WHAT ZIRCON IN OPHIOLITIC PERIDOTITES AND CHROMITITES REVEALS ABOUT UPPER MANTLE PROCESSES

On the basis of their lithology, pseudostratigraphy and geochemistry, ophiolites are considered to be fragments of oceanic lithosphere emplaced on continental margins or in island arc and accretionary complexes. Thus, they should represent samples of oceanic crust and mantle uncontaminated by crustal minerals. Yet more than 15 ophiolites investigated by our research group, including those in the Yarlung Zangbo suture zone of Tibet, the Polar Urals of Russia, the Central Asian Orogenic Belt of North China and the Tethyan belt of the eastern Mediterranean, contain abundant crustal minerals, most notably zircon. The investigated ophiolites range in age from early-middle Paleozoic to Cretaceous and include those hosting either high-Cr or high-Al chromitites. The zircons range in shape from rounded or irregular to euhedral, and display a complex textures. All of the zircon grains analysed thus far have trace element compositions indicating a continental protolith and they contain inclusions of common crystal minerals. Ages of the analysed zircons range from Archean to Cretaceous. In a few cases (Sartohay ophiolite of China and Ray-Iz ophiolite of Russia), the ages of some old zircons have been modified by post-emplacement hydrothermal activity.

The abundance of zircon and other crustal minerals in oceanic mantle rocks can only be explained by subduction of continental rocks or continentally derived, clastic sediments. Such subduction is a well-established process; however, it is unlikely that crustal minerals would survive deep subduction to the mantle transition zone. Rather we propose that these minerals are mixed with deep mantle material in suprasubduction zone mantle wedges, where they are incorporated into the peridotites and crystallizing chromitites. Any crustal minerals that survive subduction to the mantle transition zone may be incorporated into chromite grains crystallizing at that depth, along with UHP minerals, such as diamond. We suggest that podiform chromitites, particularly those composed of high-Cr magnesiochromite, are formed or modified in suprasubduction zone environments. Thus, we distinguish between chromite grains crystallized at depth in the upper mantle and podiform chromitites formed in shallow SSZ environments.