Conditions of formation of the Atbashi Ridge eclogites (South Tien Shan)

New data on the conditions and time of formation of eclogites in the Atbashi Ridge (South Tien Shan) were obtained. The PT-conditions of crystallization of high-pressure minerals from eclogites have been established: 23-25 kbar, 510-570 degrees C. Analysis of fluid inclusions showed that the metamorphic processes of the eclogite formation involved salt-aqueous fluids (up to 6-12 wt.% NaCl). Primary fluid inclusions were trapped by omphacite and garnet on their high-pressure (23-25 kbar) crystallization. The pressures determined from thermodynamic data on the inclusions (6-7.2 kbar) reflect the adaptation of the host-mineral matrix to the decrease in ambient parameters, which led to the fluid re-equilibration in the inclusions. The pseudosecondary inclusions in omphacite, quartz, and garnet reflect the conditions of regressive processes running on the ascent of eclogites at pressures of up to 4.5-6.2 kbar. The stepwise (40)Ar/(39)Ar dating showed that the high-pressure minerals from the Atbashi Ridge eclogites crystallized at 324-327 Ma. Data on trace and rare-earth elements indicate that the protoliths of two established types of eclogites (with characteristics of N-MORB and plateau basalts) formed synchronously as a result of the interaction of MOR magmatic systems with plateau-basaltic plume magmatism. These protoliths were basic paleo-oceanic complexes, whose fragments are localized as ophiolites in the Atbashi Ridge structures. The maximum time gap between the formation of ancient oceanic crust and the eclogite metamorphism in the subduction zones of the studied associations is similar to 120 Myr. (c) 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B. V. All rights reserved.