The crater-facies kimberlite system of Tokapal, Bastar District, Chhattisgarh, India

Discovery of diamondiferous kimberlites in the Mainpur Kimberlite Field, Raipur District, Chhattisgarh in central India, encouraged investigation of similar bodies in other parts of the Bastar craton. The earlier known Tokapal ultramafic intrusive body, located beyond the 19-km milestone in Tokapal village along the Jagdalpur–Geedam road, was reinterpreted as crater-facies kimberlite. Its stratigraphic position in the Meso-Neoproterozoic intracratonic sedimentary Indravati basin makes it one of the oldest preserved crater-facies kimberlite systems. Ground and limited subsurface data (dug-, tube-wells and exploratory boreholes) have outlined an extensive surface area (>550 ha) of the kimberlite. The morphological and surface color features of this body on enhanced satellite images suggest that there is a central feeder surrounded by a collar and wide pyroclastic apron. Exploration drilling indicates that the central zone probably corresponds to a vent overlain by resedimented volcaniclastic (epiclastic) rocks that are surrounded by a 2-km-wide spread of pyroclastic rocks (lapilli tuff, tuff/ash beds and volcaniclastic breccia). Drill-holes also reveal that kimberlitic lapilli tuffs and tuffs are sandwiched between the Kanger and Jagdalpur Formations and also form sills within the sedimentary sequence of the Indravati basin. The lapilli tuffs are commonly well stratified and display slumping. Base surges and lava flows occur in the southern part of the Tokapal system. The geochemistry and petrology of the rock correspond to average Group I kimberlite with a moderate degree of contamination. However, the exposed rock is intensely weathered and altered with strong leaching of mobile elements (Ba, Rb, Sr). Layers of vesicular fine-grained glassy material represent kimberlitic lava flows. Tuffs containing juvenile lapilli with pseudomorphed olivine macrocrysts are set in a talc–serpentine–carbonate matrix with locally abundant spinel and sphene. Garnet has not been observed, and phlogopite is very rare. Very limited microdiamond testing (two 18-kg samples) proved negative; however, the composition of chromite grains indicate crystallization in the diamond stability field.     

Petrogenesis of the crater-facies Tokapal kimberlite pipe,Indravati Basin,Central India

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     

Petrogenesis of the end-Cretaceous diamondiferous Behradih orangeite pipe: implication for mantle plume–lithosphere interaction in the Bastar craton,Central India

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.     

Tokapal tuff-facies kimberlite, Bastar craton, Central India: A nickel prospect?

We report the occurrence of garnierite (a general term referring to Ni-Mg bearing hydrous silicates in laterites) from the crater-facies Tokapal kimberlite of the Bastar craton, Central India. Garnierite occurs as discrete ovoid or amoeboid segregations (up to 200 mm) or as veinlets with up to 18.1 wt% NiO and high iron contents (up to 36.2 wt% FeO^T). Chemical composition of the garnierite implies its derivation from a magnesium-rich protolith. Extensive lateritisation of the large crater-facies (～2.5 km diameter) saucer-shaped kimberlite under tropical weathering conditions, aided by suitable topography, drainage and favourable structural set-up, are the factors inferred to be responsible for the formation of garnierite in the Tokapal system. As lateritic nickel ores constitute significant resources for nickel exploration, the perspective of the Tokapal kimberlite as a nickel prospect needs to be investigated.     

Petrogenesis of the Swartruggens and Star Group II kimberlite dyke swarms,South Africa: constraints from whole rock geochemistry

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/Yb_n ~ 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.     

Petrology and geochemistry of diamondiferous Mesoproterozoic kimberlites from Wajrakarur kimberlite field,Eastern Dharwar craton,southern India: genesis and constraints on mantle source regions

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.     

Precambrian continent assembly and dispersal events of South Indian and East Antarctic Shields

An Archaean continent ‘SIWA’, an acronym for South India–Western Australia, comprising the Bastar–Dharwar craton, the Yilgarn craton, the Napier Complex, and the Vestfold Hills has been identified from palaeomagnetic and spatio-temporal data. This assembly was dispersed in three phases with the development of the proto-Indian ocean. The first and second events ~2350 and ~2000 Ma were related to the separation of the Yilgarn craton and the Napier Complex, respectively, to form a proto-Indo-Antarctic ocean and the Cuddapah basin. The proto-ocean was closed ~1650 Ma by the collision of the Lambert Terrane of East Antarctica and the Bastar–Dharwar craton. This collision, associated with ultra-high temperature (UHT) granulite facies metamorphism, is identified in the southern domain of the Eastern Ghats and the Oygardens domain of East Antarctica. The third extensional event between 1500 and 1200 Ma was associated with the separation of the Vestfold Hills block and a second phase of opening of the proto-Indian ocean, and the development of a series of basins on the western side of the Eastern Ghats (the Chhatisgarh, Khariar, Ampani, Indravati, and Sabari basins). The closing of this ocean basin during the Eastern Ghats–Rayner orogeny at ~950 Ma was related to the amalgamation of India and East Antarctica to form the supercontinent Rodinia. During the Neoproterozoic, this part of Rodinia was involved in orogenic collapse/extension and deposition of the Sodruzhesvo Group. The Pan-African Prydz Bay orogeny at ~550 Ma caused the closing of the basin to form East Gondwanaland.     

Mesoproterozoic rift-related alkaline magmatism at Elchuru, Prakasam Alkaline Province, SE India

The Elchuru alkaline complex in the Prakasam igneous province represents one occurrence of several alkaline bodies within the craton–Eastern Ghats Belt contact zone in Peninsular India. Nepheline syenites and associated mafic rocks intruded the cratonic crust at ≈1321 Ma and were deformed–metamorphosed to amphibolite facies condition during Pan-African times. Trace element compositions and Sr, Nd and Pb isotopic systematics indicate that the alkaline magma was derived from an enriched mantle source in the sub-continental lithosphere. The adjacent crusts of the Eastern Dharwar craton and the Eastern Ghats Belt were not involved either as source or as contaminants. The enriched mantle source was at least 1.9–2.1 Ga old as seen from the depleted mantle model ages of the rocks. The primary parent magma was a basanitic liquid that fractionated ferrokaersutite and clinopyroxene in the mantle, lowering the density sufficiently for the residual melt to intrude the crust. Magmatic differentiation in the suite can be explained by a two stage fractional crystallization model involving the removal of amphibole, clinopyroxene, allanite, titanite, apatite and zircon. The rift-related intra-continental setting of the complex indicates that alkaline magmatism represents the manifestation of a Mesoproterozoic continental breakup. Rifting along the cratonic margin may have led to the formation of several cratogenic basins (e.g., Chattisgarh basin, Indravati basin etc.) where stable shelf-type sediments could have been deposited on the passive margin during the Proterozoic. It could also have opened an ocean where some of the sediments of the Eastern Ghats Province may have been deposited.     

Constraints on the Trace Element Composition of the Archean Mantle Root beneath Somerset Island, Arctic Canada

Peridotites that sample Archean mantle roots are frequentlyincompatible trace element enriched despite their refractorymajor element compositions. To constrain the trace element budgetof the lithosphere beneath the Canadian craton, trace elementand rare earth element (REE) abundances were determined fora suite of garnet peridotites and garnet pyroxenites from theNikos kimberlite pipe on Somerset Island, Canadian Arctic, theirconstituent garnet and clinopyroxene, and the host kimberlite.These refractory mantle xenoliths are depleted in fusible majorelements, but enriched in incompatible trace elements, suchas large ion lithophile elements (LILE), Th, U and light rareearth elements (LREE). Mass balance calculations based on modalabundances of clinopyroxene and garnet and their respectiveREE contents yield discrepancies between calculated and analyzedREE contents for the Nikos bulk rocks that amount to LREE deficienciesof 70–99%, suggesting the presence of small amounts ofinterstitial kimberlite liquid (0·4–2 wt %) toaccount for the excess LREE abundances. These results indicatethat the peridotites had in fact depleted or flat LREE patternsbefore contamination by their host kimberlite. LREE and Sr enrichmentin clinopyroxene and low Zr and Sr abundances in garnet in low-temperatureperidotites (800–1100°C) compared with high-temperatureperidotites (1200–1400°C) suggest that the shallowlithosphere is geochemically distinct from the deep lithospherebeneath the northern margin of the Canadian craton. The Somersetmantle root appears to be characterized by a depth zonationthat may date from the time of its stabilization in the Archean. KEY WORDS: Canada; mantle; metasomatism; peridotite; trace elements     

Mineralogy of primary carbonate-bearing hypabyssal kimberlite, Lac de Gras, Slave Province, Northwest Territories, Canada

A suite of fresh, Late Cretaceous to Eocene hypabyssal kimberlites from the Lac de Gras field were studied in order to understand better carbonate, silicate and oxide paragenesis. The samples have excellent preservation of textures and primary mineralogy and are archetypal or Group 1 kimberlite. Five kimberlite localities are identified as calcite-bearing based on the presence of high Sr–Ba calcite as phenocrysts, microphenocrysts and in segregations. Three kimberlite localities are identified as dolomite-bearing based on the presence of mixed calcite–dolomite segregations containing oscillatory and banded textures of calcite–dolomite solid solution and dolomite (±magnesite). Sr–Ba calcite are characterized by high X_Ca (>0.95) and are enriched in Sr (4900–11,100 ppm) and Ba (3200–14,200 ppm). The calcite–dolomite and dolomite–magnesite solid solution compositions span the X_Ca range from 0.42 to 0.95, and typically have Sr and Ba contents in the range of 1000–4000 ppm. The carbonate, silicate and oxide mineral compositions suggest that the origin of the calcite-bearing versus dolomite-bearing kimberlites studied is related to subtle differences in parent magma composition, in particular, the CO₂/H₂O ratio. Formation of the carbonates reflects the latter part of a protracted magmatic crystallization sequence, in which Sr–Ba calcite precipitates from an evolved kimberlite melt. Subsequently, calcite–dolomite solid solution and dolomite is precipitated from localized, Mg-rich carbonate fluids at relatively high temperatures (higher than serpentine stability).     

Geochemistry and geochronology of mafic rocks from the Vespor suite in the Juruena arc,Roosevelt-Juruena terrain,Brazil: Implications for Proterozoic crustal growth and geodynamic setting of the SW Amazonian craton

This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (La_n/Sm_n – 0.83 to 4.58) and (Nb_n/La_n – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Yb_n = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.     

Western Kamchatka-Koryak continental-margin volcanogenic belt: Age,composition, and sources

The isotope-geochemical study of the Eocene-Oligocene magmatic rocks from the Western Kamchatka-Koryak volcanogenic belt revealed a lateral heterogeneity of mantle magma sources in its segments: Western Kamchatka, Central Koryak, and Northern Koryak ones. In the Western Kamchatka segment, magmatic melts were generated from isotopically heterogeneous (depleted and/or insignificantly enriched) mantle sources significantly contaminated by quartz-feldspathic sialic sediments; higher ⁸⁷Sr/⁸⁶Sr (0.70429–0.70564) and lower ¹⁴³Nd/¹⁴⁴Nd(ɛ_Nd(T) = 0.06–2.9) ratios in the volcanic rocks from the Central Koryak segment presumably reflect the contribution of enriched mantle source; the high positive ɛ_Nd(T) and low ⁸⁷Sr/⁸⁶Sr ratios in the magmatic rocks from the Northern Koryak segment area indicate their derivation from isotopically depleted mantle source without significant contamination by sialic or mantle material enriched in radiogenic Sr and Nd. Significantly different contamination histories of the Eocene-Oligocene mantle magmas in Kamchatka and Koryakia are related to their different thermal regimes: the higher heat flow beneath Kamchatka led to the crustal melting and contamination of mantle suprasubduction magmas by crustal melts. The cessation of suprasubduction volcanism in the Western Kamchatka segment of the continentalmargin belt was possibly related to the accretion of the Achaivayam-Valagin terrane 40 Ma ago, whereas suprasubduction activity in the Koryak segment stopped due to the closure of the Ukelayat basin in the Oligocene time.     

The Late Pliocene mafic lavas from the Camusú Aike volcanic field (∼50°S,Argentina): Evidence for geochemical variability in slab window magmatism

The Camusú Aike volcanic field (CAVF), part of the discontinuous N–S-trending belt of Cenozoic mafic lava formations that occur in a backarc position along extra-Andean Patagonia, is located in southern Patagonia (∼50°S, Santa Cruz province), approximately 70 km east of the extensive Meseta de las Vizcachas and just south of the upper Río Santa Cruz valley. The CAVF volcanics cover a surface of ∼200 km² and occur mainly as lava flows and scoria cones. They are subdivided into two groups: Group I volcanics are high-TiO₂, low-Mg# olivine-hypersthene-normative basalts and trachybasalts that erupted at about 2.9 Ma; Group II lavas are much less abundant, more primitive basaltic andesites that erupted at about 2.5 Ma. Both groups show a within-plate geochemical signature, though it is more marked in Group I lavas.The main geochemical characteristics, age, and location of CAVF volcanics are consistent with the slab window opening model proposed by different authors for the genesis of the Miocene-Recent mafic magmatism of Patagonia south of 46.5°S. The whole-rock geochemical and Sr–Nd isotope features of Group I lavas (⁸⁷Sr/⁸⁶Sr=0.7035–0.7037; ¹⁴³Nd/¹⁴⁴Nd=0.51288–0.51291) indicate a genetic link between these lavas and the primitive basalts in southernmost Patagonia (Pali Aike volcanic field and Estancia Glencross area), which have been interpreted as melting products of an isotopically depleted asthenosphere. The relatively evolved compositions of the erupted Group I magmas are modeled by a polybaric crystal fractionation process without significant involvement of crustal contamination. The more primitive Group II lavas are strongly depleted in incompatible elements, have slightly higher (LREE+Ba+Th+U)/HFSE ratios, and have more enriched Sr–Nd isotope compositions (⁸⁷Sr/⁸⁶Sr≈0.7039; ¹⁴³Nd/¹⁴⁴Nd≈0.51277) that are more akin to the Patagonian basalts farther to the north. The most likely explanation for the geochemical features of Group II lavas is the occurrence in their mantle source of a small proportion of a subduction-related, enriched component that likely resides in the former mantle wedge or the basal continental lithospheric mantle.     

Sources and geodynamic setting of petrogenesis of the Middle Cambrian Upper Petropavlovka alkaline basic pluton (Kuznetsk Alatau,Siberia)

Early Paleozoic alkaline basic magmatism in the Kuznetsk Alatau is manifested in the Upper Petropavlovka pluton of gabbro, feldspathoid rocks (theralites, mafic foidolites, and nepheline syenites), and Ca-carbonatites. According to Sm–Nd and Rb–Sr isotope data, the pluton formed in the Middle Cambrian (509 ± 10 Ma). The silicate igneous rocks correspond in the contents of silica, alumina, and alkalies to derivates of a K–Na alkaline basic association. The Ca-carbonatites are characterized by a high-temperature (600–900 °C) paragenesis of apatite, clinopyroxene, ferromonticellite, phlogopite, and magnetite. They are enriched in P2O5 (up to 6.4 wt.%), Sr (up to 3000–4500 ppm; Sr/Ba ~ 5–7), and REE + Y (up to 800 ppm) and show evidence for liquation genesis. The predominant magmatic source (εNd(T) = 5–7) was moderately depleted PREMA, possibly combined with E-MORB and EM. According to the isotopic data ((87Sr/86Sr)T ~ 0.7024–0.7065; δ18O ~ 6.3–15.5‰; δ18C ~ –3.5 to –2.0‰), the fractionation of the melts was accompanied by their crustal contamination. The trace-element composition of the mafic rocks testifies to the participation of a substance similar to the substrata of the parental magmas of MORB, IAB, and OIB in the magma generation. This suggests intrusion in the geodynamic setting of interaction between the active continental margin and an ascending mantle diapir. Most likely, the intrusion led to the mixing of material from different sources, including the components of PREMA, enriched suprasubduction lithospheric mantle (EM), and continental crust. The assumption is made that the complexes of highly alkaline rocks and carbonatites in the western Central Asian Fold Belt are of plume origin and belong to an Early Paleozoic large igneous province.     

The geochemistry of Precambrian granulite facies rocks from the Lewisian complex of Tiree,Inner Hebrides,Scotland

The metamorphism and geochemistry of the major components of a small area of granulite facies rock are described and discussed, and a chemical model for the evolution of anomalous trace element distributions in such materials is suggested. The local complex was subjected to medium to high pressure granulite facies metamorphism between 2,900 and 2,600 m.y. All the analysed granulite facies rocks from Tiree; acid to intermediate gneisses, basic metamorphic rocks, and granitic rocks, have anomalous chemistries, being depleted in K, Rb, Nb, Y and Th, and have high K/Rb, Ba/Rb and Ca/Y ratios, and very low K/Ba and Rb/Sr ratios relative to normal portions of the upper continental crust. The gneisses seem to have been enriched in Ba and Sr.The chemical features of the rocks are considered to reflect their stable mineral assemblages in the granulite facies, and to be representative of deep-level crustal materials. The geochemical peculiarities of the complex may have been largely controlled by an upward intergranular diffusion, or “degassing” caused by high-grade metamorphism. It is suggested that such diffusion may have been active at the crust/upper mantle interface, some diffused material of mantle origin accounting for certain chemical oddities typical of Lewisian and some other Precambrian granulite facies rocks.     

Geochemical and isotopic investigation of the Laiwu–Zibo carbonatites from western Shandong Province, China, and implications for their petrogenesis and enriched mantle source

Major and trace element and Nd–Sr isotope data of the Mesozoic Laiwu–Zibo carbonatites (LZCs) from western Shandong Province, China, provide clues to the petrogenesis and the nature of their mantle source. The Laiwu–Zibo carbonatites can be petrologically classified as calcio-, magnesio- and ferro-carbonatites. All these carbonatites show a similarity in geochemistry. On the one hand, they are extremely enriched in Ba, Sr and LREE and markedly low in K, Rb and Ti, which are similar to those global carbonatites, on the other hand, they have extremely high initial ⁸⁷Sr/⁸⁶Sr (0.7095–0.7106) and very low _Nd (−18.2 to −14.3), a character completely different from those global carbonatites. The small variations in Sr and Nd isotopic ratios suggest that crustal contamination can not modify the primary isotopic compositions of LZC magmas and those values are representatives of their mantle source. The Nd–Sr isotopic compositions of LZCs and their similarity to those of Mesozoic Fangcheng basalts imply that they derived from an enriched lithospheric mantle. The formation of such enriched lithospheric mantle is connected with the major collision between the North China Craton (NCC) and the Yangtze Craton. Crustal materials from the Yangtze Craton were subducted beneath the NCC and melts derived from the subducted crust of the Yangtze Craton produced an enriched Mesozoic mantle, which is the source for the LZCs and Fangcheng basalts. The absence of alkaline silicate rocks, which are usually associated with carbonatites suggest that the LZCs originated from the mantle by directly partial melting.     

博茨瓦纳和中国含金刚石金伯利岩的地质特征及对寻找类似岩体的启示

金刚石及其寄主岩石是人类认识地球深部物质组成和性质、壳幔和核幔物质循环重要研究对象。本文总结了中国不同金刚石类型的分布,着重对比了博茨瓦纳和中国含金刚石金伯利岩的地质特征,取得如下认识:(1)博茨瓦纳含矿原生岩石仅为金伯利岩,而中国含矿岩石成分复杂,金伯利岩主要出露在华北克拉通,展布于郯庐、华北中央和华北北缘金伯利岩带,具有工业价值的蒙阴和瓦房店矿床分布于郯庐金伯利岩带中;钾镁煌斑岩主要出露在华南克拉通,重点分布在江南和华南北缘钾镁煌斑岩带中;(2)钙钛矿原位U-Pb年龄和Sr、Nd同位素显示,86～97 Ma奥拉帕金伯利岩群和456～470 Ma蒙阴和瓦房店金伯利岩均具有低87Sr/86Sr(0.703～0.705)和中等εNd(t)(-0.09～+5)特征,指示金伯利岩浆源自弱亏损地幔或初始地幔源区;(3)博茨瓦纳金伯利岩体绝大多数以岩筒产出,而中国以脉状为主岩筒次之;博茨瓦纳岩筒绝大部分为火山口相,中国均为根部相,岩筒地表面积普遍小于前者;(4)奥拉帕A/K1和朱瓦能金伯利岩体是世界上为数不多的主要产出榴辉岩捕虏体和E型金刚石的岩筒之一,而同位于奥拉帕岩群的莱特拉卡内、丹姆沙和卡罗韦岩体与我国郯庐带的金伯利岩体类似,均主要产出地幔橄榄岩捕虏体以及P型和E型金刚石;(5)寻找含矿金伯利岩重点注意以下几点:克拉通内部和周缘深大断裂带是重要的控岩构造;镁铝榴石、镁钛铁矿、铬透辉石、铬尖晶石和铬金红石等是寻找含金刚石金伯利岩重要的指示矿物;航磁等地球物理测量需与土壤取样找矿方法相结合才能取得更好效果;(6)郯庐金伯利岩带、江南钾镁煌斑岩带和塔里木地块是中国重要含矿岩石的找矿靶区,冲积型金刚石成矿潜力巨大。     

The origin of the potassic rock suite from Batu Tara volcano (East Sunda Arc, Indonesia)

atu Tara is an active potassic volcano in the eastern Sunda arc. Its leucite-bearing rock suite can be subdivided into two groups, one less evolved with Th<20 ppm, the other more evolved with Th>20 ppm. ⁸⁷Sr/⁸⁶Sr, δ¹⁸O and trace-element systematics in the less evolved group suggests that existence of parental magmas with different mantle origins. The mantle below Batu Tara is most likely heterogeneous and several source components are involved in magma genesis. Trace element and isotopic compositions of Batu Tara and adjacent volcanoes are consistent with the involvement of a subducted sedimentary/crustal component as well as MORB and OIB mantle, the latter with geochemical characteristics comparable to the mantle underlying Muriah (Java). Melt extraction from this complex mixture is envisioned as a two-stage process: partial melts of the crust-contaminated MORB mantle mix in the mantle wedge with partial melts of OIB domains. Different mixtures of these two melts provide the parental magmas that enter the volcanic plumbing system, where crystallization, hybridization and refilling processes occur. The calcalkaline volcanoes in the arc segment show stronger signatures for a subducted crustal component than Batu Tara, which displays a greater influence from the OIB mantle source. The potassium enrichment can therefore be attributed to contributions both from the enriched mantle and from subducted crustal material. Mantle-type δ¹⁸O values of the Batu Tara magmas indicate that the mantle wedge below potassic orogenic volcanoes is not necessarily strongly enriched in ¹⁸O.     

Occurrence of lamproitic dykes at the northern Margin of the Indravati Basin,Bastar Craton,Central India

Occurrence of two lamproitic dykes intruding the basement granite near Khadka village at the northern margin of the Indravati Basin, Bastar craton is reported. Combined field, microscopy, XRD, EPMA and whole-rock geochemical investigations reveal that these lamproites were subjected to high degree of hydrothermal alteration as well as possible metamorphism. However, relicts of their original textures are well-preserved thereby providing important clues as to the nature of the protolith. Quartz, carbonate, chlorite and phlogopite constitute the bulk mineralogy whereas spinel, apatite and iron oxides are the accessory phases. Chemical composition of the groundmass spinels are strikingly similar to those from the lamproites. The Khadka lamproite dykes display high abundances of compatible elements such as Ni (238–396 ppm), Cr (484–892 ppm), and V (160–200 ppm) as well as high-field strength elements such as Zr (719–2057 ppm) and Nb (92–126 ppm) that resemble those in lamproites. Khadka lamproites also have high whole-rock REE abundances (ΣREE up to 1260 ppm) and display fractionated chondrite-normalized REE patterns (La/Yb= 113–237) which together with their average compatible and incompatible trace elemental ratios (e.g. Nb/Zr, Nb/La, Ba/Rb) are strikingly similar to those of the Mesoproterozoic Krishna lamproites of the Eastern Dharwar craton. Available field evidences suggest the Khadka lamproites to be of at least Palaeoproterzoic age (1.88 Ga) which makes them some of the oldest such rocks as yet documented from the Indian shield.     

2D model of the crust and uppermost mantle along rift profile, Siberian craton

A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km.