Zonal Garnet from Eclogite-Blueschist Complexes - Recorder of the History of their Development

Abstract

Valentin Fedkin

Garnet has a unique ability to maintain its composition in the situation of changing conditions of metamorphism and fix the chemical changes in the new grains, outgrowths and replacements associated with the evolution of mineral formation parameters. These properties and capabilities of garnet were studied by the phase correspondence and mineralogical thermobarometry methods using the example of the Maksyutov (Southern Urals) and the Atbashi (South Tien Shan) complexes of crustal mafic eclogites, where they are used as PT indicators of their formation conditions. The issue is considered against the background of solving a specific petrological problem - the origin in eclogite-blueschist complexes (EBSK) of contrasting series of rocks - high-pressure eclogite blocks, boudins and interlayers in the thickness of a weakly metamorphosed gneiss-schist matrix. In the polymetamorphic Maksyutov complex, the garnet demonstrates complex multidirectional, sometimes inversion zoning, which reflects the features of mineral formation at each stage of its development. The corresponding PT trends bear signs of the determining processes of the formation of the complex - tectonic melange and coherent development. In the one-stage Atbashi eclogite-blueschist complex, inversion PT trends in the evolution of metamorphism are noted with a turn in their direction clockwise, indicating a transition from a tectonic melange setting to a coherent stage of the terrane development.

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