Wiborg Batholith Research Articles

Metal anomalies in till in the Sarvlaxviken area, Lovisa, southern Finland

Geochemical investigations of till is a widely used method in metal exploration as the till commonly inherits the geochemical signature (including the metal contents) of the parent bedrock. In this investigation, over 2000 till samples were collected in the Sarvlaxviken area, southern Finland, where several polymetallic (Cu, Zn, Pb, As, Sn, W and In) veins recently have been discovered in Proterozoic crust along the border between Late Svecofennian granites and the Wiborg Batholith. The bedrock is commonly covered by compact and poorly sorted basal till, formed during the Late Weichselian glaciation event. Several glacial-transported boulders, with high contents of Cu, Zn, As, Sn, Mo and Bi and derived from the local bedrock, have also been discovered on top of the till and provide evidence for concealed mineralisation in the local bedrock under the till cover. The frequent distribution of till in the Sarvlaxviken area provides excellent conditions for the search of such hidden mineralisation by means of systematic till sampling, even if large farm field areas, composed of clay-rich sediments, and seawater-covered areas (Sarvlaxviken bay), had to be avoided in the sampling program. The till samples were collected during university courses and training programs led by the authors and were analysed in a cost-efficient and certified laboratory. Obtained geochemical data were statistically processed by using K-means clustering algorithms which can be used to treat large sets of geochemical data. The results provided anomalies that mainly occur in till with a thickness of <1 m and are considered to be derived from a local bedrock source. The discovered anomalies provide strong evidence for numerous undiscovered veins beneath the till cover.

Rapakivi granite in the architecture of St Petersburg: a potential Global Heritage Stone from Finland and Russia

Abstract Rapakivi granites were in use during the Middle Ages in Finland. Their most spectacular use, however, was for structures built in St Petersburg between 1760 and 1917. Remarkable examples are the majestic and slender Alexander Column and the 112 columns of St Isaac's Cathedral. All Rapakivi granite was extracted from the Wiborg Rapakivi granite batholith in several quarries around the municipality of Virolahti in SE Finland (old Russia). Today, the 1640 Ma-old Wiborg batholith is the most important area for natural stone production in Finland and in the Leningrad region, Russian Federation. The main quarried stone varieties of Rapakivi granite (Baltic Brown, Baltic Green, Carmen Red, Karelia Red, Eagle Red and Balmoral Red) are regularly produced in large quantities in Finland for the global stone market due to the stone's unique qualities. Examples of applications in Rapakivi granite from Finland can be found in the USA, China, South Africa, the UK, Italy, Austria, Ireland, Spain and Germany as well as in Scandinavia and Russia. There are also quarries near Vyborg, the Russian Federation: Vozrozhdenie and Ala-Noskua.

Textural analyses of classical rapakivi granites: Texture formation through coarsening, size-selective replacement, and stirring

The well-known rapakivi texture has been extensively studied through a geochemical lens. Presented here is a detailed textural analysis of two classical rapakivi granites from the Proterozoic: The Wiborg Batholith, Finland and the Wolf River Batholith, USA. Crystal size distributions (CSDs) of mantled and unmantled ovoid populations indicate that protracted textural coarsening was important prior to mantling. An analysis of mantle thicknesses, coupled with a model replicating slices through mantled feldspars, indicates that mantling occurred in a semi-selective manner: mantle thickness on smaller K-feldspar ovoids range from 0 to 100% of the ovoid, and the variability of mantling decreases with increasing size. Selective mantling could be accomplished within a replacement front operating over a thermal gradient, whereby melting augments the ovoid population and generates the mantles, but with partial overlap of these two processes. CSDs of mantled and unmantled feldspars are exceptionally similar between randomly selected samples; suggesting stirring post-mantling and prior to emplacement was an efficient process. Results indicate that classical rapakivi granites are texturally complex and cannot be generated through a single, down temperature process. It is suggested that coarsening of ovoids, followed by mantling and homogenization, may be efficiently generated in the sub-caldera setting, where there are repeated pulses of mantle-derived magmas, or high-temperature anatectic melts acting as an intermediary for mantle heat.

Magmatic-hydrothermal evolution of the Kymi topaz granite stock, SE Finland: Mineral chemistry evidence for episodic fluid exsolution

The Kymi stock is a highly evolved topaz-bearing granite stock that has intruded into rapakivi granites in the central part of the Wiborg batholith (southwest Finland). The stock shows a well-defined zonation and intrusion sequence consisting of early porphyritic granites, subsequent equigranular granite and a late marginal stockscheider pegmatite. Miarolitic cavities occur within all rock types of the stock, but they are more abundant near the contact between the porphyritic and equigranular granites. Related hydrothermal greisen alteration, barren and Pb-Zn mineralized fluorite-bearing quartz veins crosscut the host rapakivi granites close to the Kymi stock. The major and trace element chemistry of biotite, K-feldspar, plagioclase and fluorite tracks the magmatic to hydrothermal evolution of the system. The biotite composition ranges from Al-rich annite to Li-rich siderophyllite, and the biotite chemistry shows an increase in Si, Li, AlVI, F, Mn, Rb, Cs, Zn, Be, Ga, Tl, Ta, and F, and a decrease in Fe, AlIV, Mg, Ti, Ba, Sr, Nb, and Cl along the evolution of the system. The Nb/Ta ratios in biotites suggest that magmatic fractionation is responsible for the progressive Ta enrichment in early porphyritic granites. The subsequent decrease in Nb at rather constant Ta in the more evolved equigranular granite and stockscheider pegmatite cannot be explained by magmatic fractionation, but requires selective partitioning of Nb into an aqueous fluid phase. The halogen composition of the biotites changes drastically from the early porphyritic granites to the later more evolved equigranular granite and stockscheider pegmatite. The Cl/F data of biotite from the miarolitic cavities plots at the transition between the porphyritic and equigranular granites, and is interpreted in terms of major fluid exsolution occurring at this stage. The minor and trace element data of K-feldspar and plagioclase show similar compositional trends as the biotites. The REEY patterns of fluorites from the equigranular granite and the stockscheider pegmatite, compared with those from the earlier porphyritic granites, show pronounced tetrad effects and correlated Y anomalies. This suggests that the F enrichment in the equigranular granite and the stockscheider pegmatite, in conjunction with major fluid exsolution and REEY complexing as fluoride species at this stage, have been key to development of the tetrad effect and Y anomalies. The fluorite from hydrothermal greisen alteration and quartz veins shows different chondrite normalized REEY patterns, indicating contributions from other sources such as meteoric fluids and/or interaction with the surrounding host rocks. The major and trace element data from the biotites of the Kymi stock are distinct from those of the host rapakivi granites, suggesting that the parental melts have not been entirely derived from fractionated rapakivi granite melts.

Chemical evolution and origin of the Luumäki gem beryl pegmatite: Constraints from mineral trace element chemistry and fractionation modeling

The 1928Ma old Luumäki gem beryl pegmatite is hosted by rapakivi granites of the Wiborg batholith in southeast Finland. The moderately evolved niobium-yttrium-fluorine (NYF) pegmatite system belongs to the topaz-beryl type of the rare-element pegmatite class. It has a simple major mineral assemblage of K-feldspar+plagioclase+quartz+biotite±fluorite throughout the main pegmatite zones (border, wall, intermediate, and core zone). It consists of at least two chemically related bodies (Luumäki N and Luumäki S), of which only Luumäki N contains gem beryl (heliodor) bearing miarolitic pockets. We characterize the geology, mineral assemblages, and the major and trace element chemistry of K-feldspar, plagioclase, biotite and quartz from the pegmatite. The mineral chemistry data show a progressive enrichment of Rb, Cs and Tl in K-feldspars, and depletion in Sr and Ba. The K-feldspar from the beryl-bearing pockets records the highest enrichment in incompatible elements, distinct from the data trend shown by K-feldspar from the main pegmatite zones. The REE data for plagioclase show a decrease of the positive Eu-anomaly and then a change to negative Eu-anomaly in the more evolved inner zones. This demonstrates an increase of the oxidation state of the pegmatite melt over time, consistent with the abundance of hematite in late-stage mineral assemblages including those of the miarolitic pockets. Fractional crystallization modeling is able to replicate the progressive enrichment of incompatible elements in K-feldspar, and to predict degrees of crystallization, which are in good agreement with volume estimates for the different pegmatite zones. The modeling results demonstrate that formation of the zoned pegmatite up to the quartz core can be well explained by an igneous crystallization process, leading up to considerable enrichment in incompatible elements. The melt reached saturation with an aqueous hydrothermal fluid only after more than 90% of the pegmatite melt had already crystallized. The separation of the oxidizing aqueous fluid was critical for the formation of the gem beryl pockets. The mineral trace element data of the Luumäki pegmatite and the host rapakivi granite, in conjunction with geochronological data, demonstrate that the pegmatite melt was derived from the residual melt of the rapakivi granites.

Mineralogy and geochemistry of indium-bearing polymetallic veins in the Sarvlaxviken area, Lovisa, Finland

A number of polymetallic vein mineralizations of different styles and metal associations, including base, alloy, noble and critical metals, have been discovered around the Sarvlaxviken bay in the westernmost parts of the Mesoproterozoic Wiborg Batholith, south-eastern Finland. The veins occur in two rapakivi granite varieties: coarse-grained wiborgite; and medium-grained Marviken granite. The veins are divided into five groups based on the dominant metal associations.The Li–As–W–Zn–Mn, Pb–Zn and Cu–As–In associations are hosted by wiborgite, and are strongly controlled by a NNW-trending tectonic pattern that evolved in two main stages. The Li–As–W–Zn–Mn association (generation 1) formed in a typical greisen environment with Li-bearing mica in significant alteration halos around a narrow quartz vein. This greisenization was accompanied by silicification, followed by sericitization and chloritization. The Pb–Zn association occurs in a similar vein type but without typical high-temperature minerals and is considered to have formed at a higher crustal level. Generations 2a and 2b formed under more brittle conditions leading to fracturing, quartz veining and metal precipitation of ore minerals. This metal association is characterized by very high contents of Cu, As and up to 1490ppm In but with ≤0.4% Zn, which leads to very high In/Zn ratios (up to 8400) enabling formation of abundant roquesite.The As–Sn–Cu and Mo–Bi–Be associations are hosted by alteration zones without hydrothermal quartz in the Marviken granite. Mineralization with moderately high contents of As, Sn and Cu is associated with greisenization while mineralization with spectacular contents of Be as well as high contents of Mo and Bi is associated with sericitization, chloritization and berylification.The internal age relations between the wiborgite-hosted, NNW-trending veins show a clear evolution from a typical greisen type environment (the Li–As–W–Zn–Mn and Pb–Zn associations of generation 1) to cooler and more brittle conditions governing quartz veining and precipitation of ore minerals belonging to the Cu–As–In association (generations 2a and 2b). The age relations between these wiborgite-hosted veins and the veins in the Marviken granite are more uncertain but the presence of a NS-trending granitic dyke on the eastern side of the Sarvlaxviken bay, with similar ore-fertile geochemical composition as the Marviken granite, indicates that the tectonically controlled veins formed simultaneously with the emplacement of the Marviken granite and associated hydrothermal activity.The polymetallic veins in the Sarvlaxviken bay are unique for the Fennoscandian Shield, not the least for the locally high indium grades and spectacular roquesite grains. There is an obvious exploration potential for similar veins (and hence a number of base, alloy, noble and critical metals) also elsewhere in the entire Wiborg Batholith.

Chemical characteristics of zircon from A-type granites and comparison to zircon of S-type granites

The trace element content in zircons from A-type granites and rhyolites was investigated by using back-scattered electron images and electron microprobe analyses. The studied Proterozoic (Wiborg batholith, Finland and Pará, Amazonas and Goiás states, Brazil) and Variscan (Krušné Hory/Erzgebirge, Czech Republic and Germany) plutons cover a wide range of rocks, from large rapakivi-textured geochemically primitive plutons to small intrusions of F-, Li-, Sn-, Nb-, Ta-, and U-enriched rare-metal granites. While zircon is one of the first crystallized minerals in less fractionated metaluminous and peraluminous granites, it is a late-crystallized phase in peralkaline granites and in evolved granites that may crystallize during the whole process of magma solidification. The early crystals are included in mica, quartz, and feldspar; the late grains are included in fluorite or cryolite or are interstitial. The zircon in hornblende–biotite and biotite granites from the non-mineralized plutons is poor in minor and trace elements; the zircon in moderately fractionated granite varieties is slightly enriched in Hf, Th, U, Y, and HREEs; whereas the zircon in highly fractionated ore-bearing granites may be strongly enriched in Hf (up to 10wt.% HfO2), Th (up to 10wt.% ThO2), U (up to 10wt.% UO2), Y (up to 12wt.% Y2O3), Sc (up to 3wt.% Sc2O3), Nb (up to 5wt.% Nb2O5), Ta (up to 1wt.% Ta2O5), W (up to 3wt.% WO3), F (up to 2.5wt.%F), P (up to 11wt.% P2O5), and As (up to 1wt.% As2O5). Metamictized zircons may also be enriched in Bi, Ca, Fe, and Al. The increase in the Hf content coupled with the decrease in the Zr/Hf value in zircon is one of the most reliable indicators of granitic magma evolution. In the zircon of A-type granites, the Zr/Hf value decreases from 41–67 (porphyritic granite) to 16–19 (equigranular granite) in the Kymi stock, Finland, and from 49–52 (biotite granite) to 18–36 (leucogranite) in the Pedra Branca pluton, Brazil. In the in situ strongly fractionated Cínovec cupola (Erzgebirge), the Zr/Hf value decreases from 33–51 in the protolithionite granite at a depth of 1255m to 7.5–25 in the zinnwaldite granite at a depth of 40m. At the scale of individual crystals, the Zr/Hf value decreases from 86 to 68 from the cores to the rims of the zircons from the Teplice rhyolite and from 64 to 33 in the zircons from the biotite granite at Krupka, Erzgebirge. The contents of Hf and U in zircon are dependent mainly on the degree of granite fractionation and the nature and volume of the volatile phases and are independent of the A- or S-character of the parental melt. The zircon Zr/Hf ratios 55 and 25 are proposed to approximately distinguish common, moderately evolved and highly evolved granites. Zircons from the moderately and highly evolved granites of A- and S-type can be discriminated on the basis of their HREE content and the U/Th ratios. Nb, Ta, and W are present in zircon from the highly evolved granites from all studied areas, while high As, Bi, and Sc contents are typical only for the Erzgebirge.

Emplacement and magnetic fabrics of rapakivi granite intrusions within Wiborg and Åland rapakivi granite batholiths in Finland

Anisotropy of magnetic susceptibility (AMS) studies were carried out in two areas in Finland: the Ruotsinpyhtää intrusion within the Wiborg rapakivi granite batholith and the Saltvik intrusions within the Åland rapakivi granite batholith. The main aim of this study was to understand the emplacement history of these mid-Proterozoic rapakivi granites. Aeromagnetic images reveal structures of ca. 5–10 km intrusions that build up the large rapakivi granite batholiths of Åland and Wiborg. Magnetic susceptibility data from the database of the Geological Survey of Finland, including more than 1700 samples from the Wiborg rapakivi batholith and almost 900 samples from the Åland rapakivi batholith, were compared with measurements from the present study. The mean susceptibility is ca. 1500 μSI for the Wiborg batholith and ca. 10,000 μSI for the Åland batholith. Samples taken for this study demonstrate that the mean value for the Ruotsinpyhtää intrusion is ca. 1200 μSI and for the Saltvik intrusions ca. 24,000 μSI. Thermomagnetic measurements reveal that the magnetic susceptibility is mainly derived either from paramagnetic minerals or from magnetite. The absence of solid-state deformation features such as breccia or contact deformation indicates a cauldron-type subsidence emplacement. The AMS measurements from Ruotsinpyhtää confirm these proposals, with concentric gently dipping magnetic foliations that support a ring complex structure above a piston-type subsidence system. The Saltvik area consists of a number of smaller elliptical intrusions of different rapakivi granites forming a multiple intrusive complex. The magnetic fabric shows a general westward dipping of the pyterlite and eastward dipping of the contiguous even-grained rapakivi granite, which indicates a central inflow of magma batches towards the east and west resulting from a laccolitic emplacement of magma batches, while the main mechanism for space creation was derived from subsidence. • Aeromagnetic images reveal structures within rapakivi granite batholiths in Finland • The rapakivi granites show both paramagnetic and ferromagnetic susceptibilities • The magnetic fabric results indicate a laccolitic emplacement • The main mechanism for space creation was derived from subsidence

Using a titanium-in-quartz geothermometer for crystallization temperature estimation of the Palaeoproterozoic Suursaari quartz porphyry

The Suursaari volcanic sequence represents volcanic activity related to Wiborg Batholith rapakivi intrusions in the southern part of the Fennoscandian Shield. The estimated pressure conditions for batholith granitic rocks are 1-5 kbar and crystallization temperatures range from 670 to 890 °C. To describe the temperature regime of the Suursaari volcanic system, a rock sample was taken from the Makiinpaallys Mountain outcrop and analysed with laser ablation inductively coupled plasma mass spectrometry. Sample spots were selected from quartz phenocrysts and groundmass. Quartz crystallization temperatures were calculated by the Ti-in-quartz method that takes into account rutile equilibrium and Ti activity in each phase. The calculated crystallization temperatures of the Suursaari quartz porphyry are in the range of 647-738 °C. The results show that the Suursaari quartz porphyry contains two generations of quartz which can be distinguished on the basis of crystallization temperatures: phenocrysts crystallized at higher and groundmass quartz at lower temperature.

Indium mineralisation in A-type granites in southeastern Finland: Insights into mineralogy and partitioning between coexisting minerals

Several promising exploration targets in the western parts of the Wiborg batholith, southeastern Finland, have been studied with respect to Zn, In, Ag, As, Sn and Cu. Ores occur both as massive In-bearing magnetite–sphalerite (Getmossmalmen), as greisen-style veins (Jungfrubergen) and as Cu-dominant polymetallic quartz veins (Korsvik-1 and -2, Sarvlaxviken area). The Cu-dominant (chalcopyrite–bornite), Zn-poor quartz veins are characterised by high In/Zn ratios (> 3000); roquesite (CuInS 2) being a major indium-carrier, alongside sphalerite, chalcopyrite and arsenopyrite. In contrast, sphalerite is the dominant In-carrier in the greisen veins and massive ores characterised by lower In/Zn ratios (< 100). In such cases, it appears that all indium partitioned into sphalerite at concentrations of 500–1500 ppm, and no roquesite formed. In the veins of the Sarvlaxviken area, we observe the exsolution of a high-temperature precursor sulphide to form co-existing sphalerite and roquesite. These exsolution textures extend down to the sub-microscopic scale and there is no evidence of extensive solid solution along the ZnS–CuInS 2 join beyond a couple of wt.% In (and Cu) in sphalerite. The co-existing roquesite is close to an ideal composition and contains μm-scale exsolutions of sphalerite, indicating negligible solid solution at the CuInS 2 end of the join. The new data are compared with the published data on the mineralogical distribution of the indium in sulphide ores.

Surface weathering of rapakivi granite outcrops – implications for natural stone exploration and quality evaluation

Implications of surface weathering of rapakivi granite outcrops for natural stone evaluation were studied in the Wiborg rapakivi granite batholith in southeastern Finland. The study was performed as field mapping, comprising the whole batholith and as detailed investigations on selected outcrops. The Wiborg batholith is composed of wiborgite, pyterlite, even-grained rapakivi granite, porphyritic rapakivi granite, porphyry aplite, and gabbro-anorthosite. Wiborgite is the main rapakivi granite type. Weathering affects coarse-grained rapakivi varieties: wiborgite, pyterlite, and porphyritic rapakivi granite. Weathering occurs as surficial weathering of outcrops and randomly along subhorizontal and subvertical fractures, and varies in intensity. In the weathered upper parts of the outcrops the colour of the stone is altered and soundness of the stone diminished, which has a significant impact on the natural stone evaluation. In rapakivi granite areas, subsurface evaluation methods, by which the weathered surface part of the outcrops can be recognized, should always be used. Development of subsurface methods is required in order to make them more applicable in the evaluation of natural stone in areas with weathered rock.

U–Pb and Ar–Ar geochronology of anorogenic granite magmatism of the Mazury complex, NE Poland

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/ 188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/ 206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/ 206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/ 206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.

Isotopic composition of pyterlite in Vyborg (Viipuri), Wiborg batholith, Russia

Active worldwide research on rapakivi granites and related mafic and intermediate rocks has shown that this lithologic assemblage is an important tool in the study of the composition of deep continental crust and subcontinental mantle (see Haapala & Ramo 1999 and references therein). In any one area, the radiogenic isotope composition of the rapakivi granites reflects the overall age of the unexposed lithosphere that they occupy (e.g., Ramo 1991, Neymark et al. 1994, Ramo et al. 1995, Andersson 1997, Dall’Agnol et al. 1999). This has recently been particularly well established for the ~1.5 Ga rapakivi granites of central Sweden that show a substantial Archean source component although no Archean crust has been found to be exposed in that area (Andersson 1997). In the first part of the 1990’s, the rapakivi batholiths of Russian Karelia were studied for Nd and Pb (as well as Sr) isotopes that implied a mixed Archean-Paleoproterozoic source, in contrast to the clearly younger source characteristics of the rapakivi granites farther to the west in southern Finland (Ramo 1991, Neymark et al. 1994). The latter turned out to be quite homogeneous in terms of Nd and Pb isotopes with a clear Paleoproterozoic signature for all plutons extending from the Aland Islands in the west to the Finnish-Russian border in the east (Ramo 1991). The Russian Karelian rapakivi batholiths (Salmi and Ulyalegi; Fig. 1a) are situated at the contact zone of the Archean and Paleoproterozoic crustal domains of the Fennoscandian Shield and their mixed (Archean-Proterozoic) isotopic signature is thus quite understandable. Recent studies on the Mesoproterozoic (Amantov et al. 1996, Ramo et al. 2001a) Valamo dolerite of the Jotnian Lake Ladoga basin have suggested that a Neoarchean subcontinental lithosphere (with eNd [at 1.46 Ga] of ~ –9 and initial 87Sr/86Sr of ~0.705) probably underlies the north-central part of Lake Ladoga (Upton et al. 1998, Ramo et al. 2001a; Fig. 1). This suggests that the Archean-Proterozoic boundary may continue at depth farther to the southwest than the present exposed boundary northwest of Lake Ladoga (Fig. 1a). In order to examine whether the easternmost part of the Wiborg batholith in Russian Karelia (southwest of Lake Ladoga) records a major Archean source

Crystallization conditions of the Wiborg rapakivi batholith, SE Finland: an evaluation of amphibole and biotite mineral chemistry

The wiborgite and dark wiborgite rapakivi granite phases of the Wiborg batholith in southeastern Finland compose about 80% of the total batholith area. A new study of the dominant mafic silicate minerals, in comparison with mafic silicates from more evolved granite phases, hybridized granite and mafic magmatic enclaves provide insights into the overall petrogenesis of the Wiborg batholith. All of the mafic silicate minerals are iron-rich, reflective of the whole rock compositions. Biotite is annitic, calcic amphibole is ferro-edenite to hastingsite, and subsolidus Fe-Mg amphibole is found as accessory grunerite. Temperatures derived from amphibole-plagioclase thermometry suggest crystallization at about ∼ 740 °C. Pressure estimates derived from Al in amphibole barometry range between 2.5 and 5.4 kilobars. This is noticeably higher than the previous estimates of 1 kbar for the Wiborg batholith. Oxygen fugacity estimates from biotite suggest low fO2 initial values and increase from FMQ to above NNO for late stage granite phases.

Geophysical interpretation of the crustal and upper mantle structure in the Wiborg rapakivi granite area, southeastern Finland

Abstract The rapakivis in southeastern Finland and Russian Karelia cover a total area of more than 20,000 km 2 and comprise the Proterozoic (1.6-1.3 Ga) anorogenic Wiborg, Ahvenisto, Suomenniemi, and Onas batholiths, and associated mafic rocks. A model of the crustal and upper mantle structure in this area based on gravity, aeromagnetic, deep seismic sounding, and petrophysical data is presented. The average density (2625 kg/m 3 ) of the batholiths is lower than that of the surrounding Svecofennian rocks by about 80 kg/m 3 . The density histogram of the rapakivi granites is divided into two normally distributed components with mean densities of 2625 kg/m 3 (major) and 2690 kg/m 3 (minor), respectively. There are related local gravity anomalies. The susceptibility histogram of the rapakivi granites is decomposed into five log-normally distributed components. The two major components have mean susceptibilities of (165 and 1305) × 10 −6 SI units. The former component (40%) is virtually paramagnetic, and the latter (60%) is weakly ferromagnetic. The two components with mean susceptibilities of (6000 and 13,5000) × 10 −6 SI units constitute only 3% of all the samples. Associated gabbro-anorthosites also have high susceptibilities (1450-30,370) × 10 −6 SI units. These ferrimagnetic rocks cause noticeable local aeromagnetic anomalies. An extensive Bouguer anomaly low is associated with the batholiths. A pronounced regional high surrounds this low. The range of values is nearly 60 mGal. The anomaly pattern is primarily the result of superposition of the gravity low, which is due to the relatively low-density upper and middle crust, onto the broader gravity high, that is due to the uplifted Moho. According to the published deep seismic sounding results, the crust is 41 km thick in the rapakivi region. Thus it is 6–20 km thinner than to the west, north and south of the rapakivi batholiths. The refraction model also implies a 6 km thick high-velocity body at a depth of 10 km. This body has a high v P v S -ratio and is interpreted as gabbro-anorthosite. The magnetic anomaly values in the rapakivi area are slightly higher than those in the surrounding schist areas but lower than the values in the metavolcanic belt to the west. Within the Wiborg batholith there is a zone of positive regional magnetic anomalies with a separate, well defined magnetic high of about 400 nT in the western part. This anomaly is interpreted as a pipe-like magnetic body with an abnormally strong magnetization. The upper surface of the body lies at depths between 12 and 15 km. The body extends from the upper to the lower crust. The model derived from geophysical data supports a Proterozoic mantle upwelling, which provided excess heat to cause uplift and partial melting of both the upper mantle and the lower crust. Partial melting of the upper mantle produced intrusions of gabbro-anorthosites and diabases, while extensive partial melting of the lower crust produced rapakivi magmas and caused their emplacement into the upper crust.

Tectonic setting and origin of the Proterozoic rapakivi granites of southeastern Fennoscandia

ABSTRACTThe 1·65–1·54 Ga rapakivi granites of southeastern Fennoscandia represent the silicic members of a bimodal magmatic association in which the mafic members are tholeiitic diabase dykes and minor gabbroic-anorthositic bodies. They are metaluminous to slightly peraluminous A-type granites and occur as high-level batholiths and stocks in an E-W-trending belt extending from Soviet Karelia to southwestern Finland. The Soviet Karelian granites were emplaced into the contact zone between Archaean craton and Svecofennian juvenile 1·9Ga-old crust, while the Finnish granites were intruded into the Svecofennian crust. Deep seismic soundings show that the rapakivi granites and the contemporaneous, mainly WNW or NW-trending diabase dyke swarms are situated in a zone of relatively thin crust. Below the Wiborg Batholith there exists a domal structure in the lithosphere in which a transitional zone (mafic underplate) occurs between the crust and the mantle.The Nd isotopic evolution of the rapakivi granites (εNd(T) −3·1—−0·2) corresponds to the evolution of the 1·9Ga-old Svecofennian crust, as do their Pb isotopic compositions. This implies that the Finnish granites represent anatectic melts of the Svecofennian crust. In contrast, the Soviet Karelian granites show isotopic composition indicative of substantial incorporation of Archaean lower crust material. Petrochemical modelling of one of the Finnish batholiths shows that its parental magma could have been generated byc.20% melting of a granodioritic source and that fractional crystallisation was important during the subsequent evolution of this magma.The rapakivi granites are redefined as A-type granites that show the rapakivi texture at least in larger batholiths. The field, geochemical, and seismic data indicate that the classical Finnish rapakivi granites were generated in an anorogenic extensional regime by partial melting of the lower/middle crust. The melting, and possibly also the extensional tectonics, were related to upwellings of hot mantle material which led to intrusion of mafic magmas at the base and into the crust.

(Theoretical investigation of) sound velocity in metals: Wei-Mei Shyu, Physical Review, Vol 177, No 3 (15 January 1969) pp 1041–1043 (1524)

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.

Ultrasonic attenuation in liquid metals: Sharma, K.C., Physical Review, Vol 174, No 1 (October 1968) pp 309–313 (1515)

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.

Ultrasonic attenuation in dirty dielectric crystals (using the boltzmann-equation approach of woodruff and ehrenreich): Maris, H.J., Physical Review, Vol 175, No 3 (15 November 1968) pp 1077–1081 (1499)

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.

Experiments for measuring in situ the velocity and attenuation of sound in sediments: Ulonska, A., Deutsche Hydrographische Zeitschrift Vol 21, No 2 (1968) pp 49–58 (In German) (1520)

The Mazury complex of northeastern Poland forms a 200 km long, E–W trending anorthosite–mangerite–charnockite–granite belt derived from bimodal, multiphase anorogenic magmatism and is similar to the classic anorogenic complexes of the Fennoscandian Shield (e.g. Wiborg, Salmi and Ragunda). The complex is dominated by felsic and intermediate rocks ranging from quartz monzonite through granodiorite and monzodiorite to diorite and is associated with anorthosite–norite plutons (Suwalki, Sejny and Ketrzyn). The ore deposits in Suwalki have 1559±37 and 1556±94 Ma Re–Os ages with a high initial 187Os/188Os reflecting crustal signature (Morgan et al. Miner. Deposita 35 (2000) 391). The Mazury complex crosscuts granulite and amphibolite facies metamorphic units (Pomeranian, Ciechanów and Mazovian), which are correlated with the Svecofennian rocks of Fennoscandia. U–Pb geochronology of single zircons and titanite fractions indicates that the felsic igneous activity of the Mazury complex occurred during a ∼25 m.y. period in at least three distinct pulses: (1) Krasnopol monzodiorite has a zircon 207Pb/206Pb age of 1525±4 Ma and titanite fractions dated at 1526±11 Ma (mean 207Pb/206Pb age), zircons from Bartoszyce quartz monzonite are concordant at 1522±2 Ma; (2) Boksze diorite has two concordant zircons with an age of 1512±1.1 Ma; (3) the youngest episode is represented by a 1502±2 Ma quartz monzonite from Goldap and a 1499±4 Ma quartz monzonite from Bartoszyce (Claesson et al., MAEGS meeting, St. Petersburg, 1995, p. 21). Zircon with a 207Pb/206Pb age of 1567±8 Ma from Boksze diorite points to inheritance from the older intrusions. Difference between U–Pb titanite and Ar–Ar biotite ages is ∼100 m.y. and suggests relatively slow cooling rates (∼2 °C/m.y.). The U–Pb ages indicate that the Mazury complex is roughly coeval with the Ragunda (Sweden) and Salmi (Russia) rapakivi complexes and show that the Fennoscandian rapakivi occurrences delineate a marked younging trend from north to south from the classic Wiborg batholith through the Riga batholith to Mazury.