This study reports a new rodingite type which was derived from eclogite enclosed in the ultramafic rocks of Changawuzi ophiolites, western Tianshan, China. Based on petrographical investigations, rodingite, partial rodingitized rock and completely rodingitized rock are characterized in this paper. These rocks show a continuous variation in their bulk compositions, mineralogy and their textural properties from eclogite to rodingite. The completely rodingitized rocks can be further divided into prehnite rodingites, hydrogrossular-diopside rodingite and vesuvianite rodingites on the basis of the mineral assemblage and textural character. The chemical potential path of l(SiO 2 )-l(CaO/MgO) can be used to constrain the evolution of two stages of rodingitization. The first rodingitization possibly started under conditions of 410-430°C and 7-9 kbar at upper greenschist facies, and resulted from a secondary serpentinization during exhumation of the subducted slab. A second and pervasive rodingitization took place under conditions of 250-350°C and 2-10 kbar from greenschist to subgreenschist facies. The P-T path presented shows a retrograde evolution from eclogite to rodingite. We conclude that the process of rodingitization may also take place under subduction zone conditions in addition to its more common occurrence under ocean-floor metamorphic conditions.

Geochemical data reveal the contrasting geochemical characteristics of the metasediments in the Jiepailiang and Bizigou formations of the Palaeoproterozoic Zhongtiao Group, North China Craton. High Zr/Sc and SiO 2 /Al 2 O 3 values and especially the evolved and uniform nature of ε Nd ( t ) values plus detrital zircon U–Pb ages and Hf isotopic signatures for the former are consistent with derivation from an evolved felsic source with sedimentary recycling signs. However, lower values of SiO 2 /Al 2 O 3 and Zr/Sc, and higher contents of Fe 2 O 3 * and MgO for the latter indicate input of more mafic components. These geochemical characteristics, combined with the wide range of ε Nd ( t ) values, suggest that these rocks were derived in part from old continental material, dominated by felsic rocks of magmatic origin, and in part from a continental island arc, which accounts for the variable mixture of mafic components. These results suggest that the Zhongtiao Group was deposited after c . 2110 Ma, in a back-arc basin, behind an eastward-directed subduction system. Subsequent deformation and metamorphism reflect closure of the oceanic basin between the Eastern and Western blocks of the North China Craton, along the Trans-North China Orogen, contributing to amalgamation of the supercontinental Columbia. Supplementary material: Data are available at http://www.geolsoc.org.uk/SUP18478 .