Аннотация. Приводятся уточненные данные по U-Pb (in situ) возрасту циркона из Кейвских щелочных г... more Аннотация. Приводятся уточненные данные по U-Pb (in situ) возрасту циркона из Кейвских щелочных гранитов Кольского п-ова и связанных с ними редкометалльных образований. Результаты SIMS датирования циркона из самих щелочных гранитов показывают, что его кристаллизация происходила в интервале 2660-2670 млн лет, при этом наиболее точная датировка, 2673.5 ± 0.3 млн лет, получена методом CA-ID-TIMS. U-Pb датирование по SIMS и LA-ICP-MS данным показывает, что формирование редкометалльных пегматитов, кварцолитов и метасоматитов приурочено к магматической стадии. При этом циркон из кварцолитов был значительно перекристаллизован в ходе Свекофенского метаморфизма-1770-1790 млн лет назад. Заключительная стадия ремобилизации (амазонитовые пегматиты) и/или перекристаллизации (кварцолиты) редкометалльной минерализации может быть связана с флюидо-термальной активизацией Фенноскандинавского щита в период 1670-1690 млн лет. Впервые проведенные изотопные исследования кислорода в цирконе, породообразующих минералах и валовых породах Кейвского комплекса показали, что значительную роль в эволюции его магмато-гидротермальной системы имели метеорные воды.
Аннотация. В статье приведены данные по минеральному составу, морфологии и химизму рудных и акцес... more Аннотация. В статье приведены данные по минеральному составу, морфологии и химизму рудных и акцессорных минералов гематитовых брекчий в песчаниках Терского берега. Гематитовые брекчии состоят из обломков песчаников терской свиты и гематитового цемента, состоящего из мелкокристаллического пластинчатого гематита. Описаны парагенетические REE-содержащие и сульфатные минералы (паризит-(Ce), монацит-(Ce), REE-Nb-Ti-оксид, барит). Текстурно-структурные особенности, минерализация и химический состав рудного минерала (низкотитанистый гематит с повышенным содержанием W и V) гематитовых брекчий месторождения Мыс Корабль являются характерными особенностями для месторождений IOCG-типа (Fe-оксидных Au-Cu). Это позволяет предположить их сходный механизм формирования. Предложена модель образования железооксидных рудопроявлений месторождения Мыс Корабль при участии уникальных по составу (селективно железистых) гидротермальных растворов. В связи с вышеперечисленным, изучение железооксидных рудопроявлений месторождения Мыс Корабль является весьма актуальным как с теоретической, так и с практической стороны.
The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs a... more The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs and several dikelike nepheline syenite bodies. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late-magmatic and post-magmatic (including pegmatitic) processes. The Sakharjok nepheline syenite pegmatite contains a remarkably diverse number of britholite group minerals, pointing to different physico-chemical conditions in the fluid. REE and actinides distribution in the host rock indicates that the late-magmatic (and pegmatitic) fluids were alkaline, with significant amounts of F and CO 2. From REE and F variations of the britholite group minerals possible fluid compositions at different stages are suggested. The earliest fluorbritholite-(Ce) formed locally from a late magmatic, high temperature F-rich fluid. Fluorbritholite-(Y) presumably crystallized from a Fbearing and CO 2-rich fluid; marked F saturation resulted in precipitation of abundant fluorite due to a temperature drop. Variations in REE and F contents in the most abundant fluorcalciobritholite indicate a successive decrease of F in the fluid during its evolution. The relationship between intergrown fluorapatite and fluorcalciobritholite and the presence of zones with a REE-rich fluorapatite between them indicate a continuous to sudden crystallization in this mineral sequence. The сrystallization of the latest Bcalciobritholite^is related to the input into the fluid of CO 2 and/or H 2 O.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The present work reports new mineralogical and whole rock geochemical data from the Breivikbotn s... more The present work reports new mineralogical and whole rock geochemical data from the Breivikbotn silicocarbonatite (Seiland igneous province, North Norway), allowing conclusions to be drawn concerning its origin and the role of late fluid alteration. The rock shows a rare mineral association: calcite + pyroxene + amphibole + zeolite group minerals + garnet + titanite, with apatite, allanite, magnetite and zircon as minor and accessory minerals, and it is classified as silicocarbonatite. Calcite, titanite and pyroxene (Di36-46 Acm22-37 Hd14-21) are primarily magmatic minerals. Amphibole of hastingsitic composition has formed after pyroxene at a late-magmatic stage. Zeolite group minerals (natrolite, gonnardite, Sr-rich thomsonite-(Ca)) were formed during hydrothermal alteration of primary nepheline by fluids/solutions with high Si-Al-Ca activities. Poikilitic garnet (Ti-bearing andradite) has inclusions of all primary minerals, amphibole and zeolites, and presumably crystallized metasomatically during a late metamorphic event (Caledonian orogeny). Whole rock chemical compositions of the silicocarbonatite differs from the global average of calciocarbonatites by elevated silica, aluminium, sodium and iron, but show comparable contents of trace elements (REE, Sr, Ba). Trace element distributions indicate within-plate tectonic setting of the carbonatite. The spatial proximity of carbonatite and alkaline ultramafic rock (melteigite), the presence of "primary nepheline" in carbonatite together with the trace element distributions indicate that the carbonatite was derived from crystal fractionation of a parental carbonated foidite magma. The main prerequisites for the extensive formation of zeolite group minerals in silicocarbonatite are revealed.
The present work reports on new mineralogical and whole-rock geochemical data from the Breivikbot... more The present work reports on new mineralogical and whole-rock geochemical data from the Breivikbotn silicocarbonatite (Seiland igneous province, North Norway), allowing conclusions to be drawn concerning its origin and the role of late fluid alteration. The rock shows a rare mineral association: calcite + pyroxene + amphibole + zeolite group minerals + garnet + titanite, with apatite, allanite, magnetite and zircon as minor and accessory minerals, and it is classified as silicocarbonatite. Calcite, titanite and pyroxene (Di 36-46 Acm 22-37 Hd 14-21) are primarily magmatic minerals. Amphibole of mainly hastingsitic composition has formed after pyroxene at a late-magmatic stage. Zeolite group minerals (natrolite, gonnardite, Sr-rich thomsonite-(Ca)) were formed during hydrothermal alteration of primary nepheline by fluids/solutions with high Si-Al-Ca activities. Poikilitic garnet (Ti-bearing andradite) has inclusions of all primary minerals, amphibole and zeolites, and presumably crystallized metasomatically during a late metamorphic event (Caledonian orogeny). Whole-rock chemical compositions of the silicocarbonatite differs from the global average of calciocarbonatites by elevated silica, aluminium, sodium and iron, but show comparable contents of trace elements (REE, Sr, Ba). Trace element distributions and abundances indicate within-plate tectonic setting of the carbonatite. The spatial proximity of carbonatite and alkaline ultramafic rock (melteigite), the presence of "primary nepheline" in carbonatite together with the trace element distributions indicate that the carbonatite was derived by crystal fractionation of a parental carbonated foidite magma. The main prerequisites for the extensive formation of zeolite group minerals in silicocarbonatite are revealed.
The behaviour of ThSiO4 during low-temperature alteration has significance for element mobility a... more The behaviour of ThSiO4 during low-temperature alteration has significance for element mobility and redistribution. Here we describe five types of alteration of ThSiO4 by hydrothermal fluids: (1) primary ThSiO4 associated with chevkinite-(Ce) in a quartz-epidote metasomatite; (2) during alteration of monazite-(Ce) in a quartzolite; (3) during alteration of fergusonite-(Y) in a quartz-epidote metasomatite; (4) following exsolution from chevkinite-(Ce); and (5) associated with cerite-(Ce) and with ilmenite and bastnäsite-(Ce) in late-stage veinlets in a syenitic pegmatite and a metasomatite. The great majority of crystals have been strongly altered compositionally, with variable degrees of replacement of formula elements by non-formula elements, such as Ca, Fe, P and REE. The most reliable geochemical indicators of hydrothermal alteration are low analytical totals and non-stoichiometric structural formulae. The alteration is variably ascribed to dissolution-reprecipitation and pervasi...
Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of... more Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth's history remain poorly constrained. Here we show how the climate-sensitive δ 18 O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ 18 O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ 18 O values as low as −8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ 18 O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ 17 O approach, these rocks reveal that the precipitation had a δ 18 O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.
Kolmozero-Keivy zone, NE Baltic shield, is composed mainly from basic-intermediate-acid metavolca... more Kolmozero-Keivy zone, NE Baltic shield, is composed mainly from basic-intermediate-acid metavolcanic suites of 2.83-2.87 Ga age and metamorphosed at amphibolitic facies during 1.8-1.7 Ga. Younger igneous events are represented by voluminous intrusions of 2.73 Ga plagio-microcline granites and of 2.65 Ga peralkaline A-type granites, and small stocks of 2.52 Ga tourmaline granites. Four pegmatite fields with total amount of more than 100 bodies are confined to Kolmozero subzone. The pegmatites intrude amphibolites and rarely associated gabbro-anorthosite and are of 50-700m long and of 10-35m thickness. The Kolmozero pegmatites are of complex type, spodumene subtype with Li, Cs, Be, Ta, Sn geochemical signature and belong to LCT family by classification of Cerny and Ercit [1]. They crystallized at relatively high pressure (3-4 kbar) with peralumunious S-type granite as the source magma. The Vasin Myl’k pegmatite field with the lepidolite–albite–microcline–spodumene–pollucite associatio...
low δ 18 O systems The modern hydrological cycle includes precipitation of 18 O-depleted meteoric... more low δ 18 O systems The modern hydrological cycle includes precipitation of 18 O-depleted meteoric water over continental crust exposed above sea-level. How far back in the geological record can continental exposure be documented remains uncertain, particularly for the Archean. To investigate the extent of Archean continental exposure, we document a newly discovered low-δ 18 O magmatic-hydrothermal system that was emplaced in a shallow crust of the Keivy complex, Kola craton at 2.67 Ga. This study builds a case for syn-emplacement hydrothermal exchange with the local meteoric water that had δ 18 O at least as low as −11 VSMOW. Coupled with the high-latitude position of the Kola craton at the time, this result presents a paleogeographic documentation of continental exposure and a quantitative estimate of the isotope composition of Neoarchean precipitation. We employ detailed δ 18 O mapping using mineral separates and bulk samples, as well as ion microprobe δ 18 O measurements and U-Pb dating of zircon. The spatially extensive O-isotope dataset (n = 63) spans over 120 km in length across different lithological units including the peralkaline granites of the Keivy complex, the hosting gneiss and contact hydrothermal quartzolites. All analyzed samples, including whole rock samples and mineral separates, show δ 18 O below those expected for mantle-and crust-derived magmas. The lowest δ 18 O of ca. −7 are measured in minerals and bulk rock of the altered gneisses near the intrusive contacts. Peralkaline granites returned δ 18 O values ranging between −6 and +5. Zircon crystals with preserved igneous zoning have low δ 18 O = +2.2 and yield an emplacement age for the magmatic complex of 2672 ± 7 Ma. Zircon dating from quartzolites at the contact between granites and country gneisses points to fluid-rock interaction at 2.67 and at 1.77-1.79 Ga. We propose that the spatially extensive O-isotope depletion documents a Neoarchean shallow magmatic-hydrothermal system where magmatic assimilation produced the low-δ 18 O granites and the hydrothermal cycling of meteoric water in the ambient crust resulted in formation of low-δ 18 O altered host rocks. Since the lowest δ 18 O value measured in a rock is ca. −7 , the equilibrium fluids had δ 18 O of around −9 under hydrothermal conditions (300-400 • C) characteristic for the shallow crust around cooling intrusions. Considering that hydrothermal fluids are shifted 1-2 towards higher values, the contemporaneous precipitation had δ 18 O = −11 or lower. Consequently, the 2.67 Ga Keivy complex is the earliest known intact low-δ 18 O magmatic-hydrothermal system offering a quantitative record of the Neoarchean precipitation that can be used as a tool in paleogeographic reconstructions.
Batievaite-(Y), Y 2 Ca 2 Ti[Si 2 O 7 ] 2 (OH) 2 (H 2 O) 4 , is a new mineral found in nepheline s... more Batievaite-(Y), Y 2 Ca 2 Ti[Si 2 O 7 ] 2 (OH) 2 (H 2 O) 4 , is a new mineral found in nepheline syenite pegmatite in the Sakharjok alkaline massif, Western Keivy, Kola Peninsula, Russia. The pegmatite mainly consists of nepheline, albite, alkali pyroxenes, amphiboles, biotite and zeolites. Batievaite-(Y) is a late-pegmatitic or hydrothermal mineral associated with meliphanite, fluorite, calcite, zircon, britholite-group minerals, leucophanite, gadolinite-subgroup minerals, titanite, smectites, pyrochlore-group minerals, zirkelite, cerianite-(Ce), rutile, behoite, ilmenite, apatitegroup minerals, mimetite, molybdenite, and nickeline. Batievaite-(Y) is pale-cream coloured with white streak and dull, greasy or pearly luster. Its Mohs hardness is 5-5.5. No cleavage or parting was observed. The measured density is 3.45(5) g/cm 3. Batievaite-(Y) is optically biaxial positive, α 1.745(5), β 1.747(5), γ 1.752(5) (λ 589 nm), 2V meas. = 60(5)°, 2V calc. = 65°. Batievaite-(Y) is triclinic, space group P-1, a 9.4024(8), b 5.5623(5), c 7.3784(6) Å, α 89.919(2), β 101.408(2), γ 96.621(2)°, V 375.65(6) Å 3 and Z = 1. The eight strongest lines of the X-ray powder diffraction pattern [d(Å)(I)(hkl)] are: 2.991(100)(11-2), 7.238(36)(00-1), 3.061(30)(300), 4.350(23)(0-1-1), 9.145(17)(100), 4.042(16)(11-1), 2.819(16)(3-10), 3.745(13)(2-10). The chemical composition determined by electron probe microa n a l y s i s (E P M A) i s (w t. %) : N b 2 O 5 2 .2 5 , Ti O 2 8.01, ZrO 2 2.72, SiO 2 29.
An unusual hydrothermal alteration scheme was presented for chevkinite-(Ce) from the White Tundra... more An unusual hydrothermal alteration scheme was presented for chevkinite-(Ce) from the White Tundra pegmatite (2656 ± 5 Ma), Keivy massif, Kola Peninsula. Pb-CO2-rich fluids initially removed REE and Y from the chevkinite-(Ce), with enrichment in Pb and U. PbO abundances reaching 17.35 wt%. Continued alteration resulted in the altered chevkinite-(Ce) being progressively transformed to a Pb-Ti-Fe-Si phase, which proved, upon EBSD analysis, to be almost totally amorphous. Pb enrichment was accompanied by a loss of LREE, especially La, relative to HREE, and the development of strong positive Ce anomalies. A notably U-rich aeschynite-(Y), with UO2 values ≤7.67 wt%, crystallized along with the chevkinite-(Ce). Aeschynite-(Y) with a lower UO2 value (3.91 wt%) and bastnäsite-(Ce) formed during alteration. The formation of bastnäsite-(Ce) rather than cerussite, which might have been expected in a high Pb-CO2 environment, is ascribed to the fluids being acidic.
Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in ... more Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in nepheline syenite and contain britholite-group minerals and zircon as main ore minerals. Geochemical data indicate that the formation of the britholite ores of the Sakharjok Massif was mainly controlled by magmatic differentiation and lateto post-magmatic reworking of the rocks by alkaline and F, CO2-bearing fluids. The elevated content of ore components in magma is caused by its derivation from enriched mantle source. It was established that crystallization of britholite occurred at the late and post-magmatic stages of the massif formation and was assisted by fluids with different physicochemical properties. The widest spread fluorbritholite-(Ce) typical of the trachytoid nepheline syenite crystallized mainly during albitization from highly alkaline, F-rich and CO2-bearing fluids/solutions. Britholite-(Ce) and fluorbritholite-(Y) found in the most recrystallized porphyritic nepheline syenite were formed at the later hydrothermal stage from F-bearing water-rich (metamorphic?) solutions. Fluorcalciobritholite crystallized from high-temperature pegmatite melt/solution at high CO2 activity. Postcrystallization alterations of the britholite-group minerals from the Sakharjok deposit resulted in the formation of altered zones within crystals and rims around them. The composition of overgrowth rims indicates the removal of F, Ce, and La from britholite.
Most of the complexes fall in the age range 2.66-2.70 Ga. The only examples of Archaean alkaline ... more Most of the complexes fall in the age range 2.66-2.70 Ga. The only examples of Archaean alkaline rocks from the Baltic Shield are the Siilinjarvi carbonatite and alkaline syenite with a formation age of 2.58 Ga (Patchet et al. 1981). Peralkaline granites (2760 Ma, Pb-Pb age) and associated syenites and monzonites (2512±42 Ma, Rb-Sr age) occur in the Eastern Goldfields of Western Australia (Libby & de Laeter 1980). A 2.7 Ga age is well established for peralkaline granites and associated syenites from the Superior province. These Late Archaean alkaline complexes belong to the potassic series, are depleted in LIL and HFS elements, are related to subduction and form in compressive tectonic environments (Sutcliffe et al. 1990, Blichert-Toft et al. 1995). The absence of Archaean sodic alkaline complexes forming in extensional environments was mainly ascribed to the absence of metasomatic processes in the mantle and lower lithosphere (Blichert-Toft et al. 1996).
Archean alkaline complexes are extremely rare but are of particular interest because the magmas h... more Archean alkaline complexes are extremely rare but are of particular interest because the magmas have a mantle origin. Given their high Sr and Nd contents, crustal contamination has only a minor impact on the mantle-derived isotopic signatures. Therefore, they can provide valuable information on the isotopic composition of the subcontinental mantle and geodynamics in Archean. The previously reported Late Archean alkaline complexes (2.7-2.6 Ga) from the Canadian and Australian shields belong to the potassic series, are depleted in LIL and HFS elements, and are related to subduction, thereby forming in a compressive tectonic environment and having a depleted mantle source. The absence of Archean sodic alkaline complexes forming in extensional environments was mainly ascribed to the absence of metasomatic processes in the mantle and lower lithosphere (Blichert-Toft et al., 1996). Greenland Archean alkaline complexes (2698-2664 Ma syn- to post-kinematic pyroxenites, hornblendites, norite...
Аннотация. Приводятся уточненные данные по U-Pb (in situ) возрасту циркона из Кейвских щелочных г... more Аннотация. Приводятся уточненные данные по U-Pb (in situ) возрасту циркона из Кейвских щелочных гранитов Кольского п-ова и связанных с ними редкометалльных образований. Результаты SIMS датирования циркона из самих щелочных гранитов показывают, что его кристаллизация происходила в интервале 2660-2670 млн лет, при этом наиболее точная датировка, 2673.5 ± 0.3 млн лет, получена методом CA-ID-TIMS. U-Pb датирование по SIMS и LA-ICP-MS данным показывает, что формирование редкометалльных пегматитов, кварцолитов и метасоматитов приурочено к магматической стадии. При этом циркон из кварцолитов был значительно перекристаллизован в ходе Свекофенского метаморфизма-1770-1790 млн лет назад. Заключительная стадия ремобилизации (амазонитовые пегматиты) и/или перекристаллизации (кварцолиты) редкометалльной минерализации может быть связана с флюидо-термальной активизацией Фенноскандинавского щита в период 1670-1690 млн лет. Впервые проведенные изотопные исследования кислорода в цирконе, породообразующих минералах и валовых породах Кейвского комплекса показали, что значительную роль в эволюции его магмато-гидротермальной системы имели метеорные воды.
Аннотация. В статье приведены данные по минеральному составу, морфологии и химизму рудных и акцес... more Аннотация. В статье приведены данные по минеральному составу, морфологии и химизму рудных и акцессорных минералов гематитовых брекчий в песчаниках Терского берега. Гематитовые брекчии состоят из обломков песчаников терской свиты и гематитового цемента, состоящего из мелкокристаллического пластинчатого гематита. Описаны парагенетические REE-содержащие и сульфатные минералы (паризит-(Ce), монацит-(Ce), REE-Nb-Ti-оксид, барит). Текстурно-структурные особенности, минерализация и химический состав рудного минерала (низкотитанистый гематит с повышенным содержанием W и V) гематитовых брекчий месторождения Мыс Корабль являются характерными особенностями для месторождений IOCG-типа (Fe-оксидных Au-Cu). Это позволяет предположить их сходный механизм формирования. Предложена модель образования железооксидных рудопроявлений месторождения Мыс Корабль при участии уникальных по составу (селективно железистых) гидротермальных растворов. В связи с вышеперечисленным, изучение железооксидных рудопроявлений месторождения Мыс Корабль является весьма актуальным как с теоретической, так и с практической стороны.
The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs a... more The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs and several dikelike nepheline syenite bodies. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late-magmatic and post-magmatic (including pegmatitic) processes. The Sakharjok nepheline syenite pegmatite contains a remarkably diverse number of britholite group minerals, pointing to different physico-chemical conditions in the fluid. REE and actinides distribution in the host rock indicates that the late-magmatic (and pegmatitic) fluids were alkaline, with significant amounts of F and CO 2. From REE and F variations of the britholite group minerals possible fluid compositions at different stages are suggested. The earliest fluorbritholite-(Ce) formed locally from a late magmatic, high temperature F-rich fluid. Fluorbritholite-(Y) presumably crystallized from a Fbearing and CO 2-rich fluid; marked F saturation resulted in precipitation of abundant fluorite due to a temperature drop. Variations in REE and F contents in the most abundant fluorcalciobritholite indicate a successive decrease of F in the fluid during its evolution. The relationship between intergrown fluorapatite and fluorcalciobritholite and the presence of zones with a REE-rich fluorapatite between them indicate a continuous to sudden crystallization in this mineral sequence. The сrystallization of the latest Bcalciobritholite^is related to the input into the fluid of CO 2 and/or H 2 O.
This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY
The present work reports new mineralogical and whole rock geochemical data from the Breivikbotn s... more The present work reports new mineralogical and whole rock geochemical data from the Breivikbotn silicocarbonatite (Seiland igneous province, North Norway), allowing conclusions to be drawn concerning its origin and the role of late fluid alteration. The rock shows a rare mineral association: calcite + pyroxene + amphibole + zeolite group minerals + garnet + titanite, with apatite, allanite, magnetite and zircon as minor and accessory minerals, and it is classified as silicocarbonatite. Calcite, titanite and pyroxene (Di36-46 Acm22-37 Hd14-21) are primarily magmatic minerals. Amphibole of hastingsitic composition has formed after pyroxene at a late-magmatic stage. Zeolite group minerals (natrolite, gonnardite, Sr-rich thomsonite-(Ca)) were formed during hydrothermal alteration of primary nepheline by fluids/solutions with high Si-Al-Ca activities. Poikilitic garnet (Ti-bearing andradite) has inclusions of all primary minerals, amphibole and zeolites, and presumably crystallized metasomatically during a late metamorphic event (Caledonian orogeny). Whole rock chemical compositions of the silicocarbonatite differs from the global average of calciocarbonatites by elevated silica, aluminium, sodium and iron, but show comparable contents of trace elements (REE, Sr, Ba). Trace element distributions indicate within-plate tectonic setting of the carbonatite. The spatial proximity of carbonatite and alkaline ultramafic rock (melteigite), the presence of "primary nepheline" in carbonatite together with the trace element distributions indicate that the carbonatite was derived from crystal fractionation of a parental carbonated foidite magma. The main prerequisites for the extensive formation of zeolite group minerals in silicocarbonatite are revealed.
The present work reports on new mineralogical and whole-rock geochemical data from the Breivikbot... more The present work reports on new mineralogical and whole-rock geochemical data from the Breivikbotn silicocarbonatite (Seiland igneous province, North Norway), allowing conclusions to be drawn concerning its origin and the role of late fluid alteration. The rock shows a rare mineral association: calcite + pyroxene + amphibole + zeolite group minerals + garnet + titanite, with apatite, allanite, magnetite and zircon as minor and accessory minerals, and it is classified as silicocarbonatite. Calcite, titanite and pyroxene (Di 36-46 Acm 22-37 Hd 14-21) are primarily magmatic minerals. Amphibole of mainly hastingsitic composition has formed after pyroxene at a late-magmatic stage. Zeolite group minerals (natrolite, gonnardite, Sr-rich thomsonite-(Ca)) were formed during hydrothermal alteration of primary nepheline by fluids/solutions with high Si-Al-Ca activities. Poikilitic garnet (Ti-bearing andradite) has inclusions of all primary minerals, amphibole and zeolites, and presumably crystallized metasomatically during a late metamorphic event (Caledonian orogeny). Whole-rock chemical compositions of the silicocarbonatite differs from the global average of calciocarbonatites by elevated silica, aluminium, sodium and iron, but show comparable contents of trace elements (REE, Sr, Ba). Trace element distributions and abundances indicate within-plate tectonic setting of the carbonatite. The spatial proximity of carbonatite and alkaline ultramafic rock (melteigite), the presence of "primary nepheline" in carbonatite together with the trace element distributions indicate that the carbonatite was derived by crystal fractionation of a parental carbonated foidite magma. The main prerequisites for the extensive formation of zeolite group minerals in silicocarbonatite are revealed.
The behaviour of ThSiO4 during low-temperature alteration has significance for element mobility a... more The behaviour of ThSiO4 during low-temperature alteration has significance for element mobility and redistribution. Here we describe five types of alteration of ThSiO4 by hydrothermal fluids: (1) primary ThSiO4 associated with chevkinite-(Ce) in a quartz-epidote metasomatite; (2) during alteration of monazite-(Ce) in a quartzolite; (3) during alteration of fergusonite-(Y) in a quartz-epidote metasomatite; (4) following exsolution from chevkinite-(Ce); and (5) associated with cerite-(Ce) and with ilmenite and bastnäsite-(Ce) in late-stage veinlets in a syenitic pegmatite and a metasomatite. The great majority of crystals have been strongly altered compositionally, with variable degrees of replacement of formula elements by non-formula elements, such as Ca, Fe, P and REE. The most reliable geochemical indicators of hydrothermal alteration are low analytical totals and non-stoichiometric structural formulae. The alteration is variably ascribed to dissolution-reprecipitation and pervasi...
Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of... more Given the scarcity of reliable paleoclimate record, the surface temperatures of the first half of Earth's history remain poorly constrained. Here we show how the climate-sensitive δ 18 O value of surface precipitation recorded in Archean igneous and hydrothermal formations can help to resolve the state of early Earth climate. The Keivy complex, Kola craton (Fennoscandian Shield), formed via the intrusion of granitic and mafic magmas in the shallow crust at 2.67 Ga, where circulation of meteoric water created a distinct archive of the contemporaneous water cycle. Using whole rock data, mineral separates, and in situ zircon δ 18 O measurements, we disentangle the reaction mechanisms between the shallow magma and local precipitation. Syn-emplacement hydrothermal alteration produced near-contact lithologies with δ 18 O values as low as −8‰ recorded in amphiboles, while igneous zircons from granites crystallized from melts with δ 18 O from +1‰ to +4.5‰. High-precision U-Pb geochronology constrains the granite intrusion at 2673.5 ± 0.3 Ma. Using the Δ 17 O approach, these rocks reveal that the precipitation had a δ 18 O value 18‰ lower than the hydrosphere, providing one of the earliest quantitative records of continental precipitation generally compatible with a cold climate at high latitudes.
Kolmozero-Keivy zone, NE Baltic shield, is composed mainly from basic-intermediate-acid metavolca... more Kolmozero-Keivy zone, NE Baltic shield, is composed mainly from basic-intermediate-acid metavolcanic suites of 2.83-2.87 Ga age and metamorphosed at amphibolitic facies during 1.8-1.7 Ga. Younger igneous events are represented by voluminous intrusions of 2.73 Ga plagio-microcline granites and of 2.65 Ga peralkaline A-type granites, and small stocks of 2.52 Ga tourmaline granites. Four pegmatite fields with total amount of more than 100 bodies are confined to Kolmozero subzone. The pegmatites intrude amphibolites and rarely associated gabbro-anorthosite and are of 50-700m long and of 10-35m thickness. The Kolmozero pegmatites are of complex type, spodumene subtype with Li, Cs, Be, Ta, Sn geochemical signature and belong to LCT family by classification of Cerny and Ercit [1]. They crystallized at relatively high pressure (3-4 kbar) with peralumunious S-type granite as the source magma. The Vasin Myl’k pegmatite field with the lepidolite–albite–microcline–spodumene–pollucite associatio...
low δ 18 O systems The modern hydrological cycle includes precipitation of 18 O-depleted meteoric... more low δ 18 O systems The modern hydrological cycle includes precipitation of 18 O-depleted meteoric water over continental crust exposed above sea-level. How far back in the geological record can continental exposure be documented remains uncertain, particularly for the Archean. To investigate the extent of Archean continental exposure, we document a newly discovered low-δ 18 O magmatic-hydrothermal system that was emplaced in a shallow crust of the Keivy complex, Kola craton at 2.67 Ga. This study builds a case for syn-emplacement hydrothermal exchange with the local meteoric water that had δ 18 O at least as low as −11 VSMOW. Coupled with the high-latitude position of the Kola craton at the time, this result presents a paleogeographic documentation of continental exposure and a quantitative estimate of the isotope composition of Neoarchean precipitation. We employ detailed δ 18 O mapping using mineral separates and bulk samples, as well as ion microprobe δ 18 O measurements and U-Pb dating of zircon. The spatially extensive O-isotope dataset (n = 63) spans over 120 km in length across different lithological units including the peralkaline granites of the Keivy complex, the hosting gneiss and contact hydrothermal quartzolites. All analyzed samples, including whole rock samples and mineral separates, show δ 18 O below those expected for mantle-and crust-derived magmas. The lowest δ 18 O of ca. −7 are measured in minerals and bulk rock of the altered gneisses near the intrusive contacts. Peralkaline granites returned δ 18 O values ranging between −6 and +5. Zircon crystals with preserved igneous zoning have low δ 18 O = +2.2 and yield an emplacement age for the magmatic complex of 2672 ± 7 Ma. Zircon dating from quartzolites at the contact between granites and country gneisses points to fluid-rock interaction at 2.67 and at 1.77-1.79 Ga. We propose that the spatially extensive O-isotope depletion documents a Neoarchean shallow magmatic-hydrothermal system where magmatic assimilation produced the low-δ 18 O granites and the hydrothermal cycling of meteoric water in the ambient crust resulted in formation of low-δ 18 O altered host rocks. Since the lowest δ 18 O value measured in a rock is ca. −7 , the equilibrium fluids had δ 18 O of around −9 under hydrothermal conditions (300-400 • C) characteristic for the shallow crust around cooling intrusions. Considering that hydrothermal fluids are shifted 1-2 towards higher values, the contemporaneous precipitation had δ 18 O = −11 or lower. Consequently, the 2.67 Ga Keivy complex is the earliest known intact low-δ 18 O magmatic-hydrothermal system offering a quantitative record of the Neoarchean precipitation that can be used as a tool in paleogeographic reconstructions.
Batievaite-(Y), Y 2 Ca 2 Ti[Si 2 O 7 ] 2 (OH) 2 (H 2 O) 4 , is a new mineral found in nepheline s... more Batievaite-(Y), Y 2 Ca 2 Ti[Si 2 O 7 ] 2 (OH) 2 (H 2 O) 4 , is a new mineral found in nepheline syenite pegmatite in the Sakharjok alkaline massif, Western Keivy, Kola Peninsula, Russia. The pegmatite mainly consists of nepheline, albite, alkali pyroxenes, amphiboles, biotite and zeolites. Batievaite-(Y) is a late-pegmatitic or hydrothermal mineral associated with meliphanite, fluorite, calcite, zircon, britholite-group minerals, leucophanite, gadolinite-subgroup minerals, titanite, smectites, pyrochlore-group minerals, zirkelite, cerianite-(Ce), rutile, behoite, ilmenite, apatitegroup minerals, mimetite, molybdenite, and nickeline. Batievaite-(Y) is pale-cream coloured with white streak and dull, greasy or pearly luster. Its Mohs hardness is 5-5.5. No cleavage or parting was observed. The measured density is 3.45(5) g/cm 3. Batievaite-(Y) is optically biaxial positive, α 1.745(5), β 1.747(5), γ 1.752(5) (λ 589 nm), 2V meas. = 60(5)°, 2V calc. = 65°. Batievaite-(Y) is triclinic, space group P-1, a 9.4024(8), b 5.5623(5), c 7.3784(6) Å, α 89.919(2), β 101.408(2), γ 96.621(2)°, V 375.65(6) Å 3 and Z = 1. The eight strongest lines of the X-ray powder diffraction pattern [d(Å)(I)(hkl)] are: 2.991(100)(11-2), 7.238(36)(00-1), 3.061(30)(300), 4.350(23)(0-1-1), 9.145(17)(100), 4.042(16)(11-1), 2.819(16)(3-10), 3.745(13)(2-10). The chemical composition determined by electron probe microa n a l y s i s (E P M A) i s (w t. %) : N b 2 O 5 2 .2 5 , Ti O 2 8.01, ZrO 2 2.72, SiO 2 29.
An unusual hydrothermal alteration scheme was presented for chevkinite-(Ce) from the White Tundra... more An unusual hydrothermal alteration scheme was presented for chevkinite-(Ce) from the White Tundra pegmatite (2656 ± 5 Ma), Keivy massif, Kola Peninsula. Pb-CO2-rich fluids initially removed REE and Y from the chevkinite-(Ce), with enrichment in Pb and U. PbO abundances reaching 17.35 wt%. Continued alteration resulted in the altered chevkinite-(Ce) being progressively transformed to a Pb-Ti-Fe-Si phase, which proved, upon EBSD analysis, to be almost totally amorphous. Pb enrichment was accompanied by a loss of LREE, especially La, relative to HREE, and the development of strong positive Ce anomalies. A notably U-rich aeschynite-(Y), with UO2 values ≤7.67 wt%, crystallized along with the chevkinite-(Ce). Aeschynite-(Y) with a lower UO2 value (3.91 wt%) and bastnäsite-(Ce) formed during alteration. The formation of bastnäsite-(Ce) rather than cerussite, which might have been expected in a high Pb-CO2 environment, is ascribed to the fluids being acidic.
Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in ... more Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in nepheline syenite and contain britholite-group minerals and zircon as main ore minerals. Geochemical data indicate that the formation of the britholite ores of the Sakharjok Massif was mainly controlled by magmatic differentiation and lateto post-magmatic reworking of the rocks by alkaline and F, CO2-bearing fluids. The elevated content of ore components in magma is caused by its derivation from enriched mantle source. It was established that crystallization of britholite occurred at the late and post-magmatic stages of the massif formation and was assisted by fluids with different physicochemical properties. The widest spread fluorbritholite-(Ce) typical of the trachytoid nepheline syenite crystallized mainly during albitization from highly alkaline, F-rich and CO2-bearing fluids/solutions. Britholite-(Ce) and fluorbritholite-(Y) found in the most recrystallized porphyritic nepheline syenite were formed at the later hydrothermal stage from F-bearing water-rich (metamorphic?) solutions. Fluorcalciobritholite crystallized from high-temperature pegmatite melt/solution at high CO2 activity. Postcrystallization alterations of the britholite-group minerals from the Sakharjok deposit resulted in the formation of altered zones within crystals and rims around them. The composition of overgrowth rims indicates the removal of F, Ce, and La from britholite.
Most of the complexes fall in the age range 2.66-2.70 Ga. The only examples of Archaean alkaline ... more Most of the complexes fall in the age range 2.66-2.70 Ga. The only examples of Archaean alkaline rocks from the Baltic Shield are the Siilinjarvi carbonatite and alkaline syenite with a formation age of 2.58 Ga (Patchet et al. 1981). Peralkaline granites (2760 Ma, Pb-Pb age) and associated syenites and monzonites (2512±42 Ma, Rb-Sr age) occur in the Eastern Goldfields of Western Australia (Libby & de Laeter 1980). A 2.7 Ga age is well established for peralkaline granites and associated syenites from the Superior province. These Late Archaean alkaline complexes belong to the potassic series, are depleted in LIL and HFS elements, are related to subduction and form in compressive tectonic environments (Sutcliffe et al. 1990, Blichert-Toft et al. 1995). The absence of Archaean sodic alkaline complexes forming in extensional environments was mainly ascribed to the absence of metasomatic processes in the mantle and lower lithosphere (Blichert-Toft et al. 1996).
Archean alkaline complexes are extremely rare but are of particular interest because the magmas h... more Archean alkaline complexes are extremely rare but are of particular interest because the magmas have a mantle origin. Given their high Sr and Nd contents, crustal contamination has only a minor impact on the mantle-derived isotopic signatures. Therefore, they can provide valuable information on the isotopic composition of the subcontinental mantle and geodynamics in Archean. The previously reported Late Archean alkaline complexes (2.7-2.6 Ga) from the Canadian and Australian shields belong to the potassic series, are depleted in LIL and HFS elements, and are related to subduction, thereby forming in a compressive tectonic environment and having a depleted mantle source. The absence of Archean sodic alkaline complexes forming in extensional environments was mainly ascribed to the absence of metasomatic processes in the mantle and lower lithosphere (Blichert-Toft et al., 1996). Greenland Archean alkaline complexes (2698-2664 Ma syn- to post-kinematic pyroxenites, hornblendites, norite...
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