Microcline Granite Research Articles

Nitrogen and oxygen isotopic profiles and redox structure across the Chinese Altay reveal the source rocks of granites, core of the Central Asian Orogenic Belt

Two rock series in the Chinese Altay (CA) were identified. The protoliths of the overlying series are fine sandstone and siltstone that consist of quartz and clay but contain little feldspar. The underlying series includes biotite–plagioclase gneiss and amphibolite entrained by the CA granite, migmatitic granite and the Early-Devonian Kangbutiebao Fm. as the uncovered, latest-formed southwestern margin. The CA granites include oligoclase microcline granite and biotite plagioclase granite that exhibit extensive K-Na-metasomatism of plagioclase porphyroclast. The two series both have high positive εNd-εHf values but are NO isotopically distinguishable. This study employed NO isotopes to trace source rocks of the CA granites and to explore devolatilization from metamorphism, migmatization to partial melting.The overlying series yielded much higher N content (20.2–484 ppm; average: 87.3 ± 99.0 ppm; 1σ; n = 36), δ15N (−3–19‰; average: 5.8 ± 4.3‰; n = 36) and δ18O (6.9–15.6‰; average: 13.6 ± 3.05‰; n = 29) than the underlying series that yielded N (9.9–71.1 ppm; average: 32 ± 17.1 ppm; n = 22), δ15N (−7.3–8.1‰; average: 1.1 ± 4.7‰; n = 22) and δ18O (4.5–12.1‰; average: 8.8 ± 1.9‰; n = 22). The protoliths of the overlying series were accumulated on oxic shallow sea in the CA and, hence, recorded high δ15N, whereas the protoliths of the underlying series extended to anoxic deep-ocean and experienced high-temperature seawater reaction, which resulted in low δ18O and δ15N values of the protoliths of the underlying series. The CA granites yielded much lower N content, δ15N and δ18O than the overlying series but have values overlapping those of the underlying series with a marginal increase in δ18O. The δ15N values tend to increase with decreasing element normalized-N contents from the underlying series to the CA granites, suggesting that migmatization and partial melting of the underlying series have caused 15N/14N fractionation. Quantitative modeling demonstrates that NH3 devolatilization of the underlying series at 550 to 750 °C can fit the underlying series and the CA granites, whereas 15N/14N fractionation trajectories of the overlying series miss the CA granites. These results suggest that the CA granites have been derived from the underlying series. Most data of the underlying series and the CA granites can be modeled by Rayleigh distillation suggesting escape of degassed NH3 from essentially solid-state protolith/source rock from metamorphism to migmatization. Partial conversion to N2 or N2O caused 15N-enrichment in the two series at the leading edge of the overlying series, the eastern CA. The magma derived from the underlying series mixed with magma stemming from the metasomatized mantle with contribution from country rock-derived fluid result in high δ15N of the Qinggeli granodiorite and microdioritic enclaves.

Paleoproterozoic skarn mineralization events and geodynamic background in the Jiao-Liao-Ji Belt: Geological, geochemical and chronological evidence from the Daqinggou Fe–Cu polymetallic deposit in Ji'an, NE China

The Daqinggou Fe–Cu polymetallic deposit has been discovered in the Ji'an district, east of the Jiao-Liao-Ji Belt (JLJB). It consists of the Daqinggouli and Beigou ore sections, and the ore bodies are mainly magnetite ore bodies, followed by copper polymetallic ore bodies. Six metallogenic stages have been distinguished here: The dry skarn (I1), wet skarn (I2), iron oxide (I3), early quartz-sulfide (II1), late quartz-sulfide (II2), and quartz-carbonate stages (II3). There are also three types of fluid inclusions (FIs): gas-rich (W1-type), liquid-rich (W2-type), and daughter crystal (S-type) FIs. The ore-forming fluids here belong to a H2O–NaCl ± CO2 system with a medium-high temperature and medium-high salinity during the Stages I3 and II1, in addition to a H2O-NaCl system with a medium-low temperature and medium-low salinity in the Stage II2. In-situ S isotopes indicate that the Stage II1 sulfides were derived from magmatic sources, whilst the Stage II2 sulfides were derived from a mixture of magma and stratigraphic sources and Stage II3 was dominated by the sulfur source of the sedimentary strata. Additionally, the Pb isotope data reveal that the ore-forming minerals were of crustal or orogenic origin. By comparing the metallogenic elements of typical skarn deposits, we believe that the Daqinggou deposit is a calcareous skarn deposit, which originates from dimetasomatism between the microcline granite and marble of the Mayihe Formation in the Ji'an Group. The immiscibility and mixing of ore-forming fluids are the key mechanisms of metal precipitation within the deposit. Furthermore, the U-Pb ages of single zircons from microcline granites and hydrothermal apatite in the ores constrained the formation of rocks and deposit to the late Paleoproterozoic. Elemental geochemistry and zircon Hf isotope data further divided the microcline granite into I-type granite with high potassium and differentiation, originating from the partial melting of Neoarchean crustal materials in the region, which then underwent fractional crystallization. The diagenetic tectonic environment shows the characteristics of an active continental margin, which may have resulted from regional large-scale arc-continent collisions (Longgang Block and Nangrim Block), also implying the occurrence of ocean subduction during the evolution of the JLJB.

Geochemical Reconstruction of the Provenance, Tectonic Setting and Paleoweathering of Lower Paleozoic Black Shales from Northern Europe

Lower Paleozoic black shales from Estonia, Sweden, and Russia were analyzed for major and trace elements to reconstruct the provenance, tectonic setting, and paleoweathering conditions of these shales. The black shale is highly enriched in U, V, Mo, and Pb (except in samples from Sweden where Pb is slightly enriched), slightly enriched in SiO2, Fe2O3, K2O, and TiO2 and highly depleted in CaO, Na2O, and MnO, with respect to average shales. The provenance signatures (Th/Sc versus Zr/Sc, Al2O3 versus TiO2, Zr versus TiO2 plots, and Zr/Sc ratio) of the Baltoscandian black shales suggest that they were derived from rocks of intermediate to felsic composition and from recycled sediments. The likely provenance region was the Paleoproterozoic igneous and metamorphic basement of southern central and southern Finland, which consists predominantly of felsic to intermediate metamorphic (acidic to intermediate gneisses, felsic volcanics, microcline granites and migmatites) and igneous rocks (small granitic intrusions and large rapakivi granite intrusions), and reworked older Ediacaran and Lower Cambrian sediments; however, the proportion of clastic input from these sources is not uniform in the three regions studied. The discrimination of the tectonic settings of source materials of the black shale using the SiO2 versus K2O/Na2O plot and a new discriminant method (APMdisc) favors a passive margin setting. The Chemical Index of Weathering (CIW) indicates that the clastic material in the black shale of the studied regions has experienced an intense degree of chemical weathering. Weathering indices (Chemical Index of Alteration CIA and CIW) also show that the black shale has experienced significant secondary potassium enrichment.

Uranium-thorium mineralization process of the Ukrainian shield suture zones and evolution of the composition of depth fluids in Precambrian

Тhe geochemical properties of uranium and thorium and the geological structure of the ore oc currences of the Golovanevskaya and Ingulo-Inguletskaya suture zones are correlated with changes in the composition of mantle melts and fluids in the Archean-Paleoproterozoic. It is shown that the formation of uranium and thorium deposits on the Ukrainian shield became possible in the Proterozoic, when oxidized and acidic melts prevailing in the Archean mantle were replaced by neutral and alkaline water-chloride-potassium. This led to the accumulation of tho rium (to a greater extent) and uranium (to a less extent) in granite magmas of the layer of partial melting of the crust, where they entered as chloride complexes. The ore occurrences of monazite in microcline granites and pegmatites of the Berdichev and Kirovograd complexes formed in the period 2.1-1.9 billion years ago are associated with these processes. The enrichment of crustal centers of melting with uranium occurred most intensively after 2.0 billion years ago (a maximum of 1.8-1.65 billion years ago), when uranyl-carbonate ions entered the crust as a part of mantle carbonate-fluoride-sodium fluids, they were mixed with melts of existing melting centers. A change in the pH of the medium led to the decomposition of complex compounds and the release of the uranyl group, reduction of uranium to U4+ in the presence of graphite and magnetite, and at geochemical barriers with an acid reaction, and the activity of hydroxyl ions ensured the creation of a brannerite structure.

LA–ICP–MS U–Pb DATES FOR DETRITAL ZIRCON FROM QUARTZITES OF THE TEMRYUK FORMATION (NOVOUKRAINKA IRON DEPOSIT OF THE KORSAK BLOCK, WEST AZOV AREA)

The West Azov iron district corresponds to the Korsak synclinorium or block in which iron deposits of the Kamyana Mohyla belt (Pavlivka, Novoukrainka, and Serhiivka areas), Kuksungur, and Korsak Mohyla are located. The iron– bearing units are confined to the metamorphosed volcanogenic–sedimentary rocks of the Satchky Formation of the Central Azov Group, while rocks of the Temryuk Formation form the base of the section. The group is intruded by numerous dykes of pink microcline granites and pegmatites up to 15 to 20 m in thickness. The basement for the group consists of Archean plagioclase granitoids hosting xenoliths of gneisses, crystalline schists, and amphibolites. The iron ores of the Korsak block are characterized by the same rock association and mineral assemblage as those in the Mangush and Orikhovo–Pavlohrad structures. We have conducted geochronological study of metaterrigenous rocks of the Temryuk Formation of the Novoukrainka (Kamyana Mohyla) iron deposit. The iron deposit extends along the western limb of the Korsak synclinorium for ca. 20 km. The prevailing strike of the iron deposit is NW 350°. Metamorphic rocks form separate bodies (lenses) that are continuous along the strike for 1200–1500 m with a maximum thickness of 120– 140 m. Ferruginous quartzites are represented by banded iron formation with interlayers of biotite and biotite–amphibole gneisses (Sachky Formation). Micaceous (biotite, garnet–biotite) quartzites occur in the lower part of the section, where they are interbedded with biotite, muscovite–biotite–sillimanite, biotite–amphibole, and, rarely, pyroxene–garnet–biotite gneisses and schists (Temryuk Formation). Fifty–eight grains of detrital zircon from micaceous quartzite of the Temryuk Formation were dated by LA–ICP–MS. Four crystals were dated at 3.2–3.23 Ga, three more grains have an age of 3.08– 3.11 Ga, thirty–two crystals yielded an age of 2.9–2.97 Ga, fifteen crystals had an age of 2.82–2.87 Ga, and, finally, four grains were dated at 2.76–2.78 Ga. According to the obtained data, metasedimentary rocks of the Central Azov Group, bearing iron–rich units in the Korsak synclinorium of the West Azov area were deposited on the basement with the Mesoarchean (2.9–3.1 Ga) Tonalites–Trondjemites–Granites (TTG). Archean granites with ages of 2.82–2.87 and 2.76– 2.78 Ga as well as the remnants of older TTGs provided a minor contribution from the provenance. The Central Azov Group was intruded by the 2.2 Ga Na–K–rich granites. On the Mesoarchean Azov craton, volcanogenic–sedimentary rocks of the Huliaipole Formation in the Huliaipole brachysyncline and the Vovchansk unit in the Orekhiv–Pavlohrad structure were formed during the same time interval. Deposition of this volcanogenic–sedimentary succession with thick Banded Iron Formations (BIFs) is possibly related to submarine volcanism and marine transgression.

Geochronological and Geochemical Study of Zircon from Tourmaline-Muscovite Granites of the Archaean Kolmozero–Voronya Greenstone Belt: Insights into Sources of the Rare-Metal Pegmatites

In order to determine the U-Pb crystallization age of zircon from the tourmaline-muscovite granites of the Kolmozero–Voronya greenstone belt located in the northeastern Fennoscandian Shield (Kola Peninsula), an isotope-geochronological study of the zircon grains was performed using a SHRIMP-RG microprobe. The belt is represented by the Archaean volcano-sedimentary rocks (2.9–2.8 Ga). Deposits of rare-metal pegmatites (Li and Cs with associated Nb, Ta, and Be) occur within the belt and on its margins. The age of the pegmatites within the belt was estimated at 2.7–2.6 Ga. Until now, there has been no generally accepted view on the genetic relation of the pegmatites with granite. Various authors have suggested that the pegmatites could potentially be associated with many type of granitoids within the region, i.e., plagiogranites, tonalites, amphibole-biotite granodiorites, microcline granites, alkaline granites, or muscovite-tourmaline granites. Zircon crystals from the muscovite-tourmaline granites are heterogeneous; they have less altered cores and strongly altered rims. The zircon cores are slightly enriched in U at a value of 173–1030 ppm, Th/U = 0.1–0.4. The zircons’ rims are heavily enriched in U at a value of 700–3300 ppm, Th/U = 0.03–0.08, indicating metasomatic processes. Zircon characteristics show that it crystallized from a melt enriched in a fluid phase. Fluid activity lasted after zircon crystallization as reflected in the irregular composition of the mineral and its rare earth element (REE) patterns that are typical of a metasomatic zircon. The computed zircon crystallization temperature in the tourmaline-muscovite granites is in the range of 650–850 °C. The discordant age calculated for five analyzed points of the zircon crystal cores is 2802 ± 13 Ma. The discordant age for four analyzed points of the zircon crystal rims is found to be 2728 ± 14 Ma. On the basis of the obtained isotope-geochronological data, we conclude that the tourmaline-muscovite granites located in the immediate vicinity of rare-metal pegmatite veins are the most probable source of matter for the pegmatites.

Murzinka massive at the middle urals as an example of the interformational granite pluton: magmatic sources, geochemical zonality, peculiarities of formation

Murzinka massif constitutes an interformational sheet-like body reaching up 6 km in length steeply dipping to the east. Proterozoic metamorphic rocks of predominantly granulite facies (p = 5–6 kbar, T = 750–800℃) occur at the base of the massif, with Silurian-Devonian volcanic-sedimentary rocks metamorphosed in the epidote-amphibolite facies occurring in its roof. The petrogenic elements were determined at the Laboratory for Physicochemical Research Methods of the Zava ritsky Institute of Geology and Geochemistry, UB RAS. The content of trace elements was determined at the laboratories of the University of Granada in Spain and at the Institute of Geology and Geochemistry using the ICP-MS method. In the eastern direction, the rocks change their composition from predominantly basic to granitoid as they approach the massif. The gneisses of the granitoid composition experienced a high degree of melting; the anatectic melt formed the western part of the Murzinka massif. The granites form three complexes: 1) Yuzhakovo – veins of biotite orthoclase antiperthite gra nites varying in K 2 O content in the metamorphic rocks of the base of the massif; 2) Vatikha – biotite orthoclase antiperthite granites making up the western part of the murzinka massif; 3) Murzinka – two-mica predominantly microcline granites occurring in the eastern part of the massif. Vatikha and Murzinka granites have the same isotopic age (about 255 Ma). A clear geochemical zonation is revealed in the massif: from west to east (from the base to the roof), the contents of Rb, Li, Nb, Ta increase in the granites of the Vatikha and Murzinka complexes. In the same direction, the ratios K/Rb, Zr/Hf, Nb/Ta decrease, as well as the contents of Ba and Sr. Naturally, the compositions of such rock-forming minerals as plagioclase and biotite also change. The isotopic characteristics of the granites of the Vatikha (Sr i = 0.70868–0.70923 and eNd 255 from –8.9 to –11.9) and Murzinka (Sr i = 0.70419–0.70549, eNd 255 from –2.6 to +2.3) complexes suggest that the substratum of the former was represented by Proterozoic granite-gneisses, whereas the rocks of the newly formed crust, possibly similar to the Silurian-Devonian volcanogenic-sedimentary rocks, which are at contact with the Murzinka granites, served as the substrate for the latter.

Specifics of Neoarchean Plume–Lithospheric Processes in the Kola–Norwegian Province of the Fennoscandian Shield: I. Composition and Age of the Komatiite–Tholeiite Association

The paper reports newly obtained geological and isotopic-geochemical data on the volcano-sedimentary complex of the Uraguba Neoarchean greenstone structure in the Kola–Norwegian province of the Fennoscandian Shield. New U–Th–Pb geochronologic data (SIMS) on the metadacite (2790 ± 9 Ma) from a rock unit of interbedding metadacite, komatiite tuff, and lava breccia and on veins of plagioclase–microcline granite (2697 ± 10 and 2696 ± 9 Ma) that cuts the komatiite constrain the time span when supracrustal complex of the Uraguba structure was produced and underwent tectono-metamorphic transformations to approximately 100 Ma. The metavolcanic rocks of the komatiite–tholeiite association of the Uraguba structure belong to two distinct isotopic-geochemical types, which are spatially separated from one another and were produced by melting different mantle sources. Geological and isotopic-geochemical data indicate that the Uraguba structure is analogous to such unique tectonic structures on cratons as the Neoarchean Belingwe and Bulawayo belts in the Zimbabwe Shield, Kalgoorlie Belt in the Eastern Goldfilds province at the Yilgarn Craton, Kuhmo–Tipasjarvi Belt in the Karelian epi-Archean craton, and the Warawoona Paleoarchean Belt in the Pilbara Craton.

Geochemical features of rare-metal granites of the Zashikhinsky Massif, East Sayan

The paper presents detailed geochemical data on the rocks of the Zashikhinsky Massif and mineralogical–geochemical characteristics of the ores of the eponymous deposit. The rare-metal granites are divided into three facies varieties on the basis of the degree of differentiation and ore potential: early facies represented by microcline–albite granites with arfvedsonite, middle facies represented by leucocratic albite–microcline granites, and late (most ore-bearing) facies represented by quartz–albite granites grading into albitites. Microprobe data were obtained on major minerals accumulating trace elements in the rocks and ores. All facies of the rare-metal granites, including the rocks of the fluorite–rare-metal vein, define single compositional trends in the plots of paired correlations of rock-forming and trace elements. In addition, they also show similar REE patterns and spidergrams. The latter, however, differ in the depth of anomalies of some elements. Obtained geological, petrographic, and geochemical data suggest a magmatic genesis of the rocks of different composition and their derivation from a single magma during its differentiation. On the basis of all characteristics, the Zashikhinskoe deposit is estimated as one of the largest tantalum rare-metal deposits of alkaline-granite type in Russia.

Crystallogenetic models for metasomatic replacement in zircons: implications for U–Pb geochronology of Precambrian rocks

Recrystallization of zircons under the influence of fluids was studied using examples from Precambrian rocks (microcline granites, metasedimentary, and mafic rocks) of the Kola Peninsula. All zircon crystals showed complex internal textures visible by cathodoluminescence and backscattered electron (BSE) imaging. Detailed mineralogical and geochemical studies with subsequent secondary ion mass spectrometer U–Pb dating of different zircon domains show that secondary texture formation can be interpreted in terms of metasomatic replacement of zircon crystals on the base of crystallogenetic experimental models. Mechanisms of zircon replacement and interpretation of U–Pb ages for secondary zircon domains are dependent on the degree of damage of the zircon structure and the fluid composition. The recrystallization of metamict zircon without additional supply of new zircon substance (Zr, SiO2) goes with the dissolution of amorphous domains and precipitation of new polycrystalline zircon, which preserves the U–Pb initial age, but loses radiogenic lead, and the lower intercept of Discordia lines with the Concordia curve determines the time of fluid influence. The recrystallization of metamict zircon or crystalline zircon with high contents of impurities with additional supply of Si and Zr forms monocrystalline replacements. Dissolution of primary zircon is accompanied by growth of new zircon domains differing in the composition of isomorphic impurities and zones of transitional composition, whose ages have no geological sense. The study is of particular importance for zircons from Precambrian rocks with long and complex histories.

Metasomatic replacements and isotope relationships in zircon crystals and crystallogenetic models

Secondary textures formed in natural zircon crystals under the influence of hydrothermal solutions are interpreted as products of metasomatic substitution. Such textures are clearly seen in metamict zircons and zircons enriched in radioactive elements, which are often found in potassic granites, pegmatites, and other rocks with high U and Th contents. Specific secondary textures in zircons contained in microcline granites, metasedimentary and eclogite-like rocks of the Kola Peninsula are reviewed. Detailed mineralogical and geochemical studies of zircons (CL, BSE, microprobe analysis of incompatible elements) have been carried out with the subsequent U-Pb dating of different zircon zones. The mechanism of secondary texture formation is considered in terms of metasomatic isomorphic substitution. Analogy between natural structure of zircon and products of experimental isomorphic substitution makes it possible to obtain additional information on conditions of the natural zircon growth important for interpreting geochronological data. When metamict zircons are substituted without gaining new material and with a decrease in volume, the U-Pb age of initial zircon remains unchanged and the lower intercept of discordia and concordia corresponds to the time of fluid impact. Substitution with a gain in matter is accompanied by excess volume. In this case, the timing of zones of transitional composition is devoid of geological sense.

Geochemistry of zircons from the proterozoic weathering profile of granitoids in Northern Karelia

For many years zircon has been used in geology as the main mineral-geochronometer, since it is consid� ered to be maximally stable in secondary processes. However, the methodological base of U-Pbzir� conometry (6) postulates the possibility of transforma� tion of the isotopic system in zircon during hypergen� esis. Modern experimental investigations provide evi� dence for significant evolution of the morphology, structure, and composition of zircons during their interaction with aqueous solutions and magmatic flu� ids (3-5). At the same time, ideas about the behavior of accessory zircons during continental weathering are contradictory and not systematized. In this paper we present the first results obtained for zircons from the Paleoproterozoic weathering profile in Northern Karelia. The area of investigations is located in the eastern flange of the PanaKuolayarvinskaya structure, near Kiskinlampi Lake, where Archean granite gneiss sep� arated by the crust of weathering is overlain by sedi� mentary and volcanogenic-sedimentary Jatulian for� mations (2). The basement is composed of medium� granular pink granite gneiss intruded by veins of red microcline granite with a thickness of up to 3 m. The weathering crust developed after granitoids of the basement has a thickness of ~10 m. Its structure includes a zone of physical disintegration of the sub� strate, a zone of eluvial breccia with terrigenous-car� bonate cement, which composes the main part of the hypergene profile, and a clay zone (1).

Rare-metal granitoids of the Ulug-Tanzek deposit (Eastern Tyva): Age and tectonic setting

95 Raremetal magmatism covering the age range of more than 1 billion years, from the Late Riphean to the Triassic, is widely abundant within the southern margin of the Siberian Platform and its folded sur� rounding. Late Riphean massifs of alkali-ultrabasic rocks with carbonatite of the Beloziminsk Complex, Ordovician-Triassic massifs of alkali granite (Arys� kan, Zashikhin, Ermakov, and others), as well as numerous massifs of lithium-fluorine granite includ� ing the spodumene variety, are related to the most wellknown manifestations of raremetal magmatism, which control the formation of large deposits of rare metals, rare elements, and rare earth elements. Until recently, the ideas about their age were based mainly on geological data. Only in the last few years have geo� chronological investigations, which allowed us to esti� mate the age and tectonic conditions of related mag� matic processes in geological evolution of the southern surroundings of the Siberian Platform, been carried out (1). However, the regularities of raremetal mag� matism in the structures of this region are not under� stood completely for some epochs. In particular, this relates to the epoch when the unique complex rare� metal-rareearth deposit UlugTanzek (Eastern Tyva) was formed. The existing estimations of age for host alkaline granitoids vary within very broad limits (229- 217 Ma, K-Ar method by potassic feldspar) and cover the whole Triassic (2). However, this age range in the region is amagmatic as a whole. Because of this, the reliability of age estimations obtained is doubtful and, therefore, there is a question about the geological events that controlled the formation of the deposit. In this paper we present the results of dating of alkaline granitoids of the UlugTanzek Massif by the U-Pb method for zircon and the Ar-Ar method. With account for the geochronological data obtained, we analyzed its position in the structure of magmatic areals of the same age and, on this basis, reconstructed the geodynamic environment of raremetal magma� tism in the corresponding epoch. The UlugTanzek Massif of alkaline granitoids is located in southeastern Tyva within the Sangilen part of the Tyva-Mongol microcontinent, where it intrudes metamorphosed Vendian-Cambrian terrige� nous-carbonate deposits of the Kaakhem structural- facial zone. The massif is composed of quartz-albite- microcline granite with complex (Ta, Nb, Zr, Hf, REE, Li, Th, U, and cryolite) ore.

Paleoproterozoic collisional and intraplate granitoids of the southwest margin of the Siberian craton: Petrogeochemical features and U-Pb geochronological and Sm-Nd isotopic data

Accretion of the Archean blocks of the continental crust 1.95-1.90 Ga ago that caused the formation of the Paleoproterozoic supercontinent Pangea I (1) was accompanied by metamorphism and intensive granite formation in collisional orogenes. Paleoproterozoic collisional granitoids are widely represented in the uplifts of the Eurasian platform basement: on the Ukrainian, Anabar, and AldanoStanovoi shields as well as along the southwestern margin of the Siberian craton. They form extended belts consisting of domi� nating posttectonic microcline granites with minor tonalities, granodiorites and quartz syenites predomi� nantly of sodium-potassium calcalkaline magmatic series. Their characteristic feature is enrichment by largeionic and high field strength elements, light lan� thanoids, uranium, and especially thorium (2). Fur� ther sialization of the crust and increasing of the degree of its geochemical differentiation (maturity) were important consequences of this powerful process of granite formation. The extended belt of the Paleoproterozoic grani� toids marks uplifts of the Siberian craton basement along the modern southwestern and southern margins tracing in the structures of the Yenisey ridge, Prisaya� nie, Pribaikalie, and AldanoStanovoi shield. Forma� tion of the belt was accompanied by the first occur� rence of sodiumpotassium granitoid magmatism, which was the most powerful for this segment of the Earth and occurred in the interval 1.90-1.75 Ga ago. Granite formation was caused by accretioncollisional and, as was established recently (3), subsequent intra� plate processes. In the present work by the example of the investiga� tion of the Taraka massif of the south Yenisey Ridge, the multistage character of the rock associations com� posing the massif varying in age as well as in composi� tion was proved. For the first time, the results of the petrogeochemical and isotopegeochronological investigations of the intraplate (anorogenic) granites were presented. These granites were distinguished by the authors within this massif due to their enrichment in thorium and rare earth elements. The variety of geo� dynamic conditions of granitoid formation and pro� longed multistage evolution of the Paleoproterozoic granite formation were noted. Wider development of anorogenic granites within the mentioned belt was shown.

Geochemistry and geochronology of the Neoproterozoic Pan-African Transcaucasian Massif (Republic of Georgia) and implications for island arc evolution of the late Precambrian Arabian–Nubian Shield

The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNd int = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between ∼ 750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia.

Geology, geochemistry, and U–Pb geochronology of the Archean (2.74 Ga) Serra do Rabo granite stocks, Carajás Metallogenetic Province, northern Brazil

The Carajás region, located in the southeastern part of the Amazon Craton, has been considered one of the most important mineral provinces in the world. The Serra do Rabo Granite (SRG) crops out near the eastern termination of the Carajás fault as two granite stocks, elongated approximately in an E–W direction, concordant with the regional structures. Leucomicrocline granite, hornblende–microcline granite, biotite–hornblende–microcline granite, hornblende syenogranite, and subordinate aplite are identified. The granites are grayish pink and coarse to medium grained and have mainly hypidiomorphic granular texture. Granophyric textures are common. The accessory minerals are ilmenite, apatite, zircon, allanite, and rare pyroxene. The SRG rocks are either massive or foliated, with a slightly anastomosed continuous S 1 foliation (E–W/subvertical) outlined by the preferred orientation of quartz, feldspars, and mafic minerals. Locally, decimeter- to meter-wide mylonite/ultramylonite bands ( S 1m) occur along the E–W foliation. The S 1 foliation was developed under higher temperatures than those of the S 1m mylonite foliation. The SRG structural evolution was controlled by progressive deformation under decreasing temperature, indicative of syntectonic emplacement. The SRG also has relatively high SiO 2, K 2O, and Na 2O contents; high FeO*/(FeO*+MgO) ratios; high Zr, Ba, Nb, and Ga; and very high rare-earth element contents. The chemical signature is moderately alkaline and metaluminous, comparable to that of the A-type, A2, and ALK-3 granites. The origin of the SRG magmas may be related to the partial melting of crustal sources, such as previously metamorphosed calc-alkaline granites. The SRG crosscuts supracrustal rocks, promoting low-pressure/high-temperature metamorphism. The interaction between regional compressive stresses and the ballooning effect of the granite stocks promoted slight aureole flattening and rheological changes in the supracrustal rocks. The U–Pb zircon age of 2743±1.6 Ma is interpreted as the age of zircon crystallization, granite stock emplacement, and regional horizontal shortening. Other 2.7 Ga syntectonic alkaline granites (e.g. Estrela, Plaquê, Planalto) have been reported in the region.

U-Pb ages and Nd isotope characteristics of the lateorogenic, migmatizing microcline granites in southwestern Finland

U-Pb ages and whole-rock Nd isotope data have been obtained from the Paleoproterozoic lateorogenic migmatizing microcline granites of southwestern Finland. Isotope dilution and ion microprobe U-Pb data on zircons and monazites show that the age spectrum of these granites is at least 1.85-1.82 Ga. Commonly, zircons and monazites record the same ages. The age variation in the Veikkola granite area is of the order of 25 Ma and indicates that this seemingly homogeneous granite consists of two separate intrusions. The zircons of the lateorogenic granites are pervasively altered and conventional U-Pb results are commonly discordant. The ion microprobe studies reveal that the granites contain very few inherited zircons with preserved original U-Pb isotope ratios, with the exception of the Oripaa granite. Initial e Nd values, mostly in the range of -0.5 to -1.0, imply a moderate input of older crustal material into most of the lateorogenic granites. A shift from more juvenile to less radiogenic Nd isotope composition is observed from north to south, and the variation pattern of e Nd values of the lateorogenic granites is thus similar to that of the surrounding synorogenic granitoid rocks.

Emplacement mechanisms of late-orogenic granites: structural and geochemical evidence from southern Finland

The country rock in southern Finland formed mainly during the Svecofennian orogeny ca. 1.9 Ga ago. The middle and lower crust was partially melted 1.83 Ga ago due to crustal thickening and subsequent extension. During this event, S-type migmatites and granites were formed along a 100×500 km zone. This Late Svecofennian Granite–Migmatite zone (LSGM zone) is a large crustal segment characterised by roughly E–W trending sub-horizontal migmatites and granites. Combined ductile E–W shear movements and NNW–SSE compressional movements defined a transpressional tectonic regime during the emplacement. Partial melts that moved through the crust pooled as granite sheets or froze as migmatites. Major transpressive shear zones border the LSGM zone, which forms a tectonic and metamorphic zone that crosscuts the earlier Svecofennian granitoids. Based on field observations and geochemical data from two sets of outcrops, we show that the great volumes of late-orogenic granites and migmatites in southern Finland were transported and emplaced as small chemically variable batches, possibly extracted from different protoliths. These melt batches were transported along repeatedly activated channels and collected at some horizontal level in the crust. In the Nagu area, the melt batches were trapped under a roof-layer of amphibolite and the whole complex was synchronously folded into open folds with steep axial surfaces and E–W trending fold axes. The sheets of microcline granite are, in places, strongly sheared; the microcline phenocrysts are imbricated and subsequent deformation of the microcline phenocrysts indicates syn-tectonic movements of the layers as well as a syn-tectonic mechanism for the late-magmatic fractionation. Depending on the degree of crystallisation of the individual melt batches during shearing at different intensities, the granites have slightly different appearances. Some sheared zones show a cumulate-like trace element geochemistry, indicating that melt fractions were expelled from the system, producing layers of deformation enhanced fractionated granites and cumulate layers. Our interpretation is that the Nagu area shows shear-assisted fractionation mechanisms in granitic melts, and that similar processes are responsible for the fractionation trends seen in the sub-horizontal sheeted granites in Hameenlinna at higher levels in the crust.

Timing and geochemistry of potassic magmatism in the eastern part of the Svecofennian domain, NW Ladoga Lake Region, Russian Karelia

Abstract The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.

The late Svecofennian granite-migmatite zone of southern Finland—a belt of transpressive deformation and granite emplacement

Abstract The late Svecofennian granite-migmatite (LSGM) zone in southwestern Finland is a ∼ 100 km wide and 500 km long belt transecting the southern Svecofennides from WSW to ENE. It was formed in an area of thin pillow lavas, volcaniclastic sediments and limestones. The area is interpreted as having been an early basin of crustal extension which was the locus of an inherited zone of weakness in the Proterozoic crust. Early recumbent folding was followed by crustal thickening and intrusions of ∼ 1.89-1.88 Ga old plutonics. The LSGM-zone is characterized by 1.84-1.83 Ga old rhomboidal sheets of late Svecofennian microcline granite and is bounded by ductile shears. Amongst the two major phases of deformation defined in the LSGM-zone, the earlier one (D1) affected only the supracrustals and the 1.89-1.88 Ga old early plutonics. In contrast, the later phase (D2) also deformed the late Svecofennian migmatites and granites. D1 represents a complex and long-lasting deformation event which included overturning and thrusting of the Svecofennian strata. D2 comprised ENE-WSW directed drag accompanied by NNW-SSE compression. The Svecofennian crust was thickened further and anatectic microcline granites intruded along thrusts. The rhomboidal outline of the late Svecofennian granite sheets indicates a sense of movement in agreement with measured dextral strike-slip in the shears delimiting the LSGM-zone. Imbricated feldspar megacrysts in the granites indicate thrusting towards the west during the stage of granitic magmatism. The gently dipping early Svecofennian gneisses and the late granite sheets were folded into upright F2 folds with gently plunging axes. Locally, the F2 axial surfaces were intruded by late Svecofennian granite mobilisates.