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1.
The olivine shonkinites localized among dunites and alkali gabbroids in the northern part of the alkaline ultrabasic Inagli massif (northwestern part of Central Aldan) have been studied. The obtained data on the chemical and trace-element compositions of the rocks and minerals and the results of melt inclusion study showed that the olivine shonkinites crystallized from alkaline basanite melt enriched in Cl, S, CO2, and trace elements. Clinopyroxene crystallized at 1180-1200 °C from a homogeneous silicate-salt melt, which was probably separated into immiscible silicate and carbonate-salt fractions with temperature decreasing. The composition of the silicate fraction evolved from alkaline basanite to alkaline trachyte. The carbonate-salt fraction had an alkaline carbonate composition and was enriched in S and Cl. The same trend of evolution of clinopyroxene-hosted melts and the igneous rocks of the Inagli massif suggests that the alkali gabbroids, melanocratic alkali syenites, and pulaskites formed from the same magma, which had a near-alkaline basanite composition during its crystallization differentiation. The geochemical studies showed that the olivine shonkinites and glasses of homogenized melt inclusions in clinopyroxene grains have similar contents of trace elements, one or two orders of magnitude higher than those in the primitive mantle. The high contents of LILE (K, Rb, and Sr) and LREE in the olivine shoshonites and homogenized inclusions suggest the enriched mantle source, and the negative anomalies of HFSE and Ti are a specific feature of igneous rocks formed with the participation of crustal material. The slight depletion in HREE relative to LREE and the high (La/Yb)n ratios in the rocks and inclusion glasses (10.0-11.4 and 4.7-6.2, respectively) suggest the presence of garnet in the mantle source.  相似文献   

2.
Studies of primary multiphase silicate inclusions in accessory Cr-spinels from the fine-grained dunites of the Nizhnii Tagil Pt-bearing massif reveal their similarity to melt inclusions trapped by chromite during its growth. The analyzed Cr-spinels with multiphase silicate inclusions differ in composition from ore chromites of the same massif and from chromites (with melt inclusions) from ultramafic oceanic complexes but are similar to Cr-spinels in dunites from Pt-bearing alkaline ultramafic massifs (Konder and Inagli). According to petro- and geochemical data on heated multiphase silicate inclusions, the studied Cr-spinels crystallized with the participation of subalkalic picrobasaltic melts similar to the magmas of the Konder Pt-bearing massif and having almost the same chemical composition as tylaites. The differences between the compositions of olivines formed within the multiphase silicate inclusions and of the rock-forming minerals show that the studied Cr-spinels formed from an intercumulus liquid melt in the olivine crystal interstices during the cumulate crystallization of most of the Nizhnii Tagil massif dunites in the intrusive chamber. Numerical modeling based on the compositions of heated multiphase silicate inclusions in accessory Cr-spinels demonstrates that olivines and Cr-spinels from the studied dunites crystallized at 1430 to 1310 °C and then olivine formation continued to 1280 °C during the evolution of melts.  相似文献   

3.
Obtained data shows that high-potassic dyke rocks of the Ryabinoviy massif (Central Aldan) belong to low-titanium lamproite series (Mediterranean type) and are distinct with “classic” high-titanium lamproites. Based on Al-in-olivine thermometer, temperature of olivine–chrome-spinel pair crystallization varies in range between 1100 and 1250°C. This suggests lithospheric mantle source for the parental melt and makes role of mantle plume insignificant. High-precision data on olivine composition and bulk rock traceelement composition imply mixed source for the parental melt, consisted of depleted peridotite and enriched domains, originated during ancient subduction.  相似文献   

4.
A great volume of original information on the formation of the ultrabasic rocks of the Siberian Platform has been accumulated owing to the study of melt inclusions in Cr-spinels. The inclusions show the general tendencies in the behavior of the magmatic systems during the formation of the ultrabasic massifs of the Siberian Platform, tracing the main evolution trend of decreasing Mg number with SiO2 increase in the melts with subsequent transition from picrites through picrobasalts to basalts. The compositions of the melt inclusions indicate that the crystallization conditions of the rocks of the concentrically zoned massifs (Konder, Inagli, Chad) sharply differ from those of the Guli massif. Numerical modeling using the PETROLOG and PLUTON softwares and data on the composition of inclusions in Cr-spinels yielded maximum crystallization temperatures of the olivines from the dunites of the Konder (1545–1430°C), Inagli (1530–1430°C), Chad (1460–1420°C), and Guli (1520–1420°C) massifs, and those of Cr-spinels from the Konder (1420–1380°C), Inagli (up to 1430°C), Chad (1430–1330°C), and Guli (1410–1370°C) massifs. Modeling of the Guli massif with the PLUTON software using the compositions of the melt inclusions revealed the possible formation of the alkaline rocks at the final reverse stage of the evolution of the picritic magmas (with decrease of SiO2 and alkali accumulation) after termination of olivine crystallization with temperature decrease from 1240–1230°C to 1200–1090°C. Modeling with the PLUTON software showed that the dunites of the Guli massif coexisted with Fe-rich (with moderate TiO2 contents) melts, the crystallization of which led (beginning from 1210°C) to the formation of pyroxenes between cumulate olivine. Further temperature decrease (from 1125°C) with decreasing FeO and TiO2 contents provided the formation of clinopyroxenes of pyroxenites. For the Konder massif, modeling with the PLUTON software indicates the possible formation of kosvites from picrobasaltic magmas beginning from 1350°C and the formation of clinopyroxenites and olivine–diopside rocks from olivine basaltic melts from 1250°C.  相似文献   

5.
Data obtained on melt inclusions in Cr-spinel suggest a magmatic genesis of dunite in the Konder and Inagly placer-forming platiniferous massifs in the southeastern Siberian Platform. These data make it possible to evaluate the physicochemical parameters of the magmatic processes that produced these concentrically zoned alkaline-ultrabasic complexes. The comparative analysis of the composition of the Cr-spinel with inclusions highlights remarkable differences between this mineral in the Konder and Inagli massifs, on the one hand, and in ultramafic rocks in ophiolites and the modern oceanic crust, on the other. Minute clinopyroxene crystals included in Cr-spinel from the Konder Massif have a composition and configurations of their REE patterns contrastingly different from those of clinopyroxene in basite-hyperbasite complexes of ophiolite associations but are close to those of clinopyroxene in the Kytlym and Nizhnii Tagil platiniferous massifs in the Urals. The composition of the quenched melt inclusions suggests that the chromite crystallized predominantly from picrite alkaline magmas. The concentrations of most elements in the high-Mg inclusions are close to those in biotite-pyroxene alkaline picrites, a fact testifying to the significant contribution of ultrabasic (picrite) alkaline magmatic systems to the origin of the Konder and Inagli massifs. Ion-probe analyses of the inclusions suggest that the melts were rich in water (up to 0.6 wt %). Data on the distribution of REE and other trace elements in the inclusions provide evidence of the influence of a deep plume. Our simulations with the use of the composition of the melt inclusions suggest that dunite in the Konder and Inagli massifs were produced mainly by water-bearing magmas at temperatures of 1460–1300°C. As the melts evolved to less magnesian ones, olivine continued to crystallize from them until the temperature decreased to 1230°C.  相似文献   

6.
The investigation of melt inclusions in Cr spinels yielded direct information on the physicochemical parameters of the magmatic processes responsible for the formation of the Konder platiniferous alkaline-ultrabasic massif (southeastern Aldan Shield). The comparative analysis of the composition of the chromites containing the inclusions revealed that the Cr spinels from the Konder dunites differ significantly from their counterparts in the ultrabasic complexes of ophiolites and the modern oceanic crust. In terms of their composition and REE distribution, the clinopyroxene microcrystals from the Konder chromites are significantly different from the pyroxenes in the basic-ultrabasic ophiolite complexes of associations and identical to the minerals from the Kytlym platiniferous massif (the Urals). With respect to the distribution of the major components, the high-magnesian inclusions are identical to alkaline biotite-pyroxene picrites, testifying to the active participation of ultrabasic (picritic) alkaline magmatic systems in the formation of the dunites in the Konder Massif. The results of the ion probe investigation of the inclusions indicate a high water content (up to 0.54 wt %) in the melts. The data on the distribution of the rare and rare-earth elements in the inclusions suggest that the dunites of the Konder Massif crystallized with the involvement of water-saturated magmas at minimal temperatures of about 1230°C. Such temperatures are consistent with the earlier estimates for the melt inclusions in the olivine of the Konder Massif.  相似文献   

7.
Dikes of biotitic shonkinites and minettes of the complex Ryabinovyi alkaline massif (Central Aldan) have been studied. The dikes are localized in a neck of K-picrites in the northeast of the massif, which intrudes gold-bearing microcline–muscovite metasomatites (Muscovitovyi site). The obtained data on the chemical and trace-element compositions of the rocks and minerals and study of melt inclusions in clinopyroxenes indicate that the biotitic shonkinites and minettes crystallized from the same deep-seated high-pressure alkaline ultrabasic magma during its evolution. Apparently, at the early stage of crystallization of diopside in the biotitic shonkinites, homogeneous carbonate–silicate melt was separated into immiscible fractions of silicate, carbonate–salt, and carbonate melts. The temperature of melt immiscibility was > 1120–1190 °C, i.e., higher than the homogenization temperature of silicate inclusions in the diopside. The contents of trace elements in the biotitic shonkinites and rock-forming clinopyroxenes were one or two orders of magnitude higher than the mantle values. The Eu/Eu* ratios of both the considered rocks and the clinopyroxenes were close to those of chondrites, which testifies to their crystallization from mantle magma. The HREE/LREE ratio indicates that the magma source was localized at the depths where garnet-spinel assemblages existed. The negative Nb and Ti anomalies in the trace-element spectra and the high (> 5) La/Nb ratios in the rocks and clinopyroxenes point to the influence of crustal material on the parental magma. Crystallization of magma took place in reducing conditions, which is evidenced by the low (4–7) Ti/V ratios in clinopyroxenes and the presence of chloride–sulfate inclusions in them. Since gold in the Ryabinovyi massif is associated with late sulfate–chloride and sulfate–carbonate fluids, it might have been transported by alkaline chloride–sulfate and carbonate (carbonatite) melts, found as inclusions in clinopyroxenes of the biotitic shonkinites, at the early stages of Mesozoic magmatism.  相似文献   

8.
The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous–saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. An ongonite melt crystallized with the participation of P–Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonite melt, the duration of its crystallization has been estimated for a magma chamber of the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and Rayleigh number of the ongonite melt have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonite melt in a closed magma chamber.Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and then these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P–Q magmatic fluid which coexisted with the melt in the residual chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; the silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid–magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonite melt, intense melt convection in the residual chamber, and rising fluid pressure during its degassing.  相似文献   

9.
New data are presented on the geologic structure, age, petrogeochemical composition, and conditions of formation of the Late Proterozoic Meteshikha ultramafic-mafic pluton of the Ikat complex. Mafic rocks are the main rocks of the massif, whereas ultramafic rocks are secondary; both of them correspond to two intrusive phases. The first phase includes a layered rock series enriched in intercumulus amphibole, which varies in composition from olivine gabbro to leucocratic gabbro-anorthosite; the second is composed of wehrlite, plagiowehrlite, and olivine clinopyroxenite. Mineralogical, petrographic, geochemical, and isotope studies show that the rocks of both phases crystallized from the same mantle melt; note that the PT-conditions of their formation were considerably different. We suppose that they were separated in the intermediate chamber during fractional crystallization and the accumulation of early minerals (olivine and, probably, clinopyroxene) in the lower part of the chamber. Using the COMAGMAT software, we have found the composition of the parental melt for the rocks of the first phase—normal tholeiitic basalt with 0.2-0.5 wt.% water, which might have crystallized at 3.0-3.5 kbar and the oxygen activity controlled by the QFM buffer. The differentiated series is characterized by gradual depletion with Cr and Ni and enrichment with Sr, Ti, Cu, and REE during the evolution of melt. The REE patterns for the massif rocks have a similar low-fractionation trend with domination of light lanthanides over heavy ones and (La/Yb)N = 1.25-2.75. Multielement spectra are characterized by negative anomalies of K, Th, Nb, and Zr and positive anomalies of Ba, U, Sm, and Sr. The geochemical characteristics of the rocks are similar to those of the tholeiitic basalts of present-day island arcs. Studies show that the Meteshikha massif formed in the subduction setting of the active margin of the Siberian continent in the Late Riphean (809 Ma). © 2015, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.  相似文献   

10.
Alkaline-basic dike from the Yllymakh Massif (Central Aldan) has been studied. Its partially crystallized matrix contains corroded phenocrysts of olivine and hypidiomorphic phenocrysts of clinopyroxene and pseudo-, epileucite. It was found that phenocrysts of clinopyroxene contain abundant primary inclusions, Ti-magnetite and apatite bear only single inclusions, whereas olivine is enriched in secondary inclusions, which are confined to the cleavage of host mineral (along second and third pinacoids) and its cracks. The homogenization temperatures of the primary inclusions in clinopyroxene and secondary inclusions in olivine are approximately equal and lie within 1260–1240°C. The compositions of melt inclusions in olivine and clinopyroxene are also similar and corresponded to the malignite-pseudoleucite phonolite-monzonite pulaskites, which are developed at the Yllymakh Massif. Unheated inclusions in apatite and Ti-magnetite compositionally approach monzonites and nepheline syenites—tinguaites, respectively. It was concluded that the alkaline basaltoid magma was presumably parental magma for the entire rock complex of the Yllymakh Massif. Its crystallization and differentiation presumably provided all observed rock variety from ultrabasics (early derivatives located at depth) and malignites (later derivatives) to leucite phonolites, monzonites, and alkaline pulaskites, which were obtained during subsequent stages of the melt evolution. The parental magma, and especially its derivatives, were enriched in BaO (0.8–0.1 wt %), Cl (0.1–0.3 wt %) and trace elements (primarily, LREE and MREE), which are several times higher than mantle values. At the same time, ion microprobe (SIMS) study showed that derivative melts were dry: contained only 0.01–1.13 wt % H2O. The trend of melts conserved in the minerals and the massif rocks corresponds to the evolution of alkalinebasaltoid magma with increase in Si, Al, alkalis and decrease in Mg, Ca, and Fe, i.e. the Bowen trend. The considered alkaline-basic dike was presumably formed from the derivative of leucite-phonolite melt, which during emplacement captured olivine xenocrysts from previously fractionated ultrabasic rocks. The parental magma was presumably derived by high-degree melting of garnet-spinel-facies depleted mantle at some influence of crustal material.  相似文献   

11.
This paper studies the petrology of K-alkaline lamproite-carbonatite complexes, which are widespread in Siberia. They are exemplified by the Murun and Bilibino massifs in West and Central Aldan. In these massifs, the entire range of differentiates was first found, from K-ultrabasic-alkalic rocks through basic and intermediate ones to alkali granites and unique residual calc-silicate rocks (benstonite Ba-Sr carbonatites and charoite rocks). Also, intrusive equivalents of lamproites occur in these massifs, and the Murun massif was probably formed from highly differentiated lamproite magmas. In many K-alkaline complexes, silicate and silicate-carbonate magma layering takes place. Stages of magmatism are described for both massifs. Binary and ternary petrochemical diagrams exhibit the same compositional trend from early to late rocks.In this paper, lamproites are considered from the chemical point of view; their diagnostic properties are described in terms of chemical and mineral composition. From geological, petrological, and geochemical data, formational analysis of alkaline complexes was performed, four formational types of world lamproites were first identified, and diamond content criteria were developed for them.The carbonatite problem was studied from the petrological point of view, and four formational types of carbonatites were identified using geological, geochemical, and genetic criteria. It has been suggested that for dividing carbonatite complexes into four formational types the following criteria be used: the alkalinity type (Na or K) of alkalic rocks in the complex and the time when the carbonatite liquid separates from silicate melts in different stages of primary magma differentiation. These linked parameters influence the ore content type of carbonatite complexes.A formation model for K-alkaline carbonatite complexes is given, and the Tomtor alkaline carbonatite massif with tuffaceous rare-metal ores is described to prove that they have ore reserves. The geochemistry of C, O, Sr, and Nd isotopes shows that K-alkaline complexes, depending on their geotectonic setting, can originate from three types of mantle sources: depleted mantle, enriched mantle 1 (EM1), and enriched mantle 2 (EM2). It is concluded that ore-bearing ultrabasic-alkaline complexes of lamproites and carbonatites can melt out of different types of mantle, whose composition only slightly influences their ore content. Apparently, the main factors are the low degree of selective mantle melting (less than 1%) and plumes supplying fluid and alkaline components, which stimulate this melting. Later on, the processes important for the accumulation of ore and trace elements are long-term magma differentiation and its layering during crystallization.  相似文献   

12.
This paper presents the first geochemical data on Cr-spinels from ultramafic rocks of the Alkhadyr terrane, which were obtained on a representative collection of samples using modern research methods. The compositional data on melt inclusions allowed the identification of three generations of Cr-spinels on the basis of their morphology, composition, and relationships with the rock-forming minerals. Different types of geochemical zoning were recognized in heterogeneous Cr-spinel grains. The composition of parental melt and crystallization temperatures of the minerals in ultramafic rocks were derived from the compositional data on Cr-spinels and trapped melt inclusions.  相似文献   

13.
Melt inclusions were studied in chrome diopside from the Inagli deposit of gemstones in the Inagli massif of alkaline ultrabasic rocks of potassic affinity in the northwestern Aldan shield, Yakutia, Russia. The chrome diopside is highly transparent and has an intense green color. Its Cr2O3 content varies from 0.13 to 0.75 wt %. Primary and primary-secondary polyphase inclusions in chrome diopside are dominated by crystal phases (80–90 vol %) and contain aqueous solution and a gas phase. Using electron microprobe analysis and Raman spectroscopy, the following crystalline phases were identified. Silicate minerals are represented by potassium feldspar, pectolite [NaCa2Si3O8(OH)], and phlogopite. The most abundant minerals in the majority of inclusions are sulfates: glaserite (aphthitalite) [K3Na(SO4)2], glauberite [Na2Ca(SO4)2], aluminum sulfate, anhydrite (CaSO4), gypsum (CaSO4 × 2H2O), barite (BaSO4), bloedite [Na2Mg(SO4)2 × 4H2O], thenardite (NaSO4), polyhalite [K2Ca2Mg(SO4)4 × 2H2O], arcanite (K2SO4), and celestite (SrSO4). In addition, apatite was detected in some inclusions. Chlorides are probably present among small crystalline phases, because some analyses of aggregates of silicate and sulfate minerals showed up to 0.19–10.3 wt % Cl. Hydrogen was identified in the gas phase of polyphase inclusions by Raman spectroscopy. The composition of melt from which the chrome diopside crystallized was calculated on the basis of the investigation of silicate melt inclusions. This melt contains 53.5 wt % SiO2, considerable amounts of CaO (16.3 wt %), K2O (7.9 wt %), Na2O (3.5 wt %), and SO3 (1.4 wt %) and moderate amounts of Al2O3 (7.5 wt %), MgO (5.8 wt %), FeO (1.1 wt %), and H2O (0.75 wt %). The content of Cr2O3 in the melt was 0.13 wt %. Many inclusions were homogenized at 770–850°C, when all of the crystals and the gas phase were dissolved. The material of inclusions heated up to the homogenization temperature became heterogeneous even during very fast quenching (two seconds) producing numerous small crystals. This fact implies that most of the inclusions contained a salt (rather than silicate) melt of sulfate-dominated composition. Such inclusions were formed from salt globules (with a density of about 2.5 g/cm3) occurring as an emulsion in the denser (2.6 g/cm3) silicate melt from which the chrome diopside crystallized.  相似文献   

14.
The Sm–Nd and Rb–Sr isotope characteristics were studied in clinopyroxenes (Cpx) of ultrabasic rocks (dunite, wehrlite, pyroxenite, and kosvite) from the Konder massif, which is a source of a unique placer platinum deposit. The chemical composition of the clinopyroxenes studied provides evidence for their crystallization from a single melt in the course of magmatic differentiation. The Sm–Nd isotope characteristics of Cpx in dunite from the Konder massif correspond to the regression with an age of 128 ± 40 Ma, which provides evidence for the same age of rocks of the “dunite core,” wehrlite, pyroxenite, kosvite, and alkaline rocks of the subsequent intrusive stage in the Konder massif. Variations in the Sr and Nd isotope characteristics in dunite, wehrlite, pyroxenite, and kosvite result from contamination of the picritic melt with rocks of the continental crust in the course of its cumulative evolution, which allows us to exclude the model of diapiric intrusion of mantle dunite.  相似文献   

15.
Melilite and wollastonite from the Colle Fabbri stock contain silicate melt and silicate-carbonate inclusions. The homogenization temperatures of silicate inclusions are within the magmatic temperature range of mantle ultrabasic melts: about 1,320?±?15 °С. Their composition is melilititic and evolves to the composition of leucite tephrite and phonolite. The composition of silicate-carbonate inclusions are high SiO2, Ca-rich, enriched in alkalies and are similar to that of inclusions of carbonatite melts in the minerals of melilitolites of other intrusive ultramafic complexes. They are also similar to the compositions of metasomatized travertine covering the melilitolite stock. The presence of primary silicate and silicate-carbonate inclusions evidences that the melilitite magma from which melilitolites of Colle Fabbri crystallized was associated with carbonatite liquid. This liquid was highly fluidized, mobile and aggressive. Actively interacting with overlying travertine, the liquid enriched them with alkalies, aluminosilicates and incompatible elements, which resulted in the equalization of their compositions. Heterogeneous compositional dominions were formed at the contact between melilitolite and wall pelites. In the minerals of these contact facies high-Si melt inclusions of varying composition have been observed. Their occurrence is related to the local assimilation by the high-temperature melilitite magma of pelitic country rocks. The content of incompatible elements in melilitite melts and melilitolites is higher than the mantle norm and they have peculiar indicator ratios, spectra, Eu/Eu* ratio, which suggest a peculiar mantle source.  相似文献   

16.
Chemical, mineralogical and isotopic studies have been made on nodules of the MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenolith suite in southern African kimberlites. All are ultramafic and ultrapotassic (MgO= 20–25%, K2O=4–9%), with bulk compositions reflecting the wide variation in relative proportions of the five minerals amongst the nodules. They are comparable in major element compositions to magnesian lamproites, in particular the ultrabasic olivine-lamproites of Western Australia. In a number of high pressure experimental studies on ultra-potassic rocks, the phases produced between 25–30 kbar from compositions comparable to those of MARID rocks (in the presence of additional water), were predominantly phlogopite and diopside (±K-richterite, ±ilmenite, ±rutile). Furthermore the compositions of experimental minerals produced in the synthetic-biotite-mafurite-H2O system by Edgar et al. (1976) are similar to those in MARID rocks.It is suggested on the basis of these observations and the textural appearance of MARID rocks that they are magmatic compositional equivalents of MgO-rich lamproites that crystallized at high pressures. While lamproites have higher average concentrations of incompatible elements, (including REE), some MARID rocks have comparable abundances. It is suggested that late stage vapour-rich melts carrying substantial REE and other incompatible elements escaped from crystallizing MARID magmas into surrounding subcontinental lithosphere, thus resulting in lower levels of these elements in most MARID rocks. In contrast faster crystallization of lamproitic rocks under volcanic/ hypabyssal conditions would prevent similar losses.The MARID proto-magmas are thought to be either partial melts of metasomatised phlogopite peridotite, or small volume asthenospheric melts which are modified and further enriched by incorporation of small partial melts of enriched subcontinental lithosphere during magma ascent.  相似文献   

17.
Malitch  K. N.  Kogarko  L. N. 《Doklady Earth Sciences》2011,440(2):1455-1459
This contribution firstly presents particularities of mineral chemistry of platinum-group elements (PGE) mineralization from placer deposits linked to the Bor-Uryakh massif of the Maimecha-Kotui Province, northern part of the Siberian Craton. The chemical composition of PGE mineralization has been studied by electron microprobe analysis. At Bor-Uryakh, main platinum-group minerals (PGM) comprise Os-Ir and Pt-Fe alloys represented by individual crystals, and polyphase PGM assemblages. The majority (e.g., 12 out of 19) of the Os-rich nuggets are iridian osmium, with subordinate amounts of native osmium (Os) and chengdeite (Ir3Fe). Pt-Fe alloys have a stoichiometric composition close to Pt2Fe. According to the nomen-clature by L. Cabri and C. Feather [1975] these minerals correspond to ferroan platinum. Based on geological position and geochemical features of investigated PGE mineralization the particular rock sources have been established. This study has demonstrated the similarity of chemical characteristics of Os-Ir and Pt-Fe alloys of the Bor-Uryakh massif to those of PGM from the Guli massif (Maimecha-Kotui Province), platiniferous zoned-type ultramafic massifs (e.g., Kondyor, Inagli and Chad) of the Aldan Province and Platinum belt of the Urals (Nizhny Tagil, Kytlym, etc.).  相似文献   

18.
Hectometric bodies of fresh mafic-ultramafic cumulates have been discovered within the Central Gneiss of the Zillertal massif, SW Tauern window (eastern Alps, Italy). The cumulates, intruded by the Central Gneiss granitoids, are amphibole-bearing harzburgites and norites made of cumulitic olivine (Fo73-80), spinels, sulphides and plagioclase (An79-87), included in orthopyroxene (En76-83) and Ti-pargasite (Mg#=0.73-0.81). Major and trace element geochemistry indicates that these rocks represent olivine + spinel - plagioclase cumulates, in which interstitial melt crystallized as orthopyroxene + Ti-pargasite. The parental melt has trace element patterns typical of subduction zone magmas. The crystallization sequence, mineral compositions, and modes indicate that cumulates formed from a H2O-rich basaltic andesite, which intruded at low-pressure (~2 kbar) and temperatures of 1,050-1,100 °C. SHRIMP U-Pb dating of zircons from ultramafic cumulates and adjacent metagranodiorite yielded ages of 309LJ and 295Dž Ma, respectively. In agreement with field relationships, these results show that the mafic-ultramafic cumulates represent a co-genetic, early product of the Late Carboniferous plutonic activity in the western Tauern window, which started in the Westphalian, earlier than previously thought. Our data on the most primitive rocks in the Zillertal massif permit, for the first time, insight into the parental magma and thus into the origin of this Late Carboniferous calc-alkaline magmatism, which was most likely related to slab break off during the Late Variscan convergence.  相似文献   

19.
The Dzheltula alkaline massif is located in the Tyrkanda ore region of the Chara–Aldan metallogenic zone of the Aldan–Stanovy Shield (South Yakutia). The region contains separate placer gold objects, which are being explored at the present time, and ore-bearing Mesozoic alkaline intrusions, which are weakly studied due to their poor accessibility. The Dzheltula massif (DM) is the largest exposed multiple-ring intrusion within the Tyrkanda ore region; therefore, it is considered as a typical object for geological, petrological, geochronological, and metallogenic studies. The DM consists of five magmatic phases of syenite composition. 40Ar–39Ar dating has established that the crystallization age of the oldest phase, the leucocratic syenite porphyry (pulaskite), is 121.1 ± 1.3 Ma. The crystallization age of the cross-cutting phases represented by syenite–porphyry dikes (laurvikites and pulaskites) ranges from 120.1 ± 2 to 118.3 ± 2.1 Ma. The youngest phase of the massif, trachyte, crystallized at 115.5 ± 1.6 Ma. According to the mineralogical and geochemical studies, two types of ore mineralization, namely gold and uranium–thorium–rare-earth (U–Th–REE), are established within the DM. The gold mineralization was found in the quartz–chlorite–pyritized metasomatites. It is confined to the NNE- and NNW-trending fault zones and coincides with the strike of the syenite porphyry dike belt. Uranium–thorium–rare-earth mineralization has been established in the quartz–feldspathic metasomatites localized in the outer contact of the massif. The juxtaposition of mineralization of different types in some zones of the Dzheltula syenite massif significantly increases the ore potential of the studied object within the Tyrkanda ore region.  相似文献   

20.
Olivinites of the Krestovskaya Intrusion consist of predominant amount of olivine, and minor Ti-magnetite, perovskite, and clinopyroxene (from single grain to a few vol %). Primary crystallized melt inclusions were found and studied in olivine, perovskite, and diopside of the olivinites. Daughter phases in olivine-hosted melt inclusions are monticellite, perovskite, kalsilite, phlogopite, magnetite, apatite, and garnet andradite. Perovskite-hosted melt inclusions contain such daughter phases as kalsilite, pectolite, clinopyroxene, biotite, magnetite, and apatite, while daughter phases in clinopyroxene-hosted melt inclusions are represented by kalsilite, phlogopite, magnetite, and apatite. According to melt inclusion heating experiments, olivine crystallized from above 1230°C to 1180°C. It was followed by perovskite crystallizing at ≥1200°C and clinopyroxene, at 1170°C. According to analysis of quenched glass of the melt inclusions, the chemical composition of melts hosted in the minerals corresponds to the larnite-normative alkali ultramafic (kamafugite) magma significantly enriched in incompatible elements. The high incompatible element concentrations, its distribution, and geochemical indicator ratios evidenced that the magma was derived by the partial melting of garnet-bearing undepleted mantle.  相似文献   

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