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1.
This study examines the major element composition of mantle-derived garnets recovered from heavy mineral concentrates of several Proterozoic kimberlites of the diamondiferous Wajrakarur Kimberlite Field (WKF) and the almost barren Narayanpet Kimberlite Field (NKF) in the Eastern Dharwar Craton of southern India. Concentrate garnets are abundant in the WKF kimberlites, and notably rare in the NKF kimberlites. Chemical characteristics of the pyropes indicate that the lithology of the sub-continental lithospheric mantle (SCLM) beneath both the kimberlite fields was mainly lherzolitic at the time of kimberlite eruption. A subset of green pyropes from the WKF is marked by high CaO and Cr2O3 contents, which imply contribution from a wehrlitic source. The lithological information on SCLM, when studied alongside geobarometry of lherzolite and harzburgite xenoliths, indicates that there are thin layers of harzburgite within a dominantly lherzolitic mantle in the depth interval of 115–190 km beneath the WKF. In addition, wehrlite and olivine clinopyroxenite occur locally in the depth range of 120–130 km. Mantle geotherm derived from xenoliths constrains the depth of graphite–diamond transition to 155 km beneath the kimberlite fields. Diamond in the WKF thus could have been derived from both lherzolitic and harzburgitic lithologies below this depth. The rarity of diamond and garnet xenocrysts in the NKF strongly suggest sampling of shallower (<155 km depth) mantle, and possibly a shallower source of kimberlite magma than at the WKF.  相似文献   

2.
New geochemical data of the crater-facies Tokapal kimberlite system sandwiched between the lower and upper stratigraphic horizons of the Mesoproterozoic lndravati Basin a：：e presented. The kimberlite has been subjected to extensive and pervasive low-temperature alteration. Spinel is the only primary phase identifiable, while olivine macrocrysts and juvenile lapilli are largely pseudomorphed （talc-serpentine- carbonate alteration）. However, with the exception of the alkalies, major element oxides display systematic fractionation trends; likewise, HFSE patterns are well correlated and allow petrogenetic interpretation. Various crustal contamination indices such as （SiO2 ＋ AI：：O3 ~ Na20）]（MgO ~ K20） and Si] Mg are close to those of uncontaminated kimberlites. Similar La]Yb （＇79-109） of the Tokapal samples with those from the kimberlites of Wajrakarur （73-145） and Narayanpet （72-156）, Eastern Dharwar craton, southern India implies a similarity in their genesis. In the discriminant plots involving HFSE the Tokapal samples display strong affinities to Group 1I kimberlites from southern Africa and central India as well as to ＇transitional kimberlites＇ from the Eastern Dharwar craton, southern India, and those from the Prieska and Kuruman provinces of southern Africa. There is a striking ~;imilarity in the depleted-mantle （TOM） Nd model ages of the Tokapal kimberlite system, Bastar craton, th~ kimberlites from NKF and WKE Eastern Dharwar craton, and the Majhgawan diatreme, Bundelkhand craton, with the emplacement age of some of the lamproites from within and around the Palaeo~Mesoproterozoic Cuddapah basin, southern India. These similar ages imply a major tectonomagmatic event, possibly related to the break- up of the supercontinent of Columbia, at 1.3-1.5 Ga across the l：hree cratons. The ＇transitional＇ geochemical features displayed by many of the Mesoproterozoic po~：assic-ultrapotassic rocks, across these Indian cratons are inferred to be memories of the metasomatisi  相似文献   

3.
Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite(Pipe-16) from the Wajrakarur kimberlite field(WKF), Eastern Dharwar craton(EDC),southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Tirich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of(i) liquidus mineral composition,(ii) bulk-rock chemistry, and(iii) Sr-Nd isotopic composition, we show that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The TDm Nd model age(～2.0 Ga) of the Ahobil kimberlite is(i) significantly older than those(1.5～1.3 Ga) reported for Wajrakarur and Narayanpet kimberlites of EDC,(ii) indistinguishable from those of the Mesoproterozoic EDC lamproites,and(iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. High bulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO_2, as inferred from perovskite oxybarometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.  相似文献   

4.
New mineralogical and bulk-rock geochemical data for the recently recognised Mesoproterozoic(ca.1100 Ma) and late Cretaceous(ca.90 Ma) kimberlites in the Timmasamudram cluster(TKC) of the Wajrakarur kimberlite field(WKF),Eastern Dharwar Craton,southern India,are presented.On the basis of groundmass mineral chemistry(phlogopite,spinel,perovskite and clinopyroxene),bulk-rock chemistry(SiO_2.K_2O,low TiO_2.Ba/Nb and La/Sm),and perovskite Nd isotopic compositions,the TK-1(macrocrystic variety) and TK-4(Macrocrystic variety) kimberlites in this cluster are here classified as orangeites(i.e.Group Ⅱ kimberlites),with geochemical characteristics that are very similar to orangeites previously described from the Bastar Craton in central India,as well as the Kaapvaal Craton in South Africa.The remaining kimberlites(e.g.,TK-2,TK-3 and the TK-1 microcrystic variant),are more similar to other 1100 Ma,Group Ⅰ-type kimberlites of the Eastern Dharwar Craton,as well as the typical Group Ⅰkimberlites of the Kaapvaal Craton.Through the application of geochemical modelling,based on published carbonated peridotite/melt trace element partition coefficients,we show that the generation of the TKC kimberlites and the orangeites results from low degrees of partial melting of a metasomatised,carbonated peridotite.Depleted mantle(T_(DM)) Nd perovskite model ages of the 1100 Ma Timmasamudram kimberlites show that the metasornatic enrichment of their source regions are broadly similar to that of the Mesoproterozoic kimberlites of the EDC.The younger,late Cretaceous(ca.90 Ma) TK-1(macrocrystic variant)and TK-4 kimberlites,as well as the orangeites from the Bastar Craton,share similar Nd model ages of1100 Ma,consistent with a similarity in the timing of source enrichment during the amalgamation of Rodinia supercontinent.The presence of late Cretaceous diamoncliferous orangeite activity,presumably related to the location of the Marion hotspot in southern India at the time,suggests that thick Iithosphere was preserved,at least locally,up to the late Cretaceous,and was not entirely destroyed during the breakup of Gondwana,as inferred by some recent geophysical models.  相似文献   

5.
《地学前缘(英文版)》2020,11(6):2127-2139
The Dharwar Craton in Peninsular India was intruded by a series of mafic dykes during the Paleoproterozoic and these mafic magmatic events have important implications on continental rifting and LIPs. Here we report ten precise Pb–Pb TE-TIMS age determinations on baddeleyite grains separated from seven mafic dykes and three sills, intruding into Archean basement rocks and Proterozoic sedimentary formations of the Eastern Dharwar Craton respectively. The crystallization age of the baddeleyite shows 2366.3 ​± ​1.1 ​Ma, and 2369.2 ​± ​0.8 ​Ma for the NE–SW trending dykes, 2368.1 ​± ​0.6 ​Ma, 2366.4 ​± ​0.8 ​Ma, 2207.2 ​± ​0.7 ​Ma and 1887.3 ​± ​1.0 ​Ma for the ENE–WNW to E–W striking dykes, 1880.6 ​± ​1.0 ​Ma, 1864.3 ​± ​0.6 ​Ma and 1863.6 ​± ​0.9 ​Ma for Cuddapah sills, and 1861.8 ​± ​1.4 ​Ma for the N–S trending dyke. Our results in conjunction with those from previous studies identify eight distinct stages of widespread Paleoproterozoic magmatism in the Dharwar craton. The mantle plume centres of the four radiating dyke swarms with ages of ~2367 ​Ma, ~2210 ​Ma, ~2082 ​Ma, and ~1886 ​Ma were traced to establish their proximity to the EDC kimberlite province. Though the ~2367 ​Ma and ~1886 ​Ma plume centres are inferred to be located to the west and east of the present day Dharwar craton respectively away from the kimberlite province, location of plume heads of the other two swarms with ages of ~2207 ​Ma and ~2082 ​Ma are in close proximity. In spite of the ubiquitous occurrence of dyke intrusions of all the seven generations in the kimberlite province, only few of these kimberlites are diamondiferous. Kimberlite occurrences elsewhere in the vicinity of older Large Igneous Provinces (LIPs) like the Mackenzie, Karoo, Parana-Etendeka and Yakutsk-Vilui are also non-diamondiferous. This has been attributed to the destruction of the lithospheric mantle keel (that hosts the diamonds) by the respective mantle plumes. The diamondiferous nature of the EDC kimberlites therefore suggests that plume activity does not always result in the destruction of the mantle keel.  相似文献   

6.
The Late Cretaceous (ca. 100 Ma) diamondiferous Fort à la Corne (FALC) kimberlite field in the Saskatchewan (Sask) craton, Canada, is one of the largest known kimberlite fields on Earth comprising essentially pyroclastic kimberlites. Despite its discovery more than two decades ago, petrological, geochemical and petrogenetic aspects of the kimberlites in this field are largely unknown. We present here the first detailed petrological and geochemical data combined with reconnaissance Nd isotope data on drill-hole samples of five major kimberlite bodies. Petrography of the studied samples reveals that they are loosely packed, clast-supported and variably sorted, and characterised by the presence of juvenile lapilli, crystals of olivine, xenocrystal garnet (peridotitic as well as eclogitic paragenesis) and Mg-ilmenite. Interclast material is made of serpentine, phlogopite, spinel, carbonate, perovskite and rutile. The mineral compositions, whole-rock geochemistry and Nd isotopic composition (Nd: + 0.62 to − 0.37) are indistinguishable from those known from archetypal hypabyssal kimberlites. Appreciably lower bulk-rock CaO (mostly < 5 wt%) and higher La/Sm ratios (12–15; resembling those of orangeites) are a characteristic feature of these rocks. Their geochemical composition excludes any effects of significant crustal and mantle contamination/assimilation. The fractionation trends displayed suggest a primary kimberlite melt composition indistinguishable from global estimates of primary kimberlite melt, and highlight the dominance of a kimberlite magma component in the pyroclastic variants. The lack of Nb-Ta-Ti anomalies precludes any significant role of subduction-related melts/fluids in the metasomatism of the FALC kimberlite mantle source region. Their incompatible trace elements (e.g., Nb/U) have OIB-type affinities whereas the Nd isotope composition indicates a near-chondritic to slightly depleted Nd isotope composition. The Neoproterozoic (~ 0.6–0.7 Ga) depleted mantle (TDM) Nd model ages coincide with the emplacement age (ca. 673 Ma) of the Amon kimberlite sills (Baffin Island, Rae craton, Canada) and have been related to upwelling protokimberlite melts during the break-up of the Rodinia supercontinent and its separation from Laurentia (North American cratonic shield). REE inversion modelling for the FALC kimberlites as well as for the Jericho (ca. 173 Ma) and Snap Lake (ca. 537 Ma) kimberlites from the neighbouring Slave craton, Canada, indicate all of their source regions to have been extensively depleted (~ 24%) before being subjected to metasomatic enrichment (1.3–2.2%) and subsequent small-degree partial melting. These findings are similar to those previously obtained on Mesozoic kimberlites (Kaapvaal craton, southern Africa) and Mesoproterozoic kimberlites (Dharwar craton, southern India). The striking similarity in the genesis of kimberlites emplaced over broad geological time and across different supercontinents of Laurentia, Gondwanaland and Rodinia, highlights the dominant petrogenetic role of the sub-continental lithosphere. The emplacement of the FALC kimberlites can be explained both by the extensive subduction system in western North America that was established at ca. 150 Ma as well as by far-field effects of the opening of the North Atlantic ocean during the Late Cretaceous.  相似文献   

7.
A number of limestone and metasomatised carbonate xenoliths occur in the 1,090 Ma Siddanpalli kimberlite cluster, Raichur kimberlite Field, Eastern Dharwar craton, southern India. These xenoliths are inferred to have been derived from the carbonate horizons of the Kurnool (Palnad) and Bhima Proterozoic basins and provide evidence for a connection between these basins in the geological past. A revised Mesoproterozoic age is proposed for the Bhima and Kurnool (Palnad) basins based on this kimberlite association and is in agreement with similar proposals made recently for the Chattisgarh and Upper Vindhyan sediments in Central India. The observed Bhima–Kurnool interbasinal uplift may have been caused by: (1) extension- or plume-related mafic alkaline magmatism that included the emplacement of the southern Indian kimberlites at ~1.1 Ga, (2) mantle plume-related doming of the peninsular India during the Cretaceous, or (3) Quaternary differential uplift in this region. It is not possible, with the currently available geological information to constrain the exact timing of this uplift. The deep erosion of primary diamond sources in the Raichur kimberlite Field in the upper reaches of the Krishna River caused by this uplift could be the elusive source of the alluvial diamonds of the Krishna valley. Mesoproterozoic sedimentary basins can host world class unconformity-type uranium deposits. In light of its inferred Mesoproterozoic age, a more detailed stratigraphic and metallogenic analysis of the Kurnool basin is suggested for uranium exploration.  相似文献   

8.
The Siddanpalli kimberlites constitute a newly discovered cluster (SKC) of Mesoproterozoic (1090 Ma) dykes occurring in the granite-greenstone terrain of the Gadwal area in the Eastern Dharwar Craton (EDC), Southern India. They belong to coherent facies and contain serpentinized olivines (two generations), phlogopite, spinel, perovskite, ilmenite, apatite, carbonate and garnet xenocrysts. A peculiar feature of these kimberlites is the abundance of carbonate and limestone xenoliths of the eroded platformal Proterozoic (Purana) sedimentary cover of Kurnool/Bhima age. Chemically, the Siddanpalli dykes are the most magnesium-rich (up to 35 wt.% MgO) and silica-undersaturated (SiO2?<?35 wt.%) of all kimberlites described so far from the Eastern Dharwar Craton. The La/Yb ratio in the Siddanpalli kimberlites (64–105) is considerably lower than that in the other EDC kimberlites (108–145), primarily owing to their much higher HREE abundances. Since there is no evidence of any crustal contamination by granitic rocks we infer this to be a specific character of the magmatic source. A comparison of the REE geochemistry of the Siddanpalli kimberlites with petrogenetic models for southern African kimberlites suggests that they display involvement of a wide range in the degree of melting in their genesis. The different geochemical signatures of the SKC compared to the other known kimberlites in the EDC can be explained by a combination of factors involving: (i) higher degrees of partial melting; (ii) relatively shallower depths of derivation; (iii) possible involvement of subducted component in their mantle source region; and (iv) previous extraction of boninitic magmas from their geological domain.  相似文献   

9.
The Archaean-Proterozoic Dharwar craton has many recorded occurrences of diamondiferous kimberlites. Reports of kimberlite emplacement in parts of the tectonically complex eastern Dharwar craton and a significant density contrast between kimberlites and the host peninsular gneisses motivated us to conduct gravity studies in the Narayanpet-Irladinne area of the eastern Dharwar craton. This region is contiguous with the Maddur-Narayanpet kimberlite that lies to its north, while the river Krishna lies to its south. From observed association of reported kimberlites in the Maddur-Narayanpet field with subsurface topography of the assumed three-layer earth section obtained by Bouguer gravity modelling, we developed a subsurface criterion for occurrence of kimberlites in the present study area. Using this criterion, five potential zones for kimberlite localization were identified in the Narayanpet-Irladinne region, eastern Dharwar craton.  相似文献   

10.
Olivine is the principal mineral of kimberlite magmas, and isthe main contributor to the ultramafic composition of kimberliterocks. Olivine is partly or completely altered in common kimberlites,and thus unavailable for studies of the origin and evolutionof kimberlite magmas. The masking effects of alteration, commonin kimberlites worldwide, are overcome in this study of theexceptionally fresh diamondiferous kimberlites of the Udachnaya-Eastpipe from the Daldyn–Alakit province, Yakutia, northernSiberia. These serpentine-free kimberlites contain large amountsof olivine (50 vol.%) in a chloride–carbonate groundmass.Olivine is represented by two populations (olivine-I and groundmassolivine-II) differing in morphology, colour and grain size,and trapped mineral and melt inclusions. The large fragmentalolivine-I is compositionally variable in terms of major (Fo85–94)and trace element concentrations, including H2O content (10–136ppm). Multiple sources of olivine-I, such as convecting andlithospheric mantle, are suggested. The groundmass olivine-IIis recognized by smaller grain sizes and perfect crystallographicshapes that indicate crystallization during magma ascent andemplacement. However, a simple crystallization history for olivine-IIis complicated by complex zoning in terms of Fo values and traceelement contents. The cores of olivine-II are compositionallysimilar to olivine-I, which suggests a genetic link betweenthese two types of olivine. Olivine-I and olivine-II have oxygenisotope values (+ 5·6 ± 0·1 VSMOW, 1 SD)that are indistinguishable from one another, but higher thanvalues (+ 5·18 ± 0·28) in ‘typical’mantle olivine. These elevated values probably reflect equilibriumwith the Udachnaya carbonate melt at low temperatures and 18O-enrichedmantle source. The volumetrically significant rims of olivine-IIhave constant Fo values (89·0 ± 0·2 mol%),but variable trace element compositions. The uniform Fo compositionsof the rims imply an absence of fractionation of the melt'sFe2+/Mg, which is possible in the carbonatite melt–olivinesystem. The kimberlite melt is argued to have originated inthe mantle as a chloride–carbonate liquid, devoid of ‘ultramafic’or ‘basaltic’ aluminosilicate components, but becameolivine-laden and olivine-saturated by scavenging olivine crystalsfrom the pathway rocks and dissolving them en route to the surface.During emplacement the kimberlite magma changed progressivelytowards an original alkali-rich chloride–carbonate meltby extensively crystallizing groundmass olivine and gravitationalseparation of solids in the pipe. KEY WORDS: kimberlite; olivine; partial melting; carbonatitic melt; oxygen isotopes; H2O  相似文献   

11.
We report groundmass perovskite U–Pb (SIMS) ages, perovskite Nd isotopic (LA-ICPMS) composition and bulk-rock geochemical data of the Timmasamudram diamondiferous kimberlite cluster, Wajrakarur kimberlite field, in the Eastern Dharwar craton of southern India. The kimberlite pipes gave similar Mesoproterozoic ages of 1086 ± 19 Ma (TK-1, microcrystic variant) and 1119 ± 12 Ma (TK-3). However, a perovskite population sampled from the macrocrystic variant of TK-1 gave a much younger Late Cretaceous age of ca. 90 Ma. This macrocrystic kimberlite phase intrudes the Mesoproterozoic microcrystic phase and has a distinct bulk-rock geochemistry. The Nd-isotope composition of the ~ 1100 Ma perovskites in the cluster show depleted εNd(T) values of 2.1 ± 0.6 to 6.7 ± 0.3 whereas the ~ 90 Ma perovskites have enriched εNd(T) values of − 6.3 ± 1.3. The depleted-mantle (DM) model age of the Cretaceous perovskites is 1.2 Ga, whereas the DM model age of the Proterozoic perovskites is 1.2 to 1.5 Ga. Bulk-rock incompatible trace element ratios (La/Sm, Gd/Lu, La/Nb and Th/Nb) of all Timmasamudram kimberlites show strong affinity with those from the Cretaceous Group II kimberlites from the Bastar craton (India) and Kaapvaal craton (southern Africa). As the Late Cretaceous age of the younger perovskites from the TK-1 kimberlite is indistinguishable from that of the Marion hotspot-linked extrusive and intrusive igneous rocks from Madagascar and India, we infer that all may be part of a single Madagascar Large Igneous Province. Our finding constitutes the first report of Cretaceous kimberlite activity from southern India and has significant implications for its sub-continental lithospheric mantle evolution and diamond exploration programs.  相似文献   

12.
Tokapal kimberlite is the only well preserved crater facies kimberlite intruded within sedimentary sequence of Indravati basin in Bastar craton of central India. We present detailed petrographical and whole rock geochemical studies, carried out on ten samples collected from different locations from Tokapal kimberlite to constrain its genesis and also the mantle processes involved in the origin of this earlier characterized Group I kimberlite. Geochemical studies show that only SiO2 content and the mobile trace elements Ba, Sr, and K vary in the crater facies while rest others show restricted range and can be successfully used in evaluating the petrogenetic processes. Very low abundances of Rb (<2 ppm), Sr (<28 ppm), Ba (<52 ppm) and Cs (0.02–3 ppm) are observed which show possible effects of late-stage alteration rather than significant crustal contamination. The LREE enriched REE pattern, absence of positive Eu anomalies and HREE depletion also provide further additional evidence against crustal contamination considerably modifying magma composition. We infer the presence of less enriched (metasomatised) mantle source regions and comparatively greater degrees of partial melting responsible for the genesis of Tokapal kimberlite. Present study also suggests that crater facies Tokapal kimberlite intruding the Indravati basin, Bastar craton has a Group II kimberlite (orangeite) affinity. This finding is important in light of recent identification of Mainpur kimberlites of Bastar craton as orangeites.  相似文献   

13.
Thirty-seven samples from the Swartruggens and Star Group II kimberlite dyke swarms, emplaced through the Kaapvaal craton, have been analysed for their major and trace element and Sr, Nd and Hf isotope compositions. The samples are all MgO-rich (~12–35 wt%) with high Mg# (0.72–0.90) and Ni (~610–2700 ppm) contents. The kimberlites are strongly enriched in incompatible elements (Zr = 140–668 ppm; La = 124–300 ppm; Nb = 68–227 ppm; Ba = 1500–7000), and have high and variable chondrite normalised La/Yb ratios (Swartruggens = 94 ± 21; Star = 202 ± 36). 87Sr/86Sr (0.70718–0.71050) ratios are elevated, whereas εNd (−11.95 to −7.84) and 176Hf/177Hf ratios (0.282160–0.282564) are low. Inter- and intra-dyke compositional variation is significant, and there are systematic differences between the kimberlites found at the two localities. Intra-locality differences can largely be attributed to a combination of the effects of alteration, crustal contamination, macrocryst entrainment and phenocryst fractionation. There is some evidence for distinct parental magmas formed through variable and low degrees (0.5–2%) of partial melting, as illustrated by crossing rare earth element patterns. The Star kimberlites have derived from a less radiogenic source, with higher LREE enrichment than the Swartruggens kimberlites. Inferred primary magmas at each locality have high Mg# (~0.83), are Ni-rich (850–1220 ppm) and are strongly enriched in incompatible elements. Calculated mantle source compositions are strongly enriched in incompatible elements (La/Ybn ~ 10–50), but refractory in terms of Mg# and Ni contents. Incompatible element ratios such as Ba/Nb (>13.5), La/Nb (> 1.1) and Ce/Pb (< 22) are unlike those characteristic of Group I kimberlites or ocean island basalts, but indistinguishable from calc-alkaline magmas. Taken together with extremely low εNd and εHf, these compositional characteristics are used to argue for derivation of these Group II kimberlite magmas from the deep subcontinental lithospheric mantle, metasomatised during the Proterozoic by calc-alkaline fluids/melts.  相似文献   

14.
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   

15.
We report a rare accessory groundmass mineral of K-rich titanate, having a composition close to that of potassium triskaidecatitanate (K2Ti13O27), from an underground drill-core sample of ultrapotassic rock from southwestern part of the Jharia coal field in the Damodar valley, at the northern margin of the Singhbhum craton, Eastern India. Potassium triskaidecatitanate is regarded as a typomorphic mineral of orangeites (Group II kimberlites) of Kaapvaal craton, southern Africa, and its occurrence in the Jharia ultrapotassic rock is significant since ultrapotassic suite of rocks elsewhere from the Damodar valley have been recently suggested to be peralkaline lamproites based on mineral-genetic classification. The important role played by a unique geodynamic setting (involving a thinned metasomatised lithospheric mantle and inheritance of an Archaean subduction component) at the northern margin of the Singhbhum craton in deciding the petrological diversity of the early Cretaceous ultrapotassic intrusives from the Damodar valley is highlighted in this study.  相似文献   

16.
We present mineral chemistry, geochemistry and Sr and Nd isotope data of drillcore samples from the Late Cretaceous (65 Ma), diamondiferous Behradih ultramafic pipe, Bastar craton, Central India, which is emplaced synchronous with the Deccan flood basalt eruption. The rock is affected by pervasive serpentine–talc–carbonate alteration and consists of pelletal lapilli and variously sized olivine and phlogopite macrocrysts, set in a groundmass of abundant clinopyroxene, chrome spinel, apatite, Fe-rich perovskite (<50 μm), zircon, titanite, rutile and calcite. Mineralogical studies identify the Behradih pipe as orangeite (formerly termed as Group II kimberlite) and establish the occurrence of such rocks outside the Kaapvaal craton, southern Africa. As the age of the Behradih orangeite overlaps with that of the main phase of the Deccan flood basalt magmatism, we infer a common tectonomagmatic control vis-a-vis the Deccan-related mantle plume. Trace element ratios and the Nd isotope signatures of the Behradih pipe imply that the Deccan plume has only contributed heat, but not substantial melt, to the Behradih magma with a cause-and-consequence relationship between them. Our study highlights (a) a striking similarity in the genesis of Late Cretaceous orangeites associated with the continental flood basalts in the Kaapvaal and Bastar cratons but related to different mantle plumes and (b) the role of plume–lithosphere interaction in the generation of orangeites.  相似文献   

17.
Geochemical studies on metavolcanic rocks of the Gadwal greenstone belt (GGB), eastern Dharwar craton, have documented several rock types that are indicative of subduction zone tectonics reflecting on the crustal growth processes in the Dharwar craton. The dominance of komatiites in the western Dharwar craton (WDC) and the arc volcanics in the eastern Dharwar craton (EDC) is an indication for the predominance of plume magmatism in the WDC and the intraoceanic subduction zone processes in EDC which together played a significant role in the growth and evolution of continental crust in the Dharwar craton. Boninites of GGB are high calcic type with high MgO (13–24 wt.%) and a characteristic MREE depleted U-shaped REE patterns whereas the basalts have flat REE patterns with no Eu anomalies. Nb-enriched basalts exhibit slightly fractionated REE patterns with high Nb (8–26 ppm) content compared to arc basalts. Adakites of GGB are Sr depleted with highly fractionated REE patterns and no Eu anomaly compared to rhyolites. The occurrence of boninites along with arc basalts, Nb-enriched basalts–basalt–andesite–dacite–rhyolites and adakites association in Gadwal greenstone belt indicate the intraoceanic subduction zone processes with a clear cut evidence of partial melting of metasomatized mantle wedge (boninites), melting of subducting slab (adakites) and residue of adakite–wedge hybridization (Nb-enriched basalts) which have played a significant role in the growth of continental crust in the Dharwar craton during the Neoarchaean.  相似文献   

18.
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   

19.

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).

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20.

Olivine in kimberlites can provide unique insights into magma petrogenesis, because it is the most abundant xenocrystic phase and a stable magmatic product over most of the liquid line of descent. In this study we examined the petrography and chemistry of olivine in kimberlites from different tectonic settings, including the Slave craton, Canada (Ekati: Grizzly, Koala), the Brasilia mobile belt (Limpeza-18, Tres Ranchos-04), and the Kaapvaal craton, South Africa (Kaalvallei: Samada, New Robinson). Olivine cores display a wide range of compositions (e.g., Mg# = 78–95). The similarity in olivine composition, resorption of core zones and inclusions of mantle-derived phases, indicates that most olivine cores originated from the disaggregation of mantle peridotites, including kimberlite-metasomatised lithologies (i.e. sheared lherzolites and megacrysts). Olivine rims typically show a restricted range of Mg#, with decreasing Ni and increasing Mn and Ca contents, a characteristic of kimberlitic olivine worldwide. The rims host inclusions of groundmass minerals, which implies crystallisation just before and/or during emplacement. There is a direct correlation between olivine rim composition and groundmass mineralogy, whereby high Mg/Fe rims are associated with carbonate-rich kimberlites, and lower Mg/Fe rims are correlated with increased phlogopite and Fe-bearing oxide mineral abundances. There are no differences in olivine composition between explosive (Grizzly) and hypabyssal (Koala) kimberlites. Olivine in kimberlites also displays transitional zones and less common internal zones, between cores and rims. The diffuse transitional zones exhibit intermediate compositions between cores and rims, attributed to partial re-equilibration of xenocrystic cores with the ascending kimberlite melt. In contrast, internal zones form discrete layers with resorbed margins and restricted Mg# values, but variable Ni, Mn and Ca concentrations, which indicates a discrete crystallization event from precursor kimberlite melts at mantle depths. Overall, olivine exhibits broadly analogous zoning in kimberlites worldwide. Variable compositions for individual zones relate to different parental melt compositions rather than variations in tectonic setting or emplacement mechanism.

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