Rapakivi Research Articles

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.

Chapter 10 Magmatism (1.6–1.4 Ga) and Mesoproterozoic sedimentation related to intracratonic rifting coeval with distal accretionary orogenesis

Abstract Separate pulses of magmatic activity involving the emplacement of plutons with predominantly granitic or bimodal granitic and gabbroic composition, as well as dolerite dykes, occurred in a cratonic setting in eastern Sweden at c. 1.59–1.58 Ga, c. 1.53–1.50 Ga and c. 1.47–1.44 Ga; anorthosite, monzodiorite and syenitoid rocks are locally present. Most of the granites have been compared with rapakivi granites in Finland and elsewhere. Isotopic data (Hf in zircons and ɛ Nd values) from the plutons in north-central Sweden show contamination by an Archean source. Siliciclastic rocks dominated by aeolian or deltaic sandstones overlie c. 1.58 Ga or c. 1.50 Ga plutons, are intercalated with 1.46 Ga basaltic lavas and were deposited prior to the emplacement of 1.27–1.25 Ga dolerites. The magmatic rocks are subalkaline (tholeiitic) to alkaline and the mafic components have been compared with continental flood basalts, suggesting an intracratonic, rift-related tectonic setting for the magmatism and sand deposition. These rocks constitute the westerly part of a late Paleoproterozoic to early Mesoproterozoic magmatic province in northern Europe, located along roughly north–south- and WSW–ENE-trending linear belts. This tectonic development was coeval with accretionary orogenic activity further to the west and SW.

Geochemistry of Nb and Та in magmatic rocks of the Ukrainian Shield

It was made an attempt to generalize geochemical data of Nb and Ta concentration in the most common igneous rocks of the Ukrainian Shield (USh). In the majority of widely distributed rocks of the USh (normal and subalkaline granitoids) the Nb and Ta concentration are similar to upper crust but lower the accepted Clark value for acidic rocks. In the more differentiated rapakivi granites concentrations of these elements reach or exceed the Clark’s concentrations (up to 35 ppm). Only highly differentiated alkaline rocks and alkaline feldspar granites (Perga, Kamiani mogyly, Ruska Poliana massifs) have high Nb concentrations (up to 800, 120 and 370 ppm, respectively). Medium rocks of the normal range are the least geochemically studied and typically are characterized low Nb and Ta concentrations. Almost all gabbroids as well as their metamorphosed analogues in the greenstone structures, are characterized by very low Nb (and Ta) concentration (two or more orders of magnitude) compared with Clark’s values for basic rocks (20 ppm Nb and 0.48 ppm Ta) according to A.P. Vinogradov. Against this background, increased of Nb and Ta concentration is observed in the main rocks of anorthosite-rapakivi-granite plutons. Howover even in these rocks concentration of these elements rarely reach or does not achieve the values which are typical for subalkaline and alkaline basalts of continental rifts. The regional heterogeneity in Nb and Ta distribution is observed in alkaline rocks of different composition: in the Azov Sea region these rocks are characterized by high concentrations, while in the western part of the USh the content of these elements is extremely low. The increased concentration of these elements is also revealed in kimberlites from the Azov Sea region and the Kyrovohrad megablock of the USh. The authors give some considerations and assumptions about the dependence on geochemical features of the Precambrian igneous rocks of the USh from the geodynamic conditions of their formation.

Studia nad domieszką mineralną w ceramice naczyniowej kultury amfor kulistych z terenu Kujaw

This article relates to the question, „what exactly is a mineral admixture in Globular Amphora Culture?”. My studies prove that only the granites fulfill the criteria. I designed the experiment. One of the goals was to examine which stone is the easiest to process. It was a rapakivi granite. Its minerals are pink or grey after processing. The admixture found in GAC ceramics has the same color and the same size of minerals. There was another type of admixture which was not recognizable.

High-aluminum orthopyroxene megacrysts (HAOM) in the Ahvenisto complex, SE Finland, and the polybaric crystallization of massif-type anorthosites

The occurrence of high-aluminum orthopyroxene megacrysts (HAOMs) in several massif-type Proterozoic anorthosite complexes has been used as evidence of their polybaric crystallization. Here, we report such petrographic and geochemical (XRF and EMPA) evidence from HAOMs discovered in the 1.64 Ga Ahvenisto rapakivi granite—massif-type anorthosite complex in southeastern Finland. Two different types of HAOMs were recognized: type 1 HAOMs are individual, euhedral-to-subhedral crystals, and up to 15 cm in diameter, and type 2 HAOMs occur in pegmatitic pockets closely associated with megacrystic (up to 30 cm long) plagioclase. The type 1 megacrysts in particular are surrounded by complex corona structures composed of plagioclase, low-Al orthopyroxene, iddingsite (after olivine), and sulfides. Orthopyroxene crystallization pressure estimates based on an Al-in-Opx geobarometer reveal a three-stage compositional evolution in both textural HAOM types. The Al content decreases significantly from the core regions of the HAOM (4.4–7.6 wt% Al2O3), through the rims (1.3–3.6 wt%), into the host rock (0.5–1.5 wt%). Enstatite compositions overlap, but are generally higher in the cores (En~60–70) and rims (En~50–70) of the HAOMs than in the host rock (En~45–60) orthopyroxenes. The highest recorded Al abundances in the HAOM cores correspond to crystallization pressures of up to ~ 1.1 GPa (~ 34 km depth), whereas the HAOM rims have crystallized at lower pressures (max. ~ 0.5 GPa, 20 km depth). The highest pressure estimates for the host rock orthopyroxene were ~ 0.2 GPa (< 7 km depth). These observations confirm the polybaric magmatic evolution of the Ahvenisto anorthosites and suggest that the entire 1.65–1.55 Ga Fennoscandian rapakivi suite was emplaced at a relatively shallow level (< 7 km depth) in the upper crust. Global comparison to similar rock types reveals remarkable similarities in the petrogenetic processes controlling HAOM composition and evolution of anorthosite parental magmas.

THE EFFECT OF MICROFRACTURING IN GABBRO-DOLERITES ON THEIR PHYSICAL PROPERTIES

Simple and cheap methods for the study of building stone deposits for the production of blocks of commercial size can be used to make geological work more efficient. However, physical processes in a massif are hard to understand when doing geophysical research, and the effect of microstructural parameters, mainly the microfracturing of rocks, on geophysical field indices is poorly understood.&#x0D; The authors have studied the impact of microfracturing on the physico-mechanical properties on coarse- to giant-grained rapakivi granites from the Salmi massif, Fennoscandian Shield. The goal of the present study is to assess the physical and physico-mechanical parameters in fine- and medium-grained quartz-free rocks.&#x0D; The authors have studied the various physical and physico-mechanical indices of gabbro-dolerites from the Ropruchei sill (Fennoscandian Shield, within the deposit prospected and revealed the effect of microfracturing indices on them.&#x0D; The study of these properties have shown that microfracturing in gabbro-dolerites is an essential factor which forms the physical parameters of a rock. The testing of this rock type has shown the rate of travel of an elastic wave is clearly related to the microfracturing of samples.&#x0D; The magnetic properties of gabbro-dolerite are due to high concentrations of titanomagnetite, the mineral grains of which formed upon the crystallization of magma during its interstratal intrusion. The distribution of titanomagnetite, based on size and concentration in the rock, depends on liquation conditions in the active stress field. The dependence of the microfracturing of gabbro-dolerites on their iron concentration (FeO+Fe2O3) shows that these «damages» appeared at a ‘brittle” stage in the evolution of the massif in the form of microfractures, the structural parameters of which have inherited the parameters of the stress and deformation field which existed in the massif at that time and which accumulated iron.&#x0D; As a result, microfracturing has a considerable effect on the parameters of the electromagnetic indices of the massif by accumulating iron oxides in the pores, and the higher the index, the higher the geophysical medium indices.&#x0D; Understanding of the physical problem, which accompanies the formation of a gabbro-dolerite complex, helps conduct the geophysical study of a massif, interpret data more objectively and more accurately delineate massive zones in the medium that could be used as high-quality building materials.

Detrital zircon record of the Mesoproterozoic to Lower Cambrian sequences of NW Russia: implications for the paleogeography of the Baltic interior

ABSTRACTLaser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb detrital zircon isotope data from Mesoproterozoic to Lower Cambrian strata of the St Petersburg region are used to characterize the paleogeographic and tectonic evolution of the Baltica continent. We dated fifteen samples and divided them into three groups based on their distribution of detrital zircons. The first group (comprising Lower Mesoproterozoic rocks) is dominated by Early Mesoproterozoic and Late Paleoproterozoic zircons, mostly derived from weathering of proximal source region including rapakivi granites exposed across the neighboring Baltic Shield. The second group includes Upper Ediacaran samples (Redkino and Kotlin Regional Stages), with major zircon populations ranging in age between 1970–1850 and 1600–1550 Ma, respectively, correlating with magmatic and metamorphic events within the Svecofennian Orogeny and rapakivi granite igneous activity in the interior of Fennoscandia. The third group of samples, collected from both the uppermost Ediacaran and lowermost Cambrian deposits (Kotlin, Lontova and Dominopol Regional Stages), contains older Paleo-Mesoproterozoic zircons as well as Late Neoproterozoic-earliest Cambrian zircons, indicating a Timanian source area and exhibiting a age spectra similar to spectra for coeval rocks of the Scandinavian Caledonides. Therefore, we conclude that reworking and transport of continental detritus from the Timanian Orogen began during Late Ediacaran, earlier than previously supposed, with transport of Timanian detritus not only to the marginal part of Baltica (known from the Scandinavian Caledonides), but also to the distal interior of Baltica.

The Composition of Mica from the Salmi Batholith Granites as an Indicator of the Formation of Nb–Ta Mineralization

Successive differentiates of acid rocks from the Salmi batholith are represented by the following series: biotite–amphibole rapakivi granite, biotite granite, and highly differentiated granites (Li-siderophyllite and Li–F zinnwaldite varieties). There is a gradual increase in the content of Ta and Nb and a decrease in the Nb/Ta ratio in this rock series. A regular change in the Ta and Nb partitioning coefficients between mica and the bulk rock composition is observed as well. Comparison of Li-siderophyllite granite and Li–F granite shows that the latter has a sharp decrease in the partitioning coefficients KpTa, Nb. This is due to a decrease in the content of these metals in zinnwaldite from Li–F granite and associates with the appearance of columbite in these rocks. The study of rock samples from the Salmi batholith suggests that such change of the Ta and Nb content in the mica from deeply differentiated granite and the corresponding decrease in the Kp value of these metals may be diagnostic evidence of the presence of columbite mineralization in the rock.

Interplay between random and correlated fracture processes induced by shock-waves in compressed granites

In contrast to damage accumulation in rocks subjected to constant or growing load wherein local failures increase with time, the strongest fracture events at the shock forcing occur at the very beginning of the loading. An inverse trend in the stress variation could be expected to affect some parameters of the rock behavior specific for a quasi-static pressure exposure. In this study, shock waves in uniaxially compressed granites were generated by an electric discharge, and the nucleation of microcracks was detected by the acoustic emission (AE) method. The experiment represented some conditions, under which deep underground rocks response on the impact forcing (explosions, shocks, etc.). The compression varied from zero to a pre-damage level. Fine-grained Sesam Black granite (grain size ∼1 mm) and coarse-grained Rapakivi granite (5–7 mm) were tested. The energy release distributions in all AE time series followed a Gutenberg-Richter type relation (power law) in both compressed and free samples with a departure from the log-linear dependences for the most intensive AE peaks. At the same time in a compressed fine-grained sample, a high-energy reminder of the power law distribution followed a well-defined exponential (Poisson-like) function; in coarse-grained granite, the higher-energy part of the energy distribution could not be described with any explicit expression. The specificities in the mechanical behavior of the tested fine- and coarse-grained granites were considered from the viewpoint of the hierarchical structure of rocks.

Petrografia e química do Granito Aripuanã, sul do Cráton Amazônico

O termo Granito Aripuanã refere-se a um conjunto de corpos graníticos circunscritos não deformados, intrusivos em rochas da sequência metavulcanossedimentar do Grupo Roosevelt, reconhecidos na porção sul do Cráton Amazônico. O corpo granítico principal constitui um batólito ovalado com diâmetro menor que 15 km, que ocorre a norte da cidade de Aripuanã, no noroeste do estado do Mato Grosso. É constituído de hornblenda-biotita sieno a monzogranitos porfiríticos grossos, de coloração cinza ou vermelha e localmente com texturas rapakivi. Nas áreas marginais é observada uma redução de granulação das feições porfiríticas com predomínio da fácies equigranular fina, associada a resfriamento rápido. As fácies tardias comuns são constituídas de diques equigranulares de cor cinza, aplitos róseos e cinza e veios pegmatíticos. Ocorrem veios pós-magmáticos de quartzo, fluorita, muscovita em zonas de deformação rúptil, em que as rochas mostram intensa alteração hidrotermal. Esse hidrotermalismo afeta de forma heterogênea as rochas encaixantes e as rochas magmáticas, facilitando a circulação de fluidos e favorecendo a concentração de sulfetos de zinco disseminados, sempre relacionados ao fraturamento. Os granitos hospedam xenólitos de rochas paraderivadas encaixantes, de granodioritos, quartzo dioritos e rochas micáceas. Nas bordas, ocorrem veios e apófises graníticas, além de metamorfismo de contato nas rochas encaixantes. Os monzogranitos são peraluminosos e pertencem à série de alto potássio a shoshonítica, similares a granitos “ferroan”, apresentando assinatura de granito tipo A, pós-colisionais a anorogênicos e associado à evolução do Arco Magmático Jamari.

Magma mixing in Neoarchean granite from Nalgonda region, Eastern Dharwar Craton, India: Morphological, mineralogical and geochemical evidences

We present field, petrographic, mineralogical and whole-rock geochemical data for part of the Neoarchean granite and associated microgranular enclaves (MEs) occur in the Nalgonda region, NE part of Eastern Dharwar Craton (EDC), and demonstrate the end-member magma mixing processes in the petrogenesis of the host granite. Extensive occurrence of ME and uni-directional flow band structures (N–S trend) in all the studied outcrops exposed over about 20 km depict that intensive magma mixing–mingling occurred at magma chamber scale. MEs are the portions of intermediate mafic magma that had interacted at two stages with acidic host granite. Mixing at the initial stage promoted efficient thorough mixing which resulted in rapakivi texture and mesocratic enclaves. Wispy filament structures around these enclaves indicate that mafic magma globules were mechanically diluted in the host granite magma by chaotic advection. These filamental magmas were further linearly diluted along with convection-related flow of the host magma. Smaller mafic globules also got linearly stretched along with this flow. Upon solidification of the host magma, these wispy mafic filaments were preserved as flow structures. The second stage of mafic magma mixing did not promote thorough mixing due to the large viscosity contrast with the host granite magma and preserved their physical entity. These enclaves are melanocratic with sharp boundary and devoid of gradational contact and filament structures around it. They characteristically show network of granitic vein injection which often show crenulation folding. These evidences indicate that during the second-stage interaction, the enclave magma was rigid to plastic nature with respect to the host granite magma. Coherent linear Harker variation trend of CaO, MgO, \(\hbox {TiO}_{2}\), \(\hbox {K}_{2}\hbox {O/Na}_{2}\)O, V, Y and Sc indicates that mixing has promoted to develop chemical gradient between ME and host granite magmas. Mechanical dilution by chaotic advection must have enhanced the chemical diffusion of both magmas. Smooth decreasing Harker trend of Y content in both the magmas indicates that theyhad undergone certain degree of fractional crystallisation. In contrast, elements like Rb, Ba and Sr with large value of diffusion coefficient (D) show scattered behaviour in element–element plots suggesting that diffusive fractionation was active during chaotic advection mixing. It is concluded that heterogeneity in compositional variation of Nalgonda granite can be attributed to difference in degree of mechanical dilution of mafic magma, fractional crystallisation and diffusive fractionation. The geochemical evidences indicate the role of subduction in the evolution of these rocks.

Geocronologia U-Pb, classificação e aspectos evolutivos do Granito Marajoara, Província Carajás

O Granito Marajoara (GrMj) é um stock intrusivo em granitoides mesoarqueanos do Domínio Rio Maria (DRM) da Província Carajás (PC). É formado por rochas leucocráticas: biotita monzogranito equigranular (BMzE) e heterogranular (BMzH). Textura rapakivi e enclaves de granito porfirítico (EGp) e microgranulares (EMg) são restritos à fácies BMzH. Os valores de susceptibilidade magnética (SM) e a presença de magnetita aproximam a fácies BMzH dos granitos da série magnetita, enquanto a variedade BMzE mostra afinidade com aqueles da série ilmenita por apresentar baixos valores de SM e opacos. São granitos peraluminosos similares aos ferroan. Mostram, ainda, afinidades com os granitos tipo-A oxidados (BMzH e EGp) e reduzidos (BMzE), como aquelas dos plútons das suítes Jamon e Velho Guilherme, respectivamente. Os EMg mostram afinidade com os granitos magnesianos e da série calcioalcalina, e os EGp são originados a partir da interação entre os líquidos EMg e BMzH. Os gaps composicionais entre as diversas variedades sugerem que seus magmas não são cogenéticos. Os EMg são representantes de um magmatismo básico oriundo do manto litosférico enriquecido e que teriam sido injetados na câmara magmática por underplating. As análises U-Pb em zircão (SHRIMP) fornecem idade de cristalização (1885 ± 6 Ma). O GrMj foi colocado em epizona em um ambiente de tectônica extensional. A zonalidade concêntrica do GrMj e a influência reduzida dos esforços regionais indicam que o transporte do magma se deu por diques. Dessa forma, a edificação do GrMj é resultante de ascensão vertical de magmas por meio de fraturas e acomodação ao longo dos planos da foliação regional E-W, seguida de uma mudança do fluxo vertical por um espalhamento lateral do magma.

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.

Highly varying radiogenic heat production in Finland, Fennoscandian Shield

Radiogenic heat production in Finland has been previously studied using airborne gamma-ray surveys and glacial till measurements alike. For the first time, this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country. The dataset of 6465 samples represents mostly Mesoarchean (about 2.7 Ga), Paleoproterozoic (ca. 2.2–1.8 Ga) and Mesoproterozoic (ca. 1.6–1.3 Ga) rocks. Nearly all data are from Precambrian Fennoscandian Shield area, but heat production appears to be highly variable, and above global Archean and Proterozoic averages. Spot readings show an arithmetic average of 1.34 ± 1.19 μWm−3, and a range from 0.02 to 19.4 μWm−3. The interpolated areal average of the whole area is 1.42 ± 1.41 μWm−3. The high standard deviation of data is related to the geochemical characteristics of uranium (U), thorium (Th) and potassium (K) resulting in a skewed distribution of heat production. Mesoproterozoic anorogenic rapakivi granites, and late Paleoproterozoic Svecofennian granitoids show the highest heat production values in the range of 3–5 μWm−3. The results show no distinct dependencies of heat production with geological age, metamorphic grade nor seismic P-wave velocity, but an increasing trend of heat production with SiO2 content and decreasing trends of heat production with Fe2O3 content and with rock density are evident. Surface heat flow (44 borehole data values) correlates weakly with heat production (r = 0.35). The general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in Finland and beyond.

Paleoproterozoic Underwater Volcanism and Microfossil-Like Structures in the Metasedimentary Siliceous Rocks, Hogland Island, Russia

Geological surveys showed that rhyolite and basalt strata with pillow structures typical for underwater volcanism form sheets over the Svecofennian basement. Original geochemical and isotope-geochemical data confirmed that the rhyolites were formed contemporaneously with the ra-pakivi granites of the Wiborg Massif (1 640 Ma), and the basalts are similar to gabbro-anorthosites. Abnormally high content of K2O and relatively low content of Na2O in rhyolites and basalts are interpreted as a result of hydrothermal interaction of eruptive magmas with K-enriched hot seawater. The strata of siliceous metasedimentary rocks (microquartzites) within basaltic and rhyolitic lavas were formed in processes of chemogenic sedimentation and subsequent contact metamorphism during underwater volcanism. Microquartzites showed carbon vastly depleted of heavy isotope 13C. This is typical for rocks formed with participation of living substance. The Raman spectra of the remaining carbon-containing substance have graphite bands. In the microquartzites among basalts and rhyolites we found a community of structures with external and internal morphology similar to modern or fossilized marine microorganisms:spiral cyanobacterias, amoebas, diatoms, foraminifers, virus capsids, flagellates and multicellular organisms. It is assumed that these silificated and ferruginizated microfossils represent the Paleoproterozoic community of marine microorganisms.

Geology and Mineralogy of Rare Earth Elements Deposits and Occurrences in Finland

Rare earth elements (REE) have critical importance in the manufacturing of many electronic products in the high-tech and green-tech industries. Currently, mining and processing of REE is strongly concentrated in China. A substantial growth in global exploration for REE deposits has taken place in the recent years and has resulted in considerable advances in defining new resources. This study provides an overview of the mineralogical and petrological peculiarities of the most important REE prospects and metallogeny of REE in Finland. There is a particularly good potential for future discoveries of carbonatite hosted REE deposits in the Paleozoic Sokli carbonatite complex, as well as in the Paleoproterozoic Korsnäs and Kortejärvi Laivajoki areas. This review also provides information about the highest known REE concentration in the alkaline intrusions of Finland in the Tana Belt and other alkaline rock hosted occurrences (e.g., Otanmäki and Katajakangas). Significant REE enrichments in hydrothermal alteration zones are also known in the Kuusamo Belt (Uuniniemi and Honkilehto), and occurrences of REE-rich mineralisation are also present in granite pegmatite bodies and greisens in central and southern Finland (Kovela monazite granite and the Rapakivi Granite batholith at Vyborg, respectively). REE minerals in all of the localities listed above were identified and analyzed by scanning electron microscopy (SEM) and electron microprobes (EMPs). In localities of northern and central Finland, both primary rock forming and epigenetic-hydrothermal REE minerals were found, namely phosphates (monazite-Ce, xenotime-Y), fluorcarbonates (bastnäsite-Ce, synchysite), and hydrated carbonates (ancylite-Ce), hydrated aluminium silicates (allanite-Ce, Fe-allanite, cerite, chevkinite), oxides (fergusonite, euxenite) and U-Pb rich minerals. The chondrite normalized REE concentrations, the La/Nd ratios and the REE vs. major element contents in several types of REE bearing minerals from prospects in Finland can be used to identify and define variable REE fractionation processes (carbonatites), as well as to discriminate deposits of different origins.

New rock magnetic and paleomagnetic results for the 1.64 Ga Suomenniemi dyke swarm, SE Finland

This study represents new petrophysical and paleomagnetic data obtained for the 1.64 Ga Suomenniemi diabase and quartz porphyry dyke swarm associated with Suomenniemi rapakivi granite. We also present petrophysical data for the coeval dykes of the Sipoo (1.63 Ga) and Häme (1.642 ± 0.002 Ga; 1.647 ± 0.014 Ga) swarms in order to address: (1) the discrepancy between the coeval Sipoo and Häme poles and (2) the proposed severe overprint in paleomagnetic data for rapakivi related dyke swarms in Fennoscandia.Demagnetization of Suomenniemi dyke samples revealed dominantly normal polarity magnetization carried by magnetite. For the first time, a few samples from the few sites have shown a trace of a reversed magnetization. The combined normal (N) polarity direction from nine dykes (=sites) is at D = 031.2°, I = 07.2°, (with α95 = 10.6°, k = 21.8) yielding a paleomagnetic pole at Plat = 27.8°N; Plong = 171.7°E (with A95 = 7.9°, K = 38.4). The Suomenniemi pole fulfils four of the seven quality criteria for paleomagnetic poles (Van der Voo, 1990). The new pole for the Suomenniemi swarm is similar to the coeval Sipoo swarm pole, but distinctly different from the nearly coeval Häme swarm pole. The relative position of the poles compared to other well-defined paleomagnetic poles for Baltica indicates that the Suomenniemi and the Sipoo poles are slightly younger than the Häme pole.We show here that the original magnetization in some of the 1.64 Ga dykes in Suomenniemi have been overprinted with a secondary magnetization component, but several dykes of the swarm lack this overprint. Moreover, the presumably primary Subjotnian remanence component for the Suomenniemi dykes can be clearly isolated due to its different coercivity and/or unblocking temperature from the overprinted component. The same applies to the Sipoo and Häme dyke swarms.

Magmatic evolution and source of a Proterozoic rapakivi granite complex in the North China Craton: New evidence from zircon U−Pb ages, mineral compositions, and geochemistry

Typical Proterozoic rapakivi granite of the Northern Hemisphere occur in Precambrian shield areas, and are commonly associated with anorthosites, mangerites, and charnockites, thus belonging to the AMCG (anorthosite–mangerite–charnockite–granite) suite. A similar suite is represented in the North China Craton (NCC) by the Shachang rapakivi granite and Damiao anorthosite complexes. The Shachang complex consists of amphibole–biotite rapakivi granite, porphyritic biotite granite, coarse-grained biotite granite, medium- to fine-grained biotite granite, and two-mica granite (listed from the earliest to the latest phases). New zircon U–Pb dating shows that the complex was formed over the period 1700–1682 Ma. The mafic minerals of the complex are calcic amphibole (ferroedenite–ferrohastingsite) and biotite (annite–siderophyllite), and the compositions of the amphibole suggest that the matrix of the amphibole–biotite rapakivi granite crystallized at a pressure of ∼4 kbar. The temperatures of crystallization, obtained from the compositions of amphibole, biotite, and zircon, decreased from ∼780 °C for the amphibole–biotite rapakivi granite to ∼660 °C for the two-mica granite. Our study of the Shachang complex indicates an evolution from early potassium-rich magmas to late silicon-rich magmas. The early amphibole–biotite rapakivi granite crystallized under relatively high temperatures (760–780 °C) and low fO2, while the later rocks formed under lower temperatures (760–660 °C) and high fO2. The Shachang rapakivi granite and Damiao anorthosite complexes contain different mafic minerals and show diverse trace element ratios and disparate geochemical evolutionary trends, suggesting different sources. Zircon Lu–Hf and whole-rock Sm–Nd isotopic data for the Shachang complex are relatively homogeneous, with εHf(t) and εNd(t) values varying from −9.8 to −3.3 and −6.4 to −3.4, respectively. Corresponding two-stage Hf model ages and Nd depleted mantle model ages vary from 2650 to 2850 Ma and from 2480 to 2840 Ma, respectively. These model ages are similar to the ages of Neoarchean lower crustal metamorphic rocks in the NCC (2500–2800 Ma). In addition, the compositions of the amphiboles and biotites in the granite in the Shachang complex indicate derivation from the crust. All the evidence points to the Shachang rapakivi granite complex having been formed from magmas derived from the lower crust of the NCC, rather than from the fractionation of mantle-derived mafic magmas with minor crustal contamination. This paper provides a detailed case study of the Shachang complex that will further our understanding of the sources of Proterozoic rapakivi granite in the Northern Hemisphere that belong to the AMCG suite of rocks.

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.

The age of monzonites of the eastern part of the Korsun-Novomirgorod pluton (Ingul megablock of the USh)

The article presents the results of monzonites research of the eastern part of Korsun-Novomirgorod pluton. The pluton is situated in the northwestern part of the Ingul megablock. Besides basic rocks varieties — gabbro-anorthosite and rapakivi granites the rocks of monzonite composition of different genesis are widely presented here. Small bodies of monzonites form outputs among rapakivi granites of Shpola massif at the junction of tectonic structures of different rank. Monzonite is a hybrid rock which contains xenoliths of frame rock and possibly ancient dyke formations. Zircon is represented by several generations, which are distinguishable by optical studies, as well as by different U / Th ratio. Some grains contain a nuclei and zircon inclusions, appearance of which was determined by the process of contamination. All fractions obtained discordant zircon age values. Dating calculated by isotope ratio 207Pb / 206Pb has been taken as the most reliable. Due to the presence of relict zircons received isotopic age of the figures are overstated and do not reflect the formation of monzonite. As the maximum possible monzonite age we can take a minimum age, calculated by 207Pb / 206Pb — 1.740 Ma. The assumption about the formation of monzonite melt as a result of contamination of basic magma by ancient crustal rocks of acid composition has been given, which does not contradict the results of the study of the isotopic composition of strontium, neodymium, and geochronological data.