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

Abstract

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.