Metamorphic Evolution of Late Precambrian Eclogites and Associated Metabasites, Gorny Altai, Southern Russia

Abstract

In the Vendian-Cambrian orogenic belt of the Chagan-Uzun and Kurai areas, Gorny Altai, high-pressure, low-temperature (HP/LT) metabasites and associated schists comprise a metamorphic complex less than 2 km thick. The Chagan-Uzun metabasites occur as lenticular intercalations in a recrystallized serpentinite. They include eclogite, garnet-amphibolite, amphibolite, and lower-grade schist; the first two contain glaucophane in the strict sense. The Kurai metabasites occur as a coherent body with intercalations of calcareous and siliceous schists. They are divided into four mineral zones: actinolite, hornblende, garnet-barroisite, and garnet-hornblende, based on mineral assemblages. Their metamorphic grade corresponds to greenschist to epidote-amphibolite facies, and decreases from the garnet-hornblende zone in the central part of the body to the actinolite zone in the western and eastern margins through the garnet-barroisite and hornblende zones. Estimated peak P-T conditions of the Altai metabasites from the Chagan-Uzun and Kurai areas range from 300°C at 0.4 GPa to 660°C at 2.0 GPa with successive P-T increases. On a P-T diagram, the Altai metamorphic facies series define a single curve convex toward the T axis. The Chagan-Uzun eclogites record prograde and retrograde histories, tracing a hairpin-like P-T path, which is characteristic of Mesozoic HP/LT metamorphic rocks in Pacific-type orogens. Such a P-T evolution suggests that the Altai HP/LT metamorphism occurred in conditions in which hot lithosphere subducted beneath a cool mantle wedge, or subducted lithosphere changed from hot to cold. Hot lithosphere would have been common in the Precambrian Earth with a younger average age of the subducting plate and a thicker oceanic crust, reflecting mantle activities at that time. However, the P-T evolution of the Altai metabasites indicates that the Late Precambrian Altai subduction-zone geotherm was low enough to form HP/LT rocks similar to Mesozoic analogues.