首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
The Turiy Massif, lying within the Kandalaksha Graben, and onthe southern coast of the Kola Peninsula, contains carbonatites,phoscorites, melilitolites, ijolites and pyroxenites withinone central and four surrounding satellite complexes. Sr–Ndisotopic data from the central complex phoscorites and carbonatites,and the nearby Terskii Coast kimberlites, combined with otherrecently published data on the Devonian Kola Alkaline Province,allow us to redefine the position of the Kola Carbonatite Line(KCL) of Kramm (European Journal of Mineralogy 5, 985–989,1993). We propose that the revised-KCL mantle sources includea lower-mantle plume, and a second enriched source, which alsocontributed to the Terskii Coast and Archangelsk kimberlites.The Turiy Massif silicate rocks and northern complex carbonatiteshave more enriched isotopic signatures than the distinct, anddepleted signatures of the central complex phoscorites and carbonatites,particularly with respect to  相似文献   

2.
Eighteen Cenozoic melilitite samples from Spain, France, West Germany and Czechoslovakia have been analyzed for major and trace elements (including REE) together with their Sr and Nd isotopic compositions. Leaching experiments produced significant shifts of their87Sr/86Sr ratio indicative of a contamination by a crustal component. Most samples fall within the Sr-Nd mantle array with ?Nd values in the 1.5–6 range. These values are considered as minimum for the melilitite mantle source hence demonstrating its time integrated LRE depletion. The Ni and Cr contents of the samples are typical of primary magmas and exclude extensive crystallization of olivine and pyroxene in a closed system. However, the chemical relationships suggest that dilution of the liquids by mafic minerals of the conduits during their ascent has been important. The REE patterns show some variations which are interpreted by this dilution effect. Once normalized to Yb they are closely similar and perfectly distinguishable from those of alkali basalts and kimberlites. All of these rocks have Ce/Yb ratios which are high but distinctive for each rock type: 40 to 200 times the chondritic ratio for kimberlites, 20 to 30 for melilitites, 8 to 15 for alkali basalts. As contamination is likely to have modified somewhat the isotopic characteristics of most of these rocks, there is no overwhelming evidence that their source is chemically different. The Ba and Rb contents together with the REE patterns of the melilitites would constrain the degree of melting to be very small (<0.2%). The calculation of batch melting and steady zone refining models suggests that kimberlites, melilitites and alkali basalts may have been derived by equilibration of deep melts with different upper mantle levels characterized by decreasing garnet/clinopyroxene ratios. The strongly incompatible elements are enriched in the melt during its ascent by leaching of the wall rocks. For the steady zone refining model, the degree of melting concept loses its significance and the difficult requirement of extracting small liquid fractions from a molten source disappears. Within the frame of this model, the preenrichment of the kimberlite, melilitite and alkali basalts source in incompatible elements by metasomatic fluids is no longer necessary.  相似文献   

3.
New petrogeochemical data on a collection of 138 samples taken from 101 kimberlite bodies of the Alakit region of Yakutia have been interpreted. It was concluded that all studied kimberlites are homogenous in geochemical composition and comparable with Group I kimberlites of South Africa. Based on cluster analysis, kimberlites of the region are subdivided into six clusters. From the first to sixth clusters, kimberlites show a decrease in carbonate material and increase in magnesian component. The spatial distribution of clusters allowed us to distinguish zoned areas with central parts consisting of kimberlites with elevated CaO, CO2, Rb, Sr, Ba, and lowered contents of SiO2, TiO2, Fe2O3, FeO, MgO, V, Cr, and Ni. From the center outward, the values of δNd and (87Sr/86Sr)i decrease, which indicate increasing contribution of the lithospheric source. The formation of magnesian kimberlites at the periphery was related to the intense interaction of protokimberlite melt with lithospheric mantle, which was accompanied by metasomatic reworking of mantle rocks with formation of minerals of megacryst assemblage and assimilation of mantle material. Economically viable diamondiferous kimberlites are confined to the peripheral parts of distinguished zones, i.e., to the kimberlites of 5–6 clusters.  相似文献   

4.
The petrological and geochemical characteristics of kimberlites from two Russian provinces of the northern East European craton (EEP) and the Siberian craton (SC) (especially the Yakutian diamondiferous province, YDP), and aphanitic kimberlites from the Jericho pipe (Canada) were compared for the elucidation of some aspects of the genesis of these rocks. The comparison of the EEP and YDP showed that they comprise identical rock associations with some variations in kimberlite composition between particular fields and regions, which are clearly manifested in the TiO2-K2O, TiO2-(Y, Zr, HREE), SiO2-MgO, SiO2-Al2O3, MgO-Ni, MgO-CO2, and MgO-H2O diagrams and in variations in light element ratios (Li/Yb, Be/Nd, and B/Nb). The compositions of YDP kimberlites are confined mainly to quadrant III; i.e., their source was mainly the depleted mantle, whereas the compositions of EEP kimberlites fall within all four quadrants in the fields of both enriched and slightly depleted mantle reservoirs. The initial (143Nd/144Nd) i ratio of kimberlites from the Yakutian collection is 0.5121–0.5126. The lead isotopic characteristics of the EEP and YDP kimberlites are similar to mantle values: 206Pb/204Pb of 16.19–19.14, 207Pb/204Pb of 15.44–15.61, and 208Pb/204Pb of 34.99–38.55. In the 207Pb/204Pb-206Pb/204Pb diagram, part of the kimberlites, including those from the Botuobiya pipe, fall within the lower part of the field of group I kimberlites from southern Africa near the Pb isotopic composition of the depleted mantle. It was shown that the chemical compositions of the aphanitic kimberlites of the Jericho pipe (supposedly approaching the composition of primary magmas) are similar to those of some individual kimberlite samples from the YDP and EEP. It was supposed that the initial kimberlite melt arrived from the asthenosphere and was enriched in water and other volatile components (especially CO2). During its ascent to the surface, the melt assimilated mantle components, primarily MgO; as a result, it acquired the compositional characteristics observed in kimberlites. Subsequent compositional modifications were related to diverse factors, including the type of mantle metasomatism, degree of melting, etc. We emphasized the importance of petrological and geochemical criteria (low contents of HREE and Ti in the rocks and a kimberlite source similar to BSE or EMI) for the estimation of the diamond potential of rocks.  相似文献   

5.
The petrology and geochemistry of some new occurrences of Mesoproterozoic diamondiferous hypabyssal-facies kimberlites from the Chigicherla, Wajrakarur-Lattavaram and Kalyandurg clusters of the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are reported. The kimberlites contain two generations of olivine, and multiple groundmass phases including phlogopite, spinel, calcite, dolomite, apatite, perovskite, apatite and rare titanite, and xenocrysts of eclogitic garnet and picro-ilmenite. Since many of the silicate minerals in these kimberlites have been subjected to carbonisation and alteration, the compositions of the groundmass oxide minerals play a crucial role in their characterisation and in understanding melt compositions. While there is no evidence for significant crustal contamination in these kimberlites, some limited effects of ilmenite entrainment are evident in samples from the Kalyandurg cluster. Geochemical studies reveal that the WKF kimberlites are less differentiated and more primitive than those from the Narayanpet kimberlite field (NKF), Eastern Dharwar craton. Highly fractionated (La/Yb = 108–145) chondrite-normalised distribution patterns with La abundances of 500–1,000 × chondrite and low heavy rare earth elements (HREE) abundances of 5–10 × chondrite are characteristic of these rocks. Metasomatism by percolating melts from the convecting mantle, rather than by subduction-related processes, is inferred to have occurred in their source regions based on incompatible element signatures. While the majority of the Eastern Dharwar craton kimberlites are similar to the Group I kimberlites of southern Africa in terms of petrology, geochemistry and Sr–Nd isotope systematics, others show the geochemical traits of Group II kimberlites or an overlap between Group I and II kimberlites. Rare earth element (REE)-based semi-quantitative forward modelling of batch melting of southern African Group I and II kimberlite source compositions involving a metasomatised garnet lherzolite and very low degrees of partial melting demonstrate that (1) WKF and NKF kimberlites display a relatively far greater range in the degree of melting than those from the on-craton occurrences from southern Africa and are similar to that of world-wide melilitites, (2) different degrees of partial melting of a common source cannot account for the genesis of all the EDC kimberlites, (3) multiple and highly heterogeneous kimberlite sources involve in the sub-continental lithospheric mantle (SCLM) in the Eastern Dharwar craton and (4) WKF and NKF kimberlites generation is a resultant of complex interplay between the heterogeneous sources and their different degrees of partial melting. These observations are consistent with the recent results obtained from inversion modelling of REE concentrations from EDC kimberlites in that both the forward as wells as inverse melting models necessitate a dominantly lithospheric, and not asthenospheric, mantle source regions. The invading metasomatic (enriching) melts percolating from the convecting (asthenosphere) mantle impart an OIB-like isotopic signature to the final melt products.  相似文献   

6.
Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite–K-richterite–peridotites: PKPs) or crystallize from K-rich melts, forming the mica–amphibole–rutile–ilmenite–diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U–Pb formation ages of 88 ± 2 (1σ=1 standard deviation) and 82 ± 3 Ma data for zircons from a K-richterite–phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlites in the Kimberley area dated at 83 ± 4 (2σ) and 84 ± 0.9 Ma. One exceptionally old age spot of 102 ± 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group I kimberlite emplacement by several millions of years with minor resetting of the U–Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U–Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309–323, 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks. Received: 13 December 1999 / Accepted: 13 April 2000  相似文献   

7.
Summary Mesozoic melilite-bearing ultramafic lamprophyres are developed as sill, dyke and plug-like intrusive bodies in the East Antarctic Beaver Lake area. They consist of varying amounts of olivine, melilite, phlogopite, nepheline, titanomagnetite and perovskite as major phases, accompanied by minor amounts of apatite, carbonate, spinel, glass and, rarely, monticellite. The rocks are mineralogically and geochemically broadly similar to olivine melilitites, differing in higher CO2 and modal phlogopite and carbonate contents. The ultramafic lamprophyres are MgO-rich (13.4–20.5 wt%) and SiO2-poor (32.8–37.2 wt%), indicative of a near-primary nature. Major and trace element features are consistent with minor fractionation of olivine and Cr-spinel from melts originating at depths of 130–140 km. Primary melts originated by melting of upper mantle peridotite which had been veined by phlogopite + carbonate + clinopyroxene-bearing assemblages less than 200 Ma before eruption. The presence of the veins and their time of formation is required to explain high incompatible trace element contents and growth of 87Sr/86Sr, leaving 143Nd/144Nd unaffected. The major element, compatible trace element, and most radiogenic isotope characteristics are derived from melting of the wall-rock peridotite. The depth of about 130 km is indicated by the presence of phlogopite rather than amphibole in the veins, by control of the REE pattern by residual garnet, by the high MgO content of the rocks, and by the expected intersection of the rift-flank geotherm with the solidus at this depth. The higher CO2 contents than are characteristic for olivine melilitites favoured the crystallization of melilite at crustal pressures, and suppressed the crystallization of clinopyroxene. The Beaver Lake ultramafic lamprophyres are a distal effect of the breakup of Gondwanaland, too distal to show a geochemical signature of the Kerguelen plume. Upward and outward movement of the asthenosphere-lithosphere boundary beneath the Lambert-Amery rift led first to the production of phlogopite- and carbonate-rich veins, and later to the generation of the ultramafic lamprophyres themselves. Received March 31, 2000; revised version accepted September 3, 2001  相似文献   

8.
New experimental data in CaO-MgO-SiO2-CO2 at 1 GPa define the vapor-saturated silicate-carbonate liquidus field boundary involving primary minerals calcite, forsterite and diopside. The eutectic reaction for melting of model calcite (1% MC)-wehrlite at 1 GPa is at 1100 °C, with liquid composition (by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). These data combined with previous results permit construction of the isotherm-contoured vapor-saturated liquidus surface for the calcite/dolomite field, and part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We recently determined the complete vapor-saturated liquidus surface between carbonates and model peridotites at 2.7 GPa; the peritectic reaction for dolomite (25% MC)-wehrlite at 2.7 GPa occurs at 1300 °C, with liquid composition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these two surfaces provide the road-map for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100 km. Relationships among kimberlites, melilitites, carbonatites and the liquidus phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends directly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa liquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaCO3 (90 ± 1 wt%) in terms of only carbonate components. For CO2-bearing mantle, all magmas at depth must pass through initial dolomitic compositions. Rising dolomitic carbonatite melt will vesiculate and may erupt as primary magmas through cracks from about ˜70 km. If it percolates through metasomatic wehrlite from 70 km toward the Moho at 35–40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11–18% (70–40 km) on the silicate-carbonate boundary. Received: 22 June 1998 / Accepted: 7 July 1999  相似文献   

9.
Here we present new data on the major and trace element compositions of silicate and oxide minerals from mantle xenoliths brought to the surface by the Carolina kimberlite, Pimenta Bueno Kimberlitic Field, which is located on the southwestern border of the Amazonian Craton. We also present Sr-Nd isotopic data of garnet xenocrysts and whole-rocks from the Carolina kimberlite. Mantle xenoliths are mainly clinopyroxenites and garnetites. Some of the clinopyroxenites were classified as GPP–PP–PKP (garnet-phlogopite peridotite, phlogopite-peridotite, phlogopite-K-richterite peridotite) suites, and two clinopyroxenites (eclogites) and two garnetites are relicts of an ancient subducted slab. Temperature and pressure estimates yield 855–1102 °C and 3.6–7.0 GPa, respectively. Clinopyroxenes are enriched in light rare earth elements (LREE) (LaN/YbN = 5–62; CeN/SmN = 1–3; where N = primitive mantle normalized values), they have high Ca/Al ratios (10–410), low to medium Ti/Eu ratios (742–2840), and low Zr/Hf ratios (13–26), which suggest they were formed by metasomatic reactions with CO2-rich silicate melts. Phlogopite with high TiO2 (>2.0 wt.%), Al2O3 (>12.0 wt.%), and FeOt (5.0–13.0 wt.%) resemble those found in the groundmass of kimberlites, lamproites and lamprophyres. Conversely, phlogopite with low TiO2 (<1.0 wt.%) and lower Al2O3 (<12.0 wt.%) are similar to those present in GPP-PP-PKP, and in MARID (mica-amphibole-rutile-ilmenite-diopside) and PIC (phlogopite-ilmenite-clinopyorxene) xenoliths. The GPP-PP-PKP suite of xenoliths, together with the clinopyroxene and phlogopite major and trace element signatures suggests that an intense proto-kimberlite melt metasomatism occurred in the deep cratonic lithosphere beneath the Amazonian Craton. The Sr-Nd isotopic ratios of pyrope xenocrysts (G3, G9 and G11) from the Carolina kimberlite are characterized by high 143Nd/144Nd (0.51287–0.51371) and εNd (+4.55 to +20.85) accompanied with enriched 87Sr/86Sr (0.70405–0.71098). These results suggest interaction with a proto-kimberlite melt compositionally similar with worldwide kimberlites. Based on Sr-Nd whole-rock compositions, the Carolina kimberlite has affinity with Group 1 kimberlites. The Sm-Nd isochron age calculated with selected eclogitic garnets yielded an age of 291.9 ± 5.4 Ma (2 σ), which represents the cooling age after the proto-kimberlite melt metasomatism. Therefore, we propose that the lithospheric mantle beneath the Amazonian Craton records the Paleozoic subduction with the attachment of an eclogitic slab into the cratonic mantle (garnetites and eclogites); with a later metasomatic event caused by proto-kimberlite melts shortly before the Carolina kimberlite erupted.  相似文献   

10.
The diamondiferous Letlhakane kimberlites are intruded into the Proterozoic Magondi Belt of Botswana. Given the general correlation of diamondiferous kimberlites with Archaean cratons, the apparent tectonic setting of these kimberlites is somewhat anomalous. Xenoliths in kimberlite diatremes provide a window into the underlying crust and upper mantle and, with the aid of detailed petrological and geochemical study, can help unravel problems of tectonic setting. To provide relevant data on the deep mantle under eastern Botswana we have studied peridotite xenoliths from the Letlhakane kimberlites. The mantle-derived xenolith suite at Letlhakane includes peridotites, pyroxenites, eclogites, megacrysts, MARID and glimmerite xenoliths. Peridotite xenoliths are represented by garnet-bearing harzburgites and lherzolites as well as spinel-bearing lherzolite xenoliths. Most peridotites are coarse, but some are intensely deformed. Both garnet harzburgites and garnet lherzolites are in many cases variably metasomatised and show the introduction of metasomatic phlogopite, clinopyroxene and ilmenite. The petrography and mineral chemistry of these xenoliths are comparable to that of peridotite xenoliths from the Kaapvaal craton. Calculated temperature-depth relations show a well-developed correlation between the textures of xenoliths and P-T conditions, with the highest temperatures and pressures calculated for the deformed xenoliths. This is comparable to xenoliths from the Kaapvaal craton. However, the P-T gap evident between low-T coarse peridotites and high-T deformed peridotites from the Kaapvaal craton is not seen in the Letlhakane xenoliths. The P-T data indicate the presence of lithospheric mantle beneath Letlhakane, which is at least 150 km thick and which had a 40mW/m2 continental geotherm at the time of pipe emplacement. The peridotite xenoliths were in internal Nd isotopic equilibrium at the time of pipe emplacement but a lherzolite xenolith with a relatively low calculated temperature of equilibration shows evidence for remnant isotopic disequilibrium. Both harzburgite and lherzolite xenoliths bear trace element and isotopic signatures of variously enriched mantle (low Sm/Nd, high Rb/Sr), stabilised in subcontinental lithosphere since the Archaean. It is therefore apparent that the Letlhakane kimberlites are underlain by old, cold and very thick lithosphere, probably related to the Zimbabwe craton. The eastern extremity of the Proterozoic Magondi Belt into which the kimberlites intrude is interpreted as a superficial feature not rooted in the mantle. Received: 19 March 1996 / Accepted: 16 October 1996  相似文献   

11.
《地学前缘(英文版)》2020,11(3):793-805
Detailed mineralogy,bulk rock major,trace and Sr-Nd isotope compositions,and ~(40)Ar/~(39)Ar dating of the Pipe-8 diamondiferous ultramafic intrusion in the Wajrakarur cluster of southern India,is reported.Based on the presence of Ti-rich phlogopite,high Na/K content in amphibole,Al-and Ti-rich diopside,a titanomagnetite trend in spinel and the presence of Ti-rich schorlomite garnet and carbonates in the groundmass,the Pipe-8 intrusion is here more precisely classified as an ultramafic lamprophyre(i.e.,aillikite).An aillikite affinity of the Pipe-8 intrusion is further supported by the bulk rock major and trace element and Sr-Nd isotope geochemistry.Sr-Nd isotope data are consistent with a common,moderately depleted upper mantle source region for both the Pipe-8 aillikite as well as the Wajrakarur kimberlites of southern India.A phlogopite-rich groundmass ~(40)Ar/~(39)Ar plateau age of 1115.8±7.9 Ma(2σ) for the Pipe-8 intrusion falls within a restricted 100 Ma time bracket as defined by the 1053-1155 Ma emplacement ages of kimberlites and related rocks in India.The presence of ultramafic lamprophyres,carbonatites,kimberlites,and olivine lamproites in the Wajrakarur kimberlite field requires low degrees of partial melting of contrasting metasomatic assemblages in a heterogeneous sub-continental lithospheric mantle.The widespread association of kimberlite and other mantle-derived magmatism during the Mesoproterozoic(ca.1.1 Ga) have been interpreted as being part of a single large igneous province comprising of the Kalahari,Australian,West Laurentian and Indian blocks of the Rodinia supercontinent that were in existence during its assembly.In India only kimberlite/lamproite/ultramafic lamprophyre magmatism occurred at this time without the associated large igneous provinces as seen in other parts of Rodinia.This may be because of the separated paleo-latitudinal position of India from Australia during the assembly of Rodinia.It is speculated that the presence of a large plume at or close to 1.1 Ga within the Rodinian supercontinent,with the Indian block located on its periphery,could be the reason for incipient melting of lithospheric mantle and the consequent emplacement of only kimberlites and other ultramafic,volatile rich rocks in India due to comparatively low thermal effects from the distant plume.  相似文献   

12.
A suite of spinel lherzolite and wehrlite xenoliths from a Devonian kimberlite dyke near Kandalaksha, Kola Peninsula, Russia, has been studied to determine the nature of the lithospheric mantle beneath the northern Baltic Shield. Olivine modal estimates and Fo content in the spinel lherzolite xenoliths reveal that the lithosphere beneath the Archaean–Proterozoic crust has some similarities to Phanerozoic lithospheric mantle elsewhere. Modal metasomatism is indicated by the presence of Ti-rich and Ti-poor phlogopite, pargasite, apatite and picroilmenite in the xenoliths. Wehrlite xenoliths are considered to represent localised high-pressure cumulates from mafic–ultramafic melts trapped within the mantle as veins or lenses. Equilibration temperatures range from 775 to 969 °C for the spinel lherzolite xenoliths and from 817 to 904 °C for the wehrlites.

Laser ablation ICP-MS data for incompatible trace elements in primary clinopyroxenes and metasomatic amphiboles from the spinel lherzolites show moderate levels of LREE enrichment. Replacement clinopyroxenes in the wehrlites are less enriched in LREE but richer in TiO2. Fractional melt modelling for Y and Yb concentrations in clinopyroxenes from the spinel lherzolites indicates 7–8% partial melting of a primitive source. Such a volume of partial melt could be related to the 2.4–2.5 Ga intrusion of basaltic magmas (now metamorphosed to garnet granulites) in the lower crust of the northern Baltic Shield. The lithosphere beneath the Kola Peninsula has undergone several episodes of metasomatism. Both the spinel lherzolites and wehrlites were subjected to an incomplete carbonatitic metasomatic event, probably related to an early carbonatitic phase associated with the 360–380 Ma Devonian alkaline magmatism. This resulted in crystallisation of secondary clinopyroxene rims at the expense of primary orthopyroxenes, with development of secondary forsteritic olivine and apatite. Two separate metasomatic events resulted in the crystallisation of the Ti–Fe-rich amphibole, phlogopite and ilmenite in the wehrlites and the low Ti–Fe amphibole and phlogopite in the spinel lherzolites. Alternatively, a single metasomatic event with a chemically evolving melt may have produced the significant compositional differences seen in the amphibole and phlogopite between the spinel lherzolites and wehrlites. The calculated REE pattern of a melt in equilibrium with clinopyroxenes from a cpx-rich pocket is identical to that of the kimberlite host, indicating a close petrological relationship.  相似文献   


13.
Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits.  相似文献   

14.
The Sm-Nd systematics in a variety of mantle-derived samples including kimberlites, alnoite, carbonatite, pyroxene and amphibole inclusions in alkali basalts and xenolithic eclogites, granulites and a pyroxene megacryst in kimberlites are reported. The additional data on kimberlites strengthen our earlier conclusion that kimberlites are derived from a relatively undifferentiated chondritic mantle source. This conclusion is based on the observation that the Nd values of most of the kimberlites are near zero. In contrast with the kimberlites, their garnet lherzolite inclusions show both time-averaged Nd enrichment and depletion with respect to Sm. Separated clinopyroxenes in eclogite xenoliths from the Roberts Victor kimberlite pipe show both positive and negative Nd values suggesting different genetic history. A whole rock lower crustal scapolite granulite xenolith from the Matsoku kimberlite pipe shows a negative Nd value of -4.2, possibly representative of the base of the crust in Lesotho. It appears that all inclusions, mafic and ultramafic, in kimberlites are unrelated to their kimberlite host.The above data and additional Sm-Nd data on xenoliths in alkali basalts, alpine peridotite and alnoite-carbonatites are used to construct a model for the upper 200 km of the earth's mantle — both oceanic and continental. The essential feature of this model is the increasing degree of fertility of the mantle with depth. The kimberlite's source at depths below 200 km in the subcontinental mantle is the most primitive in this model, and this primitive layer is also extended to the suboceanic mantle. However, it is clear from the Nd-isotopic data in the xenoliths of the continental kimberlites that above 200 km the continental mantle is distinctly different from their suboceanic counterpart.  相似文献   

15.
Kimberlites with different diamond grades from the Zolotitsa, Verkhotina, and Kepina occurrences of the Zimny Bereg field (Arkangel’sk oblast) have been compared in order to ascertain geochemical criteria of their diamond resource potential. A new collection of 21 core samples taken within a depth interval of 207–940 m from nine boreholes drilled in the central and western portions of the high-grade diamond-bearing Grib kimberlite pipe was subjected to comprehensive petrographic and geochemical examination, including Sr, Nd, and Pb isotopes and trace elements determined with ICP-MS. The compositional variations in kimberlites are controlled by the structural types of rocks. Porphyritic kimberlite (PK) distinctly differs from autolithic kimberlite breccia (AKB). Autoliths (Av) and PK are enriched in Th, U, Nb, Ta, La, Ce, Pr, P, Nd, Sm, Eu, Ti, LREE, and MREE, whereas HREE contents are rather uniform in all types of kimberlites. No lateral zoning was observed in pipes pertaining to the same structural type. The composition of kimberlites in the Zimny Bereg field and their diamond resource potential are variable. In the series of the Zolotitsa, Verkhotina, and Kepina occurrences, the Ti content increases, the La/Yb ratio grows from 18–44 to 70–130, and the diamond grade diminishes in the Kepina occurrence. The variations in kimberlite compositions are considered in terms of the degree of partial melting in the mantle, the role of volatiles, etc. As follows from the variation in the Ce/Y ratio, kimberlites from the Zolotitsa occurrence were formed at a lower degree of partial melting in comparison with the Kepina occurrence. Products of different degrees of partial melting are recognized within the Grib pipe; Av were likely formed at a somewhat higher degree of melting than AKB. An appreciable isotopic heterogeneity of the mantle is recorded in variable Nd and Sr isotopic compositions of kimberlites. The Kepina kimberlites were derived from a source slightly depleted relative to CHUR (?Nd(t) reaches +4) and are close to kimberlites of group I in South Africa. Kimberlites from the Grib pipe with transitional Nd isotopic composition plotted near the Bulk Silicate Earth (BSE) value in the ?Nd(t)-?Sr(t) diagram adjoin the first group. The source of kimberlites of the Zolotitsa occurrence falls in the field of enriched mantle and is considered to be a product of interaction of an asthenospheric plume with the ancient enriched lithospheric mantle. Kimberlites depleted in Ti, Zr, and Th are related to a source formed as a result of a multistage process that included mantle metasomatism with participation of fluids. Devonian kimberlites derived from sources that involve crustal material (a shift of 206Pb/204Pb, minimums of Th, U, Nb, and Ta contents) are diamond-bearing both in the East European Platform (the Zolotitsa and Verkhotina occurrences) and in the Siberian Craton (the Nakyn field).  相似文献   

16.
More than 99% of mineral inclusions in diamonds from the River Ranch pipe in the Late Archean Limpopo Mobile Belt (Zimbabwe), are phases of harzburgitic paragenesis, namely olivine (Fo92–93), orthopyroxene (Mg# = 93), G10 garnets and chromites. The diamond inclusion (DI) chemistry demonstrates a limited overlap with River Ranch kimberlite macrocrysts: the DI garnets are more Ca-undersaturated, and DI spinel and garnet are more Mg-rich. Most River Ranch diamond inclusions were equilibrated at T = 1080–1320 °C, P = 47–61 kbar, and f O2 between IW and WM buffers. The P/T profile beneath the Limpopo Mobile Belt (LMB) is consistent with a paleo-heat flow of 41–42 mW/m2, similar to calculations for Roberts Victor, but hotter than for the Finsch, Kimberley, Koffiefontein and Premier Mines. This is ascribed to the younger tectonothermal age of the LMB and its proximity to Late Archean oceans. Like diamond inclusions from all other kimberlites studied, the River Ranch DI have a lithospheric affinity and therefore indicate that an ancient, chemically depleted, thick (at least 200 km) mantle root existed beneath the Limpopo Mobile Belt 530–540 Ma ago. The mantle root might have developed beneath the continental Central Zone of the LMB as early as the Archean, and could be alien to the overthrust allochthonous sheet of the Limpopo Belt. Oxygen fugacity estimates for diamond inclusions at River Ranch are similar to other diamondiferous harzburgites beneath the Kaapvaal craton, indicating that the Kaapvaal mantle as a whole was well buffered and homogeneous with respect to f O2 at the time of peridotitic diamond crystallization. Received: 11 January 1995 / Accepted: 10 June 1997  相似文献   

17.
The geological structure, age, and genesis of sedimentary—volcanogenic, metamorphic, and metasomatic rocks from the Terskii greenstone belt fringing the southern Imandra—Varzuga structure in the southeastern Kola Peninsula are discussed with defining main stages in endogenic activity of the region in the Late Archean and Early Proterozoic. The U-Pb method (SHRIMP-II, ID-TIMS, and Pb-LS techniques) was used to determine the age of volcano-sedimentary rocks of the Imandra Group as well as that of magmatic and superimposed metamorphic and metasomatic processes. The basic—intermediate metavolcanics of the Imandra Group are dated at 2.67 Ga, which corresponds to the Lopingian Gimol’skii Superhorizon (Late Archean). The Archean metavolcanics were subjected to Early Proterozoic regional metamorphism 2.1 Ga ago and metasomatic processes in the period of 1.85 to 1.77 Ga ago. The obtained data indicate multistage evolution of rock formation in the Terskii greenstone belt located in the southern flank of the Imandra—Varzuga structure in the Kola Peninsula.  相似文献   

18.
A suite of more than 200 garnet single crystals, extracted from 150 xenoliths, covering the whole range of types of garnet parageneses in mantle xenoliths so far known from kimberlites of the Siberian platform and collected from nearly all the kimberlite pipes known in that tectonic unit, as well as some garnets found as inclusions in diamonds and olivine megacrysts from such kimberlites, were studied by means of electron microprobe analysis and single-crystal IR absorption spectroscopy in the v OH vibrational range in search of the occurrence, energy and intensity of the v OH bands of hydroxyl defects in such garnets and its potential use in an elucidation of the nature of the fluid phase in the mantle beneath the Siberian platform. The v OH single-crystal spectra show either one or a combination of two or more of the following major v OH bands, I 3645–3662 cm−1, II 3561–3583 cm−1, III 3515–3527 cm−1, and minor bands, Ia 3623–3631 cm−1, IIa 3593–3607 cm−1. The type of combination of such bands in the spectrum of a specific garnet depends on the type of the rock series of the host xenolith, Mg, Mg-Ca, Ca, Mg-Fe, or alkremite, on the xenolith type as well as on the chemical composition of the respective garnet. Nearly all garnets contain band systems I and II. Band system III occurs in Ti-rich garnets, with wt% TiO2 > ca. 0.4, from xenoliths of the Mg-Ca and Mg-Fe series, only. The v OH spectra do not correspond to those of OH− defects in synthetic pyropes or natural ultra-high pressure garnets from diamondiferous metamorphics. There were no indications of v OH from inclusions of other minerals within the selected 60 × 60 μm measuring areas in the garnets. The v OH spectra of pyrope-knorringite- and pyrope-knorringite-uvarovite-rich garnets included in diamonds do not show band systems I to III. Instead, they exhibit one weak, broad band (Δv OH 200–460 cm−1) near 3570 cm−1, a result that was also obtained on pyrope-knorringite-rich garnets extracted from two olivine megacrysts. The quantitative evaluation, on the basis of relevant existing calibrational data (Bell et al. 1995), of the sum of integral intensities of all v OH bonds of the garnets studied yielded a wide range of “water” concentrations within the set of the different garnets, between values below the detection limit of our single-crystal IR method, near 2 × 10−4 wt%, up to 163 × 10−4 wt%. The “water” contents vary in a complex manner in garnets from different xenolith types, obviously depending on a large number of constraints, inherent in the crystal chemistry as well as the formation conditions of the garnets during the crystallization of their mantle host rocks. Secondary alteration effects during uplift of the kimberlite, play, if any, only a minor role. Despite the very complex pattern of the “water” contents of the garnets, preventing an evaluation of a straightforward correlation between “water” contents of the garnets and the composition of the mantle's fluid phase during garnet formation, at least two general conclusions could be drawn: (1) the wide variation of “water” contents in garnets is not indicative of regional or local differences in the composition of the mantle's fluid phase; (2) garnets formed in the high-pressure/high-temperature diamond-pyrope facies invariably contain significantly lower amounts of “water” than garnets formed under the conditions of the graphite-pyrope facies. This latter result (2) may point to significantly lower f H2O and f O2 in the former as compared to the latter facies. Received: 25 November 1997 / Accepted: 9 March 1998  相似文献   

19.
Mica kimberlite and alkali picrite were identified in the northwestern Urik-Iya Graben of the eastern Sayan region. Typomorphism of Cr-diopside and high-Cr (up to 55.22 wt % Cr2O3) spinel from kimberlite of the Bushkanai dike indicate that the melt was generated in the mantle, composed of spinel peridotite. The high content of Cr-spinel (45–55 wt % Cr2O3) microlites in the groundmass of kimberlite and small amounts of ulvospinel and titanomagnetite in the absence of perovskite testifies to the diamond potential of this kimberlite. Picroilmenite, manganoilmenite with an anomalously high MnO content (11.37–17.78 wt %), and barium titanate with (wt %) 62.21 TiO2, 0.61 Cr2O3, 15.89 FeO, 4.05 MnO, 1.71 CaO, and 11.13 BaO close in composition to a new mineral species from the Murun pluton were identified in the groundmass for the first time. Kimberlite from the Bushkanai dike belongs to the Zolotitsa low-Ti geochemical type of kimberlites derived from the slightly enriched lithospheric mantle EM1. The distribution of trace elements, including REE, in picrite from the same dike corresponds to the slightly depleted asthenospheric mantle. Different mantle sources of kimberlite and picrite from the same dike indicate that these rocks are related to independent melts rather than to products of fractionation of a common parental alkaline ultramafic magma.  相似文献   

20.
《Gondwana Research》2016,29(4):1344-1360
Using free-board modeling, we examine a vertically-averaged mantle density beneath the Archean–Proterozoic Siberian Craton in the layer from the Moho down to base of the chemical boundary layer (CBL). Two models are tested: in Model 1 the base of the CBL coincides with the LAB, whereas in Model 2 the base of the CBL is at a 180 km depth. The uncertainty of density model is < 0.02 t/m3 or < 0.6% with respect to primitive mantle. The results, calculated at in situ and at room temperature (SPT) conditions, indicate a heterogeneous density structure of the Siberian lithospheric mantle with a strong correlation between mantle density variations and the tectonic setting. Three types of cratonic mantle are recognized from mantle density anomalies. ‘Pristine’ cratonic regions not sampled by kimberlites have the strongest depletion with density deficit of 1.8–3.0% (and SPT density of 3.29–3.33 t/m3 as compared to 3.39 t/m3 of primitive mantle). Cratonic mantle affected by magmatism (including the kimberlite provinces) has a typical density deficit of 1.0–1.5%, indicative of a metasomatic melt-enrichment. Intracratonic sedimentary basins have a high density mantle (3.38–3.40 t/m3 at SPT) which suggests, at least partial, eclogitization. Moderate density anomalies beneath the Tunguska Basin imply that the source of the Siberian LIP lies outside of the Craton. In situ mantle density is used to test the isopycnic condition of the Siberian Craton. Both CBL thickness models indicate significant lateral variations in the isopycnic state, correlated with mantle depletion and best achieved for the Anabar Shield region and other intracratonic domains with a strongly depleted mantle. A comparison of synthetic Mg# for the bulk lithospheric mantle calculated from density with Mg# from petrological studies of peridotite xenoliths from the Siberian kimberlites suggests that melt migration may produce local patches of metasomatic material in the overall depleted mantle.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号