Four Stages of the Thermal Evolution of Eclogites from the Maksyutov Complex (South Urals)

Abstract

Abstract —Based on a detailed electron probe microanalysis of the composition and zoning of coexisting minerals (garnet, clinopyroxene, and plagioclase), we have studied the P–T conditions of formation of high-pressure (HP/UHP) rocks of the Maksyutov eclogite–blueschist complex in the South Urals. We have established its periodic evolution and have determined the specific thermodynamic parameters of mineral formation at each stage of its geodynamic history, from the conditions of crystallization of the protolith to the final stages of retrograde greenschist metamorphism. The new analytical data on the composition of coexisting phases confirm the high-pressure formation of eclogites present as numerous lenses, boudins, and interlayers among blueschist and feldspar–mica schists in the lower part of the complex. Thermobarometric calculations of metamorphism parameters were performed for the Grt + Cpx ± Pl + Qz paragenesis using a Grt–Cpx geothermometer and a Pl–Cpx–Qz geobarometer. Garnet in eclogites of the Maksyutov Complex, being conservative to changes in the P–T conditions, has a direct, reverse, or inverse chemical zoning. Under equilibrium of garnet with omphacitic clinopyroxene, this zoning records conjugate progressive and regressive P–T paths reflecting the thermodynamic conditions at the certain stages of the terrane evolution. Based on the data obtained, we have recognized at least four P–T stages of progressive metamorphic transformations of the Maksyutov Complex: (1) >800–910 °C/~2.5–3.5 GPa; (2) 540–790 °C/2.0–3.5 GPa; (3) 410–690 °C/1.1–2.5 GPa; and (4) 310–520 °C/1.0–1.2 GPa. The estimated P–T parameters of the conjugate regressive stages of metamorphism are as follows: (1) 870–625 °C/3.5–2.5 GPa; (2) 745–615 °C/3.5–2.0 GPa; (3) 690–550 °C/1.5–1.0 GPa; and (4) 590–460 °C/1.2–0.6 GPa, respectively. The age data for the certain stages, along with the parameters of metamorphism, form a single P–T–t path of the complex, which determines the position of the gradient of the metamorphic field during the complex exhumation.