www.elsevier.com/locate/rgg Conditions of formation of the Atbashi Ridge eclogites (South Tien Shan) V.A. Simonov a, * , K.S. Sakiev b , N.I. Volkova a , S.I. Stupakov a , A.V. Travin a a Institute of Geology and Mineralogy, Siberian Branch of the RAS, 3 prosp. Akad. Koptyuga, Novosibirsk, 630090, Russia b Institute of Geology, National Academy of Sciences of the Kyrgyz Republic, 30 prosp. Erkindik, Bishkek, 720481, Kyrgyz Republic Received 3 October 2007; in revised form 29 January 2008; accepted 21 April 2008 Abstract 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 °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 ∼120 Myr. Keywords: Eclogites; PT-conditions; fluid inclusions; Ar/Ar dating; Atbashi Ridge; Tien Shan Introduction Eclogites of the Atbashi Ridge, localized in the Kyrgyz Republic on the northern margin of the South Tien Shan (Fig. 1), are part of a long high-pressure belt stretching westward up to the Urals (Bakirov et al., 1998; Dobretsov, 1974) and eastward into China (Gao and Klemd, 2003). The Tien Shan eclogite complexes marking the suture of the closure of the Paleozoic ocean basin resulted from the subsidence of basic rocks of ophiolite associations in subduc- tion zones. Later on, they were transported with tectonic melange and now bear traces of repeated transformations, including both progressive metamorphism and several phases of retrograde alterations (Bakirov, 1990). The formation of Tien Shan eclogites is of paramount researchers’ interest and is considered elsewhere (Bakirov, 1989, 1990; Bakirov et al., 1998; Dobretsov, 1974; Dobretsov et al., 1989; Dobretsov and Sobolev, 1970; Gao and Klemd, 2003; Shatsky and Usova, 1989; Sobolev et al., 1989a; Tagiri et al., 1995). Special attention is given to the following aspects Russian Geology and Geophysics 49 (2008) 803–815 * Corresponding author. E-mail address: simonov@uiggm.nsc.ru (V.A. Simonov) Fig. 1. Localization of ophiolite complexes in the Kyrgyz Republic territory. 1 — ophiolite and eclogite complexes of the Atbashi Ridge; 2 — other ophiolite complexes. Tien Shan: I — North, II — Central, III — South. doi:10.1016/j.rgg.2008.04.001 1068-7971/$ - see front matter D 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. © 2008, IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved.