Geologicheskii Vestnik

A comparative analysis of the composition of amphiboles from ultramafic rocks of mantle sections of two ophiolite complexes of the Urals — Kraka and Syum-Keu, which are often referred to as «lherzolite-type massifs» is carried out. Three morphological types of amphibole segregations in peridotites are recognized: 1) anhedral grains (up to 100–200 µm in size), often in association with clinopyroxene; 2) small prismatic grains (25–50 µm) in the peripheral parts or close to orthopyroxene porphyroclasts, also in association with small Opx, Cpx or Ol grains; 3) lamellae inside deformed grains of orthopyroxene, less often — clinopyroxene. The fourth amphibole type is represented by submicron inclusions in ore-forming chromite. The vast majority of the studied grains in the Kraka peridotites are represented by essentially calcium amphiboles, among which the most frequently diagnosed are pargasite, magnesian hornblende and tschermakite; tremolite and edenite are very rare. In the ultramafic rocks of the SyumKeu massif, on the contrary, the leading role belongs to edenite, in isolated cases hornblende is noted. The compositions of the amphiboles of the Syum-Keu massif are quite densely concentrated in the field of the «crust-mantle source», whereas for the compositions of the amphiboles of the Kraka massifs, a more significant range of values with a predominance of the «mantle component» is characteristic. The noted differences in the composition of amphiboles correlate with the estimated PT conditions of rock formation: higher temperatures and pressures are characteristic of the Kraka lherzolites (the boundary of the spinel and plagioclase facies of the upper mantle source), the Syum-Keu peridotites were probably recrystallized under the conditions of the lower part of the earth’s crust.

amphibole, ophiolite, ultramafic rocks, lherzolite, metamorphism, Kraka, Syum-Keu

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