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1.
Major, trace element and radiogenic isotope results are presented for a suite of hypabyssal kimberlites from a single pipe, at the Finsch Mine, South Africa. These are Group 2 kimberlites characterised by abundant phlogopite ± serpentine ± diopside; they are ultrabasic (SiO 2 < 42 wt.%%) and ultrapotassic (K 2O/Na 2O > 6.9) igneous rocks, they exhibit a wide range in major element chemistry with SiO 2 = 27.6−41.9 wt. % and MgO = 10.4−33.4 wt. %. ( 87Sr/ 86Sr) i=0.7089 to 0.7106, ε Nd is −6.2 to −9.7 and they have unradiogenic ( 207Pb/ 204Pb) i contents which ensure that they plot below the Pb-ore growth curve. They have high incompatible and compatible element contents, a striking positive array between Y and Nb which indicates that garnet was not involved in the within suite differentiation processes, and a negative trend between K/Nb and Nb contents which suggests that phlogopite was involved. In addition, some elements exhibit an unexpected order of relative incompatibility for different trace elements which suggests that the intra-kimberlite variations are not primarily due to variations in the degree of partial melting. The effects of fractional crystallization are difficult to establish because for the most part they have been masked by the entrainment of 50–60% mantle peridotite. Thus, the Finsch kimberlites are interpreted as mixtures of a melt component and entrained garnet peridotite, with no evidence for significant contamination with crustal material. The melt component was characterised by high incompatible element contents, which require both very small degrees of partial melting, and source regions with higher incompatible element contents than depleted or primitive mantle. Since the melt component was the principal source of incompatible elements in the kimberlite magma, the enriched Nd, Sr and Pb isotope ratios of the kimberlite are characteristic of the melt source region. The melt fractions were therefore derived from ancient, trace elements enriched portions of the upper mantle, most probably situated within the sub-continental mantle lithosphere, and different from the low 87Sr/ 86Sr garnet peridotite xenoliths found at Finsch. Within the sub-continental mantle lithosphere old, incompatible element enriched source regions for the kimberlite melt fraction are inferred to have been overlain by depleted mantle material which became entrained in the kimberlite magma. 相似文献
2.
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
3.
In the late 1990s, the Fazenda Largo kimberlite cluster was discovered in the Piauí State of Brazil. As with earlier known kimberlites in this area – Redondão, Santa Filomena-Bom Jesus (Gilbues) and Picos – this cluster is located within the Palaeozoic Parnaiba Sedimentary Basin that separates the São Francisco and the Amazonian Precambrian cratons. Locations of kimberlites are controlled by the ‘Transbrasiliano Lineament’. The Fazenda Largo kimberlites are intensely weathered, almost completely altered rocks with a fine-grained clastic structure, and contain variable amounts of terrigene admixture (quartz sand). These rocks represent near-surface volcano-sedimentary deposits of the crater parts of kimberlite pipes. By petrographic, mineralogical and chemical features, the Fazenda Largo kimberlites are similar to average kimberlite. The composition of the deep-seated material in the Fazenda Largo kimberlites is quite diverse: among mantle microxenoliths are amphibolitised pyrope peridotites, garnetised spinel peridotites, ilmenite peridotites, chromian spinel + chromian diopside + pyrope intergrowths, and large xenoliths of pyrope dunite. High-pressure minerals are predominantly of the ultramafic suite, Cr-association minerals (purplish-red and violet pyrope, chromian spinel, chromian diopside, Cr-pargasite and orthopyroxene). The Ti-association minerals of the ultramafic suite (picroilmenite and orange pyrope), as well as rare grains of orange pyrope-almandine of the eclogite association, are subordinate. Kimberlites from all four pipes contain rare grains of G10 pyrope of the diamond association, but chromian spinel of the diamond association was not encountered. By their tectonic position, by geochemical characteristics, and by the composition of kimberlite indicator minerals, the Fazenda Largo kimberlites, like the others of such type, are unlikely to be economic. 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 TiO 2-K 2O, TiO 2-(Y, Zr, HREE), SiO 2-MgO, SiO 2-Al 2O 3, MgO-Ni, MgO-CO 2, and MgO-H 2O 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 CO 2). 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.
Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources. Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO2 graphs as well as MgO–Cr2O3 relationships. 相似文献
6.
Early Proterozoic kimberlites of Karelia are among the most ancient diamond-bearing primary source rocks in the world. They compose the large (2.0 × 0.8 km) Kimozero body localized in the predicted Zaonezhskoe kimberlite field. The established and assumed occurrences of kimberlite magmatism are located within the Karelian craton, which was stabilized during the Early Archean. They are confined to the central part of a large geophysical anomaly detected by gravity, magnetic, seismic, and heat-flow studies and mark a deep-seated magma chamber. Kimberlite bodies occur within structural blocks bounded by zones of plicative-rupture dislocations.The Kimozero kimberlites form an extensive but thin saucer-like body cut by narrow quasi-cylindrical feeders and dikes. It consists of metamorphosed kimberlites, their breccias and tuffs with widely varying amounts of mica. The body includes fragmentary fine-layered crater formations. The rocks contain olivine and phlogopite phenocrysts in an extremely altered groundmass of serpentine, chlorite, calcite, mica, and ore minerals as well as indicator minerals of kimberlites, such as Cr-spinel, manganiferous ilmenite, Cr-diopside, and rare pyrope. About 100 diamonds were extracted from 12 samples (total weight 815 kg). The crystals are colorless resorbed octahedra and, more seldom, combined octahedra-dodecahedra and spinel twins with abundant green spots caused by natural irradiation, which often make the whole crystal surface green. The diamonds contain inclusions of Mg-rich orthopyroxene and pentlandite suggestive of peridotitic lithospheric mantle derivation and dating of the sulfide inclusion implies a late Archean mantle source. By petrochemistry, the rocks are classified as kimberlites.The Kimozero kimberlites differ from classical Phanerozoic ones in having higher Fe contents, low contents of alkalies and P 2O 5, and intense superimposed carbonate, magnetite, and amphibole mineralization. The saucer-like bodies with narrow feeders without developed diatremes have no analogs in Russia but are similar to the saucer-like kimberlite bodies in Canada (Fort a la Corne), India (Tokapal), and Central Africa (Bakwanga) and the West Kimberley lamproites in Australia. By analogy with these bodies and on the basis of some common petrographic features (presence of pyroclastics and specific amoeba-like autoliths, scarcity of fragments of the enclosing rocks, local reworking of the deposited matter), the Kimozero kimberlites are considered to be the products of subaerial volcanic central-type eruptions. 相似文献
7.
At present, 48 Late Cretaceous (ca. 70–88 Ma) kimberlitic pipes have been discovered in three separate areas of the northern Alberta: the Mountain Lake cluster, the Buffalo Head Hills field and the Birch Mountains field. The regions can be distinguished from one another by their non-archetypal kimberlite signature (Mountain Lake) or, in the case of kimberlite fields, primitive (Buffalo Head Hills) to evolved (Birch Mountains) magmatic signatures. The dominant process of magmatic differentiation is crystal fractionation and accumulation of olivine, which acts as the main criteria to distinguish between primitive and evolved Group I-type kimberlite fields in the northern Alberta. This is important from the viewpoint of diamond exploration because the majority (about 80%) of the more primitive Buffalo Head Hills kimberlites are diamondiferous, whereas the more evolved Birch Mountains pipes are barren of diamonds for the most part. Petrographically, the Buffalo Head Hills samples are distinct from the Birch Mountains samples in that they contain less carbonate, have a smaller modal abundance of late-stage minerals such as phlogopite and ilmenite, and have a higher amount of fresh, coarse macrocrystal (>0.5 mm) olivine. Consequently, samples from the Buffalo Head Hills have the highest values of MgO, Cr and Ni, and have chemistries similar to those of primitive hypabyssal kimberlite in the Northwest Territories. Based on whole-rock isotopic data, the Buffalo Head Hills K6 kimberlite has 87Sr/86Sr and Nd values similar to those of South African Group I kimberlites, whereas the Birch Mountains Legend and Phoenix kimberlites have similar Nd values (between 0 and +1.9), but distinctly higher 87Sr/86Sr values (0.7051–0.7063). The lack of whole-rock geochemical overlap between kimberlite and the freshest, least contaminated Mountain Lake South pipe rocks reflects significant mineralogical differences and Mountain Lake is similar geochemically to olivine alkali basalt and/or basanite. Intra-field geochemical variations are also evident. The K4 pipe (Buffalo Head Hills), and Xena and Kendu pipes (Birch Mountains) are characterized by anomalous concentrations of incompatible elements relative to other northern Alberta kimberlite pipes, including chondrite-normalized rare-earth element distribution patterns that are less fractionated than the other kimberlite samples from the Buffalo Head Hills and Birch Mountains. The Xena pipe has similar major element chemical signatures and high-Al clinopyroxene similar to, or trending towards, the Mountain Lake pipes. In addition, K4 and Kendu have higher 87Sr/86Sr and lower Nd than Bulk Earth and plot in the bottom right quadrant of the Nd–Sr diagram. We suggest, therefore, that the K4 and Kendu pipes contain a contribution from old, LREE-enriched (low Sm/Nd) lithosphere that is absent from the other kimberlites, are affected by crustal contamination, or both. Based on xenocryst populations, the northern Alberta kimberlite province mantle is dominated by carbonate-saturated lherzolitic mantle. Higher levels of melt depletion characterize the Buffalo Head Hills mantle sample. Despite high diamondiferous to barren pipe ratios in the Buffalo Head Hills pipes, mineral indicators of high diamond potential, such as G10 garnet, diamond inclusion composition chrome spinels and high-sodium eclogitic garnet, are rare. 相似文献
8.
Five compositional-textural types of ilmenite can be distinguished in nine kimberlites from the Eastern Dharwar craton of southern India. These ilmenite generations record different processes in kimberlite history, from mantle to surface. A first generation of Mg-rich ilmenite (type 1) was produced by metasomatic processes in the mantle before the emplacement of the kimberlite. It is found as xenolithic polycrystalline ilmenite aggregates as well as megacrysts and macrocrysts. All of these ilmenite forms may disaggregate within the kimberlite. Due to the interaction with low-viscosity kimberlitic magma replacement of pre-existing type 1 ilmenite by a succeeding generation of geikielite (type 2) along grain boundaries and cracks occurs. Another generation of Mg-rich ilmenite maybe produced by exsolution processes (type 3 ilmenite). Although the identity of the host mineral is unclear due to extensive alteration and possibility includes enstatite. Type 4 Mn-rich ilmenite is produced before the crystallization of groundmass perovskite and ulvöspinel. It usually mantles ilmenite and other Ti-rich minerals. Type 5 Mn-rich ilmenite is produced after the crystallization of the groundmass minerals and replaces them. The contents of Cr and Nb in type 2, 4 and 5 ilmenites are highly dependent on the composition of the replaced minerals, they may not be a good argument in exploration. The highest Mg contents are recorded in metasomatic ilmenite that is produced during kimberlite emplacement, and cannot be associated with diamond formation. The higher Mn contents are linked to magmatic processes and also late processes clearly produced after the crystallization of the kimberlite groundmass, and therefore ilmenite with high Mn contents cannot be considered as a reliable diamond indicator mineral (DIM) and kimberlite indicator mineral (KIM). 相似文献
9.
Using the ICP-MS method we have studied the isotope systematics of Sr and Nd as well as trace element composition of a representative collection of kimberlites and related rocks from the Siberian Platform. The summarized literature and our own data suggest that the kimberlites developed within the platform can be divided into several petrochemical and geochemical types, whose origin is related to different mantle sources. The petrochemical classification of kimberlites is based on persistent differences of their composition in mg# and in contents of indicator oxides such as FeO tot, TiO 2, and K 2O. The recognized geochemical types of kimberlites differ from one another in the level of concentration of incompatible elements as well as in their ratios.Most of isotope characteristics of kimberlites and related rocks of the Siberian Platform correspond to the earlier studied Type 1 basaltoid kimberlites from different provinces of the world: Points of isotopic compositions are in the field of primitive and weakly depleted mantle. An exception is one sample of the rocks from veins of the Ingashi field (Sayan area), which is characterized by the Sr and Nd isotopic composition corresponding to Type 2 micaceous kimberlites (orangeites).The most important feature of distribution of isotopic and trace-element compositions (incompatible elements) is their independence of the chemical rock composition. It is shown that the kimberlite formation is connected with, at least, two independent sources, fluid and melt, responsible for the trace-element and chemical compositions of the rock. It is supposed that, when rising through the heterogeneous lithosphere of the mantle, a powerful flow of an asthenosphere-derived fluid provoked the formation of local kimberlite chambers there. Thus, the partial melting of the lithosphere mantle led to the formation of contrasting petrochemical types of kimberlites, while the geochemical specialization of kimberlites is due to the mantle fluid of asthenosphere origin, which drastically dominated in the rare-metal balance of a hybrid magma of the chamber. 相似文献
10.
The North Qilian orogenic belt is a typical plate suture zone, which is studded with numerous different sizes of fragments of micro-continental crust consisting of pre-Sinian metamorphic rocks and the North Qilian orogenic belt provides important study carrier for studying the pre-orogenic tectonic process of the North Qilian. The relatively complete volcanic-clastic-carbonate rocks formation of Zhulongguan Group, Changchengian System, Mesoproterozoic are distributed in the Kawa-Qiqing area, Western North Qilian orogenic belt. Geochemical characteristics of the Aoyougou Group basalts in the North Qilian indicated that the contents of major element were relatively stable, high K 2O contents(4.27%~6.07%), TFe 2O 3(10.49%~13.01%),TiO 2(1.96%~2.90%)and middle MgO(5.37%~6.71%,Mg # 48~51), CaO(2.57%~5.51%)content. Thus, the Aoyougou Group basalts belong to potassium alkaline basalt. The contents of Cr and Ni of the basalts decreased with the decreasing of Mg #, and there was a positive correlation between CaO and CaO/Al 2O 3, Furthermore, the Eu anomaly of samples was slightly negative, high contents of high field strength elements (HFSE, Nb, Ta, Ti), and the ratios of Gd/Yb(2.5~3.0), Zr/Y(8.6~10.2), Ta/Yb(0.96~1.23), Ti/Yb(5 074~6 021), Zr/Yb(81~97), Ce/Nb(1.87), Zr/Nb(6.41)indicated that the Aoyougou Group basalts had obvious OIB-type magmatic affinity. In combination with the regional geological background, the study results showed that Aoyougou Group alkaline basalt generated from deep enrichment mantle similar to the source of OIB, with fractional crystallization of olivine and monoclinic pyroxene and a certain degree of contamination of continental crust in the process of magmatism, formed in the background of continental rift, might response to the Proterozoic Columbia supercontinent rifting, which further provides important petrological evidences for the relationship between North China Craton rifting events and Columbia supercontinent rifting during Proterozoic. 相似文献
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