Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle S. G. SONG 1,2 , L. F. ZHANG 1 , Y. NIU 2 , C. J. WEI 1 , J. G. LIOU 3 AND G. M. SHU 1 1 MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China (sgsong@pku.edu.cn, shuguang.song@durham.ac.uk) 2 Department of Earth Sciences, Durham University, Durham DH1 3LE, UK 3 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA ABSTRACT Low-temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high- pressure low-temperature (HPLT) subduction complex (me´lange) in the North Qilian oceanic-type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75– 1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite– phengite–glaucophane schist and various chloritoid-bearing schists. Mg-carpholite was identified in some high-Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp– Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg-carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi-pelagic sediments have been subducted to the mantle depths (‡75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction-zone gradients (6–7 °C km )1 ), and this cold subduction brought a large amount of H 2 O to the deep mantle in the Early Palaeozoic times. Key words: eclogite; eclogite facies metapelite; lawsonite and carpholite; North Qilian suture zone; oceanic subduction and exhumation. INTRODUCTION High-pressure (HP) low-temperature (LT) rocks inclu- ding blueschist and eclogite in Pacific-type subduction complexes recorded a complete process of ancient oceanic lithosphere consumption and closure at an active convergent margin. These rocks are thus consid- ered to be the key markers for a palaeosuture zone within orogenic belts and have received a great deal of attention from geoscientists for their tectonic signifi- cance (e.g. Ernst, 1970, 1988; Ernst & Liou, 1995; Maruyama et al., 1996). Differences in protoliths and in metamorphic assemblages between oceanic-type sub- duction zones and continental-type ultra-high-pressure (UHP) metamorphic belts enable us to unravel the evolution of a subduction zone from oceanic subduction to continental collision (e.g. Chopin et al., 1991; Ernst & Liou, 1995; Guillot et al., 1997; Agard et al., 2001; Ernst, 2001; Song et al., 2006). The North Qilian Mountains are considered to be a typical Pacific-type subduction zone in China (e.g. Xiao et al., 1978; Wu et al., 1993) as it comprises ophiolite suites, calc-alkaline volcanic and granitic rocks, and subduction-zone complexes including HPLT rocks and me´lange. Recent studies revealed that this belt is an Early Palaeozoic suture zone (c. 460 Ma) between the North China Craton (NCC) and the Qilian–Qaidam microcontinent, a fragment of the Rodinia superconti- nent (Wan et al., 2001; Song et al., 2006). Wu et al. (1993) reported occurrences of high-grade blueschist with the assemblage Grt + Gln + Ep + Phn + Ab + Qtz and a low-grade blueschist characterized by Gln + Lws + Pmp + Ab + Chl + Arg + Qtz in two distinct areas (Fig. 1a), and gave P–T estimates of 340 °C and 0.8 ± 0.1 GPa for eclogite lenses within high-grade blueschists. Mineral abbreviations are after Kretz (1983). In this paper, we report a new discovery of Mg- carpholite + Mg-chloritoid-bearing metapelite and document in detail petrological characteristics and their metamorphic P–T conditions for various proto- liths of the subduction complex in the high-grade J. metamorphic Geol., 2007, 25, 547–563 doi:10.1111/j.1525-1314.2007.00713.x Ó 2007 Blackwell Publishing Ltd 547