are garnet bearing peraluminous, S-type anatectic granites. New data on the mafic and the felsic intrusions yielded the same U–Pb zircon age of 1815 Ma. Therefore, the mafic and felsic intrusions are coeval but not cogenetic. Narrow contact metamorphic aureoles around the mafic intrusions contain garnet–orthopyroxene bearing assemblages, and thermobarometry indicates an intrusion depth of at least 15 km. Hence, there was little or no unroofing after peak regional metamorphism at 4–6 kbar. The geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during Svecofennian subduction. It is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle. Uprising mafic melts increased the already high temperatures at mid-crustal levels and caused granulite facies metamorphism, crustal anatexis and production of granitic melts.

Svecofennian magmatic and metamorphic evolution in southwestern Finland as revealed by U-Pb zircon SIMS geochronology

Abstract 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 Orijarvi 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.

P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland: II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods

Exposed blocks of lower crustal Archaean granulites separated by Palaeoproterozoic faults occur near the western boundary of the Karelian craton in central Finland. The southwesternmost granulite block in the study area, the Jonsa block, differs from other granulites by having younger zircon U–Pb and Sm–Nd TDM model ages. The U–Pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the Jonsa block are ca. 2.63 Ga, which is interpreted as the age of granulite metamorphism. Igneous enderbites, which make up a considerable part of the bedrock in granulite blocks, are ca. 50 Ma older. Outside the Jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3.2–3.1 Ga, which are the protolith ages. Sm–Nd TDM model ages of the mafic granulites are 2.7–2.9 Ga in the Jonsa block but ca. 3.2 Ga in the other granulites. The age differences are interpreted to represent terrane accretion that juxtaposed 3.2 Ga rocks next to younger rocks. All granulites were metamorphosed in similar PT conditions at 2.63 Ga; granulite metamorphism affected the whole area but was not able to reset zircons in older rocks. In all blocks the Sm–Nd garnet-whole rock ages are younger, ranging from 2.48–2.59 Ga. This reflects relatively low closure temperature of the Sm–Nd system in garnet in these rocks. The U–Pb age of a Palaeoproterozoic dolerite which cuts the granulites is 2.3 Ga, close to the K–Ar age of amphibole in a retrogressed fracture.

Petrology and geochemistry of mafic granulite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Abstract The Lahtojoki kimberlite pipe in Kaavi, eastern Finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx−amph−pl±grt±opx±bt. The TWEEQU thermobarometry indicates crystallization at ca. 800–900°C and 0.75–1.25 GPa, which corresponds with crustal depths of ca. 22–38 km. Reaction textures and results of thermobarometry in some xenoliths indicate post-peak decompression with cooling. Chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts, with some K-enrichment. Their REE patterns are slightly LREE-enriched, some xenoliths having small negative Eu anomalies. The U-Pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith, between ca. 2.6 and 1.7 Ga. These ages correspond with main late Archaean and Palaeoproterozoic orogenic events in the Fennoscandian shield. The Sm–Nd garnet–clinopyroxene isochron age from one dated sample is 1.6 Ga, which represents either a cooling or reheating of the lower crust by rapakivi magmatism. Because the petrology, geochemistry and ages of xenoliths do not correlate well with the nearest exposed Archaean mafic granulites, it is evident that the present lower crust, underlying the Archaean rocks, has a considerable Palaeoproterozoic component.

Evolution of the Archaean Karelian Province in the Fennoscandian Shield in the light of U–Pb zircon ages and Sm–Nd and Lu–Hf isotope systematics

Abstract: In situ Lu–Hf (laser ablation microprobe–inductively coupled plasma mass spectrometry (LAM-ICPMS)) and U–Pb (LAM-ICPMS, secondary ionization mass spectrometry (SIMS)) analyses of zircon, and whole-rock Sm–Nd isotope analyses were performed on rocks formed during magmatic events in three Archaean complexes in the Karelian Province of Fennoscandia (Pudasjärvi, Koillismaa and Iisalmi). These complexes have U–Pb ages ranging from 3.5 to 2.6 Ga. In Pudasjärvi, sparse xenocrystic cores give ages of 3.6–3.7 Ga and initial 176Hf/177Hf suggesting influence of a crustal component T ≥ 4.0 Ga (assuming a CHUR-like mantle source). Ages and Nd and Hf isotope patterns indicate magmatic events at 3.6–3.7 Ga (Siurua, Pudasjärvi with ≥4.0 Ga precursor), 3.2 Ga (Iisalmi, Koillismaa), 2.8 Ga (Pudasjärvi) and 2.7 Ga (Pudasjärvi, Iisalmi). In the Meso- and Palaeoarchaean events, there is no evidence of sources equivalent to present-day depleted mantle; such sources were, however, involved in the 2.8–2.7 Ga events. εHf and εNd are strongly correlated. Contrasts between the Archaean complexes indicate that they evolved separately until c. 2.7 Ga. The age and εHf pattern of ≤2.8 Ga rocks in the Karelian Province is compatible with a scenario in which the Karelia, Superior, Yilgarn and Slave cratons were part of a late Archaean supercontinent, but does not constitute proof of the existence of such a supercontinent. Supplementary material: U–Pb and Lu–Hf data are available at http://www.geolsoc.org.uk/SUP18430.

U–Pb dating of zircons and monazites from Archean granulites in Varpaisjärvi, Central Finland:: Evidence for multiple metamorphism and Neoarchean terrane accretion

Abstract U–Pb age data from Archean lower crustal granulites in the Varpaisjarvi 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.

Geochemical characteristics of granulite facies rocks in the Archean Varpaisjärvi area, central Fennoscandian Shield

Abstract Archean lower crustal granulites are exposed as fault-bounded blocks in the Varpaisjarvi area, central Finland. The prevailing rock types in these blocks are igneous enderbites, tonalitic-trondhjemitic-granodioritic migmatites and mafic granulites. Although metamorphosed at moderate pressures (8–10 kbar), the Varpaisjarvi granulites are not, in a geochemical sense, good representatives of depleted restitic lower crust, having invariably negative or negligible Eu anomalies; only the enderbites have K Rb ratios showing depleted granulite trends. The enderbites are orthopyroxene-bearing igneous rocks, ranging in composition from diorite to tonalite. They have distinctive flat LREE patterns with La Sm from 2.03-2.86, which may be due to the removal of a LREE-rich phase during fractionation, if not the result of infiltration by a CO 2 rich fluid. Mafic granulites occur as intercalations within the tonalitic-trondhjemitic-granodioritic gneisses, and while all are tholeiitic in composition, they fall into two groups on the basis of major and trace element abundances. The Group 1 tholeiites are mostly basaltic, with steeply increasing alkali contents over a short range in SiO 2 ; Ti Zr ratios are close to the chondritic value of 110. The Group 2 tholeiites vary from picrobasaltic to andesitic in composition, with the latter being dominant. Ti Zr ratios lie below the chondritic value. The Group 1 rocks have higher Ni and Cr and lower Zr Y ratios than their Group 2 counterparts. The Varpaisjarvi area represents at least two accreted blocks or terranes that exhibit marked lithological and geochemical differences, with the Jonsa block being the most distinctive. The main differences are the two contrasting groups of tholeiitic mafic granulites, the Jonsa block having only the Group 2 metatholeiites, and the presence of quartz-cordierite and cordierite-orthoamphibole/orthopyroxene rocks only in the Jonsa block, where they are found as interlayers amongst the Group 2 tholeiites. The Ti Zr and Zr Y ratios of these latter lithologies are closely comparable with those of the Group 2 tholeiites. The Jonsa block cordierite-orthoamphibole/orthopyroxene rocks have high MgO MgO + FeO tot ratios compared to most cordierite-orthoamphibole rocks produced by seawater-basalt interaction; their major and trace element geochemistry nevertheless indicates that they were derived from a mafic igneous precursor.

High-resolution X-ray computed microtomography: A holistic approach to metamorphic fabric analyses

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 Orijarvi 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 Orijarvi area formed by bulk north-south crustal shortening and associated vertical stretching.

The Archaean Karelia and Belomorian Provinces, Fennoscandian Shield

The Archaean bedrock of the Karelia and Belomorian Provinces is mostly composed of granitoids and volcanic rocks of greenstone belts whose ages vary from c. 3.50 to 2.66 Ga. Neoarchaean rocks are dominant, since Paleoarchaean and Mesoarchaean granitoids (> 2.9 Ga) are only locally present. The granitoid rocks can be classified, based on their major and trace element compositions and age, into four main groups: TTG (tonalite-trondhjemite-granodiorite), sanukitoid, QQ (quartz diorite-quartz monzodiorite) and GGM (granodiorite-granite-monzogranite) groups. Most ages obtained from TTGs are between 2.83–2.72 Ga, and they seem to define two age groups separated by a c. 20 m.y. time gap. TTGs are 2.83–2.78 Ga in the older group and 2.76–2.72 Ga in the younger group. Sanukitoids have been dated at 2.74–2.72 Ga, QQs at c. 2.70 Ga and GGMs at 2.73–2.66 Ga. Based on REE, the TTGs fall into two major groups: low-HREE (heavy rare earth elements) and high-HREE TTGs, which originated at various crustal depths. Sanukitoids likely formed from partial melting of subcontinental metasomatized mantle, whereas the GGM group from partial melting of pre-existing TTG crust.

Metamorphic evolution of the Ilomantsi greenstone belt in the Archaean Karelia Province, eastern Finland

Abstract The Ilomantsi greenstone belt is a Neoarchaean, c. 2.75–2.70 Ga volcanic–sedimentary complex in which metamorphic grade increases from staurolite grade in the SW of the belt to sillimanite grade in the NE. In the staurolite zone, prograde garnet zoning indicates pressure and temperature increases from 480–500°C at 2–4 kbar to 560–570°C at 6–7 kbar. Within the sillimanite zone temperatures peaked at 660–670°C at pressures of around 6 kbar. The U–Pb age determinations on monazite from the sillimanite zone yielded both Archaean and Proterozoic ages. One sample contains an exclusively Archaean monazite population of 2620±24 Ma, while another sample has two generations of monazite, with ages of 2664±33 Ma and 1837±13 Ma. The monazite data confirm that the Ilomantsi greenstone belt was metamorphosed simultaneously with the surrounding Neoarchaean migmatite complexes. The apparent clockwise PT path and medium P/T-type metamorphism are consistent with collisional tectonic settings, but the two distinct metamorphic events recorded by monazite indicate that a second, Palaeoproterozoic thermal event caused recrystallization and new mineral growth, in line with previous evidence from other isotopic systems. Accordingly, great care is necessary in defining metamorphic evolutionary P–T–t paths in rocks with complex mineral assemblages, to ensure correct identification of truly coeval mineral assemblages.