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Abstract

The Maksyutov complex (Southern Urals, Russia) is a well-preserved example of subduction-related high pressure metamorphism. One of its two litho-tectonic units consists of rocks which experienced eclogite facies conditions. Published 40Ar/39Ar data on phengite, U/Pb data on rutile and Sm/Nd mineral data define a cluster of ages around 370 to 380 Ma. Nevertheless, no consensus exists as to the detailed interpretation of data and the exact age of eclogitization. We present new, high-precision internal mineral Rb/Sr isochrons for eclogite-facies metabasites, felsic eclogites and eclogite-facies quartz veins. Nine isochrons, mainly controlled by omphacite and white mica phases, give concordant ages with an average value of 375 ¡Ó 2 Ma (2ƒã). Microtextural features, like prograde growth zoning in eclogite-facies phases, suggest that the assemblages dated formed at a stage of prograde metamorphism. Sr-isotopic equilibria among eclogite-facies phases, and among eclogite facies fluid veins and the host rocks, indicate that our ages reflect crystallization ages, related to the prograde-metamorphic, probably fluid-mediated eclogitization reactions. This interpretation is reinforced by data from fluid-precipitated quartzitic eclogites, whose modal composition, together with intergrowth relationships, conclusively imply closed-system behavior after crystallization. The possible occurrence of a pre-375 Ma event of ultra-high pressure metamorphism in the Maksyutov complex is disproved by isotope systematics, microtextures and mineral zoning patterns.