Geologicheskii Vestnik

This article presents the results of studying the compositions of rock-forming and accessory ultramafic minerals of izrandite samples from Alexandrovsk metamorphic complex. It was found that petrographical, mineralogical and compositional features indicate igneous cumulative origin of studied rocks but amazing freshness of izrandite contradict with their “oldest age”. Together with igneous, numerous occurrences of subsolidus structures were found in studied rocks. These are following: reaction rims on the olivine-plagioclase boundaries, orthopyroxene-pargasite symplectite intergrowths on the olivine-augite and olivine-plagioclase boundaries, ilmenite and Ti-magnetite lamellae in clinopyroxene and amphibole grains, Ti-magnetite solid-solution breakdown structures. It is shown that spinel group mineral grains have complex structure including different blocks: Ti-magnetite with ilmenite exsolution lamellae, homogenous ilmenite and high-Al spinel, and these were probably formed by subsolidus processes. We have considered critically some scenarios of izrandite origin. We infer that izrandites are similar to cumulative rocks from some cratonic layered intrusions, but it is necessary to note that there is no hard evidence of that izrandite is relict of the oldest protolith of metamorphic rocks.

Alexandrovsk complex, reaction structure, subsolidus, solid-solution breakdown, exsolution, lamellae, olivine, augite

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