Trace elements and Hf isotope composition as indicators of zircon genesis due to the evolution of alkaline-carbonatite magmatic system (Ilmeny-Vishnevogorsky complex, Urals, Russia)

We present results of investigation of the trace-element (REE, HFSE) and Hf isotope compositions and U-Pb age of single zircons crystallized from alkaline-carbonatite magmas of the Ilmeny-Vishnevogorsky complex (IVC) (Urals, Russia). It has been established that the geochemical characteristics of the early zircon (U-Pb age of 430-410 Ma) from alkaline rocks and carbonatites of this complex are determined mainly by the magmatic evolution of parental fluid-saturated alkaline-carbonatite melts and are highly associated with the cocrystallization of zircon and uranium-containing rare-metal minerals (gatchettolite and pyrochlore) at the final stages of the magmatic-system activity. The early zircons have a moderately depleted Hf isotope composition (eHf from + 11.3 to + 4.7), confirming the mantle nature of the magma source and indicating the participation of DM-like and enriched-source (probably, lower-crust component) substances in the magma generation. The considerable variations in the initial Hf isotope composition of the early zircons testify to the multistage zircon crystallization involving new portions of melts with different isotope compositions controlled by mixing of substances at their source. Late IVC zircons (250-350 Ma) have strongly disturbed “rejuvenated” isotope systems and a geochemical composition different from that of the magmatic zircons. They formed apparently at the metamorphic stage of the IVC evolution without a significant input of additional material.16 page(s

Hf isotopes and trace elements as indicators of zircon genesis in the evolution of the alkaline-carbonatite magmatic system (Il'meno-Vishnevogorskii Complex, Urals, Russia)

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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

The Late Cretaceous diamondiferous pyroclastic kimberlites from the Fort à la Corne (FALC) field, Saskatchewan craton, Canada: petrology, geochemistry and genesis

The Late Cretaceous (ca. 100 Ma) diamondiferous Fort a 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 (T_DM) 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

Zircon U-Pb-ages, Hf isotope and trace element composition in the evolution of the IVAC Complex (Urals, Russia)

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Petro-geochemistry, Sr Nd isotopes and 40Ar/39Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

International audienceMount Girnar is one of the most conspicuous alkaline complexes (gabbro-diorite-syenite-lamprophyre), which intrude some of the earliest erupted basalts (ca. 69 Myr) of the Deccan Large Igneous Province (Deccan LIP) in the Kathiawar plateau of NW India. Petrography, bulk-rock geochemistry, Sr and Nd isotopes and 40Ar/39Ar mineral ages of fractionated (Mg#: 36.3–43.6) lamprophyre dykes (younger intrusives) are reported from two widely separated domains from Mt. Girnar. Petrography and mineral chemistry reveal that major mineral assemblages (pargasite and kaersutite varieties of amphibole, diopside, biotite and feldspar) in the lamprophyres are pristine and devoid of alteration. The lamprophyres belong to the alkaline variety in general and the camptonite - monchiquite series in particular. The bulk-rock major and trace elements of the Girnar lamprophyres display very good correlation with each other and also with those of associated rocks (syenites, diorites and gabbros) which support their genetic relationship. Trace-element ratios do not evidence crustal contamination and reveal derivation of the lamprophyres from partial melting of a lithospheric mantle source significantly modified by interaction with asthenospheric-derived melts, and resembles other alkaline rocks from the Deccan LIP in this regard. Initial 87Sr/86Sr (0.7052–0.7053) and 143Nd/144Nd (0.5125–0.5127) of the Girnar lamprophyres and associated rocks such as syenite, diorite and gabbro are tightly clustered and further attest to their derivation from a cogenetic parental melt. Their positive εNdi values (+0.8 to +3.4) require a mantle source that has experienced moderate long-term depletion of light rare-earth elements. Neoproterozoic/Early Cambrian depleted-mantle (TDM) Nd model ages of ~414–588 Myr are closer to the timing of break-up of the Rodinia supercontinent as well as coincide with the Ediacaran-Cambrian Malagasy orogeny. 40Ar/39Ar dating of three mineral separates (amphibole and biotite) from the lamprophyres gave precise plateau ages of 65.9 ± 0.3 Myr to 66.1 ± 0.4 Myr demonstrating that the emplacement of the Mt. Girnar igneous complex was close to the Cretaceous-Paleogene boundary. The emplacement of a range (110 Myr to 68.5 Myr) of other spatially related alkaline as well as silicic plutonic complexes, such as the Mundwara, Sarnu-Dandali, and Barda complexes prior to the main flood basalt event at ca. 66.0–65.1 Myr, highlights the role of extensional events in pre-existing rift/fault zones preceding the plume-lithosphere interaction in the Deccan LIP