Ekaterina Reguir MSc thesis abstract

Thesis Title: 
Aspects of the mineralogy of the Murun alkaline complex, Yakutia, Russia
Ekaterina
Reguir
MSc
2001

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)5Si12O30(OH,F).  3H2O. X-ray diffraction patterns of mosaic-fibrous and schistose charoite are indexed using a primitive monoclinic cell with the following parameters: a =19.86(1) D,  b =32.13(2) D,  c =7.952(9) D,  $=97.24E (mosaic-fibrous) and a =32.13(2) D,  b =19.64(2) D, 
c =8.509(6) D,  $=95.23E (schistose).  Some of the patterns of mosaic-fibrous charoite can also be refined on a triclinic cell (a =19.96(1) D,  b =32.17(2) D,  c =7.258(4) D,  "=93.51E, $=99.45E, (=89.50E).  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 of SiO4tetrahedra 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 of 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 of 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 of the crystal.  The composition of the primary carbonates becomes progressively depleted in Ba + Sr (calcite) or Sr (barytocalcite) towards the margin, suggesting that interstitial fluid played an important role in the onset of exsolution processes promoting an outward diffusion of components in the peripheral zones of carbonate crystals.  In contrast to the calcite-carbonatite, the phlogopite-calcite carbonatite [type (iii)] is devoid of clinopyroxene and Ba-Sr-Ca carbonates, and rarely exhibits exsolution textures.  Potassium feldspar and phlogopite from the carbonatite of type (iii) are Ba-rich, and shoe core-to-rim zonation pattern of decreasing Ba content.  The mineralogical differences between the carbonatites of types (i) and (iii) clearly indicate that the latter crystallized from a volatile-rich magma depleted in Na, under more reducing conditions, and upon crystallization, underwent a rapid loss of residual volatiles.  Textural and mineralogical features of the quartz-fledspar-carbonate rock suggest that the rock is transitional from carbonatites to quartz-calcite rocks locally known as "torgolites".
 
The youngest igneous suite at Murun is represented mostly by dykes and sills.  Studied hypabyssal and volcanic rocks are divided into eight petrographic groups: lamprophyre, pseudolueucite syenite, pseudoleucite italite, kalsilite melasyenite, eudialyte-bearing syenite, "potassium-batisite" syenite, lamprophyllite- and barytolamprophyllite-bearing syenite, and aegirinite.  The compositional trends of mafic minerals from different groups clearly indicate that their parental magmas could not be derived from the same source, and that the magma mixing cannot account for the diversity of the hypabyssal rocks found at Murun.  The compositional data of mafic minerals are in agreement with the previous findings, supporting the consanguineous nature of lamprophyres, phlogopite clinopyroxenites and skonkinites (including melasyenites).  The syenitic rocks containing major proportions of such "agpaitic" minerals as eudialyte, "potassium batisite", and lamprophyllite-group minerals clearly represent the most evolved magmas.  Compositions of their characteristic accessory minerals crystallized early in the evolutionary history suggest enrichment of parental magmas in Sr, Ba and Zr.  The compositions of mafic minerals from pseudoleucite syenites fall off major evolutionary trends, therefore the source of these rocks is uncertain.
  
Clearly, further studies of the geochemistry and mineralogy of the alkaline ultramafic rocks, unique Ba-Sr-rich carbonatites and charoitites of the Murun complex are required. 
 
A copy of the thesis can be downloaded here