Kosmochlor-bearing jadeite rocks from Kenterlau-Itmurundy (Lake Balkhash, Kazakhstan)

Petrographic, petrologic and geochemical investigations on Cr-jadeite rocks from Kenterlau-Itmurundy near Lake Balkhash (Kazakhstan) reveal five different rock groups. Jadeitites show irregular, medium-grained white sections made up of decussate jadeite crystals and fine-grained sections made up of aligned Cr-jadeite. Omphacite jadeitites show a homogeneous, pale green colour with randomly oriented jadeite crystals, which are overgrown by omphacite on their rims and along fractures. Phlogopite-analcime jadeitites show decussate, white sections made up of jadeite and foliated, partially microfolded green layers and spots consisting of Cr-jadeite. Analcime and phlogopite formed late in fractures in the jadeite-rich sections. Phlogopite-omphacite jadeitites reveal an inhomogeneous mineral distribution with a dark-green matrix made up of sheaf-like aggregates of Cr-omphacite and white spots of decussate prismatic jadeite crystals. Post-crystallization deformation is evident by bent and twisted pyroxenes while phlogopite formed post-tectonically on fractures and grain boundaries. Due to the extreme textural inhomogeneity, transitions between these four groups are blurred. Kosmochlor-analcime-albite-omphacite jadeitites reveal an inhomogeneous fabric with decussate, white sections, pale-green shear bands, and dark green layers. Again, white jadeite-rich sections formed pre-tectonically while shear bands display aligned Cr-omphacite, and in rare cases small aggregates of kosmochlor (identified by Raman spectroscopy, see Fig. 1). The main occurrence of kosmochlor is in the dark green layers, where it mantles strongly corroded chromite grains (Fig. 2). Microprobe investigations of two kosmochlor-bearing rocks and one phlogopite omphacite jadeitite reveal extreme mineral compositional variability. Pyroxene zoning with jadeite cores and rim sections of omphacite or Cr-omphacite are wide spread. Similarly, kosmochlor aggregates are very inhomogeneous covering a wide range in the pyroxene plots (Figs. 3 A&B). Similar textures have been observed by other investigations (e.g. Shi et al., 2005) and are due to pre-, syn- and post-tectonic crystallization processes. Distinct inhomogeneities are also revealed by ED-XRF spot analyses, which show a strong compositional variability from section to section.