Granulite facies metamorphism and metasomatism in the gabbro-anorthosites of the Kolvitsa massif,Kola Peninsula

The investigation of the Kolvitsa gabbro-anorthosite massif showed that its melanocratic layers conformable with metamorphic banding are mafic differentiates transformed into eclogite-like rocks during prograde metamorphism. During the peak and retrograde stage of the Svecofennian metamorphism in the White Sea region at t = 910–750°C and P = 14-7 kbar, the infiltration of Fe-, CO₂-, Si-, and Na-bearing fluids with X_{H2 O} < 0.4X_{H_2 O} < 0.4 resulted in metasomatic alterations of the melanocratic gabbro-anorthosite interlayers, dissolution of a number of elements, and their reprecipitation with the formation of cross-cutting zoned metasomatic veins with abundant magnetite and ilmenite. The high content of hematite in the ilmenite suggests that the veins were formed at an increase in oxygen fugacity from logf_O2 = - 14.5\log f_{O_2 } = - 14.5 to logf_O2 = - 11\log f_{O_2 } = - 11, which caused the Fe²⁺ → Fe³⁺ transition and iron precipitation. The increase in at the conditions corresponding to the metamorphic peak was probably related to the neutralization of solutions during their infiltration through the gabbro-anorthosites. The reprecipitation of components and the formation of cross-cutting veins occurred owing to interaction between the melanocratic layers in the gabbro-anorthosites and a fluid phase and, contrary to previous models, did not involve the fluid transport of components from the zones of charnockite formation and granitization located far away from the sites of reprecipitation. This is demonstrated by the similarity of mineral compositions and major component contents in the melanocratic gabbro-anorthosite layers and cross-cutting metasomatic veins and regular distribution of trace elements.     

Local disequilibrium of plagioclase in high-temperature shear zones of the Ivrea Zone, Italy

Abstract Microstructural and chemical analysis of plagioclase in 20 superficially similar amphibolite facies ductile shear zones in metagabbors and amphibolites of the Ivrea Zone in Italy reveals significant differences in An and Ba contents. Plagioclase, which was deformed at P-T conditions lower than those of the wall rocks, occurs in the following four different microstructural situations with different chemical compositions: (i) relatively undeformed porphyroclasts, (ii) dynamically recrystallized grains and subgrains rimming the porphyroclasts, (iii) infill of microcracks cross-cutting the porphyroclasts and (iv) fine-grained recrystallized grains in the matrix of the shear zones. The differences in the An and Ba contents are caused by partial chemical equilibration of plagioclase in the shear zones during and partly after deformation. Changes in An and Ba contents were caused by fluid-assisted grain-boundary migration recrystallization, as well as by solid-state diffusion, while fluid activity was high. The relation between the composition and microstructures of the plagioclase in the shear zones indicates that in the different shear zones, fluids ceased to be active during different stages in the late shear zone deformation history.
The interpretation of the variations in composition and microstructures reveals that only grains that developed by grain-boundary migration recrystallization and that are not adjacent to porphyroclasts reflect P-T conditions during the dominant shear-zone deformation.     

P-T paths and tectonic evolution of shear zones separating high-grade terrains from cratons: examples from Kola Peninsula (Russia) and Limpopo Region (South Africa)

Summary ?The petrology and P-T evolution of mica schists from two regional scale tectonic (shear) zones that separate high grade terrains (“mobile belts”) from cratons are described. These are the 2.4–1.9 Ga Tanaelv Belt, a suture zone that separates the Lapland granulite complex from the Karelian craton (Kola Peninsula–Fennoscandia), and the 2.69 Ga Hout River Shear Zone that separates the > 2.9 Ga Kaapvaal craton from the 2.69 Ga South Marginal Zone of the Limpopo high-grade terrain (South Africa). Two metamorphic zones are identified in strongly deformed mica schists from the 1.9 Ga Korva Tundra Group of the Tanaelv belt: (1) a chlorite-staurolite zone tectonically overlaying gneisses of the Karelian craton, and (2) a kyanite-biotite zone tectonically underlying garnet amphibolites of the Tanaelv Belt, which are in tectonic contact with the Lapland granulite complex. The prograde reaction Chl+St+Ms ↠ Ky+Bt+Qtz+H₂O clearly defines a boundary between zones (1) and (2). Rotated garnet porphyroblasts from zone (1) contain numerous inclusions (Otz, Chl, Ms), and show clear Mg/Fe chemical zoning, suggesting garnet growth during prograde metamorphism. The metamorphic peak, T = 650°C and P = 7.5 kbar, is recorded in the kyanite-biotite zone and characterized by unzoned snowball garnet. In many samples of mica schists euhedral garnet porphyroblasts of the retrograde stage are completely devoid of mineral inclusions while N _Mg decreases from core to rim, indicating a decrease in P-T from 650°C, 7.5 kbar to 530°C, 5 kbar. The Hout River Shear Zone (South Africa) shows metamorphic zonation from greenschists through epidote amphibolites to garnet amphibolites. Rare strongly deformed mica schists (Chl+Grt+Pl+Ms+Bt+Qtz) occur as thin layers among epidote-amphibolites only. Garnet porphyroblasts in the schists are similar to that of the Tanaelv Belt recording a prograde P-T path with peak conditions of T = 600°C and P∼ 5.5 kbar. The retrograde stage is documented by the continuous reaction Prp+2Ms+Phl ↠ 6Qtz+3East recording a minimum T = 520°C and P ∼ 3.3 kbar. Similar narrow clock-wise P-T loops recorded in mica schists from both studied shear zones suggest similarities in the geodynamic history of both shear zones under consideration.

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Zusammenfassung ?P-T Pfade und tektonische Entwicklung von Scherzonen, die hochgradige Terranes von Kratonen trennen: Zwei Beispiele von der Halbinsel Kola (Russland) und der Limpopo-Region (Südafrika) Die Petrologie und P-T Entwicklung von Glimmerschiefern aus zwei regionalen tektonischen Scherzonen, die hochgradige Terranes (“mobile belts”) von Kratonen trennen, werden beschrieben. Diese sind der 2.4−1.9 Ga Tanaev Belt, eine Suturzone, die die Lappland Granulite vom karelischen Pluton (Halbinsel Kola - Fennoskandien) trennt, sowie die 2.69 Ga Hout River Shear Zone, die den > 2.9 Ga Kaapvaal Kraton von der 2.69 Ga South Marginal Zone des hochgradigen Limpopo Terranes (Südafrika) trennt. Zwei metamorphe Zonen sind in stark deformierten Glimmerschiefern der 1.9 Ga Korva Tundra Group zu unterscheiden: (1) eine Chlorit-Staurolith-Zone, die den Gneisen des karelischen Kratons auflagert, und (2) eine Kyanit-Biotit-Zone, die die Granatamphibolite des Tanaev Belt unterlagert und in tektonischem Kontakt mit dem Lappland Granulitkomplex steht. Die prograde Reaktion Chl+St+Ms ↠ Ky+Bt+Qtz+H₂O trennt die beiden Zonen. Rotierte Granatporphyroblasten aus der Zone (1) enthalten zahlreiche Einschlüsse (Qtz, Chl, Ms) und zeigen eine Mg/Fe Zonierung, die Granatwachstum w?hrend des prograden Metamorphosestadiums nahelegen. Der Metamorphoseh?hepunkt (650°C, 7.5 kbar) wurde in der Kyanit-Biotit-Zone erreicht und ist durch nicht zonierte Schneeballgranate charakterisiert. In vielen Glimmerschieferproben sind die euhedralen Granatporphyroblasten des retrograden Stadiums vollkommen einschlu?frei und N _Mg nimmt vom Kern zum Rand hin ab. Das zeigt eine Abnahme der P-T Bedingungen von 650°C, 7.5 kbar auf 530°C, 5 kbar an. Die Hout River Shear Zone in Südafrika zeigt eine metamorphe Zonierung von Grünschiefern, über Epidotamphibolite zu Granatamphiboliten. Selten kommen stark deformierte Glimmerschiefer (Chl+Grt+Pl+Ms+Bt+Qtz) als dünne Lagen zwischen den Epidotamphiboliten vor. Die Granatporphyroblasten sind ?hnlich wie die aus dem Tanaev Belt und belegen eine prograde P-T Entwicklung mit Peak-Bedingungen von 600°C und ≈ 5.5 kbar. Das retrograde Stadium ist durch die kontinuierliche Reaktion Prp+2Ms+Phl ↠ 6Qtz+3East mit minimal 530°C und ≈ 3.3 kbar dokumentiert. Die sehr ?hnlichen P-T Pfade der Glimmerschiefer belegen ?hnlichkeiten in der geodynamischen Geschichte der beiden bearbeiteten Scherzonen.

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Received January 29, 1999;/revised version accepted August 10, 1999     

The formation conditions of labuntsovite-group minerals in the Kovdor massif,Kola Peninsula

In the Kovdor massif, labuntsovite-group minerals occur in dolomite carbonatite veins (labuntsovite-Mg), in a natrolite-calcite vein (lemmleinite-Ba and labuntsovite-Fe), and in calcite pockets and veinlets cutting fenites (late labuntsovite-Mg). They are closely intergrown with paragenetic carbonates, and this makes it possible to estimate their crystallization temperature from the fluid inclusions entrapped in dolomite (≥265°C) and calcite (175–225°C). The earlier labuntsovite-Mg was formed under relatively acidic conditions, whereas later labuntsovite-calcite mineralization was derived from alkaline solutions.     

Chromium mineralization in the Khibiny alkaline massif (Kola Peninsula,Russia)

This paper presents new data on chromium mineralization in a fenitized xenolith in Mt. Kaskasnyunchorr in the Khibiny alkaline massif (Kola Peninsula, Russia) and summarizes data on Cr mineralogy in the Khibiny Mountains. Protolith silicates that contained Cr³⁺ admixture are believed to be the source of this element in the fenite. Cr-bearing (maximum Cr₂O₃ concentrations, wt %, are in parentheses) aegirine (5.8), crichtonite-group minerals (2.1), muscovite (1.3), zirconolite (1.1), titanite (1.0), fluorine-magnesioarfvedsonite (0.8), biotite (0.8), ilmenite (0.6), and aenigmatite (0.6) occur in the fenite. The late-stage spinellides of the FeTi-chromite-CrTi-magnetite series, which are very poor in Mg and Al and which formed after Crrich aegirine and ilmenite, are the richest in Cr (up to 42% Ct₂O₃). Cr concentrations grew with time during the fenitization process. Unlike minerals in the Khibiny ultramafic rocks where Cr is associated with Mg, Al (it is isomorphic with Cr), and with Ca, chromium in the fenites is associated with Fe, Ti, and V (with which Cr³⁺ is isomorphic) and with Na in silicate minerals. Cr³⁺ Mobility of Cr³⁺ and the unique character of chromium mineralization in the examined fenites were caused by high alkalinity of the fluid.     

Loparite-(Ce) from the Khibiny Alkaline Pluton,Kola Peninsula,Russia

Data on the occurrence, morphology, anatomy, composition, and formation conditions of loparite-(Ce) in the Khibiny alkaline pluton are given. Loparite-(Ce), (Na,Ce,Sr)(Ce,Th)(Ti,Nb)2O6, resulted from metasomatic alteration and assimilation of metamorphic host rocks at the contact with foyaite as well as foyaite on the contact with foidolite. This alteration was the highest in pegmatite, and albitite developed there. A decrease in temperature resulted in enrichment of the perovskite and tausonite endmembers in loparite-(Ce) owing to a decrease in the loparite and lueshite endmembers. La and Ce sharply predominate among rare earth elements in the composition of loparite-(Ce).     

K-rich fluid metasomatism at high-pressure metamorphic conditions: Lawsonite decomposition in rodingitized ultramafite of the Maksyutovo Complex, Southern Urals (Russia)

Fine grained rodingite‐like rocks containing epidote, clinozoisite, garnet, chlorite, phengite and titanite occur within antigorite serpentinite boudins from the high‐pressure metamorphic Maksyutovo Complex in the Southern Urals. Pseudomorphs after lawsonite, resorption of garnet by chlorite and phengite and stoichiometry suggest the reaction lawsonite + garnet + K‐bearing fluid → clinozoisite + chlorite + phengite, and define a relic assemblage of lawsonite + garnet + chlorite + titanite ± epidote as well as a later post‐lawsonite assemblage of clinozoisite + phengite + chlorite + titanite. The reaction lawsonite + titanite → clinozoisite + rutile + pyrophyllite + H₂O delimits the maximum stability of former lawsonite + titanite to pressures >13 kbar. P–T conditions of 18–21 kbar/520–540 °C result, if the average chlorite, Mg‐rich garnet rim and average epidote compositions are used as equilibrium compositions of the former lawsonite assemblage. These estimates indicate a similar depth of formation but lower temperatures to those recorded in nearby eclogites. The metamorphic conditions of the lawsonite assemblage are considerably higher than previously suggested and, together with published structural data, support a model in which a normal fault within the Maksyutovo complex acted as the major transport plane of eclogite exhumation. The maximum Si content of phengite and minimum Fe content in clinozoisite constrain the metamorphic conditions of the later pseudomorph assemblage to be >4.5 kbar and <440 °C. Rb–Sr isotopic dating of the pseudomorph assemblage results in a formation age of 339 ± 6 and 338 ± 5 Ma, respectively. These results support the recent exhumation models for this complex.     

Pleistocene marine deposits in the coastal areas of Kola Peninsula (Russia)

On the basis of field data, datings from both electron spin resonance – and optically stimulated luminescence, and micro- and macrofauna, in addition to presence of diatoms, three Late Pleistocene marine units have been identified in the coastal areas of the Kola Peninsula. The stratigraphically lowest sequence is correlated to the Ponoi Beds and the Boreal transgression, attributed to the marine isotope stages (MIS) 5e to 5d in the White Sea depression and to MIS 5e to 5c in the Barents Sea. Thermophilic fauna and diatoms indicate normal water salinity and a water temperature above zero. The second marine unit, referred as the Strel'na Beds, can be correlated with the Early Weischselian transgression, termed the Belomorian transgression. With low water salinity and a water temperature similar or colder than the present times, Belomorian transgressions are reliably detected in the White Sea and are not clearly found in the Barents Sea. The results obtained from the sediments of the Ponoi and Strel'na Beds indicate a continuously existing marine reservoir from 130 to 80–70 ka ago (entire MIS 5) in the White Sea depression. The early Middle Weichselian Barents–Kara ice-sheet invasion and its recession might have caused the glacioeustatic Middle Weichselian (MIS 3) transgression, and the third Late Pleistocene marine sequence has been deposited in the regressing shallow cold sea with less saline waters. The results help in the understanding of the history of Late Quaternary ice sheets in North Eurasia and provide evidence for the debatable Early and Middle Weichselian marine events.     

Rb-Sr age of metasomatism and ore formation in the low-temperature shear zones of the Fenno-Karelian CRATON,Baltic Shield

Manifestations of the main types of metasomatites (beresite, propylite, listwaenite, aceite, and gumbeite) were identified in the shear zones of the Fenno-Karelian craton on the basis of the previously proposed systematics of metasomatic facies. These metasomatites were formed in shear stress environments, which determined their morphological features, in particular, finely banded texture. Comparatively low-temperature conditions of infiltration process and salting out effect (reduction of CO₂ solubility with increasing salt content) lead to the heterogenization of fluid into two phases: aqueous salt solution and almost pure CO₂. This results in more aggressive and mobile behavior of the fluid, and, correspondingly, more intense differentiation of the matter and contrast in metasomatic banding. Relations between metasomatic parageneses indicate an evolution trend of the processes from propylite, beresite and listwaenite to alkaline varieties and their repeated manifestation in the same shear zones. The results of Rb-Sr isochron dating of ore metasomatites from eight deposits and occurrences of the Fenno-Karelian craton (more than 100 samples of rocks and minerals) confirm previous assumptions. In general obtained data show that the shear zones controlling the distribution of the studied occurrences operated as fluid pathways during a long time period, up to 200 Ma, after the Svecofennian orogeny completion and did not show any correlation with Paleoproterozoic and Neoarchean magmatism. Rb-Sr isotope data on the metasomatites indicate three peaks of the post-Svecofennian metallogenic activity: 1700–1780, 1600–1650, and 1400–1500 Ma. Since the studied ore deposits were formed within tectonic structures, which evolved on the Archean crust and have a long prehistory, and fluid flows were subjected to intensive contamination by ancient crustal material, a relatively high initial Sr isotope ratios of formed ore-metasomatic systems were developed. High variablity of this value in the studied rocks ((⁸⁷Sr/⁸⁶Sr)₀ from 0.706 to 0.750) is related to the heterogeneity of crustal protolith and to the relative storage and manifestation of the juvenile component of the fluid, which was responsible for the metasomatic transformation of the Archean and Paleoproterozoic rocks and ore-deposit formation.     

Deformation-enhanced metamorphic reactions and the rheology of high-pressure shear zones, Western Gneiss Region, Norway

Microstructural and petrological analysis of samples with increasing strain in high‐pressure (HP) shear zones from the Haram garnet corona gabbro give insights into the deformation mechanisms of minerals, rheological properties of the shear zone and the role of deformation in enhancing metamorphic reactions. Scanning electron microscopy with electron backscattering diffraction (SEM–EBSD), compositional mapping and petrographic analysis were used to evaluate the nature of deformation in both reactants and products associated with eclogitization. Plagioclase with a shape‐preferred orientation that occurs in the interior part of layers in the mylonitic sample deformed by intracrystalline glide on the (0 0 1)[1 0 0] slip system. In omphacite, crystallographic preferred orientations indicate slip on (1 0 0)[0 0 1] and (1 1 0)[0 0 1] during deformation. Fine‐grained garnet deformed by diffusion creep and grain‐boundary sliding. Ilmenite deformed by dislocation glide on the basal and, at higher strains, prism planes in the a direction. Relationships among the minerals present and petrological analysis indicate that deformation and metamorphism in the shear zones began at 500–650 °C and 0.5–1.4 GPa and continued during prograde metamorphism to ultra‐high‐pressure (UHP) conditions. Both products and reactants show evidence of syn‐ and post‐kinematic growth indicating that prograde reactions continued after strain was partitioned away. The restriction of post‐kinematic growth to narrow regions at the interface of garnet and plagioclase and preservation of earlier syn‐kinematic microstructures in older parts layers that were involved in reactions during deformation show that diffusion distances were significantly shortened when strain was partitioned away, demonstrating that deformation played an important role in enhancing metamorphic reactions. Two important consequences of deformation observed in these shear zones are: (i) the homogenization of chemical composition gradients occurred by mixing and grain‐boundary migration and (ii) composition changes in zoned metamorphic garnet by lengthening diffusion distances. The application of experimental flow laws to the main phases present in nearly monomineralic layers yield upper limits for stresses of 100–150 MPa and lower limits for strain rates of 10^?12 to 10^?13 s^?1 as deformation conditions for the shear zones in the Haram gabbro that were produced during subduction of the Baltica craton and resulted in the production of HP and UHP metamorphic rocks.     

Free hydrogen-hydrocarbon gases from the Lovozero loparite deposit (Kola Peninsula,NW Russia)

The Lovozero nepheline-syenite massif in the north-eastern Fennoscandian Shield, well-known to mineralogists and petrologists, is also interesting with its high contents of hydrogen-hydrocarbon gases in different forms of presence, which is untypical of magmatic rocks. The article systematizes and generalizes little known and unpublished data on the composition, location, character and scale (intensity) of the free gases (FG) emission within a major loparite deposit confined to the massif. СН₄ and Н₂ are dominant in the FG composition. The molecular weight distribution of hydrocarbon gas components corresponds to the classic Anderson–Schulz–Flory distribution with a steep gradient. Carbon and hydrogen of the gases are characterized by rather heavy isotope compositions, becoming lighter from the transition of methane to ethane. The FG volume has been estimated as 0.2–1.6 m³ of gas per 1 m³ of undisturbed rock. The gas recovery of walls in underground workings has been up to 0.2 ml/min/m² for СН₄ and 0.5 ml/min/m² for Н₂ in several years after their heading. The discharge of some shot holes that characterizes the gas emission intensity (1.8–2 m deep and 40 mm in diameter) is up to 300 ml/min, but its 1–2 orders lesser values dominate. The discharge time in some sections varies from several days to 20 years. The overpressure of gases towards the air mainly does not increase 100 hPa, sometimes reaching 120 kPa. It has been defined, that FG distribute irregularly (at the distance of centimeters to hundreds of meters) and their composition and particularly emission intensity perform different temporal fluctuations. The abiogenic origin of FG has been proposed, with FG appearing as a mixture of gases in various proportions: (a) gases remaining in microfissures at the massif's consolidation after the capture by fluid inclusions and those lost during degassing and (b) gases occurred in mechanic-chemical reactions, partial emission and concentration of occluded and diffusely scattered gases under the unstable stress-strain mode of the rock mass. Combustible and explosive hydrogen-hydrocarbon FG can accumulate in the air of underground workings and cause accidents, disrupting the workflow. The background for using characteristics of spatial-temporal variations of the FG emission as precursors of dangerous geodynamic phenomena has been indicated.     

Structural and compositional zoning and formation conditions of the Greater Eastern Litsa BIF occurrence, Kola Peninsula

New geological and geochronological data on the Greater Eastern Litsa banded iron formation (BIF) occurrence demonstrate its similarity to the BIF of the Olenegorsk iron district in geology (lenticular orebodies with exponential distribution of their sizes), age (2.8 Ga), and typical structural and compositional zoning of orebodies. The temperature of ore formation (600–780°C) and BIF composition depend on the intensity of folding expressed in the fractal dimension D = 1.0–1.3 of a single layer. All BIF deposits of the Kola-Norwegian Megablock, including the Greater Eastern Litsa occurrence pertain to the one system. Variation m their composition is controlled by the size of orebodies (capacity of oxygen buffer) and the energy of metamorphic reactions, which strongly depend on the intensity of folding.