ИСТИНА

Metamorphic fluids released from the subducting plates are crucial for metamorphic, metasomatic and magmatic processes operated in subduction zone setting. A record of these processes in the mantle wedge rocks is limited however because the ultramafic rocks are not widespread in (U)HP metamorphic complexes or they have been affected by considerable alteration during the retrogression. Here we report results of the detail petrologic study of the metaultramafic rocks from the Middle Paleozoic Maksyutov Complex. The Complex is divided in three fault-bounded lithologic units: polymetamorphic HP-UHP Unit #1 consisting of micaschists and quartzites (including diamond-bearing ones) containing boudins and lenses of eclogites and metaultramafic rocks; a meta-ophiolitic Unit #2 was experienced blueschist-facies metamorphism and greenschist-facies retrogression; and a metasedimentary blueschist-facies Yumaguzinskaya unit. Ultramafic boudins (from 10 m to 200 m in size) are concordant with the country rocks (metasediments) of the HP-UHP Unit. Fresh samples are composed predominantly of a granoblastic aggregate of the olivine grains with the accessory chromian spinel. The olivine contains inclusions of Ti-clinohumite and ilmenite. Most of the ultramafic rocks are characterized by three types of high-pressure fluid controlled replacements of olivine: (1) orthopyroxene prismatic crystal often with radial aggregation; (2) intergrowth of orthopyroxene and magnesite; (3) aggregate of Cr-rich chlorite and magnesite. Based on the textural observations, all the replacements were operated in the rock simultaneously at the beginning of exhumation stage thus implying that activities of CO2 and H2O in the COH fluid have been variable in a local scale (less than cm) likely due to the buffering of the fluid components by the mineral reactions. Growth of the orthopyroxene in expense of olivine might be explained by the high pressure reaction between olivine with silica (in fluid), whereas development of orthopyroxene-magnesite association is due to the reaction of olivine with CO2 fluid. The most retrogressed samples are represented by tremolite- chlorite schists thus indicating influx of Ca and Al by bearing hydrous fluid at LP/LT conditions. Complex petrotectonic evolution of the metaultramafic rocks and sources of COH fluid (in carbonate-free complex) will be discussed in scope of the development of subduction channel.