Chemical Heterogeneity of Garnet in Eclogite-Blueschist Complexes

Abstract

V. V. Fedkin and L. S. Kotova

In mafic eclogite-blueschist complexes, the unique properties of garnet were studied - to preserve its composition under conditions of a changing metamorphism regime and to record these changes in the process of their development. In the key paragenesis with omphacite, plagioclase, and quartz, garnet, within the limited space of one sample and even one grain, is able to record multidirectional and opposite trends in changes in metamorphism parameters. The studies were carried out on two known complexes of mafic eclogites, similar in origin, but contrastingly different in terms of formation conditions: the Maksyutov (Southern Urals) and Atbashi (Southern Tien Shan) eclogite-blueschist complexes. For the deeper Maksyutov complex, a 4-stage RT trend of its retrograde development from the stability field of ultrahigh-pressure (SHB) minerals (diamond and coesite) to low-temperature diaphthoresis of the greenschist facies (T=870-310°C, P=3.5-0, 6 GPa). For the less deep Atbashi complex, a one-stage prograde-retrograde development trend was recorded within T=300-650°C and P=8-13 GPa. In parallel, in the same complexes, a study was carried out and the conditions for the formation of contrasting series of rock - high-pressure eclogite inclusions (of boudins, blocks, interlayers and lenses) in the strata of weakly metamorphosed metasedimentary rocks were identified. It has been established that the processes of tectonic mélange and coherent development of the host matrix are independent stages of a single process of metamorphic evolution of the complex. In the multi-stage history of the Maksyutov terrane, they are separated in time, but sometimes they repeat in the form of a cycle at different stages of development. In the one- act Atbashi eclogite-blueschist complex, both processes proceed sequentially in the same key of metamorphic evolution.

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