40Ar/39Ar data on the age and metamorphism of the Ottfjället dolerites,Särv Nappe,Swedish Caledonides

The age of the Ottfjället dolerites, an extensive dyke swarm within the Särv Nappe of the central Scandinavian Caledonides, has earlier been suggested from K/Ar and Rb/Sr data to be either at least 1800 Ma or about 700 Ma. New K/Ar data on mineral separates confirm that the rock contains excess Ar while ⁴⁰Ar/³⁹Ar analyses of plagioclase indicate that intrusion and alteration of the dykes took place 600–700 Ma ago. Reassessment of previous K/Ar data confirms this and provides a more precise age of 665±10 Ma. The K/Ar data indicate that, despite Caledonian orogeny and tectonic transport, much of the Särv Nappe has not been above 300°C since dyke emplacement. Using neutron produced Ar and Kr the excess Ar in plagioclase is shown to be associated with anion sites containing Br and Cl.     

The RbSr age of the Mashonaland dolerites of Rhodesia and its significance for palaeomagnetic correlation in Southern Africa

Palaeomagnetic correlation in southern Africa predicts that the age of the Mashonaland dolerites of Rhodesia is confined within the limits of the age of the Waterberg System of South Africa, viz., between 1,950 m.y. and about 1,750 m.y. Rb-Sr data from the dolerites confirm this prediction. One sample gave a mineral isochron of 1,850 ± 20 m.y., which may be the true age of emplacement and is certainly a reliable minimum estimate for it. Total rock samples from nine dykes define an isochron of age 1,910 ± 280 m.y. and initial ⁸⁷Sr/⁸⁶Sr of 0.705 ? 0.002. In addition to variation in initial ⁸⁷Sr/⁸⁶Sr between dykes, there is also variation between minerals within single dykes presumeably due to contamination during crystallization and/or deuteric alteration.     

Single zircon evaporation ages: Evidence for the Mesoproterozoic crust in the southeastern Nigerian basement complex

The Ukwortung area of the Obudu Plateau exposes high-grade metamorphic rocks of upper amphibolite to granulite facies. Zircon grains from three locations of Southwest Obudu Plateau were dated by the Pb-Pb dating method. This single zircon radiometric dating method confirms the occurrence of Mesoproterozoic crust in the southeastern Nigeria basement complex. The pyroxene gneiss (granulite facies) sample from Biereberi near Ukpe yielded a mean age of 2061.4±0.4 Ma, suggesting the presence of Paleoproterozoic crust. The combined 207Pb/206Pb ratios from the garnet-biotite gneiss in the area gave a mean age of 1794.4±0.4 Ma. This further confirms the pres-ence of Paleoproterozoic crustal components within the Obudu Plateau. The Mesoproterozoic (1548.8±0.5 Ma) and Neoproterozoic (619.8±0.9 Ma) ages were obtained from two populations of zircon grains from the leucogranite in Okordem, Southwest Obudu. These "signatures" collaborate the occurrence of Mesoproterozoic ages in the Afar region and Cameroon basements along with the southeastern Nigerian basement complex, constituting the major portion of the West Central African mobile belt. The zircon age of 1794.4±0.4 Ma may probably be the maximum depositional age of the meta-sediment (garnet-biotite gneiss) and the age of 1548.8±0.5 Ma may be interpreted as the time of emplacement of the granitoid. The Neoproterozoic age (619.8±0.9 Ma) probably records the metamorphic event that was prevalent during the Pan-African period and thus affected the area.     

Mantle plume: the dynamic setting of the origin of Early Paleozoic mafic dykes in Ziyang,Shaanxi Province,Southern Qinling Block,China

The mafic dykes (dolerites) during the Early Paleozoic are widely spread in Langao-Ziyang, southern Qiling Block, and the investigation on these dykes are very important. Previous studies have mainly focused on the Silurian mafic dykes; however, research on the Earlier Paleozoic mafic dykes is relatively weak at present. Therefore, the overall understanding of the mantle source and genetic dynamic setting during the Early Paleozoic in this area is lacking. To study the accurate age and origin of the Early Paleozoic mafic dykes in Ziyang, southern Shaanxi Province, the mafic dykes from dabacunand Qinmingzhai were selected and the petrology, zircon U–Pb chronology, geochemistry, and Sr–Nd–Hf isotopes were studied. Analysis indicates that the mafic dykes studied are mainly composed of dolerite, and they are the products of the Early Ordovician (475.8–480.7 Ma). Furthermore, the dolerites belong to alkaline rock series, and they are characterized by enrichment in LREE, Rb, Ba, Sr, Nb, (⁸⁷Sr/⁸⁶Sr)_i = 0.7020–0.7050, ε_Nd(t) = 3.0–4.0), ε_Hf (t) = 4.5–12.1,¹⁷⁶Hf/¹⁷⁷Hf = 0.282681–0.282844. This suggests that the mafic dyke were derived from the partial melting of a depleted lithospheric mantle, and the genetic process is mainly controlled by the mantle plume based on the discussion of the genetic model. Furthermore, the genetic process experienced the separation and crystallization of olivine and clinopyroxene at the same time, with little crustal contamination.

     

Precambrian mafic magmatism of Shillong plateau,Meghalaya and their evolutionary history

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low-pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im= +57 (α₉₅= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.     

New radiometric dating of the dykes from the Hurd Peninsula, Livingston Island, South Shetland Islands

Fifteen new K–Ar ages in the range of 79–31 Ma are partially confirmed by three ⁴⁰Ar/³⁹Ar plateaus and isochron data of 64.9±0.4, 55.5±0.1 and 52.8±0.6 Ma. The new geochronological data reveal a much more detailed picture of the subduction imprint in the Hurd Peninsula. Using cutting relationships, the dyke emplacement history is divided into four episodes. The Late Cretaceous–Paleocene dykes in the range of 80–60 Ma are related to the main magmatism in Livingston Island and most likely reflect the final stages of subduction of the proto-Pacific oceanic crust. The Early Eocene dykes (56–52 Ma) fill the gap in volcanic activity 70–50 Ma ago. They are the only magmatic event manifested at this time in the region. The 45–42 Ma dykes may be related to the intrusion of the Barnard Point tonalite. Three samples of Oligocene age appear to represent the last igneous activities on the Hurd Peninsula prior to the opening of the Bransfield Strait.     

Âge 40K/40Ar,Carbonifère inférieur,du magmatisme basique filonien du synclinal paléozoïque de Tin Serririne,Sud-Est du Hoggar (Algérie)

Palaeozoic formations of the Tassilis Oua-n-Ahaggar (southeastern Hoggar) include magmatic rocks in the Tin Serririne syncline. Slight contact metamorphism of the overlying bed and studies of anisotropy of magnetic susceptibility of these rocks show that the latter correspond to sills and NW–SE or north–south dykes. ⁴⁰K/⁴⁰Ar dating of separated feldspars and whole rock for one sample and of whole rock for two other samples give a mean age of $347.6\pm 16.2\text{Ma}$ (at the 2-σ level), thus corresponding to a Lower Carboniferous (Tournaisian) age. Taking into account both the age of this magmatism and the stratigraphic and structural data for this region suggests that dolerites were emplaced within distensive zones that are related to the reactivation of Panafrican faults. To cite this article: H. Djellit et al., C. R. Geoscience 338 (2006).     

Middle Jurassic subduction-related ophiolite fragment in Triassic accretionary complex (Mamu Dağı ophiolite,Northern Turkey)

ABSTRACT

The Mamu Da?? ophiolite, ca. 13 km long and 5 km across (Tokat, Sakarya Zone), consists of peridotites, pyroxenites, gabbros, and basalts, which are crosscut by dolerite dykes. These rocks show variable degrees of serpentinization and alteration. Gabbroic rocks consisting of plagioclase + clinopyroxene ± orthopyroxene ± olivine ± amphibole ± sphene ± opaque minerals have commonly the ophitic and the cumulate textures. Similar mineral paragenesis is observed in the basalts and the dolerites, which are commonly characterized by the sub-ophitic and the microlitic porphyric textures.

Primitive mantle-normalized rare earth and trace element diagrams of gabbros and basalts display subduction-related geochemical characteristics such as high Th concentrations, negative Nb, Zr, and Ti anomalies. Some of the gabbros are interpreted to be the cumulate rocks. They have mostly positive europium anomaly (Eu/Eu* 1.77–0.83) and relatively low SiO₂ and incompatible element (e.g. Zr, Ti) contents. The initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd values of gabbro/dolerite and basalt samples vary between 0.7036 and 0.7049, between 0.51259 and 0.51278, respectively. The isotope data and the whole rock geochemistry suggest that the Mamu Da?? ophiolite was derived from a mantle source that was affected by the subduction component rather than MORB or depleted mantle source.

Hornblendes from a gabbro sample of the Mamu Da?? ophiolite yielded ⁴⁰Ar/³⁹Ar plateau age of 159 ± 1 Ma. This age data is similar to those of many ophiolites located along the ?zmir-Ankara-Erzincan suture zone but is different from the ages reported for the Tokat Massif.     

New data on the age of dolerites and basalts of Mendeleev Rise (Arctic Ocean)

We present results of ⁴⁰Ar/³⁹Ar isotopic investigations concerning the dating of dolerites and basalts that were sampled during the Arctica-2012 polar expedition. Basalts were sampled by means of deep underwater drilling with wells up to 2 m in outcrops on the seafloor (basalts), and dolerite samples were obtained from the bottom of an escarp of Mendeleev Rise using a manipulator on the research submarine. The analysis results of the obtained mono-mineral fractions (amphibole, plagioclase, pyroxene) from the studied rocks yielded an Early Paleozoic age of the dolerites and basalts from Mendeleev Rise. The oldest ages obtained for amphibole reach 471.5 ± 18.1 and 466.9 ± 3.3 Ma, which corresponds to the Early-Middle Ordovician. The isotopic composition of argon was measured on two mass spectrometers: the Micromass Noble Gas 5400 (UK) and the Thermo Scientific Argus (Germany). The determined Early Paleozoic age of igneous rocks of Mendeleev Rise and seismic data obtained during the last Russian expedition Arctica-2012 [2] let us suppose that this continental block of the Earth’s crust has a Precambrian basement similar to the basement identified for the New Siberian islands including the De Long archipelago.     

Structural and geochronological studies on the Liba goldfield of the West Qinling Orogen, Central China

The Liba goldfield, located to the northeast of the Zhongchuan Granite in the West Qinling Orogen (WQO) of mainland China, contains the largest known gold resource of 2.8 Moz in the Zhongchuan area. Devonian metasedimentary rocks host the structurally controlled gold mineralization, which is associated with silica–sericite–chlorite–carbonate alteration. Two major styles of mineralization occurred at the goldfield, which are disseminated sediment-hosted and quartz vein hosted types. Pyrite, arsenopyrite, and arsenian pyrite are major gold carriers and gold also occurs as native gold grains and electrum spatially associated with the sulfides. Numerous felsic/intermediate dykes have a similar structural control as the mineralization, and their contacts with host rocks are recognized as favorable zones for mineralization. Detailed fieldwork in conjunction with geochronological studies has helped to define the deformation history and gold metallogenesis of the goldfield. Three major phases of deformation have been recognized in the Zhongchuan area. The first deformation (D₁) event was compressional in broadly a N–S orientation, the second (D₂) event was also compressional and orientated in a NE–SW direction, and the third (D₃) event was post-mineralization and was associated with the emplacement of barren calcite and anhydrite veins. Compression related to D₂ is the key process that controlled the distribution of igneous dykes and gold mineralization in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local trans-extension, which took place during the late stage of D₂. Precise dating with high-resolution ion microprobe (SHRIMP) U–Pb on zircon and ⁴⁰Ar/³⁹Ar on muscovite, biotite, hornblende, and plagioclase of crosscutting pre-mineralization granitic porphyry and diorite dykes have constrained the mineralization age to after ca. 227 Ma. ⁴⁰Ar/³⁹Ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralization indicates that there was a widespread ca. 216 Ma hydrothermal event that affected almost all lithologies in the area. This detailed investigation is the first study to tightly constrain the timing of gold mineralization in the WQO. The broadly overlapping timing and similar structural control of the mineralization and igneous dykes show a promising correlation, which could be potentially used to map this Late Triassic gold mineralization event in the WQO.     

Age of ultramafic high-K rocks from Planalto da Serra (Mato Grosso,Brazil)

The Planalto da Serra ultramafic-alkaline rocks are represented by several plugs, lava-flows and N60–80E trending dykes which are mainly represented by glimmerites, harzburgites and carbonatites. These rocks intrude the Cryogenian basal unit of the Northern Paraguay Belt in the Rio dos Cavalos rift (southeastern part of the Amazonian Craton). ⁴⁰Ar/³⁹Ar dating on the phlogopite from glimmeritic dykes and plugs yielded an age of ca. 600 Ma, which is consistent with Rb/Sr and Sm/Nd determinations on the other rock-types. These age determinations make it difficult to establish a geochronological and genetic relationship between the Planalto da Serra rocks and the Cretaceous bodies of the “Azimuth 125° Lineament”, and it suggests there is a relationship between its emplacement and the beginning of the extensional tectonics responsible for the separation of the Amazonian and Laurentia Cratons. This age also indicates that the onset of the deformation of the Cuiaba Group may be older than 600 Ma.     

Precambrian mafic magmatism in south Indian granulite terrain

South Indian granulite terrain had witnessed significant part of Precambrian mafic igneous activity in the form of episodic mafic dyke intrusions of the Palaeoproterozoic period. Strike trends of these dykes are not uniform over the region and the dykes are generally fresh, massive, black dolerites except in the Bhavani shear zone bordering the southern fringes of Nilgiri massif. In Agali-Coimbatore area of our study in the western Bhavani shear zone, the dykes appear to be penecontemporaneous with shearing. Isotopic data place age of Agali-Coimbatore dyke intrusions at about 2.1 Ga. The age of these dykes is significant to constrain an early Palaeoproterozoic age for major shearing event in the Bhavani shear zone. Other dyke emplacement ages are placed at about 1.8 Ga and 1.65 Ga based on the Ar/Ar and K-Ar isotopic results of dykes in Dharmapuri and Tiruvannamalai areas. Older ages comparable to those of the Dharwar craton are not known and in this respect future isotopic dating is vital. Geochemically, these dykes are quartz/hypersthene normative subalkalic tholeiites. An attempt is made here to provide insights into the general petrogenetic history of the Precambrian dykes. Compositional trends are explained by the fractional crystallization of ferromagnesian phases and plagioclase control is conspicuous at the advanced stages of fractionation. Geochemical characteristics suggest that the dykes have tapped Fe-rich non-pyrolite mantle sources with LIL and LREE enrichment as in many continental basalts. The data suggest that role of crustal contamination is limited in petrogenesis; crustal signatures are noticed in the more mafic end members formed in early stage of evolution suggesting that contamination was temperature controlled with most primitive high temperature magmas being most vulnerable to the process. Nd-Sr isotopic data, at present restricted to Agali-Coimbatore dykes, suggest that Palaeoproterozoic magmas tapped subcontinental lithosphere that may have stabilized in the Archaean times at about 3 Ga during the major crustal building activity in the shield region. Further work coupled with isotopic and mineral chemistry will improve our knowledge on the petrological evolution of the dyke magmas and mafic magmatism in general.     

Geochemistry of the Mesozoic regional basic dykes of western Dronning Maud Land,Antarctica

Regional dolerite dykes of Mesozoic age in western Dronning Maud Land are variable in both major and trace element composition and include picritic types (MgO>18 wt%). The range in incompatible element concentrations is considerable (e.g. Zr 40–478 ppm) and shows little correlation with MgO content. Both high-and low-Ti, Zr (HTZ and LTZ) magma types are present and there is a spread of compositions between these types. Major element oxide variations in dykes having MgO>10 wt% indicate that olivine and orthoyproxene fractionation occurred, presumably at an early high-pressure stage of magma evolution. Major element oxide variations in dykes having MgO<10 wt% indicate control by olivine and clinopyroxene. A minority of the more evolved dykes are compositionally similar to the nearby Kirwan basalts, but the majority cannot be related to the Kirwan basalts by any simple petrogenetic process as they contain higher concentrations of incompatible elements and have higher Mg-numbers. The HTZ Dronning Maud Land dolerites have incompatible trace element concentrations which are very similar to the HTZ basalt magma types of the Karoo of southern Africa with the exception of lower K and Rb in DML dolerites. The HTZ dolerites occur in the part of Dronning Maud Land which appears to have been tectonically stable since the Archaean and are not found to intrude the surrounding high-grade (about 1000 Ma) metamorphic rocks of the Sverdrup Group. These data provide qualified support for models which seek to relate spatially the HTZ Mesozoic basalt types of Gondwana to sources beneath stable Archaean cratons.     

Magnetic susceptibility used in mapping of amphibolite facies recrystallisation in basic dykes

The dense tholeiitic Koster dyke swarm, west Sweden, has undergone progressive deformation under amphibolite facies metamorphic conditions. The varying influence of this event on the dyke rock forms the basis for a regional division of the swarm into three sectors: the sectors of igneous dolerites, partly recrystallised metadolerites, and lineated amphibolites, respectively. Systematic measurements of the magnetic susceptibility in the dykes have demonstrated a correlation between the regional pattern of recrystallisation and the pattern of mean susceptibility given as mean susceptibility per 0.5 km². The transformation of dolerites to metadolerites and finally to lineated amphibolites is accompanied by a marked decrease in the susceptibility, which is related essentially to the degree of alteration of titanomagnetite to sphene + Fe-ions incorporated in the silicates.     

Single zircon ages of migmatitic gneisses and granulites in the Obudu Plateau: Timing of granulite-facies metamorphism in southeastern Nigeria

A single zircon geochronological study of gneisses from the Obudu Plateau of southeastern Nigeria, using the evaporation technique, indicates that zircons recorded several Precambrian high-grade metamorphic events (Eburnean and Pan-African). Igneous and multifaceted metamorphic zircons yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2062.4 ± 0.4 Ma, 1803.8 ± 0.4 Ma and 574 ± 10 Ma, respectively and confirm for the first time that granulite-facies metamorphism affected the basement of southeastern Nigeria, resulting in the formation of charnockites and granulitic gneisses. The Pan-African high-grade event was coeval with the formation of granulites in Cameroon, Togo and Ghana and resulted from collisional processes during continental amalgamation to form the Gondwana supercontinent. The sources of the sediments, which were deposited at ≈605 Ma and metamorphosed at 574 Ma, comprise older igneous and metamorphic protoliths (including inherited xenocrystic zircons up to 2.5 Ga in age). The Palaeoproterozoic zircons seem to have survived Pan-African melting.     

40Ar/39Ar laser probe evidence concerning the age and associated hazards of the Lake Nyos Maar,Cameroon

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent ⁴⁰Ar/³⁹Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic ⁴⁰Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a ¹⁴C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria.     

Geology and age of mafic magmatism on Jeannette Island (De Long archipelago)—Implications for paleotectonic reconstructions for the Arctic

The paper presents the first data on the geology of a series of mafic dikes emplaced in the volcanogenic sedimentary sequence composing the structure of Jeannette Island. We show that the island hosts two generations of dikes consisting of fine-grained dolerites. Both magmatic stages were followed by fold deformations. The youngest and weakly deformed dikes are none the less chloritized and serpentinized. However, we were able to obtain the first ever precise geochronological ⁴⁰Ar/³⁹Ar data for them. According to this analysis, the emplacement age of the intrusive complex of Jeannette Island is close to the Ediacaran (553.6 ± 10.3 Ma). The ages of several superimposed tectonothermal events were determined: Middle Ordovician (463.3 ± 11.7 Ma), Late Silurian (425.3 ± 8.7 Ma), Late Devonian (360.7 ± 8.3 Ma) and Early Carboniferous (341.3 ± 6.8 Ma).     

Geochronological and geochemical constraints on the petrogenesis of alkaline ultramafic dykes from southwest Guizhou Province,SW China

Geochronological, geochemical and whole-rock Sr–Nd isotopic analyses have been completed on a suite of alkaline ultramafic dykes from southwest (SW) Guizhou Province, China with the aim of characterising their petrogenesis. The Baiceng ultramafic dykes have a LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age of 88.1 ± 1.1 Ma (n = 8), whereas two phlogopites studied by ⁴⁰Ar/³⁹Ar dating methods give emplacement ages of 85.25 ± 0.57 Ma and 87.51 ± 0.45 Ma for ultramafic dykes from Yinhe and Lurong, respectively. In terms of composition, these Late Mesozoic ultramafic dykes belong to the alkaline magma series due to their high K₂O (3.31–5.04 wt.%) contents. The dykes are characterised by enrichment of light rare earth element (LREE) and large-ion lithosphile elements (LILEs) (Rb and Ba), negative anomalies in high field strength elements (HFSEs), such as, Nb, Ta and Ti relative to primitive mantle, low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7060–0.7063) and positive ε_Nd(t) values (0.3–0.4). Such features suggest derivation from low degree (< 1%) partial melting of depleted asthenospheric mantle (garnet-lherzolite), and contamination to various degrees (～ 10%) by interaction with upper crustal materials.     

Geochronological (Rb-Sr and Sm-Nd) studies on intrusive gabbros and dolerite dykes from parts of Northern and Central Indian cratons: Implications for the age of onset of sedimentation in Bijawar and Chattisgarh basins and uranium mineralisation

The Dargawan gabbros intrusive into the Moli Subgroup of Bijawar Group, yielded Rb-Sr whole rock isochron age of 1967 ± 140 Ma. Based on the oldest age from overlying Lower Vindhyan (1.6Ga) and the underlying youngest basement ages (2.2 Ga), the time range of Bijawar sedimentation may be assigned as 2.1–1.6 Ga (Paleoproterozoic). Sm-Nd Model ages (T_DM), obtained, for Dargawan gabbros, is c. 2876–3145 Ma. High initial ⁸⁷Sr/ ⁸⁶Sr ratio of 0.70451 (higher than the contemporary mantle) and negative ɛ_Ndi (at 1.9 Ga) value of −1.5 to − 4.5, indicate assimilation of Archaean lower crustal component by the enriched mantle source magma at the time of gabbroic intrusion. The dolerite, from Damdama area, which is intrusive into the basement and overlying sediments of Chandrapur Group in the central Indian craton, yielded Rb-Sr internal isochron age of 1641 ± 120 Ma. The high initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7098 and ɛ_Ndi value of −3.5 to −3.7 (at 1.6 Ga) is due to contamination of the mantle source magma with the overlying sediments. These dolerites have younger Sm-Nd Model ages (T_DM) than Dargawan gabbros as c. 2462–2675 Ma, which is similar to the age of the Sambalpur granite, from which probably sediments to this part of Chattisgarh basin are derived. Hence mixing of sediments with the Damdama dyke during its emplacement, gives rise to high initial ⁸⁷Sr/⁸⁶Sr and low initial ¹⁴³Nd/¹⁴⁴ ratios for these dykes. The c. 1600 Ma age indicates minimum age of onset of the sedimentation in the Chandrapur Group of Chattisgarh basin. Both the above mafic intrusions might have taken place in an intracratonic rift related (anorogenic) tectonic setting. This study is the first reliable age report on the onset of sedimentation in the Chandrapur Group. The total minimum time span of Chandrapur and Raipur Group may be 1.6 Ga to 1.0 Ga (Mesoproterozoic). The unconformably underlying Shingora Group of rocks of Chhattisgarh Supergroup thus indicates Paleoproterozoic age (older than 1.6 Ga). Most part of the recently classified Chattisgarh Supergroup and Bijawar-Vindhyan sequence are of Mesoproterozoic-Paleoproterozoic age and not of Neoproterozoic-Mesoproterozoic age as considered earlier. Petrographic study of basic dykes from Damdama area (eastern margin of Chattisgarh Supergroup) indicated presence of primary uranium mineral brannerite associated with goethite. This is the evidence of mafic intrusive providing geotherm and helping in scavenging the uranium from the surrounding and later alterations causing remobilisation and reconcentration of pre-existing uranium in host rocks as well as in mafic dyke itself otherwise mafic rocks are poor source of uranium and can not have primary uranium minerals initially. It can be concluded that mafic dykes have role in uranium mineralisation although indirectly.     

Palaeomagnetism of metadolerite dykes and sills from Proterozoic Shillong basin,NE India: Implications related to the age and magmatism

The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α₉₅ = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite.