Aspects of the mineralogy of the Murun alkaline complex, Yakutia, Russia

The Murun complex contains a number of unusual and mineralogically unique rocks. Many of them are problematic in terms of their genesis and petrographic interpretation. These enigmatic rocks include charoite assemblages, as well as unique Ba-Sr-rich carbonatites and alkaline ultramafic dykes referred to as lamproites or lamprophyres. Charoitites occur in about 25 localities along the southern margin of the Little Murun intrusion. Obtained compositional data for charoite suggests, in general, the empirical formula: (K,Na)3(Ca,Sr,Ba,Mn)5Si1203o(OH,F)* SH2O. X-ray diffraction patterns of mosaic-fibrous and schistose charoite are indexed using a primitive monoclinic cell with the following parameters: (see document) Comparison of the compositional and structural data of charoite and “tube”-chain silicates of similar composition (canasite and miserite) suggests that charoite has a significantly higher relative proportion o f Si04 tetrahedra to octahedrally-coordinated cations than two other minerals. Possible similarities between the structural motifs of charoite and phyllosilicates are demonstrated. The carbonatite bodies are confined to the aegirine-microcline fenite aureole in the southern contact zone of the Little Murun massif with the Precambrian crystalline basement. Three mineralogical types o f the studied carbonatites are distinguished: calcite carbonatite (i), Ba-Sr-Ca carbonatite (ii), and phlogopite-calcite carbonatite (iii). In addition, the quartz-feldspar- carbonate rock is classified as a distinct lithological type. Carbonatites of types (i) and (ii) noticeably differ in terms o f the composition of rock-forming silicates (potassium feldspar and clinopyroxene) and primary carbonates (Sr-rich calcite or barytocalcite, respectively). In both cases, the primary carbonates underwent complex exsolution processes. Typical exsolution textures are represented by primary carbonates in a core, and the subsolvus mineral assemblage confined to marginal parts o f the crystal. The composition of the primary carbonates becomes progressively depleted in Ba + Sr (calcite) or Sr (barytocalcite) towards the margin, suggesting that interstitial fluids played an important role in the onset of exsolution processes promoting an outward diffusion o f components in the peripheral zones of carbonate crystals

Zircon Macrocrysts from the Drybones Bay Kimberlite Pipe (Northwest Territories, Canada): A High-Resolution Trace Element and Geochronological Study

Zircon macrocrysts in (sub)volcanic silica-undersaturated rocks are an important source of information about mantle processes and their relative timing with respect to magmatism. The present work describes variations in trace element (Sc, Ti, Y, Nb, lanthanides, Hf, Ta, Pb, Th, and U) and isotopic (U-Pb) composition of zircon from the Drybones Bay kimberlite, Northwest Territories, Canada. These data were acquired at a spatial resolution of ≤100 µm and correlated to the internal characteristics of macrocrysts (imaged using cathodoluminescence, CL). Six types of zircon were distinguished on the basis of its luminescence characteristics, with the majority of grains exhibiting more than one type of CL response. The oscillatory-zoned core and growth sectors of Drybones Bay zircon show consistent variations in rare-earth elements (REE), Hf, Th, and U. Their chondrite-normalized REE patterns are typical of macrocrystic zircon and exhibit extreme enrichment in heavy lanthanides and a positive Ce anomaly. Their Ti content decreases slightly from the core into growth sectors, but the Ti-in-zircon thermometry gives overlapping average crystallization temperatures (820 ± 26 °C to 781 ± 19 °C, respectively). There is no trace element or CL evidence for Pb loss or other forms of chemical re-equilibration. All distinct zircon types are concordant and give a U-Pb age of 445.6 ± 0.8 Ma. We interpret the examined macrocrysts as products of interaction between a shallow (<100 km) mantle source and transient kimberlitic melt

Petrogenesis of the late Paleoproterozoic Gleibat Lafhouda dolomite carbonatite (West African Craton Margin, Moroccan Sahara) and its relevance to the onset of fragmentation of the Columbia supercontinent

The Gleibat Lafhouda dolomite carbonatites of the Moroccan Sahara occur as three separate cone-shaped plugs intruding an autochthonous succession of Archean supracrustal basement rocks. Geochemically, the Gleibat Lafhouda dolomite carbonatites are characterized by a compositional range of 11.3–27.1 wt% MgO, 3.1–29.7 wt% CaO, 3.5–38.0 wt% FeOtot and < 0.1–7.5 wt% SiO2, and enrichment in large-ion lithophile elements (LILE), particularly Sr (2173–11,310 ppm), Ba (174–4537 ppm), U (0.1–296 ppm) and light REEs (LREEs) (131–1295 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, and Hf. Nb and Ta show, however, much higher concentrations ranging from 0.5 ppm to 1.0 wt%, and < 0.0 to 199 ppm, respectively, which set them apart from naturally occurring carbonatites and the experimentally derived carbonated melts. The combined stable (δ13CV-PDB = −2.5 to −6.6‰, δ18OV-SMOW = 6.0 to 20.7‰) and radiogenic 87Sr/86Srin (0.7032–0.7046), 143Nd/144Ndin (0.5105–0.5106) or εNd(t) (+ 3 to +6), and 206Pb/204Pb (19.06–49.05), 207Pb/204Pb (15.90–18.87), and 208Pb/204Pb (37.87–38.50) isotope compositions are consistent with low degree partial melting, at convecting upper mantle conditions, of a predominantly depleted mantle source in a rift-related environment. Based on these geochemical features, it is suggested that the Gleibat Lafhouda dolomite carbonatites represent the earliest manifestation of rifting processes related to the fragmentation of the Columbia supercontinent at 1.85 Ga. Accordingly, we propose that these carbonatitic rocks represent the initial Mg-rich melt in the mantle plume head that derived from decompressional adiabatic melting of a depleted mantle source