Abstract
The rocks of the Armovka Formation (the Fore Range zone, Greater Caucasus) have undergone low-grade metamorphism that partially erased information about initial rock formation conditions. We discovered high-pressure mineral inclusions such as omphacite, phengite, garnet, and paragonite enclosed by pyrite and chalcopyrite. Mineral inclusions in sulfides may provide important information about metamorphic pressure−temperature conditions because they are shielded by the host minerals and isolated from significant low-grade overprinting. Calculations performed on phengite inclusions using the phengite Si-content barometry indicate a pressure ranging from 1.7 ± 0.2 to 1.9 ± 0.2 GPa for temperature of 600 ± 40 °C. These data are consistent with estimations obtained for eclogite bodies embedded in rocks of the Armovka Formation. Geothermobarometry of the latest yielded conditions of 680 ± 40 °C and a minimum pressure of 1.6 ± 0.2 GPa to upper pressure boundary at 2.1 GPa. This fact allows us to assume that the metamorphic rocks of the Armovka Formation were immersed in the subduction zone to the conditions of the eclogite facies of metamorphism, forming a coherent subduction complex together with eclogites.
Highlights
A reliable determination of the thermodynamic parameters of metamorphic rocks processes, especially an estimation of their depth, is of high importance for identification of the Earth’s crust structure
Sulfides in the Armovka Formation rocks are mainly pyrite (Py, FeS2 ) and rare chalcopyrite (Ccp, CuFeS2 ). Chalcopyrite occurs both as grains in the matrix and as inclusions in pyrite
The petrological and electron microprobe study of sulfide-bearing gneisses and amphibolites of the Armovka Formation (Fore Range zone of the Greater Caucasus) show that pyrite and chalcopyrite can be considered as a new type of mineral container capable of preserving information about the initial mineral composition of rocks and their metamorphic evolution
Summary
A reliable determination of the thermodynamic parameters of metamorphic rocks processes, especially an estimation of their depth, is of high importance for identification of the Earth’s crust structure. It is generally accepted that the temperature conditions of metamorphism depend on the proximity and power of the heat source and can change much faster in the crust than pressure, which is determined, first of all, by the lithostatic pressure [1]. A sharp change in the P-parameters of metamorphic rocks is an evidence of tectonic disturbances such as large-scale nappes or thrust sheets in the crystalline crust. The task of determining pressure is far from simple, especially for complexes considered as of high pressure. Overprinting by retrograde processes can almost completely change the mineral assemblage of rocks, making difficult to recognize earlier metamorphic conditions or removing all information about peak conditions. Minerals stable in a wide range of Minerals 2019, 9, 701; doi:10.3390/min9110701 www.mdpi.com/journal/minerals
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