The origin and tectonic setting of ophiolites in China

Abstract

Ophiolites are key features for unravelling the geologic history of orogenic belts, especially in China, which was formed by amalgamation of numerous tectonic blocks from Precambrian time to the present. Although much has been learned about ophiolites over the last 30 years, questions remain as to their definition, composition, classification, origin and tectonic significance. Most ophiolites, particularly those in China are incomplete bodies composed of serpentinized peridotite, minor cumulate rocks and sparse pillow lavas. Sheeted dikes are rare in most ophiolites and the structural relationships between the different lithologies are commonly unclear. Such bodies should be considered 'possible' ophiolites unless the lithologies can be clearly linked structurally or compositionally. The former classifications of ophiolites into Cordilleran- or Tethyan-types and Harzburgite- or Lherzolite-types have been superseded by a more process-oriented classification proposed by Dilek (2003) that relates them to different magmatic and tectonic processes. This classification makes it easier to reconstruct paleoenvironments and processes in the geologic record. Although ophiolites were originally interpreted as fragments of normal ocean lithosphere, most such bodies have been shown to contain clear suprasubduction zone geochemical fingerprints. Some ophiolites consist entirely of suprasubduction zone components whereas others are compound bodies formed originally at mid-ocean spreading centers and then modified by later suprasubduction zone melts. Percolation of suprasubduction zone melts through the overlying mantle wedge can produce dunite pods and dikes, commonly associated with podiform chromitites, and may be responsible for the formation of transitional zone dunites. Ophiolite emplacement is thought to require subduction of lower density material beneath the body, a process that is easy to explain in subduction zone environments. Emplacement can occur either during subduction rollback or by final closure of the ocean basin in which the ophiolite formed. Much new work on Chinese ophiolites is currently underway and new interpretations of these bodies are being developed. This Special Issue presents the results of recent studies of ophiolites from most of the major orogenic belts in China. © 2007 Elsevier Ltd. All rights reserved.