Ultrahigh-pressure (UHP) associations in ultramafites of the Maksutov complex (Southern Urals)

Abstract

or irregular form bodies from 2-3 to 20-30 m in size, which are located mainly in contact zones between eclogites and the surrounding quartzites and schists. Olivine-enstatite and enstatite rocks have yellowish gray or greenish gray color, giantgrained or coarse� grained texture, and starlike structure. Enstatite occurs as rosettes or single elongated crystals in the matrix, made of finegrained forsterite and magnesite. Schistic rocks are surrounded by rims composed of the following associations: talk + phengite, talk + tremo� lite ± winchite (asbestoslike), tremolite + chlorite ± garnet, and others. The rocks are composed of forster� ite, enstatite, magnesite, and clinohumite. Ore minerals are chromite and pentlandite; secondary minerals are serpentine and chlorite (Table 1). Forsterite contains 11-17% of fayalite molecule, which is typical of for� sterite of the Fe-Ti type from olivine peridotites. According to some peculiarities of its chemical com� position (Mg content (0.89-0.91), a higher content of Al (1.86-2.87 wt%), and Ni (up to 0.20 wt%)), enstatite is close to the theoretical composition. Mag� nesite makes up to 25% of the total volume of the sam� ple. The characteristic peculiarity of the magnesite composition is a low Fe and Ca content in olivine- enstatite and enstatite rocks (4.3 and 3.8 wt%, 0.19 and 0.11 wt %, respectively). Titanoclinohumite occurs as small grains in a ser� pentine or magnesite matrix and contains Ti (4.1- 4.5 wt %) and Ni (0.18-0.75 wt %) with insignificant variations in the Mg content (0.88-0.90). Antigorite is characterized by a high Mg content (0.89-0.95) and Cr content, varying from 0.5 to 1.9 wt%. Magnesial chlorite ( ~0.94) has up to 2.5 wt% Cr and 0.6 wt% Ni. Chromite and pentlandite occur as irregular grains in forsterite. The paragenesis of Fo-En-Mgs-Ti-Chu makes it possible to clarify the conditions of formation of the rocks considered. This mineral association is typical of ultramafic rocks of the Dabie Suite and Dabie Shau ultrahighpressure complexes and diamondcontent "calcsilicate" rocks of the Kokchetav massif (3, 5, 8). The experimental data for the CaO-MgO-SiO 2 -